Loudspeaker module and system

Abstract

There is provided a loudspeaker module for producing sound at bass frequencies, including: a loudspeaker frame; a diaphragm suspended from the loudspeaker frame; a drive unit to move the diaphragm along a movement axis of the loudspeaker module to produce sound from a first radiating surface and a second radiating surface of the diaphragm; the first and second radiating surfaces on opposite sides of the diaphragm; wherein the drive unit includes a magnet unit to generate magnetic flux across an air gap and a voice coil to be located in the air gap when the diaphragm is rest; and an enclosure to receive sound produced by the second radiating surface of the diaphragm; wherein the loudspeaker module has a total quality factor Qtc of at least 1.8 when the diaphragm is at rest; and wherein a magnetic flux density at a first perimeter of the voice coil is 50% or less of a magnetic flux density at a second perimeter of the voice coil when the diaphragm is at rest.

Description

[0001] LOUDSPEAKER MODULE AND SYSTEM

[0002] This application claims priority to GB2314915.6, filed 28 September 2023.

[0003] Field of the Invention

[0004] The present invention relates to a loudspeaker module for producing sound at bass frequencies and a loudspeaker system including the loudspeaker module.

[0005] Background

[0006] A conventional loudspeaker typically includes a diaphragm moveable by an electrodynamic drive unit. The drive unit includes a magnet unit and a voice coil which in use magnetically interact to cause movement of the diaphragm, which acts as a piston, to produce sound.

[0007] To produce optimised sound quality, loudspeakers are carefully designed to meet certain design parameters, including the damping of movement of diaphragm. Such damping is caused by the mechanical suspension of the acoustic components and the counter-electromotive force caused by the electrodynamic drive unit. Moreover, for a loudspeaker in an enclosure, the acoustic suspension of the air enclosed behind the diaphragm also contributes to the damping and affects the response of the loudspeaker.

[0008] Damping of a loudspeaker in an enclosure is quantified using the so-called total quality factor Qtc. The total quality factor of a loudspeaker in its enclosure, Qtc, is a dimensionless parameter that is defined as the ratio of initial energy divided by lost energy per cycle. As such, higher dissipation of energy of movement means a lower quality factor Qtc, while less dissipation of energy means a larger total quality factor Qtc. Consensus in the art is that a total quality factor Qtc with a value in a range of 0.5 to 1 .2 is generally acceptable, with a value of 0.707 regarded as particularly desirable by at least some in the art since 0.707 results in the flattest roll-off. By contrast, a total quality factor Qtc outside of the range of 0.5 to 1.2 is considered undesirable. Notably, a value greater than 1.2 results in ‘ringing’ at resonance, which means that the duration for which the diaphragm continues moving is substantial. For low-frequency reproduction the ‘ringing’ is known as a ‘booming’ bass.

[0009] Increasing the signal level, i.e. driving the loudspeaker at higher output volumes (corresponding to larger displacements of the drive unit), causes the total quality factor Qtc to increase also. As such, it is important to consider the total quality factor Qtc for a large signal to avoid degradation of the audio reproduction at high output volume, such as the aforementioned ‘ringing’ at resonance. A typical aim is to provide ‘tight’ bass reproduction.

[0010] The present invention has been devised in light of the above considerations. Summary of the Invention

[0011] The present inventors observed that a loudspeaker module with an electrodynamic drive unit is operable to compensate a comparatively large total quality factor Qtc provided that the magnetic flux density in the air gap drops comparatively sharply across the voice coil.

[0012] According to the invention, there is provided a loudspeaker module for producing sound at bass frequencies, the loudspeaker module including: a loudspeaker frame; a diaphragm suspended from the loudspeaker frame; a drive unit configured to move the diaphragm along a movement axis of the loudspeaker module to produce sound from a first radiating surface and a second radiating surface of the diaphragm, wherein the first radiating surface and the second radiating surface are on opposite sides of the diaphragm; wherein the drive unit includes a magnet unit to generate magnetic flux across an air gap and a voice coil configured to be located in the air gap when the diaphragm is rest; and an enclosure arranged to receive sound produced by the second radiating surface of the diaphragm; wherein the loudspeaker module has a total quality factor Qtc of at least 1 .8 when the diaphragm is at rest; and wherein a magnetic flux density at a first perimeter of the voice coil is 50 percent or less of a magnetic flux density at a second perimeter of the voice coil when the diaphragm is at rest.

[0013] In combination, the large total quality factor Qtc and the sharp drop of the magnetic flux density of at least 50 percent across the voice coil provide a loudspeaker module that is operable to conveniently control at least some sound characteristics, for example to achieve a tighter bass or a warmer bass (e.g. according to user preference), by suitably adjusting an input signal fed to the drive unit of the loudspeaker module, for example by using suitable feed-forward filtering. Furthermore, the comparatively large total quality factor Qtc means that the loudspeaker module is “underdamped” and thus that there is less resistance to operating the loudspeaker module than in a conventional loudspeaker. Hence, less power is required to move the diaphragm for a given output volume than would be required for a lower total quality factor Qtc.

[0014] The loudspeaker module may be operable to energise the drive unit to cause movement of the diaphragm relative to the loudspeaker frame along the movement axis of the loudspeaker module to produce sound.

[0015] The diaphragm may be moveable along the movement axis to a position of nominal maximum displacement. Nominal maximum displacement may correspond to an intended maximum output volume. In some examples, nominal maximum displacement may be a position along the movement axis where the force factor BL is half of the force factor BL when the diaphragm is at rest in the air gap. The force factor BL is defined as the product of the magnetic flux density B permeating the voice coil and the total length L of windings of the voice coil and quantifies the strength of the magnetic interaction between the magnet unit and the voice coil.

[0016] The drive unit may include a moveable part attached to the diaphragm and a stationary part attached to the loudspeaker frame. The loudspeaker module may be operable to cause the moveable part and the stationary part to magnetically cooperate to cause movement of the moveable part along the movement axis. A moveable assembly, which may include the moveable part of the drive unit and the diaphragm, may thus be caused to move along the movement axis of the loudspeaker module to produce sound. The moveable assembly may also be referred to as a “moving” assembly.

[0017] The terms “stationary” and “moveable” are relative terms and in principle dependent on the particular frame of reference. In this context, the terms “stationary” and “moveable” are intended to refer to a conventional frame of reference according to which the diaphragm is considered to be moveable, and in use to be moving, whereas other parts of the loudspeaker module are considered to be stationary. As such those parts of the loudspeaker module which in use move with the diaphragm, such as the moveable part of the drive unit, are referred to as moveable, while other parts of the loudspeaker module, which in use do not move with the diaphragm (and which may be stationary with respect to an external apparatus to which the loudspeaker module is attached), are referred to as stationary, e.g. the stationary part of the drive unit. Those parts of the loudspeaker module which are moveable, i.e. the diaphragm and parts that in use move with the diaphragm, may collectively be referred to as the moveable assembly.

[0018] The magnet unit of the drive unit may be included in one of the stationary part and the moveable part, and the voice coil may be included in the other of the stationary part and the moveable part. When the drive unit is energised (e.g. by supplying a time-varying electric current to the voice coil), the moveable part of the drive unit and the stationary part of the drive unit may magnetically interact to cause the movement of the moveable part relative to the stationary part.

[0019] In some examples (referred to herein as “moveable voice coil” examples), the moveable part of the drive unit may be the voice coil and the stationary part of the drive unit may be the magnet unit. In such examples, when the drive unit is energised, magnetic flux generated by the flow of electric current in the (moveable) voice coil may interact with magnetic flux generated by the (stationary) magnet unit of the drive unit to cause the movement of the moveable part, which is the voice coil, relative to the stationary part, which is the magnet unit.

[0020] In other examples (referred to herein as “moveable magnet unit” examples), the moveable part of the drive unit may be the magnet unit and the stationary part of the drive unit may be the voice coil. In such examples, when the drive unit is energised, magnetic flux generated by the flow of electric current in the (stationary) voice coil may interact with magnetic flux generated by the (moveable) magnet unit of the drive unit to cause the movement of the moveable part, which is the magnet unit, relative to the stationary part, which is the voice coil.

[0021] The magnetic flux density may be a radial magnetic flux density. The radial magnetic flux density may be measured in a radial direction, which is perpendicular to the movement axis.

[0022] A distance between the inner perimeter and outer perimeter of the voice coil measured in the radial direction may be taken as a winding thickness of the voice coil. Accordingly, the magnetic flux density permeating the voice coil may drop by at least 50 percent over the winding thickness of the voice coil.

[0023] If the voice coil is substantially circular, the inner perimeter of the voice coil may be referred to as the inner diameter of the voice coil, and the outer perimeter of the voice coil may be referred to as the outer diameter of the voice coil. As set out above, the total quality factor Qtc is a parameter known to the skilled person and as such the Qtc value of a loudspeaker module when the diaphragm is at rest is determinable by the skilled person using known means. For example, the total quality factor Qtc can be determined through computer simulation of the loudspeaker module or based on physical measurement performed on the loudspeaker module (see, for example, R.H. Small, “Closed-Box Loudspeaker Systems, Part I: Analysis”, J. Audio Eng. Soc., vol. 20, pp. 798-808 (1972 Dec.)). Physical measurement of the total quality factor Qtc may involve, for example, a distortion analyser or an optical setup for measurement of the position of the diaphragm along the movement axis when the loudspeaker module is being operated. See, for example, W. Klippel, U. Seidel, "Fast and Accurate Measurement of Linear Transducer Parameters," presented at the 110th Convention of the Audio Engineering Society, Amsterdam, May 12-15, 2001 , preprint 5308; or W. Klippel, “Distortion Analyzer - a New Tool for Assessing and Improving Electrodynamic Transducers,” presented at the 108th Convention of the Audio Engineering Society, Paris, February 19-22, 2000, preprint 5109.

[0024] The total quality factor Qtc when the diaphragm is at rest is a small signal parameter. A small signal parameter is a concept known in the art, describing an operational parameter at small input signal (see, for example, V. Dickason, Loudspeaker Design Cookbook, 7th Edition, KCP Media Corp., 2006; R.H. Small, “Closed-Box Loudspeaker Systems, Part I: Analysis”, J. Audio Eng. Soc., vol. 20, pp. 798-808 (1972 Dec.)). Accordingly, physical measurement of the total quality factor Qtc when the diaphragm is at rest may be carried out when operating the loudspeaker module at small voltage such that small displacement of the diaphragm is caused, or any other suitable means known to the skilled person.

[0025] The total quality factor Qtc of the loudspeaker module may be at least 2.0, optionally at least 2.5, when the diaphragm is at rest.

[0026] The total quality factor Qtc of the loudspeaker module may be up to 10, optionally up to 5, when the diaphragm is at rest.

[0027] The total quality factor Qtc may have a first value when the diaphragm is at rest and a second value when the diaphragm is at a nominal maximum displacement. The second value may be up to 3 times the first value; optionally up to 2.5 times the first value; optionally up to 2 times the first value; optionally up to 1 .5 times the first value, and optionally up to 1 .25 times the first value. The second value may be at least 1 .00 times the first value.

[0028] With the total quality factor Qtc at nominal maximum displacement being up to three times the total quality factor Qtc at rest, distortion across output volumes, and especially at high output volumes, may be reduced compared to a conventional loudspeaker module. For a conventional loudspeaker module, the total quality factor Qtc of the conventional loudspeaker module may increase significantly with the output volume. For example, the total quality factor Qtc of a conventional loudspeaker at nominal maximum displacement may be seven times the total quality factor Qtc at rest. Accordingly, for the conventional loudspeaker significant distortion of sound reproduction may occur across output volumes and especially at high output volume. Qts is used to indicate the total quality factor of a loudspeaker without its enclosure (as opposed to Qtc, which is used to indicate the total quality factor of a loudspeaker with its enclosure),

[0029] The total quality factor Qtc of the loudspeaker module is known to be proportional to the total quality factor Qts of the loudspeaker, i.e. Qtcoc Qts. The total quality factor Qts of the loudspeaker is known to be dependent on an electrical quality factor Qes (associated with the drive unit) and a mechanical quality factor Qms (associated with suspension elements). More particularly, Qts is equal to the product of Qes and Qms divided by the sum of Qes and Qms, i.e. Qts= Qes■ Qms / Qes+ Qms).

[0030] The electrical quality factor Qms of the drive unit may be at least 3, optionally at least 4, when the diaphragm is at rest in the air gap.

[0031] The electrical quality factor Qms of the drive unit may be up to 8, optionally up to 6, when the diaphragm is at rest in the air gap.

[0032] The electrodynamic quality factor Qes of the drive unit may be at least 1 .2, optionally at least 2.0, when the diaphragm is at rest in the air gap.

[0033] The electrodynamic quality factor Qes of the drive unit may be up to 2.5, optionally up to 2.0, when the diaphragm is at rest in the air gap.

[0034] The loudspeaker module may have a free-air resonant frequency fs which has a first resonant frequency value when the diaphragm is at rest and a second resonant frequency value when the diaphragm is at nominal maximum displacement. The second resonant frequency value may be up to 1 .20 times the first resonant frequency value; optionally up to 1.10 times the first resonant frequency value. The second resonant frequency value may be at least 1 .00 times the first resonant frequency value.

[0035] The free-air resonant frequency fs of the loudspeaker module may be taken to be the resonant frequency of the loudspeaker module when the enclosure is omitted / taken away.

[0036] It is known that the free-air resonant frequency fs of a loudspeaker may not be constant across output volumes and may instead be higher at a higher output volume. The free-air resonant frequency fs when the diaphragm is at rest is a small signal parameter. As mentioned above, small signal parameters are a concept known in the art, and a small signal parameter may be measured when operating the loudspeaker using a small voltage, i.e., at small output volume. For more information see, for example, V. Dickason, Loudspeaker Design Cookbook, 7th Edition, KCP Media Corp., 2006.

[0037] Accordingly, the free-air resonant frequency fs at nominal maximum displacement may be up to 20 percent higher than the resonant frequency fs at rest (for measurement of the free-air resonant frequency at nominal maximum displacement see, for example, W. Klippel, “Nonlinear Large-Signal Behavior of Electrodynamic Loudspeakers at Low Frequencies,” J. Audio Eng. Soc. , vol. 40, pp. 483-496 (1992) ). By contrast, for a conventional loudspeaker the free-air resonant frequency fs may increase by as much as 50 percent. Thus, the present loudspeaker may provide a free-air resonant frequency fs which is comparatively constant across the normal working range of the loudspeaker and, correspondingly, of the loudspeaker module. A free-air resonant frequency fs which is comparatively constant across the normal working range of the loudspeaker helps to improve audio reproduction across output volumes and may provide for reduced power consumption, since the resonant frequency fs of the present loudspeaker module may more closely match the spectral power density of audio contents across output volumes.

[0038] The skilled person will appreciate that the choice of suspension may affect the extent to which the resonant frequency fs of a loudspeaker changes over the normal working range of the loudspeaker. Moreover, the skilled person will be able to select an appropriate choice of suspension that does not introduce a significant change to the resonant frequency fs. For examples of suitable suspension, see GB2212935.7 and PCT / EP2023 / 073013.

[0039] The loudspeaker may have a free-air resonant frequency fs in a range of 20-100Hz (Hertz), optionally 40- 80Hz when the diaphragm is at rest.

[0040] The resonant frequency fc of the loudspeaker module, which is also referred to as the in-box resonant frequency (as it is the resonant frequency when the enclosure is present), is known to be proportional to the free-air resonant frequency fs of the loudspeaker, i.e. fc<x fs. The resonant frequency of the loudspeaker module fc is higher than the free-air resonant frequency fs.

[0041] The resonant frequency fc of the loudspeaker module may have a first resonant frequency value when the diaphragm is at rest and a second resonant frequency value when the diaphragm is at nominal maximum displacement. The second resonant frequency value may be up to 1 .20 times the first resonant frequency value; optionally up to 1.10 times the first resonant frequency value. The second resonant frequency value may be at least 1 .00 times the first resonant frequency value.

[0042] The resonant frequency fc of the loudspeaker module may be in a range of 28-160Hz (Hertz), optionally 30-100Hz, optionally 40-80Hz.

[0043] The loudspeaker module, which may be a subwoofer, is configured to produce sound at bass frequencies. The bass frequencies may be included in a bass frequency range which may include 60- 80Hz, more preferably include 40-100Hz. By way of example, the bass frequency range may be 20Hz- 100Hz.

[0044] The loudspeaker module may have a force factor BL which has a first BL value when the diaphragm is at rest and a second BL value when the diaphragm is at nominal maximum displacement. The second BL value may be at least 0.5 times the first BL value.

[0045] The force factor BL at maximum nominal displacement being at least 0.5 times the force factor BL at rest means that the motor force drops to no less than 50 percent of the value at rest. As such, sufficient control by the drive unit over the movement of the diaphragm may be ensured even at nominal maximum displacement, whereas in a conventional loudspeaker module the diaphragm may move to a position where the drive unit may not sufficiently control the movement of the diaphragm and control thereover is predominantly by the suspension.

[0046] Also, the force factor BL dropping to no less than 50 percent of the value at rest means that the magnetic flux density is comparatively uniform, since in a conventional loudspeaker the force factor BL may drop to 15 percent. The comparatively uniform magnetic flux density ensures that the electrical quality factor is comparatively constant across output volumes when compared to a conventional loudspeaker.

[0047] The loudspeaker module may have a box stiffness Kb. The box stiffness Kb may be calculatable as: rz _ P° „2 c2 ^■b —TZcVB

[0048] Here, p0is the density of air; c is the speed of sound in air; Sdis the effective radiating area of the loudspeaker; and VBis the acoustic volume enclosed by the enclosure.

[0049] The loudspeaker module may have a driver stiffness Kms. The driver stiffness Kms is a known parameter which quantifies the mechanical stiffness of the suspension of the loudspeaker based on the Thiele-Small parameter measurements of the loudspeaker, as described in R.H. Small, “Closed-Box Loudspeaker Systems, Part I: Analysis”, J. Audio Eng. Soc., vol. 20, pp. 798-808 (1972 December). The driver stiffness Kms may be measured using a distortion analyser (see, for example, W. Klippel, “Distortion Analyzer - a New Tool for Assessing and Improving Electrodynamic Transducers,” presented at the 108th Convention of the Audio Engineering Society, Paris, February 19-22, 2000, preprint 5109). A ratio of driver stiffness Kms over box stiffness Kb is known to be used for quantifying a stiffness acting on the moveable assembly. Where the box stiffness Kb is larger than the driver stiffness Kms, i.e. the ratio of driver stiffness Kms over box stiffness Kb is less than 0.5, the box stiffness Kb is considered to be dominant.

[0050] The ratio of driver stiffness Kms over box stiffness Kb may be at least 0.25 and may be up to 0.75, and optionally up to 0.5. Thus, the driver stiffness Kms may be about a quarter to three quarter (optionally half) of the box stiffness Kb provided by the air volume acting on the diaphragm Sd.

[0051] The mass of the moveable part of the drive unit may be at least 60%, more preferably 70%, of the mass of the moveable assembly. That is to say, the moveable part of the drive unit may have a first mass, the moveable assembly may have a second mass, and the first mass may be at least 60% (preferably 70%) of the second mass.

[0052] In this context, the moveable part of the drive unit may be taken to include only the parts of the moveable assembly which magnetically interact with the stationary part of the drive unit, when the drive unit is energised. Thus, in a “moveable voice coil” example (see above), the moveable part of the drive unit may include only the voice coil (and may in some examples be taken as including only the metal wire of the voice coil). Similarly, in a “moveable magnet unit” example (see above), the moveable part of the drive unit may include only the magnet unit (e.g. only a permanent magnet and flux guiding elements of the magnet unit).

[0053] Having the mass of the moveable part of the drive unit be at least 60%, more preferably 70%, of the mass of the moveable assembly, may mean that an unusually large proportion of the mass of the moveable assembly is due to the moveable part of the drive unit.

[0054] The present inventors have observed that forces due to a comparatively large mass of the moveable assembly can be absorbed by mounting the loudspeaker module to a heavy body (e.g. 100kg+) or cancelled where a pair of loudspeaker modules is mounted in back-to-back configuration, leading to good sound performance. A ratio of the mass of the moveable assembly to the effective radiating area Sd of the diaphragm may be at least 5 kilograms per square-metre, optionally at least 7 kilograms per squaremetre. The ratio of the mass of the moveable assembly to the effective radiating area Sd of the diaphragm may be at most 20 kilograms per square-metre, optionally at most 15 kilograms per squaremetre, optionally at most 12 kilograms per square-metre.

[0055] For example, the ratio of the mass of the moveable assembly to the effective radiating area Sd of the diaphragm may be in a range of 5 to 20 kilograms per square-metre, optionally in a range of 5 to 15 kilograms per square-metre, optionally in a range of 7 to 12 kilograms per square-metre.

[0056] Such a ratio, particularly when combined with the mass of the moveable part of the drive unit being at least 60%, more preferably 70%, of the mass of the moveable assembly, may represent a compact yet heavy moveable assembly. Such a moveable assembly may in use generate large forces, but as noted above, the present inventor has observed that such forces can be absorbed by mounting the loudspeaker module to a heavy body (e.g. 100kg+) or cancelled where a pair of loudspeaker modules is mounted in back-to-back configuration, leading to good sound performance (despite the significant forces).

[0057] The effective radiating area Sd is a concept known in the art. For a round diaphragm, the effective radiating area Sd is quantified using the half-roll-to-half-roll diameter and hence the suspension contributes to the effective radiating area Sd. Particularly for a loudspeaker with a comparatively small diaphragm, the contribution of the suspension to the effective radiating area Sd may not be negligible. For a diaphragm having a circular perimeter which is suspended from the loudspeaker frame by a roll suspension having an outer diameter do (“d_o”) and an inner diameter di (“dj”), the effective radiating

[0058] Zd\2area of that diaphragm may be estimated as Sd = it I -) , (“Sd equals pi times the square of d over the square of 2”) where d is the half-diameter of the roll suspension, (do + d) / 2, (“d_o dj over 2”).

[0059] In other examples, the effective radiating area of the diaphragm Sd may be measured using known techniques, see e.g. “Dynamical Measurement of the Effective Radiating area SD”, Klippel GmbH (https: / / www.klippel.de / fileadmin / klippel / Files / Know How / Application Notes / AN 32 Effective Radiation Area.pdf).

[0060] The indicated ratio of the mass of the moveable part of the drive unit and the effective radiating area Sd of the diaphragm may provide for comparatively heavy moveable part when compared against the effective sound radiating area Sd and, thus, for a comparatively powerful drive unit and a comparatively low resonant frequency.

[0061] The loudspeaker module may be operable to cause displacement of a volume VD by the diaphragm. The volume VD displaced by the diaphragm may be defined as the volume of air displaced by the diaphragm as it moves from its rest position in the air gap to a position of nominal maximum displacement. As such, twice the volume VD is displaced by the diaphragm when the diaphragm is moved from nominal maximum displacement inwards to nominal maximum displacement outwards or vice versa, i.e. a peak- to-peak stroke. The loudspeaker module, and in particular the enclosure, encloses an acoustic effective volume VB. The acoustic effective volume VB, which hereinbelow is referred to as the volume VB, is a concept known in the art (see, for example, L.L. Beranek, Acoustics, McGraw-Hill, 1954). It is known that the volume VB is not a purely structural volume as enclosed by the enclosure, but that the volume VB can be increased by stuffing the structural volume, e.g. with wool, by typically up to 20 percent, in some cases up to 40 percent.

[0062] The volume VD displaced by the diaphragm may be at least 0.05 times the volume VB (5 percent of the volume VB) enclosed by the enclosure, i.e. the ratio of VD over VB is greater than 0.05. This may result in a loudspeaker module having a comparatively large diaphragm and a comparatively small volume.

[0063] Where the loudspeaker module includes a plurality of loudspeakers sharing the same volume VB, the volume of air VD displaced by the plurality of loudspeakers may be at least 5 percent of the volume VB.

[0064] A ratio of the volume VB of the enclosure over the effective radiating Sd may be in a range of 0.01 to 0.5 metres. This ratio of VB over Sd may provide an alternative parametrisation of a comparatively large diaphragm and a comparatively small volume.

[0065] According to a second aspect, there is provided a loudspeaker system which includes the loudspeaker module as described above. The loudspeaker system further includes a signal processor, e.g. a filter such as an infinite-impulse-response filter, configured to adjust an electrical signal to be supplied to the voice coil of the loudspeaker module. For example, the signal processor may be configured to carry out frequency-dependent adjustment of the electrical signal so as to provide feedforward filtering of the electrical signal; e.g. the signal processor may apply different gains at different frequencies.

[0066] Feedforward filtering to modify the frequency response of a loudspeaker module is well-known and discussed, for example, in “Active control of loudspeakers: An investigation of practical applications” (Bright et al, 2002, Chapter s, https: / / core.ac.uk / download / pdf / 13746312.pdf).

[0067] The present inventor has found that the combination of a loudspeaker module according to the first aspect and a signal processor configured to provide feedforward filtering of the electrical signal to be supplied to the voice coil of the loudspeaker module can surprisingly provide a subjectively good sound quality (e.g. a tight bass or a warm bass according to user preference), despite the comparatively large total quality factor Qtc of the loudspeaker module (which would normally be thought to result in a worse sound quality). Without wishing to be bound by theory, it is thought this useful result is obtained because the high Qtc of the loudspeaker module provides a flatter Qtc curve, to which it is easier to apply feedforward filtering, as discussed in more detail in relation to the figures, below.

[0068] Here it is observed that the comparatively large total quality factor Qtc of the loudspeaker module according to the first aspect of the invention affects the time response of the loudspeaker module. Where the time response of a loudspeaker module having a large Qtc is generally considered undesirable, the present inventor has found that suitable frequency-dependent adjustment of the electrical signal may be carried out to compensate for this undesirable time response. The signal processor may be configured to decrease the group delay at and around the resonant frequency of the loudspeaker module.

[0069] The signal processor may be user configurable between a first frequency-dependent adjustment to the electrical signal and a second frequency-dependent adjustment to the electrical signal. The first and second frequency-dependent adjustments may correspond to different user preferences, e.g. a “tighter” bass and “warmer” bass, respectively.

[0070] The loudspeaker module may include one or more suspension elements, wherein the diaphragm is suspended from the loudspeaker frame by the one or more suspension elements. Preferably, the loudspeaker module includes at least two suspension elements.

[0071] The one or more suspension elements may include a first suspension element, e.g. a surround (which may also be referred to as the roll suspension), which attaches to the loudspeaker frame at a first landing surface on the loudspeaker frame. The first suspension element may be attached directly or indirectly to the diaphragm. In some examples, the first suspension element may attach (directly) to an outer edge of the diaphragm. In other examples, the first suspension element may be attached to another element of the moveable assembly.

[0072] The one or more suspension elements may include a second suspension element, e.g. a damper, which attaches to the loudspeaker frame at a second landing surface on the loudspeaker frame. The second suspension element may be attached directly or indirectly to the diaphragm. In some examples, the second suspension element may be attached (directly) to the diaphragm at a location inwardly located with respect to the outer edge of the diaphragm. In other examples, the second suspension element may be attached to another element of the moveable assembly (e.g. the moveable part of the drive unit, particularly where the moveable part is the magnet unit).

[0073] The centre of gravity of the moveable part of the drive unit may have a position along the movement axis that is between the first landing surface and the second landing surface.

[0074] By locating the centre of gravity of the magnet unit between the first landing surface and the second landing surface, rocking of the moveable assembly may be inhibited. More particularly, the rocking modes of the loudspeaker module may be pushed outside of the working frequency range of the loudspeaker module.

[0075] The magnet unit may include a permanent magnet and one or more flux guiding elements for guiding magnet flux produced by the permanent magnet in a magnetic circuit and across an air gap, wherein the voice coil is configured to be located in the air gap when the moveable part of the drive unit is at rest.

[0076] Herein, a reference to the moveable part of the drive unit being at rest, or the diaphragm being at rest, may be taken to correspond to a state in which the drive unit is not energised (e.g. an electric current is not being supplied to a voice coil of the drive unit) with the moveable part of the drive unit and the diaphragm having been given time to reach a state of rest. The magnet unit may include at least two flux guiding elements. The at least two flux guiding elements may be configured to guide magnet flux produced by the permanent magnet across the air gap. The air gap may be formed between the at least two flux guiding elements.

[0077] The at least two flux guiding elements may include a washer and a yoke. The permanent magnet may be located between the washer and the yoke. The washer and the yoke may be arranged to define the air gap between the washer and the yoke.

[0078] The yoke may include a base and an upright portion extending from the base. In some examples, the yoke may be a U-yoke wherein the upright portion is an annular sidewall. In some examples, the yoke may be a T-yoke wherein the upright portion is a central post.

[0079] The magnet unit may form a magnetic circuit. The magnetic circuit may provide a substantially closed circuit (or loop) for the magnetic flux. The magnetic circuit may have a comparatively high magnetic reluctance. For example, the magnetic reluctance of the magnetic circuit may be at least 2.5 x 10A6 [1 / H] or even 3 x 10A6 [1 / H], where “H” represents the physical unit “Henry”. The majority of the magnetic reluctance of the magnetic circuit may be attributed to the air gap. For example, the air gap may have a magnetic reluctance of at least 2 x 10A6 [1 / H],

[0080] Further detail relating to high-reluctance magnetic circuits in the context of loudspeakers is described in GB 2209544.2 and PCT / EP2023 / 067426.

[0081] The voice coil may have a first perimeter and a second perimeter. The first perimeter may be an outer perimeter or an inner perimeter of the voice coil, and the second perimeter may corresponding be the inner perimeter or the outer perimeter.

[0082] The distance between the first perimeter and the second perimeter may be referred to as a winding thickness of the voice coil. Accordingly, the magnetic flux density permeating the voice coil may drop over the winding thickness of the voice coil as specified above.

[0083] If the voice coil is generally circular, the inner perimeter of the voice coil may be referred to as the inner diameter of the voice coil, and the outer perimeter of the voice coil may be referred to as the outer diameter of the voice coil.

[0084] When the voice coil is located in the air gap, i.e. the moveable part of the drive unit is at rest, a magnetic flux density at the first perimeter of the voice coil may be 50% or less of a magnetic flux density at the second perimeter of the voice coil.

[0085] In an arrangement in which the at least two flux guiding elements include a U-yoke, the magnetic flux density at the outer perimeter of the voice coil may be 50% or less of the magnetic flux density at the inner perimeter of the voice coil.

[0086] In an arrangement in which the at least two flux guiding elements include a T-yoke, the magnetic flux density at the inner perimeter of the voice coil may be 50% or less of the magnetic flux density at the outer perimeter of the voice coil. According to a third aspect of the invention, there is provided a vehicle comprising a loudspeaker module according to the first aspect or a loudspeaker system according to the second aspect.

[0087] The vehicle according to the third aspect may include any one or more features as described in relation to the first aspect of the invention or the second aspect of the invention.

[0088] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0089] Summary of the Figures

[0090] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0091] Figure 1 is a cross-sectional view of a loudspeaker module including a loudspeaker and an enclosure.

[0092] Figure 2 is an enlarged view of a portion of the loudspeaker.

[0093] Figure 3 is a cross-sectional view of the loudspeaker, with some components not shown.

[0094] Figure 4 is a graph illustrating magnetic flux lines of the loudspeaker.

[0095] Figure 5 is a graph illustrating magnetic flux density of the loudspeaker.

[0096] Figure 6 is a graph illustrating the total quality factor Qts and Qtc of the loudspeaker.

[0097] Figure 7 is a graph illustrating the total quality factor Qts of a conventional loudspeaker.

[0098] Figure 8 is a graph illustrating the resonant frequency fs and fc of the loudspeaker.

[0099] Figure 9 is a graph illustrating the resonant frequency fs of the conventional loudspeaker.

[0100] Figure 10 is a graph illustrating the step response of the loudspeaker module with raw signal input.

[0101] Figure 11 is a graph illustrating the step response of the loudspeaker module with a filtered signal input.

[0102] Figure 12 is a graph illustrating the measured magnitude frequency response of the transfer function of the loudspeaker module with raw signal input.

[0103] Figure 13 is a graph illustrating the measured magnitude frequency response of the transfer function of the loudspeaker module with filtered signal input.

[0104] Figure 14 is a graph illustrating the measured magnitude frequency response of the transfer function of the loudspeaker module with raw signal input.

[0105] Figure 15 is a graph illustrating the measured magnitude frequency response of the transfer function of the loudspeaker module with filtered signal input.

[0106] Figure 16 is a schematic illustration of a loudspeaker system including the loudspeaker module.

[0107] Figure 17 is a graph illustrating the real power input of the loudspeaker module and the conventional loudspeaker module. Figure 18 is a flowchart illustrating a method for use in manufacturing the loudspeaker module.

[0108] Figure 19 is a graph showing relating the mass Mms of a moveable assembly of the loudspeaker module, a volume VB enclosed by the loudspeaker module, and the resonant frequency fc of the loudspeaker module.

[0109] Figure 20 is a cross-sectional view of another loudspeaker module which includes two loudspeakers as shown in Figure 1 in a force-cancelled back-to-back configuration.

[0110] Figure 21 is a cross-sectional view of yet another loudspeaker module which includes two loudspeakers in a force-cancelled back-to-back configuration.

[0111] Figure 22 shows a vehicle including the loudspeaker module of Figure 1 .

[0112] Detailed Description of the Invention

[0113] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0114] Figure 1 is a cross-sectional view of an exemplary loudspeaker module 20. The loudspeaker module 20 includes a loudspeaker 200 and an enclosure 300.

[0115] The loudspeaker 200 includes a loudspeaker frame 210, a diaphragm 220, and a drive unit 230. The drive unit 230 includes a moveable part 240, which is attached to the diaphragm 220, and a stationary part 250, which is attached to the frame 210.

[0116] The loudspeaker module 20 is operable to energise the drive unit 230 to cause movement of the moveable part 240 relative to the stationary part 250. Suitably, the moveable part 240 and the stationary part 250 magnetically interact with each other to effect the movement of the moveable part 240. The diaphragm 220 and the moveable part 240 of the drive unit 230 are joined together to define a moveable assembly 260, such that they move together ‘as a unit’ along a movement axis 202 when the moveable part 240 is caused to move. When causing the moveable assembly 260 to move, the diaphragm 220 acts as a piston and produces sound.

[0117] The moveable assembly 260 is suspended from the frame 210 by at least a first suspension element 262 and a second suspension element 264. The first suspension element 262 is attached to the frame 210 at a first landing surface 211 on the frame 210 and the second suspension element 264 is attached to the frame 210 at a second landing surface 212 on the frame 210. The first suspension element 262 and the second suspension element 264 are secured to the moveable assembly 260 such that the centre of gravity of the moveable part 240 of the drive unit 230 is located between the first landing surface 211 and the second landing surface 212 when the moveable assembly 260 is at rest. More particularly, the centre of gravity of the moveable part 240 has a position along the movement axis 202 that is between the first landing surface 211 and the second landing surface 212. The diaphragm 220 has a first radiating surface 221 and a second radiating surface 222. The first radiating surface 221 faces in a forward direction (away from the frame 210) and is utilised for producing sound. The second radiating surface 222 faces in a rearward direction (into the frame 210). The forward direction and the rearward direction are opposite directions parallel to the movement axis 202.

[0118] The enclosure 300 is arranged to receive sound produced by the second radiating surface 222 of the diaphragm 220. When the loudspeaker module 20 is in operation, the enclosure 300 separates the sound produced by the second radiating surface 222 from the sound produced by the first radiating surface 221 to inhibit cancellation of the sound produced by the first radiating surface 221 . Suitably, a sealed volume 310 is formed in the loudspeaker module 20 into which the sound produced by the second radiating surface 222 is received.

[0119] In some examples, the enclosure 300 is separate from the loudspeaker frame 210. In other examples, part or the whole of the enclosure 300 is formed integrally with the loudspeaker frame 210; or part or the whole of the loudspeaker frame 210 is formed integrally with the enclosure 300.

[0120] As shown in Figure 1 , the moveable part 240 of the drive unit 230 is a voice coil 242 while the stationary part 250 is a magnet unit 252.

[0121] The magnet unit 252 is configured to generate magnetic flux and to guide the magnetic flux across an air gap 253. Suitably, the magnet unit 252 includes a permanent magnet 254 to generate the magnetic flux and at least two flux guiding elements 255, 256 to guide the magnetic flux across the air gap 253. The permanent magnet 254 is a rare earth magnet and may comprise more than one structural element. The flux guiding elements 255, 256 are provided as a (magnetic) yoke 255 and a (magnetic) washer 256. In Figure 1 , the yoke 255 is a U-yoke which has a base and a wall, which extends from the base and is arranged about the movement axis 202. In some examples, the permanent magnet 254 and the flux guiding elements 255, 256 are axially symmetric about the movement axis 202, though other arrangements are possible.

[0122] The magnet unit 252 and the air gap 253 form a magnetic circuit. The magnetic circuit provides a closed loop for the magnetic flux generated by the permanent magnet 254 and guided across the air gap 253 by the two flux guiding elements 255, 256. In some examples, the air gap 253 has a magnetic reluctance of 5.3 x 10A6 [1 / Henry] and the magnetic circuit has a total magnetic reluctance slightly greater than the magnetic reluctance of the air gap 253.

[0123] The loudspeaker module 20 makes use of a comparatively large voice coil 242 using many layers in the magnetic circuit. In some examples, the mass of the voice coil 242 is 60% or more of the total mass of the moveable assembly 260. In view of the large mass of the voice coil 242, the connection between the voice coil 242 and the diaphragm 220 is improved, for example as described with reference to Figures 2 and 3, to prevent forces acting on the moveable assembly during operation from damaging the moveable assembly 260. Figures 2 and 3 are views of the loudspeaker 200. Figure 2 is an enlarged view of a particular section of Figure 1 (indicated by a dotted circle in Figure 1). Figure 3 shows the loudspeaker 200 with the moveable part 240 of the drive unit 230 removed.

[0124] The voice coil 242 has a first perimeter 243 and a second perimeter 244. The distance between the first perimeter 243 and the second perimeter 244 is a winding thickness 245 of the voice coil 242. Here, the voice coil 242 is a substantially circular coil such that the first perimeter 243 is an inner radial boundary of the voice coil 242, and the second perimeter 244 is an outer radial boundary of the voice coil 242.

[0125] The diaphragm 220 includes a neck portion 223 forming a diaphragm aperture 224 (shown in Figure 3). The neck portion 223 is (when viewed in cross-section) a straight segment of the diaphragm 220. The diaphragm 220 further includes an inner diaphragm portion 225 and a curved portion 226 (or ‘curved segment’). The inner diaphragm portion 225 is another segment of the diaphragm 220 which (in crosssection) is straight. The curved portion 226 is located between and connects the neck portion 223 and the inner diaphragm portion 225, and (in cross-section) is curved.

[0126] The moveable assembly 260 also includes a voice coil former 290 which carries the voice coil 242. The voice coil former 290 extends through the diaphragm aperture 224 such that a first gap 266 is formed between the voice coil former 290 and the neck portion 223 of the diaphragm 220. Also, a second gap 268 is formed between the neck portion 223 and the voice coil 242. An adhesive body 269 of hardened adhesive occupies the first gap 266 between the voice coil former 290 and the neck portion 223 of the diaphragm 220 and the second gap 268 between the neck portion 223 and the voice coil 242 so as to attach the neck portion 223 of the diaphragm 220 to the voice coil former 290. The adhesive body 269 may occupy the entire first gap 266 and / or the entire second gap 268, i.e. fill either or both gaps 256, 258. In some examples, the adhesive body 269 may not be bonded to the voice coil 242, which in use may get very hot such that a bond between the adhesive body 269 and the voice coil 242 may be damaged. Nevertheless, the bond between the diaphragm 220 and the voice coil former 290 may be improved over a traditional loudspeaker.

[0127] The adhesive body 269 of hardened adhesive, e.g. a monolithic body of hardened / cured adhesive, occupies a volume of space both above and below the neck portion 223. More particularly, the adhesive body 269 of hardened adhesive extends all the way to the voice coil 242 (even if in use not bonded to the voice coil 242). As such, a greater volume of adhesive is provided than may be present in a traditional loudspeaker. Moreover, the volume of adhesive is both above and below the neck portion 223, whereas in the case of a traditional loudspeaker the adhesive may be exclusively on the diaphragm.

[0128] Figures 4 and 5 illustrate the magnetic flux generated by the magnet unit 252. In particular, Figure 4 shows a sectional view of the drive unit 230 and illustrates magnetic flux lines generated by the magnet unit 252, while Figure 5 is a graph showing the magnetic flux density across the voice coil 242 (“COIL ID” to “COIL OD”, i.e. inner diameter / perimeter 243 to outer diameter / perimeter 244).

[0129] In Figure 4, the magnetic flux follows curved paths spreading out in the air gap 253 across the voice coil 242. Correspondingly, the magnetic flux density decreases across the voice coil 242. Figure 5 shows the radial flux density Br over the winding thickness of the voice coil 242, which shows a substantial drop typically undesired in loudspeaker design. In a typical loudspeaker the magnetic flux density may be almost constant and may drop to approximately 85 to 90% (percent) of the initial value. By contrast, the radial flux density of the loudspeaker module 20 may drop by at least 50% over the winding thickness of the voice coil 242. For example, the magnetic flux density at the outer perimeter 244 of the voice coil 242 may be approximately 37% of the magnetic flux density at the inner perimeter 243 of the voice coil 242. In other words, the magnetic flux density may drop by approximately 63% from the inner perimeter 243 to the outer perimeter 244 of the voice coil 242.

[0130] The comparatively large decrease in magnetic flux density is related to the comparatively high reluctance of the magnet unit 252 of the loudspeaker module 20. The reluctance can be estimated by calculations derived from simulations, e.g. using the Finite Element Method of the static magnetic circuit.

[0131] It has been found that having a reluctance above 2.5 x 10A6 [1 / H] (or “2.5E6 [1 / H]”), preferably above 3 x 10A6 [1 / H] (or “3E6 [1 / H]”) may lead to highly efficient loudspeakers of low weight. By contrast, carrying out the same calculations on magnet systems of conventional loudspeakers shows that their reluctance may not exceed 1.5 x 10A6 [1 / H] (or“1.5E6 1 / H”).

[0132] Where the flux guiding elements 255, 256 have a relative permeability » 1 , e.g. for flux guiding elements made from steel, the reluctance of the flux guiding elements can be neglected and all reluctance can be assigned to the air gap 253.

[0133] Figures 6 and 7 illustrate the total quality factor Qts for the loudspeaker 200 and a conventional loudspeaker, respectively. Figure 6 further shows the total quality factor Qtc for the loudspeaker module 20.

[0134] In Figure 6, the lower solid line represents the total quality factor Qts of the loudspeaker 200 as physically measured for displacements up to plus / minus 10 millimetres. The lower dotted line represents a quadratic fitting curve 0.015xA2+0.0052x+0.894 which approximates the total quality factor Qts; here x represents displacement of the moveable part 240 of the drive unit 230 (in this example the voice coil 242) in millimetres and ranges between -10mm and +10mm, i.e. nominal maximum displacement of 10mm.

[0135] The total quality factor Qts of the loudspeaker 200 (at rest) is comparatively large with a value of approximately 0.9. Moreover, at nominal maximum displacement of plus / minus 10 millimetres, the total quality factor Qts is approximately 2.7 times the value of Qts at rest. Therefore, the total quality factor Qts is more uniform across the normal working range of the loudspeaker 200 when compared to the total quality factor Qts of the conventional loudspeaker shown in Figure 7.

[0136] The normal working range of the loudspeaker 200 is understood to include operating the loudspeaker 200 at intended output volumes and so causing displacement of the moveable assembly 260 along the movement axis 202. When operating the loudspeaker 200 at a maximum output volume intended for the loudspeaker 200, a nominal maximum displacement of the moveable assembly 260 along the movement axis 202 is caused. As shown in Figure 6, for the loudspeaker 200 the maximum output volume corresponds to a nominal maximum displacement of plus / minus 10 millimetres. In Figure 6, the upper solid line represents the total quality factor Qtc of the loudspeaker module 20 as physically measured for displacements up to plus / minus 10 millimetres. The upper dotted line represents a quadratic fitting curve 0.0183xA2+0.0262x+1 .9172 which approximates the total quality factor Qtc.

[0137] The total quality factor Qtc of the loudspeaker module 20 (at rest) has a value of approximately 1 .9 and so is even larger than the total quality factor Qts (at rest). At nominal maximum displacement the total quality factor Qtc is approximately twice as large as the value of Qtc at rest. As such, the total quality factor Qtc is more uniform than for a loudspeaker module including the conventional loudspeaker.

[0138] In Figure 7, the solid line represents the total quality factor Qts of the conventional loudspeaker as physically measured. The dotted line represents a quadratic fitting curve 0.0555xA2-0.0117x+0.8628 which approximates the total quality factor Qts.

[0139] The conventional loudspeaker has a total quality factor Qts of approximately 0.86 at rest, but the value of the total quality factor Qts changes more significantly across the normal working range of the conventional loudspeaker and, in particular, sharply increases as coil displacement is increased. At nominal maximum displacement, the total quality factor of the conventional loudspeaker is more than 7 times as large as the total quality factor Qts of the conventional loudspeaker at rest.

[0140] The total quality factor Qtc of a loudspeaker module including the conventional loudspeaker is not shown in Figure 7. It is noted that the total quality factor Qtc would be represented by a curve substantially identical to that of the total quality factor Qts but shifted upwards. The size of shift depends on the particular choice of enclosure. As set out in the Background above, the particular choice of enclosure (sometimes referred to as “alignment”) is conventionally made to achieve a desired loudspeaker response, which typically involves a total quality factor Qtc in the range 0.5 to 1 .2, noting that a total quality factor Qtc exceeding 1 .2 is conventionally deemed undesirable. As such, the skilled person implementing these conventional teachings would want to avoid a total quality factor Qtc as large as 1 .9 as shown in Figure 6.

[0141] The total quality factor Qtc is a dimensionless parameter which quantifies the damping of the movement of the moveable assembly 260 in the loudspeaker module 20. The total quality factor Qtc is defined as the ratio of initial energy divided by lost energy per cycle and as such a lower total quality factor Qtc is associated with higher damping, while a higher total quality factor Qtc is associated with less damping.

[0142] The total quality factor Qtc of the loudspeaker module 20 is calculatable based on the total quality factor Qts of the loudspeaker 200 (i.e. without the enclosure 300), the equivalent air volume Vas of the loudspeaker 200, and the volume VB of the loudspeaker module 20:

[0143] Qtc = (Qtcequals Qts times the square root of the sum of Vas over VB and unity)

[0144] The equivalent air volume Vas and the volume VB are parameters known to the skilled person.

[0145] The proportionality factor linking Qts and Qts, i.e. the square root, is strictly greater than unity and so Qtc is greater than Qts. Thus, when a loudspeaker with a large total quality factor Qts is mounted in an enclosure, the resulting loudspeaker module has an even larger total quality factor Qtc. In practice Qms is typically not constant, but instead changes with displacement and this becomes particularly pronounced as the box stiffness Kb becomes (much) greater than the driver stiffness Kms. The change in Qms with increasing displacement is attributed mainly to losses caused by the suspension. In free air, i.e. considering the loudspeaker 200, that is only Kms. When used in box, i.e. considering the loudspeaker module 20, the suspension is dominated by Kb which typically has much lower losses than the mechanical suspension Kms, and so Qms increases. The small losses of the air are substantially constant with displacement whereas this is not necessarily the case with the mechanical suspension parts. As a result, Qtc is generally not identical to a Qts curve shifted by the factor sqrt(Vas / Vb+1). However, the indicated relationship between the total quality factors Qtc and Qts is nevertheless deemed useful even if approximate.

[0146] The total quality factor Qts is calculatable based on the electrical quality factor Qes and the mechanical quality factor Qms of the loudspeaker. The relationship is:

[0147] =Qes Qms(Qts equals the product of Qes and Qms over the sum of Qes and Qms) Qes Qms

[0148] The mechanical quality factor Qms is associated with the mechanical suspension of the moveable assembly 260 and determined by the suspension elements, such as the first suspension element 262 and the second suspension element 264. Generally, the suspension elements are carefully designed towards achieving that the mechanical quality factor Qms is substantially constant across the intended working range of the loudspeaker, i.e. provides a linear suspension, meaning that the restoring force by the suspension elements exerted on the moveable assembly scales linearly with displacement of the moveable assembly along the movement axis.

[0149] The electrical quality factor Qes is associated with the electrodynamic drive unit 230. When the voice coil 242 is caused to move relative to the magnet unit 252, the magnetic flux density which permeates the voice coil 242 changes and a back-EMF is caused that opposes the change in magnetic flux density. Thus, Qes generally increases with relative displacement since the motor force factor BL decreases with relative displacement. However, the loudspeaker module 20 has an air gap 252 with comparatively high magnetic reluctance (or, correspondingly, a comparatively sharp decrease of the magnetic flux density across the voice coil 242 when at rest in the air gap 253), which results in an increased stray flux density outside of the air gap 252. Accordingly, for the loudspeaker 200 the magnetic flux density has increased uniformity and as such the back-EMF is reduced, providing a substantially constant electrical quality factor Qes and a more linear suspension.

[0150] The substantially constant mechanical factor Qms and electrical quality factor Qes in combination mean that the total quality factor Qts of the loudspeaker 200 is relatively uniform, i.e. less dependent on relative displacement of the voice coil 242 and the magnet unit 252. By contrast, in a conventional loudspeaker the electrical quality factor Qes may be highly dependent on relative displacement of the voice coil and the magnet unit and, therefore, so is the total quality factor Qts of the conventional loudspeaker. General consensus is in the art is that the magnetic reluctance of the air gap should be as low as possible to ensure efficient use of the magnetic circuit. It follows that the radial magnetic flux density across the air gap in a conventional loudspeaker is as constant as possible; the magnetic flux density may typically drop from an initial 100% to a value of 90%, or maybe as low as 85%, across the voice coil in the air gap. As such, in a conventional loudspeaker module the magnetic flux density within the air gap is comparatively high and the stray magnetic flux density outside of the air gap is comparatively low. A voice coil therefore experiences a sharp decrease when moving outside of the air gap, and a correspondingly large back- EMF is generated. The strong displacement dependency of the electrical quality factor Qes for the conventional loudspeaker is shown in Figure 7. At rest position, and for small signals, the total quality factor Qts is around 0.8, which would generally be considered an acceptable value. However, as soon as the conventional loudspeaker produces sound at higher output volume, causing the moveable assembly to displace plus / minus 5mm or even as much as plus / minus 10mm at nominal maximum displacement, the total quality factor Qts increases and reaches values of 1 .5 and even 2. Figures 8 and 9 illustrate the resonant frequency fs of the loudspeaker 200 and the conventional loudspeaker, respectively. Figure 8 further shows the resonant frequency fc of the loudspeaker module 20.

[0151] The free air resonance frequency fs is the fundamental, mechanical resonance of the moving assembly of a loudspeaker. As shown in Figure 9, the conventional loudspeaker has a comparatively displacementdependent resonant frequency fs. By contrast, Figure 8 shows that the loudspeaker 200 has a resonant frequency fs which is substantially constant across the nominal range of coil displacement, i.e. across output volumes.

[0152] In some examples, the loudspeaker 200 may include a suspension element, in particular the second suspension element 264, which the skilled person has chosen to provide a linear response, i.e. the stiffness of the suspension element(s) is described by a substantially flat KMS curve over the normal working range of the loudspeaker 200. Using conventional suspension elements, the skilled person will be able to devise a suspension wherein the KMS value at nominal maximum displacement may be, for example, no greater than 1.3 times the value at rest position.

[0153] The resonant frequency fc of the loudspeaker module 20 is the fundamental mechanical resonance of the moving assembly 260 of a loudspeaker module 20. The resonant frequency fc of the loudspeaker module 20 is calculatable based on the free-air resonant frequency fs of the loudspeaker 200 (i.e. without the enclosure 300), the equivalent air volume Vas of the loudspeaker 200, and the volume VB of the loudspeaker module 20:

[0154] As before, the proportionality factor is greater than unity and as such fc is greater than fs. Thus, when the loudspeaker 200 with resonant frequency fs is mounted in the enclosure 300, the resulting loudspeaker module 20 has a resonant frequency fc which is greater than the loudspeaker resonant frequency fs.

[0155] As shown in Figure 8, the loudspeaker 200 has a resonant frequency fs of less than 40Hz and moreover is substantially constant across the normal working range of the loudspeaker 200, i.e. from negative to positive nominal maximum displacement of 10mm. Similarly, the loudspeaker module 20 has a resonant frequency fc of less than 70Hz and moreover is substantially constant across the normal working range of the loudspeaker module 20.

[0156] By contrast, Figure 9 shows that the resonant frequency fs of the conventional loudspeaker is comparatively dependent on displacement across the normal working range of the conventional loudspeaker. The resonant frequency of the conventional loudspeaker has a minimum at approximately 50Hz and rises to more than 70Hz for high output volume. A loudspeaker module including the conventional loudspeaker would have a similar curve for the resonant frequency of the loudspeaker module, but shifted upwards depending on the choice of enclosure.

[0157] The loudspeaker module 20 has a total quality factor Qtc and resonant frequency fc which are less dependent on displacement of the moveable assembly than for a loudspeaker module including the conventional loudspeaker. Moreover, the inventor found that the total quality factor Qtc and the resonant frequency fc are sufficiently constant such that the loudspeaker module 20 provides a linear time-invariant system in first approximation and as such linear signal processing may suitably be utilised to adjust the sound produced by the loudspeaker module 20 uniformly across output volumes. By contrast, the conventional loudspeaker module has a total quality factor Qtc and resonant frequency fc which are highly dependent on displacement of the moveable assembly and therefore cannot readily be approximated as a linear system across the normal working range of the conventional loudspeaker module.

[0158] Many modern audio systems allow for digital signal processing before amplifying the signal for playback by the loudspeaker. Still, many loudspeakers are designed independent of the signal processing stage assuming a constant voltage vs frequency transfer function of the electronics.

[0159] There are two main types of signal processing. In a feedforward-type design, the signal to the loudspeaker is altered based on parameters chosen by the loudspeaker system designer during the development phase. In a feedback-type system, state parameters of the loudspeaker are fed back to control electronics, such as the acceleration or position of the loudspeaker diaphragm or the current through the voice coil, and signal processing is adjusted dynamically towards achieving a certain loudspeaker response. Despite many efforts in the field, feedback systems are not widely implemented due to their complexity. Examples of feedback systems are described, for example, in US3334184A (DE KONING) and DE4332804C2 (KLIPPEL).

[0160] For economic reasons, a balance is sought between the cost of the control electronics and the loudspeaker. Ideally, a loudspeaker for producing sound at bass frequencies would be designed to perform well at small signal levels, including tight bass response, while also being capable of handling larger signal levels, which cause larger diaphragm displacement and produce louder bass, at the same quality level as for small signal levels. However, for conventional loudspeaker designs this may be difficult to achieve and may make for a costly loudspeaker design. For a feedback system, the requirements on the linearity can be somewhat loosened since the amplifier may compensate the loudspeaker’s performance using a suitably increased voltage output to the loudspeaker based on the feedback parameters, but the increased voltage output may not be desirable and may be impractical to maintain for typical amplifiers and loudspeakers.

[0161] The loudspeaker module 20 has a low-cost loudspeaker design and allows for low-cost feed forward signal processing, e.g. using a conventional amplifier having standard voltage output, and is capable of providing tight and loud bass reproduction across the normal working range of the loudspeaker module 20. To exemplify the effectiveness of this approach, the step response was measured for the loudspeaker module 20 using a raw input signal and with signal processing of the input signal. This is described with reference to Figures 10 and 11 .

[0162] Figures 10 and 11 illustrate the step response of the loudspeaker module 20. The step response is the generated output pressure over time in response to supplying the loudspeaker module 20 with a DC impulse that is maintained, i.e. a step function.

[0163] In Figure 10, the step response of the loudspeaker module 20 is shown. Multiple pronounced oscillations until about 50 milliseconds indicate ‘ringing’ in the time domain. Such ringing is associated with poor transient behaviour of a loudspeaker module with a total quality factor exceeding 1 .2.

[0164] In Figure 11 , the step response of the loudspeaker module 20 is shown but, in contrast to Figure 10, with suitable signal processing performed to adjust the step response. Notably, the ringing present in Figure 10 is absent in Figure 11 due to the signal processing. The step response shown in Figure 11 is substantially identical to that of a loudspeaker module with a total quality factor Qtc equal to 1 , for example as described in the seminal paper by R. H. Small, “Closed-Box Loudspeaker Systems-Part 1 : Analysis”, J. Eng. Soc. (December 1972).

[0165] The use of signal processing, in particular linear filtering, to remove ringing at resonance, may be considered surprising since in the art there is a belief that ringing at resonance of a loudspeaker cannot be equalized. In particular, for the loudspeaker module 20 ringing at resonance is removed across output volumes., e.g. the nominal working range of the loudspeaker module 20. By contrast, for a conventional loudspeaker it may be possible to remove ringing for small signals using linear filtering, but the same linear filtering would not remove ringing for larger signals.

[0166] By contrast, it is taught in the art that in the context of low-frequency reproduction it is exclusively alignment, i.e. selection of an appropriate enclosure to receive a loudspeaker, which finalises the total quality factor. For example, Vance Dickason writes in his popular textbook Loudspeaker Design Cookbook, 7th Edition, KCP Media Corp., 2006 that “The whole point of fitting a woofer to a particular box volume is to control the response characteristic of the combination. The objective method is to measure and align the Q-factor. [...] Qtc greater than 1.2 or so, however, should be regarded as undesirable.” Pages of alignment tables follow the reproduced section of text, instructing the loudspeaker designer how to perfectly fit a loudspeaker to a closed box to avoid “misalignment” since a large total quality factor degrades the audio performance and leads to “tubby” bass reproduction instead of the desired “tight” bass reproduction. As such, the loudspeaker module 20 and the use of signal processing for achieving a step response substantially identical to that of a loudspeaker module with a total quality factor Qtc equal to 1 may be considered surprising.

[0167] Figures 12 and 13 illustrate the measured magnitude frequency response of the transfer function, showing the roll-off behaviour for low frequencies from about 30Hz to about 300Hz for the loudspeaker module 20, i.e the loudspeaker 200 mounted in the enclosure 300 of 1 .5 litres volume (solid line). The dashed line shows a typically desired low frequency roll-off target corresponding to a 2nd-order Butterworth target with a total quality factor Qtc of 1 . In Figure 12, the loudspeaker module 20 overshoots by approximately 12dB at 70Hz when fed a raw input signal.

[0168] The loudspeaker module 20 responds more strongly to the raw input signal around the resonant frequency fc of the loudspeaker module 20. Accordingly, the magnitude frequency response shown in Figure 12 is related to the resonance behaviour shown in Figure 10. As before, the addition of the signal processing aligns the measured magnitude frequency response of the loudspeaker module 20. This is shown in Figure 13, wherein signal processing has aligned the measured magnitude frequency response of the loudspeaker module 20 with the 2nd-order Butterworth target. The measured magnitude frequency response shown in Figure 13 substantially coincides with the 2nd-order Butterworth target across the measured frequency range.

[0169] Figures 14 and 15 illustrate the group delay of the loudspeaker 200 versus frequency from about 30Hz to about 300Hz. Group delay is a measure of delay between input of a signal and output of the signal.

[0170] Group delay may be understood as frequency-dependent lagging leading to “slow” and unprecise bass reproduction. The upper threshold for acceptable group delay may be subject to discussion in the art, but there seems to be consensus about the 1 -cycle limit in that group delay should be below one cycle time at any given frequency, and should be as little as possible for “tight” bass reproduction.

[0171] In Figure 14, the measured group delay is plotted for a raw input signal. At around 70Hz, the group delay has a local maximum of approximately 14ms. The cycle time at 70Hz is 1 / 70s=14ms and as such the graph of Figure 14 would be considered to represent a loudspeaker unsuitable for high quality playback. This agrees with Figure 10, which shows the resonance behaviour, and with Figure 12, which shows the overshot of the magnitude frequency response at 70Hz. As before, application of signal processing addresses this. In Figure 15, the application of signal processing reduces the group delay at and around the resonant frequency fc of the loudspeaker module 20. More particularly, the signal processing reduces the group delay at 70Hz to as little as 4.5ms. The resulting group delay versus frequency increases to 10ms at 40Hz, which is less than half of the 1 -cycle limit.

[0172] Figure 16 illustrates an audio system 1000 for implementing the signal processing as described with reference to Figures 11 , 13 and 15. The audio system 1000 includes the loudspeaker module 20, a signal processor 1100, a data storage medium 1200, and an amplifier 1300.

[0173] The signal processor 1100 is configured to carry out the signal processing resulting in the graphs of Figures 11 , 13 and 15. In particular, the signal processor 1100 is configured to carry out feed-forward filtering (without feedback). Analog components may be used for the signal processing, such as capacitors and inductors, which exhibit linear behaviour. In the present example, the signal processor 1100 is a digital signal processor using a set of infinite impulse response filters, which may be considered digital counterparts to the analogue components. The signal processor 1100 may be, for example, an Analog Devices (TM) ADAU1701 , and is configured to apply the following peak equalizers:

[0174] The data storage medium 1200 stores the value of the peak equalisers.

[0175] By means of the signal processing applied by the signal processor 1100, the frequency response is adjusted to become smooth as desired; the group delay at 70Hz is adjusted to be as little as 4.5ms; and both adjustments are uniform across output volumes, i.e. even at high input levels and large displacements, which is as expected knowing the loudspeaker behaviour as shown in Figures 6 and 8.

[0176] It is noted that the above illustrates one possible set of filters, but that different filter settings leading to a similar transfer function are possible. In some examples, good alignment may be found using fewer than 5 peak filters. Here “alignment” refers to the process of so-called alignment, which is discussed above and conventionally carried out by choosing a particular enclosure in order to align towards a target response. Here, the target response was chosen to be a 12dB Butterworth roll-off and implemented by means of the described signal processing.

[0177] Figure 17 is a graph illustrating real input power for the loudspeaker module 20 and a conventional loudspeaker module over a frequency range of 40Hz to 60Hz and target displacement of plus / minus 8mm. Note that the real input power is independent of the voltage profile to the loudspeaker (DSP settings) and RDC (DC resistance) of the loudspeaker, and can be calculated for any set of parameters for a given target displacement and as such is a useful figure of merit.

[0178] The parameters of the two loudspeakers are as follows:

[0179] The loudspeaker module 20 draws an average power of 130W [Watts] to reach the desired peak displacement. The conventional loudspeaker module requires 360W on average, with close to 450W at 40Hz. Such high input power is impractical as power handling becomes difficult and costly.

[0180] For the loudspeaker module 20, the amplifier 130 does not require high peak voltages and the real input power sunk into the voice coil 242 is comparatively low due to several reasons. Due to comparatively high mass of the moveable assembly 260, the resonance frequency fc is in a frequency range where audio content to be reproduced, such as modern music, has the highest spectral content and consequently the highest power requirement. Around resonance frequency fc, the impedance magnitude is high and voltage and current are out of phase. The real input power is low. Further, due to the large total quality factor Qtc, the voltage efficiency around resonance frequency fc is very high. Returning to Figure 12 and comparing the solid line with the dashed line in 12, the overshoot at 70Hz corresponds to approximately 12dB. To achieve the desired smooth frequency roll-off, the voltage can be decreased by a factor of 10A(12 / 20)=4 and, accordingly, provides a reduction in power.

[0181] Accordingly, the loudspeaker module 20 very effective at low-frequency radiation.

[0182] Figure 18 is a flowchart illustrating a method for use in manufacturing of a loudspeaker module and is hereinbelow discussed using the example of the loudspeaker module 20.

[0183] The method includes a step S110 of choosing a volume VB of the enclosure 300. The volume VB may be chosen, for example, based on constraints on the available space depending on the intended application.

[0184] The method includes a step S120 of determining the mass Mms of the moveable assembly 260:

[0185] Here, p0is the density of air; c is the speed of sound in air; Sdis the effective radiating surface of the loudspeaker module 20; the resonant frequency fcmay be chosen, for example, to be in the centre of the intended passband. The mass Mms of the moveable assembly 260 so determined may be relatively large compared to conventional low-frequency systems.

[0186] The method includes a step S130 of determining the mass of the moveable part 240 of the drive unit 230.

[0187] To allow for a relatively high moving mass Mms and a low-cost drive unit 230, the mass of the moveable part 240 of the drive unit 230 is chosen to make up a substantial portion of the mass of the moveable assembly 260, e.g. at least 60 percent of the moving mass Mms. Where the moveable part 240 of the drive unit 230 is the voice coil 242, such as in Figure 1 , the mass of the voice coil 242 is suitably increased. Increasing of the mass of the voice coil 242 may typically include increasing of the number of layers of the voice coil 242 and / or using a wire with large cross-section and / or non-round cross-section (e.g. rectangular or square). As regards the non-round cross-section, this may be particularly useful as typically a large number of layers are used, so that the advantages of non-round cross-section are likewise increased (high packing factor, increased mass, decreased resistance RDC for given coil dimensions).

[0188] Conveniently, accommodating the voice coil 242 in the air gap 253 means that the air gap 253 will be comparatively wide and, hence, have a comparatively a high reluctance. This means that the total flux in the magnetic circuit is decreased and the flux guiding steel parts can be decreased in cross- section. The result is a powerful yet lightweight drive unit.

[0189] Where the moveable part 240 of the drive unit 230 includes the magnet unit 252 (see the below discussion of Figure 21), mass of the magnet unit 252 may typically be sufficiently large and may not need increasing.

[0190] The method includes a step S140 of constructing the drive unit 230 for substantially constant Qts(x) as function of displacement x across the intended normal working range of the loudspeaker module 20. Here, a high value for Qts(O), i.e. at rest in the air gap 242, is acceptable.

[0191] As mentioned above, the air gap 253 being comparatively wide for accommodating the unusually large voice coil 242 means that the stray magnetic flux density is comparatively high. This leads to a relatively smooth force factor BL(x) curve and so provides a relatively flat Qts(x) curve. As such, the high mass allocated to the voice coil 242 (in a moving voice coil example) may already contribute towards achieving a substantially constant Qts across the intended normal working range of the loudspeaker module 20. For a moving magnet unit example, the air gap 253 may be made wider to smooth the force factor BL(x) curve and to compensate the voice coil 242 may be made wider, such that a desired mass of the moveable part of the drive unit is achieved.

[0192] The method includes a step S150 of constructing the suspension of the moveable assembly 260 for substantially constant fs across the intended normal working range of the loudspeaker module 20.

[0193] For improved axial movement along the movement axis 202 and inhibiting of rocking, the suspension elements 262, 264 are positioned such that the centre of gravity of the moving assembly 260 has a position along the movement axis 202 which is between the landing surfaces 211 , 212 of the suspension elements 262, 264. This allows the suspension elements 262, 264 to be compliant and linear with respect to movement along the movement axis 202, leading to a linear fs(x) curve and dominance of the air suspension across the nominal working range of the loudspeaker module.

[0194] The method may further include a step S210 of choosing a target total quality factor Qtc. The target total quality factor Qtc may be chosen during manufacturing of the loudspeaker module 20 based on an intended application. The method further includes a step S220 of applying a set of filters to the input signal of the loudspeaker module 20 to alter the loudspeaker response in order to produce sound matching the target total quality factor Qtc.

[0195] The method further includes a step S310 of obtaining the loudspeaker module with the target total quality factor Qtc.

[0196] Figure 19 is a graph illustrating parameter ranges applicable to loudspeaker modules according to the present disclosure, such as the loudspeaker module 20. As such, Figure 19 may be used to estimate parameters during the design process, for example as described with reference to Figure 18.

[0197] The horizontal axis represents a normalised moving mass Mms / Sd, which corresponds to the mass Mms of the moveable assembly over the effective radiating surface Sd and ranges from 0 to 20 [kg / m2]. The vertical axis represents a normalised volume VB of the enclosure, which corresponds to the volume VB over the effective radiating surface Sd and ranges from 0 to 1 [m3 / m2]. Accordingly, Figure 19 is Sd normalised; and Kms / Kb is equal to 0.5.

[0198] A set of frequency curve (solid lines) corresponds to different in-box resonant frequencies of 20Hz, 30Hz, 40Hz, 50Hz, 60Hz, 70Hz and 80Hz, respectively. As such, Figure 19 relates the mass Mms of the moveable assembly, the volume VB of the loudspeaker module, and the resonant frequency fc.

[0199] The grey area indicates preferred parameter ranges, with darker areas being more preferred. Thus, Mms / Sd is preferably in a range of 5 to 15 [kg / m2], and more preferably in a range from 7 to 12 [kg / m2], VB / Sd is preferably greater than 0 and up to 0.5 [m], and more preferably in a range from 0.03 to 0.3 [m]. The in-box resonant frequency is preferably in a range from 30Hz to 80Hz, and more preferably in a range of 40Hz to 80Hz.

[0200] The top left corner of Figure 19 may be considered to represent conventional loudspeaker design where either the moving mass is small, or the volume VB is large, or both are small. The lower right corner is a preferred area, and relates to comparatively high masses Mms, such as 180g, in a volume VB such as 6 litres. These parameters may not be preferred for music reproduction but may be preferred for cinema applications.

[0201] As an example, when choosing a volume VB of 1 .5 litres, then in order to achieve a target in-box resonant frequency fc of 60Hz, according to Figure 19 the normalised moving mass Mms / Sd is estimated at approximately 8.5 Mms / Sd. For an effective radiating surface Sd of 110cm2, this would correspond to a moving mass of approximately 93.5g.

[0202] Such estimation may give a first indication of approximate parameter values during the design process. It is noted that effects of the mechanical suspension contribution may in practice not be negligible and have been taken into account in Figure 19 (Kms / KB=0.5).

[0203] Figure 20 shows a cross-section of a loudspeaker module 30 including two loudspeakers 200 as described above. Suitably, the loudspeaker module 30 further includes an enclosure 400 configured to receive the two loudspeakers 200. The enclosure 400 is arranged to receive sound produced by the second radiating surface 222 of the diaphragm 220 of each loudspeaker 200. Suitably, a volume VB 410 is formed in the loudspeaker module 20 into which the sound produced by the second radiating surface 222 is received. The volume VB 410 is 3 litres. The surface area of the diaphragm 220 of each loudspeaker 200 is 110cm2. Each loudspeaker 200 has a moving mass of 160g leading to an in-box resonance frequency fc of 51 Hz.

[0204] The loudspeakers 200 are provided in a force-cancelled arrangement. The first loudspeaker 200 and the second loudspeaker 200 are arranged in back-to-back configuration such that the first loudspeaker 200 and the second loudspeaker 200 face in opposite directions. The loudspeaker module 30 is operable to energise the voice coil 242 of the first loudspeaker 200 and the voice coil 242 of the second loudspeaker 200 to cause the voice coils 242 to move along the movement axis 202 in opposite directions, thereby moving the diaphragm 220 of the first loudspeaker 200 and the diaphragm 220 of the second loudspeaker 200 to produce sound.

[0205] Figure 21 shows a cross-section of another loudspeaker module 40 including two loudspeakers 700 mounted in the enclosure 400 in force-cancelled arrangement. The arrangement is generally the same as in Figure 20, but while Figure 20 illustrates a “moveable voice coil” example, Figure 21 illustrates a “moveable magnet” example.

[0206] Each loudspeaker 700 includes a loudspeaker frame 710, a diaphragm 720 suspended from the loudspeaker frame 710, and a drive unit 730 configured to move the diaphragm 720 along a movement axis 702 of the loudspeaker module 40 to produce sound from a first radiating surface 721 and a second radiating surface 722 of the diaphragm 720. A moveable part 740 of the drive unit 730 includes a magnet unit 742. A stationary part 750 of the drive unit 730 includes a voice coil 752 of the drive unit 730.

[0207] As described with reference to Figure 20, the loudspeakers 700 are arranged in back-to-back configuration such that they face in opposite directions. The loudspeaker module 40 is operable to energise the voice coil 752 of the first loudspeaker 700 and the voice coil 752 of the second loudspeaker 700 to cause the magnet unit 742 of the first loudspeaker 700 and the magnet unit 742 of the second loudspeaker 700 to move along the movement axis 702 in opposite directions, thereby moving the diaphragm 720 of the first loudspeaker 700 and the diaphragm 720 of the second loudspeaker 700 to produce sound.

[0208] The mass of the voice coil 752 is comparably high due to many layers and high winding height, whereas the mass of the magnet unit 742 is comparably low due to the small cross-section needed for the flux guiding elements of the magnet unit 742, a viable loudspeaker solution may be provided by including the magnet unit 742 in the moveable part 740 of the drive unit 730 and including the voice coil 752 in the stationary part 750 of the drive unit 730. In the present example, the moving mass is 100 grams for each loudspeaker 700, and the in-box resonant frequency fc is 64Hz.

[0209] Figure 22 is a schematic view of an automobile 2000. In this example, the automobile 2000 includes the loudspeaker module 20 but more generally any loudspeaker 20, 30, 40 or loudspeaker system 1000 as described above may be installed in the automobile 2000, or a combination thereof. In this example, the loudspeaker module 20 is provided between the footwells 2100 of the automobile 2000. Other locations are also envisaged.

[0210] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0211] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0212] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0213] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0214] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0215] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

[0216] References

[0217] A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.

[0218] 1. US3334184A (DE KONING)

[0219] 2. DE4332804C2 (KLIPPEL) R.H. Small, “Closed-Box Loudspeaker Systems, Part I: Analysis”, J. Audio Eng. Soc., vol. 20, pp. 798-808 (1972 December) W. Klippel, U. Seidel, “Fast and Accurate Measurement of Linear Transducer Parameters” presented at the 110th Convention of the Audio Engineering Society,

[0220] Amsterdam, May 12-15, 2001 , preprint 5308 W. Klippel, “Distortion Analyzer - a New Tool for Assessing and Improving Electrodynamic Transducers,” presented at the 108th Convention of the Audio Engineering Society, Paris, February 19-22, 2000, preprint 5109 V. Dickason, Loudspeaker Design Cookbook, 7th Edition, KCP Media Corp., 2006 PCT / EP2023 / 073013 (VUINE) GB2212935.7 (VUINE) L.L. Beranek, Acoustics, McGraw-Hill, 1954

Claims

Claims:1 . A loudspeaker module for producing sound at bass frequencies, the loudspeaker module including: a loudspeaker frame; a diaphragm suspended from the loudspeaker frame; a drive unit configured to move the diaphragm along a movement axis of the loudspeaker module to produce sound from a first radiating surface and a second radiating surface of the diaphragm, wherein the first and second radiating surfaces are on opposite sides of the diaphragm; wherein the drive unit includes a magnet unit to generate magnetic flux across an air gap and a voice coil configured to be located in the air gap when the diaphragm is rest; and an enclosure arranged to receive sound produced by the second radiating surface of the diaphragm; wherein the loudspeaker module has a total quality factor Qtc of at least 1 .8 when the diaphragm is at rest; and wherein a magnetic flux density at a first perimeter of the voice coil is 50% or less of a magnetic flux density at a second perimeter of the voice coil when the diaphragm is at rest.

2. The loudspeaker module according to any preceding claim, wherein the loudspeaker module has a total quality factor Qtc of at least 2.5 when the diaphragm is at rest; optionally at least 3 when the diaphragm is at rest.

3. The loudspeaker module according to any preceding claim, wherein the drive unit has a total quality factor Qtc of up to 10 when the diaphragm is at rest; optionally up to 5 when the diaphragm is at rest.

4. The loudspeaker module according to any preceding claim, wherein the drive unit has a mechanical quality factor Qms in a range of 3 to 8 when the diaphragm is at rest.

5. The loudspeaker module according to any preceding claim, wherein the drive unit has an electrodynamic quality factor Qes in a range of 1 .2 to 2.5 when the diaphragm is at rest.

6. The loudspeaker module according to any preceding claim, wherein the total quality factor Qtc has a first Qtc value when the diaphragm is at rest and a second Qtc value when the diaphragm is at a nominal maximum displacement; wherein the second Qtc value is in a range of 1 .00 to 2 times the first Qtc value, and optionally in a range of 1 .00 to 1 .25 times the first Qtc value.

7. The loudspeaker module according to any preceding claim, wherein the loudspeaker module has a resonant frequency fc which has a first resonant frequency value when the diaphragm is at rest and a second resonant frequency value when the diaphragm is at a nominal maximum displacement; wherein the second resonant frequency value is in a range of 1 .00 to 1 .10 times the first resonant frequency value.

8. The loudspeaker module according to any preceding claim, wherein wherein the loudspeaker module has a force factor BL which has a first BL value when the diaphragm is at rest and a second BL value when the diaphragm is at nominal maximum displacement; wherein the second BL value is at least 0.5 times the first BL value.

9. The loudspeaker module according to any preceding claim, wherein a ratio of driver stiffness Kms over box stiffness Kb is in a range of 0.25 to 0.75.

10. The loudspeaker module according to any preceding claim, wherein the loudspeaker module includes a moveable assembly which includes the diaphragm and a moveable part of the drive unit.11 . The loudspeaker module according to claim 10, wherein the loudspeaker module is operable to cause the moveable part and a stationary part of the drive unit to magnetically cooperate to cause the moveable assembly to move along the movement axis to produce sound; and a ratio of the mass of the moveable part of the drive unit and an effective sound radiating area Sd of the diaphragm is at least 5 kilograms per square-metre, optionally at least 7 kilograms per squaremetre.

12. The loudspeaker module according to claim 9 or 10, wherein the mass of the moveable part of the drive unit may be at least 60% of the mass of the moveable assembly.

13. The loudspeaker module according to any preceding claim, wherein the loudspeaker module encloses an acoustic effective volume VB; the ratio of the volume VB over an effective sound radiating area Sd of the diaphragm is in a range of 0.01 to 0.5 metres.

14. A loudspeaker system that includes the loudspeaker module according to any preceding claim, wherein the loudspeaker system further includes a signal processor configured to adjust an electrical signal to be supplied to the voice coil,wherein the signal processor is configured to carry out frequency-dependent adjustment of the electrical signal.

15. The loudspeaker module according to claim, wherein the signal processor is the signal processor is configured to carry out frequency-dependent adjustment of the electrical signal so as to provide feedforward filtering of the electrical signal.

16. The loudspeaker module according to claim 14 or 15, wherein the signal processor is configured to decrease the group delay at and around resonant frequency of the loudspeaker module.

17. The loudspeaker module according to any one of claims 14 to 16, wherein the signal processor is user configurable between a first frequency-dependent adjustment and a second frequency-dependent adjustment.

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