Loudspeaker systems and methods

Abstract

There is provided a loudspeaker system comprising a housing; a first loudspeaker and a second loudspeaker mounted in the housing; wherein the loudspeaker system is operable to cause a first diaphragm of the first loudspeaker and a second diaphragm of the second loudspeaker to move along a common movement axis in opposite directions to produce sound; a separation sheet arranged to divide an internal volume enclosed by the housing into a first portion accommodating the first loudspeaker and a second portion accommodating the second loudspeaker; wherein an inwardly-facing radiating surface of the first diaphragm is arranged to radiate sound into the first portion of the internal volume enclosed by the housing and an inwardly-facing radiating surface of the second diaphragm is arranged to radiate sound into the second portion of the internal volume enclosed by the housing; wherein a thickness of the separation sheet is 2 millimetres or less.

Description

[0001] LOUDSPEAKER SYSTEMS AND METHODS

[0002] This application claims priority to GB2302099.3, filed 14 February 2023.

[0003] Field of the Invention

[0004] The present invention relates to loudspeaker systems and methods.

[0005] Background

[0006] A typical conventional loudspeaker has a diaphragm and a drive unit for the reproduction of sound. In use, the drive unit causes the diaphragm, which acts as a piston, to move forwards and backwards to generate pressure waves, i.e. sound.

[0007] In some applications, one or multiple conventional loudspeakers are mounted in a housing, such as a wooden cabinet. A loudspeaker system with two loudspeakers in a shared enclosure may be of particular interest as these can be arranged in a force-cancelled configuration. In this configuration, the two loudspeakers are positioned on opposite sides of the housing and face in opposite directions such that the forces generated by drive units cancel out as they are of equal magnitude and in opposite direction.

[0008] Where multiple loudspeakers are mounted in the same housing, the loudspeakers may share a single large internal volume or be in separated individual volumes, whereby a separation wall divides the internal volume among the loudspeakers. Figure 1 is a sectional view of a loudspeaker system with two loudspeakers 1 , 2 mounted in a housing 3 (or ‘box’) and sharing a single internal volume 4. Figure 2 is another sectional view showing the same configuration but additionally with a separation wall 5 splitting the internal volume 4 and the loudspeakers 1 , 2 in separate portions 6, 7 of the internal volume.

[0009] The separation wall 5 is added to strengthen the housing so as to better withstand internal pressure changes as a result of operation of the loudspeakers 1 , 2. However, the separation wall 5 comes at increased material usage, weight, and cost. Addition of the separation wall 5 also decreases the available net volume for given outside dimensions of the housing 3. Typically, the separation wall 5 is made from the same material and same material thickness as the rest of the housing 5; for wooden cabinets this may be in the range of 8mm to 25mm (millimetres), for small injection-moulded plastic enclosures it may be in the range of 2mm to 6mm.

[0010] Summary of the Invention

[0011] It is believed by the present inventors to be an accepted teaching in the art that loudspeaker performance of a loudspeaker system is not affected by whether the loudspeakers act on a shared internal volume or on individual internal volumes of half the size, i.e. loudspeaker performance is deemed independent of the presence or absence of a separation wall. The present inventors further believe that the accepted teaching is that identical loudspeakers driven by the same signal will show identical behaviour and, hence, not interfere with each other. For at least some loudspeaker systems, calculated parameters such as the in-box resonance frequency for each loudspeaker are identical for configurations with and without a separation wall.

[0012] However, the present inventors have observed that the accepted teaching as described above may not be applicable to some loudspeaker configurations. Such loudspeaker configurations may involve comparatively high input power, high moving mass, or a small housing; and especially those loudspeaker configurations with high input power and high moving mass and a small housing, e.g. small low-frequency loudspeaker systems. The inventors observed that for such loudspeaker configurations small differences between two loudspeakers may result in different behaviour between the loudspeakers that may become relevant to loudspeaker performance. Notably, a difference in loudspeaker performance may be observable even for loudspeakers considered ‘identical’, e.g. loudspeakers from the same production batch with differences arising from manufacturing tolerances and the assembly process. The inventors observed that, when the loudspeakers share a common volume, these differences may in use cause interaction between the diaphragms and so decrease overall performance of the loudspeaker system. For example, there may be an audible and measurable effect at high input power. Thus, splitting the internal volume between the loudspeakers may be preferable to optimise loudspeaker performance.

[0013] Accordingly, the present inventors consider it may be desirable to split the volume, whilst also addressing the issues described above relating to material usage, weight, cost for a traditional separation wall. The present inventors have found that this may be achieved by replacing the typically thick separation wall with a substantially thinner structure, thereby it may be possible to reduce material usage, weight and cost, whilst still deriving a benefit from splitting the volume. The substantially thinner structure may have such a small thickness that the structure is more aptly described as a separation sheet or foil. Without wishing to be bound by theory, the present inventors believe that in operation the pressure on each side of the separation wall is almost the same and any load experienced by the structure is due to the differences in pressure as a result of the differences in loudspeaker motion, which allows a thinner wall to be used.

[0014] According to a first aspect of the invention, there is provided a loudspeaker system comprising a housing, a first loudspeaker and a second loudspeaker mounted in the housing. The loudspeaker system is operable to cause a first diaphragm of the first loudspeaker and a second diaphragm of the second loudspeaker to move along a common movement axis in opposite directions to produce sound. A separation sheet of material is arranged to divide an internal volume enclosed by the housing into a first portion accommodating the first loudspeaker and a second portion accommodating the second loudspeaker. An inwardly-facing radiating surface of the first diaphragm is arranged to radiate sound into the first portion of the internal volume enclosed by the housing and an inwardly-facing radiating surface of the second diaphragm is arranged to radiate sound into the second portion of the internal volume enclosed by the housing. The separation sheet has a thickness of 2mm or less, where “mm” represents the physical unit “millimetre”. As set out above, the load experienced by the separation sheet may be relatively small since such a load is the result of any pressure differences between the first portion of the internal volume and the second portion of the internal volume. As such, a comparatively thin separation sheet may suffice to withstand these loads at reduced material usage, weight and cost compared to a traditional separation wall, whilst providing an improvement in sound reproduction as in a traditional loudspeaker with separation wall.

[0015] The thickness of the separation sheet may be uniform, i.e. constant across the separation sheet, or may differ across regions of the separation sheet. As such, the thickness of the separation sheet is understood to be 2mm or less in at least some region of the separation sheet.

[0016] The thickness of the separation sheet may be in a range of 0.1 mm to 2mm.

[0017] The thickness of the separation sheet may be 1 .5mm or less. For example, the thickness of the separation sheet may be in a range of 0.1 mm to 1 .5mm.

[0018] The thickness of the separation sheet may be 1 mm or less. For example, the thickness of the separation sheet may be in a range of 0.1 mm to 1 mm.

[0019] The thickness of the separation sheet may be measured in a direction parallel to the common movement axis.

[0020] In some examples, the thickness of the separation sheet may have a thickness as described herein (e.g. 2mm or less) across substantially the entirety of the separation sheet.

[0021] The separation sheet may extend across the internal volume of the housing in a transverse direction relative to the movement axis, optionally in a direction perpendicular to the common movement axis.

[0022] The material of the housing and the material of the separation sheet may be different.

[0023] The separation sheet may be manufactured separately from the housing and hence a suitable choice of material for the separation sheet may be made independently of the choice of material for the housing.

[0024] The material of the separation sheet may be metal, for example a ferromagnetic metal.

[0025] By providing a ferromagnetic separation sheet, magnetic interaction between the drive units of the first loudspeaker and the second loudspeaker may be reduced. This may be particularly desirable in examples in which the drive units include stationary voice coils and moveable magnet units. In such examples, the ferromagnetic separation sheet may become saturated with magnetic when flux when the magnet units of approach the separation sheet, thereby causing mutual repulsion of the magnet units. By providing the ferromagnetic separation sheet and allowing for magnetic saturation thereof, performance of the loudspeaker system may be improved and, at the same time, safe operation ensured even when operated at peak power operation.

[0026] The material of the separation sheet may be a polymer, for example a fibre-reinforced polymer.

[0027] The loudspeaker system may further comprise a reinforcement structure which extends across the internal volume of the housing. The reinforcement structure may extend across the internal volume of the housing in a transverse direction relative to the movement axis, optionally in a direction perpendicular to the common movement axis.

[0028] The reinforcement structure may extend across the separation sheet and may be configured to inhibit movement of the separation sheet along the common movement axis. For example, the reinforcement structure may extend across the separation sheet such that the reinforcement structure is in contact with the separation sheet, e.g. with the reinforcement structure being attached to the separation sheet or pressing against the separation sheet.

[0029] By providing the reinforcement structure to inhibit movement of the separation sheet, it may be ensured that the separation sheet does not radiate sound or introduce unwanted conditions for the loudspeakers, e.g. through resonances in the working range of the loudspeakers acting on the split volumes.

[0030] The reinforcement structure may include multiple portions formed separately and which may be arranged separately about the separation sheet. For example, the reinforcement structure may include a central structure and an outer structure, which may be separate.

[0031] The central structure may be formed by either or both of the first loudspeaker and the second loudspeaker. For example, the central structure may be formed by either or both of a first frame of the first loudspeaker and a second frame of the second loudspeaker.

[0032] The outer structure may extend from the housing towards the central structure. In some examples, the outer structure is formed integrally with the housing.

[0033] The reinforcement structure may include a rib structure which contacts the separation sheet. For example, the rib structure may be arranged to press against the separation sheet or the rib structure may be configured to adhere to the separation sheet, e.g. through adhesive, to provide contact between the rib structure and the separation sheet.

[0034] The rib structure may form a plurality of apertures extending through the reinforcement structure in a direction along the common movement axis.

[0035] By contacting the separation sheet with the rib structure including the plurality of apertures, the separation sheet is divided into a corresponding plurality of cells (or ‘sheet portion’). That is to say, a cell is formed where the material of the separation sheet extends across an aperture. Thus dividing the separation sheet by the rib structure may cause the drum resonance of each cell to be above the working range of the loudspeakers acting on the volumes.

[0036] The first eigenfrequency of each cell may be at least 50Hz, where “Hz” represents the physical unit “Hertz”. Preferably, the first drum mode of each cell may be at least 150Hz, and more preferably at least 300Hz. The first drum mode of each cell may be above the operating range of the loudspeaker system. By sizing the cells of the rib structure to achieve this effect, it is possible to reduce the extent to which the first drum mode of each cell interferes with sound production by the loudspeakers, particularly if the loudspeakers are configured to produce sound with frequencies in a bass frequency range. The area of an aperture as projected onto a plane perpendicular to the common movement axis may correspond to the size of said aperture.

[0037] A plurality of reinforcement structures as described above may be provided. A first reinforcement structure of the plurality of reinforcement structures may be located in the first portion of the internal volume. A second reinforcement structure of the plurality of reinforcement structures may be located in the second portion of the internal volume.

[0038] The separation sheet may be sandwiched between the first reinforcement structure and the second reinforcement structure.

[0039] By sandwiching the separation sheet between first and second reinforcement structures, movement of the separation sheet along the common movement axis may be inhibited. Here “sandwiching” is understood to describe a configuration wherein the separation sheet is located between the reinforcement structures, without the reinforcement structures necessarily exerting pressure on the separation sheet. In some examples, the reinforcement structures clamp the separation sheet.

[0040] The housing may include a first housing portion and a second housing portion which are substantially symmetric about a plane of symmetry coinciding with the separation sheet.

[0041] The separation sheet may be sandwiched between the first housing portion and the second housing portion. For example, the separation sheet may be attached to the first housing portion and the second housing portion, e.g. using suitable adhesive. The first housing portion and the second housing portion may directly engage each other in at least some locations or may engage only the separation sheet.

[0042] The loudspeaker system may further comprise a waveguide configured to receive sound radiated by an outwardly-facing radiating surface of the second diaphragm and to guide the sound to a waveguide outlet.

[0043] The waveguide may be attached to the housing or may be a separate structure arranged about the housing, e.g. the waveguide may provide a surface on which sound radiated by the second diaphragm is incident. In some examples, the separate structure may be a portion of an automobile extending about the loudspeaker system mounted in said automobile.

[0044] When the loudspeaker system is in use, the waveguide may result in an air load to the second diaphragm which exceeds an air load of the first diaphragm. That is to say, the air load experienced by the first loudspeaker and the second loudspeaker may be different when the loudspeaker system is in use, (at least in part) as a result of the waveguide, such that the air load of the second loudspeaker is greater (when the loudspeaker system is in use).

[0045] In some examples, there may be an additional waveguide configured to receive sound radiated by an outwardly-facing radiating surface of the first diaphragm, such that also the air load experienced by the first diaphragm is increased as a result of the waveguide but less so than for the second diaphragm. In some examples, the waveguide configured to receive sound radiated by an outwardly-facing radiating surface of the second diaphragm and (if present) the additional waveguide configured to receive sound radiated by an outwardly-facing radiating surface of the first diaphragm may form part of the same structure (in other examples they may be separate structures).

[0046] An outwardly-facing radiating surface of the first diaphragm and the waveguide outlet may be arranged to radiate sound in the same direction. Alternatively, the outwardly-facing radiating surface of the first diaphragm and the waveguide outlet may be arranged to radiate sound into different directions, e.g. perpendicular directions.

[0047] A mechanical moving mass of the first loudspeaker and a mechanical moving mass of the second loudspeaker may be different. The mechanical moving mass may be understood to refer to the mass of a moving assembly of a loudspeaker (i.e. an assembly which moves relative to the housing when the loudspeaker is in use), as measurable by weighing the moving assembly, and may include the mass of the diaphragm. The mechanical moving mass may further include the mass of a moveable part of a drive unit of the loudspeaker.

[0048] The mechanical moving mass of the moving assembly of the first loudspeaker may be larger than the mechanical moving mass of the moving assembly of the second loudspeaker, which may reduce (preferably minimise) vibrations of the housing when the loudspeaker system is in use.

[0049] By having a larger mechanical moving mass of the first loudspeaker, it may be possible to offset (e.g. compensate for) an increased air load on the moving assembly of the second loudspeaker caused by the presence of the waveguide when the loudspeaker system is in use. In an asymmetric loudspeaker system with two loudspeakers, the air load experienced by (the moving assembly of) one loudspeaker may be greater than the air load experienced by (the moving assembly of) the other loudspeaker, when the loudspeaker system is in use. For example, the waveguide described above may result in an air load experienced by (the moving assembly of) the second loudspeaker which may exceed an air load experienced by (the moving assembly of) the first loudspeaker, when the loudspeaker system is in use. Thus, it may be desirable to have a larger mechanical moving mass of the first loudspeaker configured to balance the additional air load experienced by the second loudspeaker, when the loudspeaker system is in use.

[0050] The skilled person will be familiar with the notion of the “air load”, which is a physical quantity used for describing the response a moving assembly moving in air as an additional mass of said moving assembly.

[0051] According to a second aspect, there is provided a loudspeaker system comprising a housing; a first loudspeaker and a second loudspeaker mounted in the housing; wherein the loudspeaker system is operable to cause a first moving assembly of the first loudspeaker and a second moving assembly of the second loudspeaker to move along a common movement axis in opposite directions to produce sound; a waveguide configured to receive sound radiated by the second moving assembly and to guide the sound to a waveguide outlet; wherein a mechanical moving mass of the first moving assembly is larger than a mechanical moving mass of the second moving assembly. As set out above, by having a larger mechanical moving mass of the first loudspeaker, it may be possible to offset (e.g. compensate for) an air load on the second loudspeaker caused by the presence of the waveguide when the loudspeaker is in use.

[0052] The mechanical moving mass of the first moving assembly may be larger than the mechanical moving mass of the second moving assembly by a mass difference provided by a balance weight included in the first moving assembly.

[0053] The balance weight may be a cap attached to the first diaphragm.

[0054] The balance weight may form part of the first diaphragm, e.g. integrally formed with the first diaphragm. For example, the first diaphragm may be thicker than the second diaphragm.

[0055] The mechanical moving mass of the first loudspeaker may be larger than the mechanical moving mass of the second loudspeaker by at least 1g, where “g” represents the physical unit “gram”. Preferably, the difference in mechanical moving mass may be at least 3g and, more preferably, at least 4g. In some examples, the difference in mechanical moving mass between the first loudspeaker and the second loudspeaker may be in a range of 1g to 50g, preferably 3g to 30g and more preferably 4g to 15g.

[0056] The loudspeaker system according to the second aspect may include a separation sheet described in relation to the first aspect.

[0057] The loudspeaker system may include a separation sheet of material arranged to divide an internal volume enclosed by the housing into a first portion accommodating the first loudspeaker and a second portion accommodating the second loudspeaker; the first moving assembly may include a first diaphragm and the second moving assembly may include a second diaphragm; wherein an inwardly-facing radiating surface of the first diaphragm may be arranged to radiate sound into the first portion of the internal volume enclosed by the housing and an inwardly-facing radiating surface of the second diaphragm may be arranged to radiate sound into the second portion of the internal volume enclosed by the housing; wherein a thickness of the separation sheet may be 2 millimetres or less.

[0058] The loudspeaker system according to the first aspect or the second aspect may be configured to produce sound with frequencies in a bass frequency range. The bass frequency range may include 60-80Hz, where “Hz” represents the physical unit “Hertz”. More preferably, the bass frequency range may include 40-100Hz. By way of example, the bass frequency range may be 20Hz-100Hz.

[0059] The loudspeaker system according to the first aspect or the second aspect may be provided in an automobile. More particularly, the loudspeaker system may be provided at a footwell or under the seat of the automobile, or indeed in any other location suitable for packaging a bass loudspeaker system in the automobile.

[0060] Each drive unit may include a permanent magnet and a voice coil. The permanent magnet may have a mass which is smaller than a mass of the voice coil. In some examples, the mass of the permanent magnet may be smaller than the mass of the voice coil by at least a factor of two, preferably at least a factor of 2.5, e.g. 2.8. The drive unit may form a magnetic circuit with a magnetic reluctance of at least 2.5 x 10A6 [1 / H] or even 3 x 10A6 [1 / H], where “H” represents the physical unit “Henry”.

[0061] The drive unit may include an air gap with a magnetic reluctance of at least 2 x 10A6 [1 / H].

[0062] By utilising a magnetic circuit with high magnetic reluctance, and particularly a high-reluctance air gap, it is possible to utilise comparatively small flux guiding elements. Thus, it is possible to reduce the weight of the magnet unit. This weight reduction of the magnet unit may more than compensate for the weight of a large voice coil, meaning that the comparatively high magnetic reluctance of the magnetic circuit enables designing of particularly lightweight loudspeakers. Such considerations may be relevant especially for applications in, for example, the automobile industry.

[0063] The internal volume may have a size (or ‘volume’) in a range of 0.25 litres to 5 litres. For example, the size of the internal volume may be approximately 3 litres, and the first portion and the second portion of the internal volume may have a size of approximately 1 .5 litres each.

[0064] According to a third aspect, there is provided a method of reducing vibrations of a loudspeaker system. The method comprises providing a loudspeaker system including a housing comprising a first loudspeaker, a second loudspeaker and a waveguide arranged to guide sound generated by the second loudspeaker, wherein the loudspeaker system is operable to cause a first moving assembly of the first loudspeaker and a second moving assembly of the second loudspeaker to move along a common movement axis in opposite directions to produce sound; operating the loudspeaker system and measuring vibrations of the housing with a first balance weight included in the first moving assembly; operating the loudspeaker system and measuring vibrations of the housing with a second balance weight included in the first moving assembly; identifying a balance weight mass corresponding to a reduction in vibrations by comparing vibrations measured with the first balance weight added to the first loudspeaker system against vibrations measured with the second balance weight added to the first loudspeaker system.

[0065] By measuring vibrations of the housing with different balance weights included in the first moving assembly, e.g. the first balance weight and the second balance weight, it is possible to compare vibration measurements in order to identify a reduction in vibrations. As such, it is possible to iteratively reduce vibrations until a balance weight mass is identified which achieves a desired reduction of vibrations. The loudspeaker system may then be adjusted to use a balance weight having that mass, so as to operate the loudspeaker system with reduced vibrations.

[0066] The method may further comprise balancing of another loudspeaker system. The method of balancing may comprise balancing the other loudspeaker system by adding a balance weight with the identified balance weight mass to a first moving assembly of the other loudspeaker system.

[0067] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. In particular, any feature described in relation to the first aspect of the invention may be combined with any feature described in relation to the second aspect of the invention. Summary of the Figures

[0068] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0069] Figure 1 is a cross-sectional view with of a loudspeaker system believed to be known.

[0070] Figure 2 is another cross-sectional view of the loudspeaker system of Figure 1 , additionally including a separation wall.

[0071] Figure 3 is a cross-sectional view of a loudspeaker system according to the present disclosure.

[0072] Figure 4 is another cross-sectional view of the loudspeaker system of Figure 3, showing part of the loudspeaker system.

[0073] Figure 5 is a cross-sectional view of a part of a reinforcement structure.

[0074] Figure 6 is a cross-sectional view of another part of the reinforcement structure.

[0075] Figure 7 is a plan view of the reinforcement structure and a separation sheet.

[0076] Figure 8 is an exploded view of a part of the separation sheet sandwiched between reinforcement structures.

[0077] Figure 9 illustrates a mode shape of a cell of the separation sheet bounded by the reinforcement structure.

[0078] Figure 10 is a graph relating to a first loudspeaker of a loudspeaker system without a separation sheet.

[0079] Figure 11 is a graph relating to a second loudspeaker of the loudspeaker system of Figure 10.

[0080] Figure 12 is a graph relating to a first loudspeaker of a loudspeaker system with a separation sheet.

[0081] Figure 13 is a graph relating to a second loudspeaker of the loudspeaker system of Figure 12.

[0082] Figure 14 is a graph relating to the loudspeaker system of Figure 12.

[0083] Figure 15 is a cross-sectional view of another loudspeaker system according to the present disclosure.

[0084] Figure 16 is a front view of the loudspeaker system of Figure 15.

[0085] Figure 17 is a graph relating to the loudspeaker system of Figure 15.

[0086] Figure 18 illustrates a method of measuring vibrations of a loudspeaker system.

[0087] Figure 19 illustrates a method of balancing a loudspeaker system.

[0088] Figure 20 illustrates a simplified loudspeaker system for finite element analysis.

[0089] Figure 21 is a graph relating to the simplified loudspeaker system of Figure 20.

[0090] Figure 22 is another graph relating to the simplified loudspeaker system of Figure 20.

[0091] Figure 23 is a cross-sectional view of another exemplary loudspeaker system.

[0092] Figure 24 is a cross-sectional view of another exemplary loudspeaker system. Figure 25 is a cross-sectional view of another exemplary loudspeaker system.

[0093] Figure 26 illustrates an automobile with the loudspeaker system of Figure 15.

[0094] Detailed Description of the Invention

[0095] 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.

[0096] Figure 3 is a cross-sectional view of an exemplary loudspeaker system 100. The exemplary loudspeaker system 100 is configured to produce sound with frequencies in a bass frequency range including 60- 80Hz, where “Hz” represents the physical unit “Hertz”.

[0097] The loudspeaker system 100 includes a housing 102 (or ‘enclosure’).

[0098] The loudspeaker system 100 includes an internal volume 104 enclosed by the housing 102.

[0099] The loudspeaker system 100 includes a separation sheet of material 110 (or ‘separation foil’) arranged in the housing 102 such that the internal volume 104 is divided into a first portion 106 and a second portion 108. As is further discussed below, fluid communication between the first portion 106 and the second portion 108 of the internal volume 104 is inhibited by the separation sheet 110.

[0100] In Figure 3, the separation foil 110 is located at the symmetry plane between the loudspeakers 120, 130 and splits the internal volume 104 into substantially equal halves, i.e. the first portion 106 and the second portion 108 of the internal volume 104 are substantially identical.

[0101] The loudspeaker system 100 includes a first loudspeaker 120 and a second loudspeaker 130. The first loudspeaker 120 and the second loudspeaker 130 are mounted in the housing 102 such that the first loudspeaker 120 is accommodated in the first portion 106 of the internal volume 104 and the second loudspeaker 130 is accommodated in the second portion 108 of the internal volume 104.

[0102] The loudspeaker system 100 is operable to cause the first loudspeaker 120 and the second loudspeaker 130 to move along a common movement axis 109 in opposite directions to produce sound. A portion of the sound so generated is radiated into the internal volume 104. More particularly, the first loudspeaker 120 radiates into the first portion 106 of the internal volume 104 and the second loudspeaker 130 radiates into the second portion 108 of the internal volume 104.

[0103] A thickness of the separation sheet is 2 millimetres or less. In operation, each loudspeaker 120, 130 generates pressure in the respective portion 106, 108 of the internal volume 104. The net load on the separation sheet 110 is the difference between these pressures in the first volume 106 and the second volume 108. Accordingly, the separation sheet 110 experiences a load which is small when compared to the individual pressures and pressure changes in the portions 106, 108 of the internal volume 104.

[0104] Therefore, the separation sheet 110, despite being comparatively thin, is sufficient for splitting the internal volume 104. Such a configuration involving the comparatively thin separation sheet 110 may be particularly desirable for small loudspeaker systems intended for low frequency radiation. In such systems, a traditional separation wall, with greater thickness, would substantially decrease the available net volume in addition to increasing material usage, weight and cost. By contrast, the loudspeaker system 100 includes a thin and lightweight volume separation which may involve very little additional cost, virtually no decrease in volume, or increase in product weight.

[0105] Figure 4 shows one half of the loudspeaker system 100, i.e. one of the loudspeakers 120, 130 in the corresponding portion of the housing 102. For the exemplary loudspeaker system 100, the loudspeakers 120, 130 are identical and therefore Figure 4 is applicable to both loudspeakers 120, 130. The separation sheet 110 is not shown in Figure 4.

[0106] Each loudspeaker 120, 130 includes a frame 150; a diaphragm 160 suspended from the frame 150; and a drive unit 170.

[0107] The diaphragm 160 of each loudspeaker 120, 130 has an inwardly-radiating surface 162 (or ‘first sound radiating surface’) and an outwardly-radiating surface 164 (or ‘second sound radiating surface’). The inwardly-radiating surfaces 162 are arranged to radiate sound into the internal volume 104 in the housing 102. More particularly, the inwardly-facing radiating surface 162 of the diaphragm 160 of the first loudspeaker 120 is arranged to radiate sound into the first portion 106 of the internal volume 104, and the inwardly-facing radiating surface 162 of the diaphragm 160 of the second loudspeaker 130 is arranged to radiate sound into the second portion 108 of the internal volume 104. The internal volume 104 is closed, such that sound radiated into the internal volume 104 is captured. The outwardly-radiating surfaces 164 are arranged to radiate sound away from the loudspeaker system 100.

[0108] The drive unit 170 of each loudspeaker 120, 130 has a stationary part 171 and a translatable part 172. In Figure 3, the loudspeaker system 100 has the two loudspeakers 120, 130 symmetrically mounted in a back-to-back configuration such that the stationary parts 171 of the drive units 170 are brought together.

[0109] The stationary part 171 of the drive unit 170 is secured to the frame 150 while the translatable part 172 is secured to the diaphragm 160 and arranged to move with the diaphragm 160. The translatable part 174 and the diaphragm 160 of each loudspeaker 120, 130 form a moving assembly 180. Each moving assembly 180 is suspended from the frame 150 by a first suspension element and a second suspension element. In this example, the first suspension element is provided as a surround and a second suspension element is provided as a damper.

[0110] The stationary part 171 of the drive unit 170 includes a voice coil 173 and the translatable part 172 includes a magnet unit 174 configured to produce a magnetic field in an air gap 175. When the diaphragm 160 is at rest, the voice coil 173 sits in the air gap 175.

[0111] The loudspeaker system 100 is operable to cause the moving assembly 180 of the first loudspeaker 120 and the moving assembly 180 of the second loudspeaker 130 to move along the common movement axis 109 in opposite directions to produce sound. More particularly, the diaphragm 160 of the first loudspeaker 120 and the diaphragm 160 of the second loudspeaker 130 move to produce sound. Figures 5, 6, 7 and 8 illustrate a reinforcement structure 140. The reinforcement structure 140 comprises a central structure 142 shown in Figure 5; an outer structure 144 shown in Figure 6; Figure 7 shows the reinforcement structure 140 arranged across the separation sheet 110; Figure 8 is an exploded view showing the separation sheet 110 sandwiched between two reinforcement structures 140.

[0112] The reinforcement structure 140 is arranged to inhibit excursion of the separation sheet 110 along the common movement axis 109, as may otherwise be caused by a pressure difference between the portions 106, 108 of the internal volume 104. Suitably, the reinforcement structure 140 is stationary, e.g. secured to the housing 102 or the frame 150 of either loudspeaker 120, 130.

[0113] In this example, the reinforcement structure 140 extends across the internal volume 104 in the housing 102 in a direction perpendicular to the common movement axis 109. In Figure 3, the central structure 142 and the outer structure 144 span the whole internal volume 104.

[0114] In this example, the reinforcement structure 140 includes multiple portions 142, 144 formed separately. More particularly, the reinforcement structure 140 includes the central structure 142 and the outer structure 144, which are formed separately and, when assembled, both contact the separation sheet 1 10.

[0115] The central structure 142 corresponds to a portion of the loudspeaker frame 150 of the loudspeakers 120, 130. That is to say, the back of the frame 150 is shaped to provide the central structure 142, e.g. the one shown in Figure 5. In other words, the loudspeaker frame 150 is in the direct vicinity of the separation sheet 1 10 such that the central structure 142 of the frame 150 provides a stiffening structure for the separation sheet 110.

[0116] In this example, the outer structure 144 is an integral part of the housing 102.

[0117] The reinforcement structure 140 includes a rib structure 146 which contacts the separation sheet 110. The rib structure 146 which forms a plurality of apertures 148 extending through the rib structure 146 in a direction along the common movement axis 109.

[0118] The separation sheet 110 and the rib structure 146 are in contact such that the separation sheet 110 is divided into a plurality of cells 1 12. Each cell 112 corresponds to a region where the separation sheet 110 extends across one of the apertures 148. Each cell is preferably sized such that the first eigenfrequency of each cell 112, i.e. the first harmonic of each cell 112, is above the working range of the loudspeakers 120, 130. Here “first” eigenfrequency is understood, as is conventional, to refer to the lowest eigenfrequency.

[0119] By raising the first eigenfrequency of each cell 112 above the working range of the loudspeakers 120, 130, this may prevent driving the separation sheet 1 10 at resonance when operating the loudspeakers 120, 130 at a frequency within the working range of the loudspeakers 120, 130, since the first eigenfrequency of each of the cells 112 exceeds the working range of the loudspeakers 120, 130.

[0120] In other words, the separation sheet 1 10 is fixed and clamped from both sides and so mechanically split into smaller portions, i.e. the cells -112. That is, the structure is preferably designed such that the first eigenfrequency of each of the cells 112 of the separation sheet 110 is above the working range of the loudspeakers 120, 130 acting on the portions 106, 108 of the internal volume 104.

[0121] It is noted that some traditional loudspeaker frames and cabinets have ribs to improve mechanical stiffness. Here, the rib structure is preferably further arranged so that the rib structure supports and contacts the separation sheet 110 such that no cell 112 of the separation sheet 110 is subject to modal behaviour within the working range of the loudspeaker system 100.

[0122] Sizing of the cells 112 to suitably increase the first eigenfrequency of said cells 112 may depend on the separation sheet 110, such as the material choice and thickness of the separation sheet 110. Sizing of the cells 112 may also be affected by pre-tensioning of the separation sheet 110.

[0123] Figure 9 illustrates oscillation of a single cell 112 of the separation sheet 110 at the first eigenfrequency of the cell 112. As shown in Figure 9, the ribs 144 bound the aperture 148 which determines the size of the cell 112. Here, oscillation of the cell 112 is illustrated by the crossed solid lines (representing displacement of the cell 112 in an upwards direction in Figure 9) and the crossed dashed lines (representing displacement of the cell 112 in a downwards direction in Figure 9).

[0124] As outlined above, the cells 112 are configured such that the first eigenfrequency of each cell 112 is above the working range of the loudspeaker system 100. As such, the oscillation shown in Figure 9 is not expected when operating the loudspeaker system 100 within the working range of frequencies.

[0125] As shown in Figures 3 and 8, a reinforcement structure 140 is provided on both sides of the separation sheet 110. That is to say, a first reinforcement structure 140 is provided in the first portion 106 of the internal volume 104 and a second reinforcement structure 140 is provided in the second portion 108 of the internal volume 104.

[0126] Each reinforcement structure 140 extends across the separation sheet 110 located therebetween such that the separation sheet 110 is sandwiched between the reinforcement structures 140.

[0127] In other examples (not shown), a single reinforcement structure 140 may be used, with the separation sheet 110 attached to the reinforcement structure 140 (e.g. by adhesive) so as to provide the desired performance.

[0128] Although in the example described herein each reinforcement structure 140 is provided in part by a frame of a loudspeaker (and in part by a separate outer structure), in other examples (not shown) each reinforcement structure 140 may be provided by a structure which is separate from the frames of the loudspeakers.

[0129] The loudspeaker system 100, and in particular the housing 102 and the separation sheet 110, can be manufactured using any suitable known processes.

[0130] For example, the housing 102 can be conveniently manufactured by injection moulding and may be formed integrally with the loudspeaker frames 150.

[0131] The housing 102 may be manufactured as two halves (such as the housing portion shown in Figure 4), e.g. each half including one of the frames 150, and subsequently joined together (to provide the arrangement of Figure 3). Manufacturing and joining of the housing halves may be achievable with high accuracy, such that the symmetry plane is accurately located where the two halves of the housing 102 are joined. At this symmetry plane, the separation sheet 110 is placed to separate the internal volume 104 into two equal portions 106, 108. Thus, the separation sheet 110 is at the natural split line of the loudspeaker system 100 and the process of adding it is simple. Joining these components together can be done e.g. via hotplate welding or with an adhesive.

[0132] The separation sheet 110 can be made from any suitable materials, e.g. polymers (such as PP, PC , PET or boPET), metals, or a composite (such as a laminate of glass fibre + epoxy). The material thickness may be kept at no more than 2mm, preferably no more than 1 .5mm, and the reinforcement structure 140 may be arranged such that the first eigenfrequency of any cell 112 (or ‘clamped portion’) of the separation sheet 110 are at sufficiently high frequency, eigenfrequencies of the cells 112 can be increased by tensioning the separation sheet 140 before fixing it to the housing 102 and the frames 150.

[0133] Where the separation sheet 110 is made from ferromagnetic metal, the separation sheet 110 may be designed (e.g. by selection of thickness) such that at rest position, there is no net attraction force of the drive units 170 towards the separation sheet 110 as otherwise the magnet units 174 may creep towards the separation sheet 110. A suitable choice of thickness may ensure that there is no tendency for creep of the magnet units 174, for example by ensuring a small repelling force remains when the magnet units 174 are at rest positions.

[0134] Figures 10, 11 , 12 and 13 compare performance parameters of the loudspeaker system 100 and a traditional loudspeaker system. More particularly, Figures 10 to 13 show graphs of the peak displacement transfer function against frequency curves at constant voltage corresponding to full power (solid line) and at a quarter of the full power (dashed line) of each loudspeaker in a loudspeaker system with the loudspeakers back-to-back acting on the same volume with and without separation.

[0135] Figure 10 and Figure 11 correspond to graphs for each loudspeaker in a loudspeaker system without the separation sheet 110, whereas Figure 12 and Figure 13 correspond to graphs for each loudspeaker 120, 130 of the loudspeaker system 100 which includes the separation sheet 110.

[0136] In the graphs, positive values represent movement of the diaphragm 160 away from the housing, also referred to as outwards, while negative values represent movement towards the symmetry plane, also referred to as inwards.

[0137] In Figure 10, maximum peak displacement of the diaphragm in outwards and inwards direction is not reached at the same frequency and the peak displacement in inwards and outwards direction is not symmetrical. Figure 11 shows the same curve for the other, opposing loudspeaker diaphragm. Again, the maximum peak displacement occurs at different frequencies. Not only is each loudspeaker’s displacement asymmetric, the two loudspeakers do not behave identically but interact. Both diaphragms experience a so-called DC shift, meaning the moving assemblies drift in the same direction relative to the housing. This increases distortion and may ultimately lead to bottoming and damage of the driver shifted inwards. This can be a particular problem for a loudspeaker system that uses loudspeakers with high mechanical compliance relying mainly on the stiffness of the air for the restoring force. If the air is not effectively compressed and rarefacted between the diaphragms but shifts towards one side, the displacement is not well controlled anymore. None of the discussed curves is smooth for low frequencies and does not behave as expected from theory in the stiffness governed region below the in-box resonance frequency.

[0138] Figures 12 and 13 show displacement vs frequency curves for the loudspeaker system 100. The loudspeaker system 100 is identical to that of Figures 10 and 1 1 , but additionally includes the separation sheet 1 10. The behaviour of each loudspeaker 120, 130 is now independent and as expected from a single loudspeaker in a single volume. The loudspeaker system 100 acts according to accepted theory even at full power.

[0139] It is noted that in Figures 12 and 13 no adverse effect introduced by the presence of the separation sheet 110 is visible. Thus, the separation sheet 110 may lead to a substantial improvement in sound quality and a dramatic improvement in reliability.

[0140] Figure 14 shows the acceleration transfer function of the cabinet for 1Vrms (root mean square at 1 Volt) at the loudspeaker terminal of the loudspeaker system 100 with the separation sheet 110.

[0141] When only one loudspeaker 120, 130 is powered (dotted line), the acceleration has a maximum at 60Hz, around the resonance frequency of the loudspeaker system, with a value of 2.7m / s2 (meters over seconds squared). When both loudspeakers are connected in parallel (solid line), the acceleration at the resonance of 60Hz is completely cancelled, and values above and below resonance comparatively small.

[0142] For reference, measurement of a traditional, single loudspeaker with similar bass output but much lower moving mass is also shown in Figure 14 (dashed line). By comparing the results for the back-to-back arrangement against the single loudspeaker, it can be seen how effective the back-to-back arrangement is for cancelling out vibrations. Even when fully powered with music, the housing 102 remains perfectly calm to the touch. As such, the remaining 0.25m / s2 per volt below resonance can be seen as a baseline representing “perfect” cancellation of the vibrations.

[0143] Figures 15 and 16 show another exemplary loudspeaker system 200. Figure 15 is a cross-sectional view of the loudspeaker system 200. Figure 16 is a front view with reinforcement ribs shown in the middle of the sound exits.

[0144] The loudspeaker system 200 is similar to the loudspeaker system 100 described above. As such, detailed description of identical features is omitted.

[0145] The loudspeaker system 200 includes a housing 202 with an internal volume 204 divided into a first portion 206 and a second portion 208 by a separation sheet 210.

[0146] The loudspeaker system 200 includes a first loudspeaker 220 and a second loudspeaker 230 in back-to- back configuration with the separation sheet 210 at the symmetry plane. The first loudspeaker 220 is also referred to as a front loudspeaker 220, and the second loudspeaker 230 as a rear loudspeaker 230. The loudspeaker system 200 includes a reinforcement structure 240 to inhibit movement of the separation sheet 210 in a direction along the common movement axis 209.

[0147] Each loudspeaker 220, 230 includes a diaphragm 260.

[0148] Each loudspeaker 220, 230 includes a drive unit 270. For the loudspeaker system 200, voice coils 273 are moveable with the diaphragms 260, while magnet units 274 are stationary relative to the frames 250.

[0149] In Figure 15, the loudspeaker system 200 is shown with large voice coils 273 accommodated in large air gaps 275. The moving mass per loudspeaker 220, 230 is 100g with an effective radiating area of 113cm2 (square centimetres); the acoustic air volume per loudspeaker 220, 230 is 1 .5 litres. The portions 206, 208 of the internal volume 204 are separated by a polycarbonate sheet (or ‘foil’) of 0.2mm thickness, with square outside dimensions of 22cm x 22cm.

[0150] The simulated first eigenfrequency of the separation sheet 210 occurs in the corners at above 350Hz but the areas are so small and the damping large enough to neither see a resonance in the electrical impedance curve nor in the frequency response.

[0151] The housing 202 is made from a PC / ABS blend with 3 to 4 mm wall thickness and well stiffened by many ribs connecting the halves of the housing 202 on either side of the separation sheet 210, effectively fixing the separation sheet 210 by bonding with an adhesive.

[0152] The loudspeaker system 200 further includes a waveguide 290. The waveguide 290 is configured to guide sound from the rear loudspeaker 230 along the housing 202 and towards a waveguide exit 292 in the same exit plane as the front loudspeaker 220 is mounted in. The loudspeaker system 200 may be mounted, e.g., in a baffle or a bulkhead or even a firewall of a vehicle with the front loudspeaker 220 directed towards the car cabin.

[0153] The waveguide 290 has a distance to the rear baffle of 3cm with a clearance between the housing 202 and the inner walls of the side portion of the waveguide 290 of 2cm.

[0154] The implementation of the separation sheet 210 allows the loudspeakers 220, 230 to be measured independently, e.g. a measurement of the electrical input impedance vs frequency. The inventors observed that the resonance frequency of the rear loudspeaker 230 (radiating upwards in Figure 15) is somewhat lower than the front loudspeaker 220 (radiating downwards in Figure 15) due to the added air load of the waveguide 290. That is to say, when directing the sound of the rear speaker 230 to the front, the radiation impedance of the rear diaphragm 260 is substantially different to the radiation impedance of the front diaphragm 260. While the front diaphragm 260 directly radiates into free space, the rear diaphragm 260 is loaded by the waveguide 290. The skilled person will be familiar with the phenomenon of “air load”, which can be understood as a parameter used to describe the response of a moving assembly, and in particular a diaphragm, moving in air as an additional mass of said moving assembly. For example, according to Acoustics, Beranek, L.L, McGraw-Hill, 1954, a low frequency approximation for the air load on one side of a diaphragm mounted in an infinite baffle is Mair= 2.67 a3p0(M_air=2.67aA3 p_0), where a is the radius of the diaphragm and p0(rho naught) is the density of air. It is noted that the units work out as kilograms, i.e. a mass is quantified. Without wishing to be bound by theory, the inventors believe that, to a degree of approximation for low frequencies, the imbalance in mass resulting from the waveguide 290 is independent of the total moving mass. If the moving mass for the front loudspeaker 220 is 100g and the moving mass for the rear loudspeaker 230 including air load due to the waveguide 290 is 105g, the vibration generated is such as if 5g were moving. Thus, it may not be the percentage difference as one may reckon. However, the observed change may be small since the resonance frequency is generally calculated as w=sqrt(k / m), where k represents the wavenumber and m the mass, and m may only change by 5 percent. Depending on the total moving mass and the stiffnesses involved, this effect maybe small and measurement system may not be able to resolve it without sufficient measurement accuracy.

[0155] The interaction between the waveguide 290 and the loudspeaker 230 is intricate; the additional mass due to the air load is not merely the air volume inside the waveguide 290 multiplied by the density of air. It may be considered surprising that the air load increases when the dimensions of the waveguide 290 are decreased, i.e. the size of the passage through the waveguide is decreased. In practice, there may be a limit to how small the dimensions of the waveguide are made, since it may be desirable for the sound exit 292 to be a substantial portion of the radiating surface area (e.g. 30 percent, or more preferably 50 percent) of the radiating surface area of the rear diaphragm 260; and the waveguide 290 may be configured so that losses along the walls remain acceptable and the mass loading remains the dominant effect. Too small dimensions of the waveguide 290 may result in substantial losses and chuffing sound from the waveguide exit 292.

[0156] Having the waveguide 290 loading the rear loudspeaker 230 but not the front loudspeaker 220 may lead to a substantial imbalance between the two loudspeakers 220, 230 and this may exceed production tolerances and other previously described effects. As such, the loudspeakers 220, 230 effectively seize to be identical, because the effective moving mass of the rear speaker 230 has increased due to the waveguide 290. This may in use lead to vibration of the housing 202 and any application it is mounted to. It may be desirable to restore the balance between the two loudspeakers 220, 230 by adding a balance weight 222 (or ‘compensation weight’) to the front loudspeaker 220. This may minimise vibration and lead to symmetric behaviour of the loudspeakers 220, 230.

[0157] For completeness it is noted that also the moving assembly of the front loudspeaker 220 experiences an air load, i.e. behaves in use as if the moving assembly had a higher mass exceeding the mechanical moving mass of the moving assembly. However, this effect may be small compared to the air loading effect of the waveguide and may not need to be considered independently. Indeed, when adding a compensation mass to the moving assembly of the front loudspeaker 220 while the loudspeakers 220, 230 are mounted in the housing 202 and radiating in air, this effect is already considered.

[0158] Figure 17 illustrates measured acceleration of the loudspeaker system 200 with the balance weight 222 (solid line) and without the balance weight 222 (dashed line), and in both cases both drive units 270 operated in parallel.

[0159] Without the balance weight 222 to compensate, the measured vibrations are substantially greater than in Figure 14, but may already be better than for a comparable traditional loudspeaker system. Adding the compensation weight 222, the measured vibrations are roughly halved, with a maximum value close to 0.25m / s2. With reference to Figure 14 it had been noted that a value of 0.25m / s2 is considered “perfect” cancellation for the described loudspeaker systems, and may be dramatically better than a single conventional loudspeaker.

[0160] To determine the balance weight 222 experimentally, the vibration on the housing 202, which is rigidly coupled with both loudspeaker frames 250, is measured with an accelerometer along an axis parallel to the principal axis 209 of the loudspeakers 220, 230.

[0161] The experimentally added balance weight 222 to initially identical loudspeakers is 6g to minimise the vibration on the housing 202.

[0162] Once the mass of the balance weight 222 is known, it is possible to replace a component of the front loudspeaker 220 with a corresponding but heavier component. For example, the thickness of a metal dust cap of the front driver may be increased by the mass of the determined balance weight 222. Hence, a mechanical moving mass imbalance between rear and front driver of 6g towards the front driver is reached which then leads to mass balance once the drivers are mounted in their respective position.

[0163] Figure 18 illustrates a method of reducing vibrations of a loudspeaker system. A method of reducing vibrations of a loudspeaker system.

[0164] The method includes a step S110 of providing a first loudspeaker system including a housing comprising a first loudspeaker, a second loudspeaker and a waveguide arranged to guide sound generated by the second loudspeaker, wherein the first loudspeaker system is operable to cause a first moving assembly of the first loudspeaker and a second moving assembly of the second loudspeaker to move along a common movement axis in opposite directions to produce sound.

[0165] The method includes a step S120 of operating the first loudspeaker system and measuring vibrations of the housing with a first balance weight included in the first moving assembly.

[0166] The method includes a step S130 of operating the first loudspeaker system and measuring vibrations of the housing with a second balance weight included in the first moving assembly.

[0167] The method includes a step S140 of identifying a balance weight mass corresponding to a reduction in vibrations by comparing vibrations measured with the first balance weight added to the first loudspeaker system against vibrations measured with the second balance weight added to the first loudspeaker system. It is possible to iteratively reduce vibrations until a balance weight mass is identified which achieves a desired reduction of vibrations. As such, any number of balance weights may be added and associated vibrations measured to identify a particular balance weight mass.

[0168] Figure 19 illustrates a method of balancing a second loudspeaker system. The method of Figure 19 includes a step S210 corresponding to the method of Figure 18. The method of Figure 19 further includes a step S220 of balancing the second loudspeaker system by adding a balance weight with the identified balance weight mass to a first moving assembly of the second loudspeaker system. The second loudspeaker system may be substantially identical to the first loudspeaker system, such that similar vibrations may be expected and, accordingly, a similar correction by way of balancing may be desirable.

[0169] Figure 20 shows an axisymmetric simplified model of two volumes V1 and V2 separated (for some simulations) by a sound hard boundary. Also, the walls of the cylindrical cabinet and the waveguide are sound hard. The front and rear loudspeakers are represented by boundaries modelled as lumped loudspeakers. The system is fixed and surrounded by air in all directions.

[0170] In this case, the loudspeakers are modelled to have a mechanical moving mass of 10g at a BL of 6Tm and an effective radiating diameter of 10.8cm. The mechanical suspension stiffness is set to 0.36N / mm, a low value impractical for real life application but useful for the purpose of this exercise. These parameters lead to a free-air resonance frequency of 30Hz.

[0171] Using acoustic finite element simulation, the required compensation weight for a physical system can be estimated based on this simplified system. An advantage of using a computer simulation as starting point for estimating the mass may be that the initial moving mass of the diaphragms can be chosen freely and identical and are not subject to tolerances. Also, there is no noise and inaccuracy from a measurement system estimating the parameters based on real life data.

[0172] Figures 21 and 22 show graphs generated through finite element simulation for the simplified model of Figure 20. Figure 21 relates to a scenario without separation sheet and shows the electrical input impedance for both loudspeakers when driven in parallel when the simulation is carried out without separation sheet (V1 and V2 are connected to a common volume) and without balance weight. Because the mechanical moving mass is chosen so low, the shifts in resonance frequency for the drivers are large. The front loudspeaker has an in-box resonance frequency of 170Hz whereas the rear loudspeaker has a lower in-box resonance frequency at higher damping of approximately 150Hz. In view of Figure 19, it is obvious that two speakers with such vastly different impedance curves are best not used in a common volume. The electrical current through the voice coils at given frequency is different, and so are the Lorentz forces acting on the respective moving assemblies. This consequently leads to different displacement and imbalance of the loudspeakers, particularly when the loudspeakers are driven to their non-linear region.

[0173] The graphs of Figure 22 are also based on the loudspeaker arrangement of Figure 20 but V1 and V2 are separated by a sound hard boundary representing the separation sheet. A balance weight of 2.7g was chosen to be added to the front diaphragm as this led to the best match between the shown impedance curves. This balance weight can be calculated from the shift in resonance frequencies at constant stiffness. As the impedance curve is descriptive of the linear low frequency behaviour of a loudspeaker, the two loudspeakers may now act identical and generate the same sound pressure level with the same displacement from the same current.

[0174] It is noted that the simulated balance weight may differ slightly depending on the initially chosen mass and stiffness. Without wishing to be bound by theory, this is believed to originate from the fact that the change in radiation impedance due to the waveguide - and particularly the reactive part of the radiation impedance - of the rear driver is only in first approximation a constant mass. The reactive part of the radiation impedance changes with frequency and so does the associated mass. However, for many applications, and especially low frequency applications, this effect may not be large. Moreover, when carrying out the accelerometer measurements in practice, the mass of the balance weight can be iteratively adjusted until the vibration is minimised. It is worth noting that the chosen mass does not necessarily correspond to the lowest vibration value for constant voltage excitation vs frequency. Depending on the application and music choice with associated spectral power distribution, a slightly different compensation mass may be preferred.

[0175] Figures 23, 24 and 25 show further exemplary loudspeaker systems 300, 400, 500. These loudspeaker systems are similar to the loudspeaker systems 100, 200 described above. Detailed description of corresponding features is therefore omitted.

[0176] The loudspeaker system 300 includes a housing 302 with a single internal volume 304. Two loudspeakers 320, 330 are mounted in the housing 302. A balance weight 322 is attached to the first loudspeaker 320 of the two loudspeakers, and a waveguide 390 is provided to guide sound from the second loudspeaker 330 to a sound outlet 392 at the first loudspeaker 320.

[0177] The loudspeaker system 400 includes a housing 402 with an internal volume 404 divided into a first portion 406 and a second portion 408 by a separation sheet 410. Two loudspeakers 420, 430 are mounted in the housing 402. A balance weight 422 is attached to the first loudspeaker 420 of the two loudspeakers, and a waveguide 490 is provided to guide sound from the second loudspeaker 430 to a sound outlet 492 at the first loudspeaker 420.

[0178] The loudspeaker system 500 includes a housing 502 with an internal volume 504 divided into a first portion 506 and a second portion 508 by a separation sheet 510. Two loudspeakers 520, 530 are mounted in the housing 502. A balance weight 522 is attached to the first loudspeaker 520 of the two loudspeakers, and a waveguide 590 is provided to guide sound from the second loudspeaker 530 to a sound outlet 592. In Figure 23, the waveguide 590 is flared and the sound outlet 592 opens in a direction perpendicular to a common movement axis 509 of the loudspeaker system 500.

[0179] Figure 26 is a schematic view of an automobile 1000 including a loudspeaker system as described above, in this example the loudspeaker 200. Any exemplary loudspeaker system as described above may be installed in the automobile 1000. In this example, the loudspeaker system 200 described above is provided between the footwells 1100 of the automobile 1000. Other locations are also envisaged.

[0180] 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.

[0181] 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.

[0182] 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.

[0183] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0184] 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.

[0185] 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%.

[0186] References

[0187] Acoustics, Beranek, L.L, McGraw-Hill, 1954

Claims

Claims:1 . A loudspeaker system comprising: a housing; a first loudspeaker and a second loudspeaker mounted in the housing; wherein the loudspeaker system is operable to cause a first diaphragm of the first loudspeaker and a second diaphragm of the second loudspeaker to move along a common movement axis in opposite directions to produce sound; a separation sheet of material arranged to divide an internal volume enclosed by the housing into a first portion accommodating the first loudspeaker and a second portion accommodating the second loudspeaker; wherein an inwardly-facing radiating surface of the first diaphragm is arranged to radiate sound into the first portion of the internal volume enclosed by the housing and an inwardly-facing radiating surface of the second diaphragm is arranged to radiate sound into the second portion of the internal volume enclosed by the housing; wherein a thickness of the separation sheet is 2 millimetres or less.

2. The loudspeaker system according to claim 1 , wherein the separation sheet has a thickness of 1 .5 millimetres or less, preferably 1 millimetre or less.

3. The loudspeaker system according to claim 1 or 2, wherein a material of the housing and the material of the separation sheet are different; wherein the material of the separation sheet is a metal, optionally a ferromagnetic metal, or a polymer, optionally a fibre-reinforced polymer.

4. The loudspeaker system according to any preceding claim, further comprising a reinforcement structure which extends across the internal volume of the housing; wherein the reinforcement structure extends across the separation sheet and is configured to inhibit movement of the separation sheet along the common movement axis.

5. The loudspeaker system according to claim 4, wherein the reinforcement structure includes a central structure and an outer structure; the central structure includes a first frame of the first loudspeaker or a second frame of the second loudspeaker; and the outer structure extends from the housing towards the central structure.

6. The loudspeaker system according to claim 4 or 5, the reinforcement structure includes a rib structure which contacts the separation sheet; wherein the rib structure forms a plurality of apertures extending through the reinforcement structure in a direction along the common movement axis; andwherein the portion of separation sheet of material extending across each aperture has a first eigenfrequency of at least 50Hz, more preferably at least 100Hz, more preferably at least 150Hz, more preferably at least 300Hz.

7. The loudspeaker system according to any one of claims 4 to 6, wherein the reinforcement structure is a first reinforcement structure located in the first portion of the internal volume; wherein the loudspeaker system includes a second reinforcement structure located in the second portion of the internal volume; wherein the separation sheet is sandwiched between the first reinforcement structure and the second reinforcement structure.

8. The loudspeaker system according to any preceding claim, wherein the housing includes a first housing portion and a second housing portion which are substantially symmetric about a plane of symmetry coinciding with the separation sheet; and wherein the separation sheet is sandwiched between the first housing portion and the second housing portion.

9. The loudspeaker system according to any preceding claim, further comprising a waveguide configured to receive sound radiated by an outwardly-facing radiating surface of the second diaphragm and to guide the sound to a waveguide outlet.

10. The loudspeaker system according to claim 9, wherein an outwardly-facing radiating surface of the first diaphragm and the waveguide outlet are arranged to radiate sound in the same direction.

11. The loudspeaker system according to claim 9 or 10, wherein a mechanical moving mass of a first moving assembly of the first loudspeaker is larger than a mechanical moving mass of a second moving assembly of the second loudspeaker.

12. A loudspeaker system comprising: a housing; a first loudspeaker and a second loudspeaker mounted in the housing; wherein the loudspeaker system is operable to cause a first moving assembly of the first loudspeaker and a second moving assembly of the second loudspeaker to move along a common movement axis in opposite directions to produce sound; a waveguide configured to receive sound radiated by the second moving assembly and to guide the sound to a waveguide outlet; wherein a mechanical moving mass of the first moving assembly is larger than a mechanical moving mass of the second moving assembly.

13. The loudspeaker system according to claim 11 or 12, wherein the mechanical moving mass of the first moving assembly is larger than the mechanical moving mass of the second moving assembly by a mass difference provided by a balance weight included in the first moving assembly.

14. The loudspeaker system according to claim 13, wherein the balance weight is a cap attached to the first diaphragm or the balance weight forms part of the first diaphragm.

15. The loudspeaker system according to any one of claims 11 to 14, wherein the mechanical moving mass of the first loudspeaker is larger than the mechanical moving mass of the second loudspeaker by at least 1g, preferably at least 3g, and more at least 4g.

16. The loudspeaker system according to any preceding claim, wherein the loudspeaker system is provided as a subwoofer system configured to produce sound with frequencies in a bass frequency range, the bass frequency range including 60-80Hz.

17. The loudspeaker system according to any preceding claim, wherein each loudspeaker comprises a drive unit including an air gap with a magnetic reluctance of at least 2 x 10A6 [1 / H],18. An automobile including a loudspeaker system according to any preceding claim.

19. A method of reducing vibrations of a loudspeaker system, comprising: providing a first loudspeaker system including a housing comprising a first loudspeaker, a second loudspeaker and a waveguide arranged to guide sound generated by the second loudspeaker, wherein the loudspeaker system is operable to cause a first moving assembly of the first loudspeaker and a second moving assembly of the second loudspeaker to move along a common movement axis in opposite directions to produce sound; operating the first loudspeaker system and measuring vibrations of the housing with a first balance weight included in the first moving assembly; operating the first loudspeaker system and measuring vibrations of the housing with a second balance weight included in the first moving assembly; identifying a balance weight mass corresponding to a reduction in vibrations by comparing vibrations measured with the first balance weight added to the first loudspeaker system against vibrations measured with the second balance weight added to the first loudspeaker system.

20. A method of balancing a loudspeaker system, comprising the method of reducing vibrations according to claim 19, and further comprising balancing a second loudspeaker system by adding a balance weight with the identified balance weight mass to a first moving assembly of the second loudspeaker system.

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