Loudspeaker

Abstract

To provide a speed detection device and method for a loudspeaker driver motion feedback.SOLUTION: A loudspeaker driver includes a voice coil 114 and a detection winding 118 coaxially wound around a voice coil former 110, and is driven so as to reciprocate along a reciprocating axis 112 by the application of an electrical signal to the voice coil, and has a first magnetic field arranged to couple with the voice coil and a second magnetic field arranged to couple with the detection winding, and detects a voltage induced in the second detection winding during reciprocating motion in the axial direction in the second magnetic field. The second magnetic field has an orientation periodicity that is circumferential with respect to the reciprocating axis, and the orientation periodicity extends over a portion of the length of the voice coil former along the reciprocating axis.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to the field of loudspeakers, and in particular to detecting the instantaneous velocity of a voice coil and a voice coil former that reciprocate to drive an acoustic diaphragm from which acoustic waves are emitted. The present invention relates to a method for designing a loudspeaker, a loudspeaker and its voice coil former. [Background technology]

[0002] There are many types of conventional acoustic loudspeakers that convert electrical audio signals into corresponding sounds.

[0003] A loudspeaker typically includes one or more drivers, a housing, and electrical connections and often also circuitry such as a crossover network. The drivers to which this invention pertains are voice coils, i.e. coils of conducting wire wound in a spiral shape around a rigid, usually cylindrical former. These drivers are located within a magnetic field, and when an electrical audio signal is passed through the voice coil, they reciprocate, driving an acoustic diaphragm to radiate sound waves. Such arrangements have been in use for over a century and are the subject of, for example, US 1707570.

[0004] Ideally, in a loudspeaker, the motion of the voice coil is linearly related to the electrical signal applied to the driver terminals of the loudspeaker, so that when a signal consisting of one or more sine waves is applied to the loudspeaker, the resulting motion of the voice coil consists of only a set of identical sine waves. However, in real devices, this is not the case and there are significant nonlinearities in the transfer function. As a result, when one or more sine waves are applied to the driver terminals of a loudspeaker, the resulting motion of the voice coil contains harmonics at multiples and sum and difference frequencies of the applied sine waves. This characteristic is undesirable for high quality sound reproduction.

[0005] The nonlinear characteristics are due to modulation of the transfer function parameters during operation of the loudspeaker, in particular as a function of voice coil current, voice coil temperature, voice coil and diaphragm displacement. The main mechanisms causing nonlinearity are typically:

[0006] Suspension and ambient stiffness which vary as a function of voice coil position, motor system strength (BL) which varies as a function of voice coil position, voice coil wire resistance which varies as a function of voice coil temperature, and the change in voice coil inductance as a function of voice coil position as a function of the state of the motor system magnetic circuit, which is itself a function of the present and past voice coil position and voice coil current.

[0007] One approach taken in loudspeaker design is to try to minimize these mechanisms, but this approach usually adds significant additional cost and complexity. Another approach to reduce the nonlinearity of loudspeaker drivers is to detect the motion of the voice coil and use this detected signal in a negative feedback loop (using appropriate amplifiers and control electronics). This second approach, commonly referred to as "motion feedback", is well known, but is not commonly used in commercial loudspeakers, mainly due to the complexity of the placement, and the cost and performance of available motion sensors. US3941932 is an example of motion feedback that detects the acceleration of the voice coil using a piezoelectric accelerometer placed under the driver dust cap. Another well-known example of motion feedback places an additional sensing winding in the magnet motor gap, wound coaxially with the voice coil, as shown in Figure 1.

[0008] In a conventional rate sensing motion feedback arrangement, shown generally in Figure 1 as a cross-sectional side view, a loudspeaker has a ferrite ring magnet 2, a steel yoke 4 and a steel front plate 6 which combine to define a magnetic gap 8 within which a voice coil former 10 reciprocates along an axis 12 to drive a diaphragm (not shown). A voice coil 14 is wound around the voice coil former 10 in a conventional manner and extends along the axis 12 a sufficient distance to accommodate the reciprocating motion of the voice coil former 10, and the voice coil former is driven by energizing the voice coil 14 with an electrical signal.

[0009] A voltage applied to voice coil 14 generates a magnetic field which interacts with the magnetic field in magnetic gap 8 to drive voice coil former 10 to move along axis 12 in accordance with well-known principles of electromagnetic induction. A fine wire sensing coil 16 is wound coaxially with voice coil 14.

[0010] As the voice coil former 10, carrying the detection coil 16 with it, moves back and forth within the magnetic gap 8, a voltage is induced in the detection coil 16 by the magnetic field within the gap 8, and this voltage can be measured and used to determine the instantaneous velocity of the voice coil former 10.

[0011] The sensing winding moves with the voice coil and theoretically, an induced motion EMF, ε, is generated as the sensing winding passes through the magnetic field in the magnet motor gap according to the following equation: motion The oscillator generates an output voltage proportional to the velocity of the voice coil.

[0012]

number

[0013] where (BL)sc is the sensitivity coefficient (which is the average magnetic flux density (B) across the sensing voice coil winding multiplied by the length of the speed sensing winding (L));

number

[0014] The secondary winding motion feedback approach has several advantages over the accelerometer approach:

[0015] The detected velocity will be nearly exact axial voice coil motion up to very high frequencies and may not include parasitic self-resonances or resonances from the sensor mount that are often problematic with other sensors such as accelerometers.

[0016] The speed sensing winding has a low impedance output and does not require proximate electrical amplification in the moving parts of the driver.

[0017] The result is that half of the speed sensing winding moves rearward and the other half moves forward, so any rocking motion of the moving driver parts (i.e. motion not parallel to the axis of reciprocation) tends to go undetected (this is an advantage as a small amount of rocking is common in many drivers and has little effect on performance).

[0018] Because the speed sensing windings do not carry large currents, very thin wire can be used, which means that they take up very little space and add very little mass to the moving assembly.

[0019] The conventional speed sensing winding approach shown in Figure 1 has two major problems that severely hinder performance and limit the use of this approach. First, for the sense voltage Vs to be linearly related to the voice coil speed, it is essential that the sensitivity coefficient (BL)sc in Equation 1.1 is constant. Figure 1 is drawn with an overhanging speed sensing winding arrangement such that the length A of the magnet motor gap is shorter than the length D of the speed sensing winding. This means that while the driver's excursion (movement of the driver away from the "rest" position as shown in Figure 1) is less than the distance C, the average flux density experienced by the sense winding is approximately constant. If the excursion is greater than the distance C, the sensitivity of the speed sensing winding drops dramatically as the average flux density changes (since only a portion of the winding is in the motor gap). To address this source of nonlinearity, the winding height of the speed sensing winding could be increased beyond that of the voice coil, but this is not useful as it would require increasing the length of the former, increasing the weight of the driver and the size of the loudspeaker, and requiring an increased clearance between the former and the steel yoke to avoid collisions during operation.

[0020] Secondly, an additional mutual emf exists at the terminals of the speed-sensing winding due to transformer-like coupling with the current flowing in the voice coil. A more accurate description of the speed-sensing winding voltage is:

[0021]

number

[0022] where M is the mutual inductance between the voice winding and the speed-detection winding, and i is the current flowing in the voice coil. This mutual inductance effect is well known and contaminates the detected signal in typical drivers to the extent that the effective feedback bandwidth is severely limited. Figure 2 shows a simulation of the speed-detection winding voltage and component EMF for a typical low-inductance loudspeaker with the speed-detection winding wound directly on the voice coil, clearly showing how the mutual inductance dominates the detected voltage at high frequencies. At frequencies where the motion EMF and the mutual EMF have the same value, there is a large dip in the detected signal.

[0023] The value of the mutual inductance depends on the self-inductance of each coil and the magnetic coupling between the two coils, as follows:

[0024]

number

[0025] Here, L e is the self-inductance of the voice coil, and L esc is the self-inductance of the speed-sensing winding, and k is a coupling coefficient with a value between 0 and 1 that represents the proportion of magnetic flux coupled from one coil to the other. The mutual emf is proportional to the square of the number of turns in the voice and speed-sensing windings, while BL and (BL)sc are proportional to the number of turns. This means that the mutual emf is particularly high in drivers with a high number of voice coil turns. For example, Figure 3 (showing the measured speed-sensing winding voltage and the actual speed (measured by a laser) for a loudspeaker with high inductance and strong coupling between the voice coil and the speed-sensing winding) shows that in some cases the speed signal is completely drowned in the mutual emf, resulting in poor sound quality and making this arrangement unusable for speed detection within the audible range. There is a need for a speed-sensing motion feedback approach that is suitable for all loudspeakers with high inductance voice coils and that addresses or improves upon the challenges of conventional systems. Summary of the Invention

[0026] The present invention is based on the realization that by providing a primarily radial magnetic field to energize and move a voice coil axially, and by providing an auxiliary magnetic field along at least a portion of the voice coil axis that has a higher order of circular periodicity than the primarily radial magnetic field, it is possible to provide a speed sensing apparatus that is relatively small, lightweight, easy to manufacture, and does not suffer to the same extent from the problems of conventional speed sensing motion feedback devices.

[0027] The present invention therefore provides a method of measuring the instantaneous velocity of a loudspeaker driver reciprocating in a magnetic field, the driver having a voice coil wound coaxially on a former and a detection winding wound coaxially on the former, the driver being driven to reciprocate along a reciprocation axis by application of an electrical signal to the voice coil, the method comprising providing a first magnetic field positioned and adapted to couple primarily with the voice coil, providing a second magnetic field positioned and adapted to couple primarily with the detection winding, and detecting a voltage induced in the second detection winding as it reciprocates axially in the second magnetic field, the second magnetic field having an orientation periodicity that is circumferential with respect to the reciprocation axis, the orientation periodicity extending over at least a portion of the length of the former along the reciprocation axis.

[0028] In this way, the magnetic circuit provides two magnetic flux distributions, one that interacts almost independently with each of the voice coil and the detection winding. The first magnetic flux distribution corresponds to the magnetic gap of a normal motor system optimized for maximum coupling with the voice coil, and has a substantially radial magnetic field. The second magnetic flux distribution is optimized for maximum coupling with the detection winding. The detection winding is designed for minimum coupling to the first region of concentrated magnetic flux, and the second magnetic gap is designed and positioned relative to the voice coil such that the voice coil is minimally coupled to the second region of concentrated magnetic flux. The two magnetic fields can be embodied in a number of different ways, some of which are described below, allowing for there to be an overall magnetic field that varies in a plane perpendicular to the loudspeaker axis at one or more locations along the loudspeaker axis, which in turn allows for different detection winding arrangements to be used to detect the instantaneous speed of the voice coil former when the loudspeaker is in use, an improvement over conventional speed-detecting motion feedback loudspeakers. A loudspeaker in accordance with the principles of the present invention exhibits reduced nonlinearity in the detected voltage / voice coil velocity relationship and extremely low mutual inductance.

[0029] The first and second magnetic fields may be located at different positions along the axis. Additionally or alternatively, the first and second magnetic fields may be superimposed. Offsetting the two magnetic fields with respect to the reciprocating axis helps to increase the linearity range of the detection winding. Superimposing the magnetic fields allows for a more compact design.

[0030] Such an arrangement allows the detection winding to be configured such that it does not interact with the magnetic drive field generated by the voice coil when an electrical signal is applied to drive the voice coil (e.g., by ensuring that the detection winding is substantially perpendicular at all points to the axis of reciprocation). As a result, the coupling coefficient between the voice coil and the speed detection winding is close to zero, and the mutual inductance is also close to zero. As a result, the mutual emf is also close to zero, and the speed detection winding voltage is dominated by the motion emf signal, even for drivers with very high inductance voice coils. The detection winding is preferably configured to have a circumferential periodicity that coincides with the second magnetic field.

[0031] The first magnetic field may be primarily radial with respect to the axis of reciprocation and the second magnetic field may have a small circumferentially varying component, such that when the two magnetic fields are superimposed there is a reference radial magnetic flux and when moving circumferentially there are regions of radial magnetic flux slightly higher than the reference radial magnetic flux and regions of radial magnetic flux slightly lower than the reference radial magnetic flux. If the two magnetic fields are not superimposed but instead are axially separated, the first magnetic flux is substantially constant around the circumference of the magnetic gap over a first axial distance and the second magnetic field has regions of slightly positive radial magnetic flux and regions of slightly negative radial magnetic flux over the circumferential direction and the second axial distance.

[0032] In another aspect, the invention provides a loudspeaker comprising a voice coil wound coaxially on a former, which together are adapted to reciprocate along a reciprocating axis within a gap in a magnet arrangement to radiate acoustic energy when an electrical signal is applied to the voice coil, causing an acoustic diaphragm connected to the former to reciprocate along the reciprocating axis, the voice coil extending a first distance along the axis and the former, and a detection winding disposed coaxially on the former and extending a second distance along the reciprocating axis. and the magnet arrangement is adapted and configured to generate two magnetic fields, a first magnetic field primarily driving the voice coil to reciprocate and disposed axially adjacent to the first distance, and a second magnetic field primarily adapted to couple with the detection winding, the detection winding being disposed around the former in the form of an even number of loops, the even number of loops being separately disposed around the former but electrically connected to form a single winding, each loop extending around a loop axis substantially perpendicular to the reciprocation axis.

[0033] The position of the loop around the former provides the detection winding with a circumferential periodicity of sensitivity which preferably matches the periodicity of the second magnetic field.

[0034] The first and second magnetic fields and the first and second distances may not overlap in the direction of the reciprocation axis (but may instead be offset along the reciprocation axis), or the first and second magnetic fields and the first and second distances may overlap in the direction of the reciprocation axis. As discussed above, offsetting or overlapping the magnetic fields each have advantages.

[0035] The second detection winding may be formed on the outer surface of the former in one or more layers including a plurality of separate coils arranged circumferentially around the former, each coil including a plurality of adjacent turns extending about a loop axis. The outer surface of the former may include the radially outermost surface of the former and / or its radially innermost surface. The circumferentially adjacent coils may turn in alternating directions about their respective axes to match the change in radial magnetic flux magnitude, which may be caused by alternating radial magnetic polarity in the second magnetic field. The circumferentially invariant primary magnetic field and voice coil magnetic field may result in induced voltages in the individual loops, but because the loops alternate in polarity, there is no net EMF from the secondary windings formed by the loops.

[0036] Two or more coils may be provided in overlapping adjacent layers, or they may be overlapping such that the majority of the turns are provided on a single layer and a small portion of each turn provides a crossing path on a second adjacent layer where the turns of one coil cross the turns of another coil.

[0037] The detection winding may comprise two or more printed layers, with the printed coils in one layer being aligned circumferentially around the reciprocation axis with the printed coils in an adjacent layer. Preferably, a portion of each coil is aligned perpendicular to the reciprocation axis, and the turns in said portion of each coil may be spaced apart from each other relative to the reciprocation axis by a greater distance than the turns forming the remainder of the coil. This allows the sensitivity factor (BL)sc to remain substantially constant over a very wide axial range of detection winding positions. The first distance is preferably smaller than the second distance, which helps to keep the sensitivity of the detection winding constant along the reciprocation axis.

[0038] The magnet arrangement may include separate first and second magnets for generating the first and second magnetic fields, or there may be a single integral magnet adapted to generate a magnetic field that corresponds to the combined first and second magnetic fields.

[0039] In a further aspect, the present invention also provides a former around which a loudspeaker voice coil is coaxially wound, wherein the former and the voice coil are adapted to reciprocate along a reciprocating axis within a gap in a magnet arrangement such that an acoustic diaphragm connected to the former reciprocates along the reciprocating axis to radiate acoustic energy, the former including a detection winding formed on an outer and / or inner surface of the former in one or more printed circuit layers including a plurality of separate detection coils arranged circumferentially around the former, each detection coil including a plurality of adjacent turns, the detection coils being separately arranged around the former but electrically connected to form a single winding, each detection coil extending around a loop axis substantially perpendicular to the reciprocating axis.

[0040] Such an arrangement is ideal for operation as a speed detection winding in a magnetic gap formed by a magnet arrangement adapted and configured to generate two magnetic fields: a first magnetic field for primarily driving the voice coil to reciprocate, and a second magnetic field disposed axially adjacent the first magnetic field and adapted to primarily couple with the detection winding. The former may comprise two or more printed layers, the printed coils in one layer being aligned circumferentially about an axis with the printed coils in an adjacent layer. A portion of each coil may be aligned perpendicular to the reciprocation axis, and the turns of the portion of each coil are spaced apart from each other a greater distance relative to the reciprocation axis than the turns forming the remainder of the coil.

[0041] An example of the present invention will now be described with reference to the accompanying drawings. [Brief description of the drawings]

[0042] [Figure 1(a)] FIG. 1 is a schematic diagram of a loudspeaker having a conventional velocity sensing winding. [Diagram 2] 2 is a graph showing a simulation of the velocity winding voltage and component EMF frequency response of the loudspeaker of FIG. 1; [Diagram 3] 1 is a graph showing measured speed sensing winding voltage and actual speed for a conventional loudspeaker having high inductance and strong coupling between the voice coil and the speed sensing winding. [Figure 4] 1 is a schematic cross-sectional view of an embodiment of a velocity sensing loudspeaker according to the present invention; [Diagram 5] FIG. 5 is a schematic plan view of the loudspeaker of FIG. 4. [Figure 6a-b] 6a and 6b are schematic diagrams of the magnetic field of the loudspeaker of FIG. 4, showing the direction of the magnetic field at different points along the axis of the loudspeaker. [Figure 6c-e] 6c and 6d are schematic diagrams of the magnetic field of the loudspeaker of FIG. 4, showing two magnetic fields which combine at one or more points along the axis of the loudspeaker to produce the magnetic field shown in FIG. 6e. [Figure 7] FIG. 1 is a schematic diagram of a detection winding arrangement having two layers. [Figure 8] 8 shows one layer of the sense winding arrangement of FIG. 7. [Figure 9] 8 illustrates another layer of the sense winding of FIG. 7. [Figure 10] 4 is a graph showing the detected winding voltage in a prior art loudspeaker and the detected winding voltage and actual speed of a loudspeaker according to the present invention; [Figure 11] 1 is a graph showing fundamental SPL (sound pressure level) and THD (total harmonic distortion) SPL of a loudspeaker with a conventional voltage amplifier compared to a current amplifier with negative feedback from a speed sensor. [Figure 12] FIG. 13 is a schematic diagram of another example of a detection winding arrangement. [Figure 13] 13a and 13b are schematic diagrams of possible loudspeaker drive magnet configurations for providing the magnetic field of FIG. 6e, without and with a voice coil former respectively. [Figure 14]14a and 14b are schematic diagrams of another possible loudspeaker drive magnet configuration for providing the magnetic field of FIG. 6e, without and with a voice coil former respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] Figures 1 to 3 relate to the prior art and are described in the introduction above.

[0044] 4 and 5 show an embodiment of a velocity sensing loudspeaker according to the invention.

[0045] The loudspeaker has a ferrite ring magnet 102, a steel yoke 104, and a steel front plate 106 which combine to form a magnetic gap 108 within which a voice coil former 110 reciprocates along an axis 112 to drive a diaphragm (not shown). A voice coil 114 is wound around the voice coil former 110 in a conventional manner and extends along the axis 112 a distance sufficient to accommodate the reciprocating motion of the voice coil former 110, and the voice coil former is driven by energizing the voice coil 114 with an electrical signal (the electrical signal is typically generated by electrical and / or electronic circuitry external to the loudspeaker enclosure (although there may be crossover circuitry, etc., internal to the enclosure).

[0046] Those skilled in the art will understand the principles and apparatus involved in generating and transmitting these signals to loudspeakers, which are not directly relevant to the present invention and will not be described further herein).

[0047] A voltage applied to the voice coil 114 generates a magnetic field that interacts with the magnetic field in the magnetic gap 108, driving the voice coil former 110 to move along the axis 112. The sense winding 118 is formed as a PCB (printed circuit board) that extends around the voice coil former 110 (on either or both of the inner and outer surfaces of the voice coil former 110), under the voice coil 114, and axially beyond the voice coil 114 (the laminated sandwich of conductive and insulating layers that form the PCB may itself constitute the former). At the front end of the magnetic gap 108, an even number (four are shown) of neodymium (NdFeB) magnets 120 are provided, arranged as shown, above the T-yoke poles, with alternating magnetic polarity. As with conventional speed detection designs, as the voice coil former 110 with the detection winding 118 reciprocates through the magnetic gap 108, a voltage is induced in the detection winding 118 by the magnetic field generated by the NdFeB magnet 120, which can be measured and used to determine the instantaneous speed of the voice coil former 10. In the illustrated embodiment, four cylindrical neodymium magnets 120 are added in a magnetic quadrupole orientation to create a secondary magnetic field that generates a motion EMF in the speed detection winding proportional to the speed of the voice coil former 110. The detection winding is printed on the voice coil former 110 using flexible PCB technology. This approach is lightweight and allows for complex winding patterns (described in more detail below). The axial extent of the voice coil and speed detection track is indicated by dimensions D1 and D2, the secondary magnetic gap is indicated by A2, and the overhang of the detection winding is indicated by C2. Because the location of the detection field is above the primary field and the field height A2 can be lower, the overhang C2 can be substantially higher than in conventional arrangements, which allows (BL)sc to be linear over a much wider range of voice coil displacement than in conventional loudspeakers.

[0048] Figures 6a and 6b show the magnetic field orientations occurring in the region of the ring magnet 102 and front plate 106 and in the region of the neodymium magnet 120, respectively. In this particular embodiment, the two regions are axially separated, so that there is little interaction between the two magnetic fields, and the first magnetic field in the primary gap 108 (Figure 6a) is nearly completely radial, thus operating in the same way as a conventional motor system. The secondary magnetic field has a circumferential periodicity.

[0049] In this embodiment, the magnetic field generated by the secondary magnetic circuit has a generally quadrupole orientation with the radial magnetic field polarity changing twice around the circumference of the magnetic gap. Note that many other secondary magnetic field shapes are possible, e.g., generally dipole, generally octapole, etc., essentially any shape with an even number of radial magnetic field polarity changes. The secondary magnetic field must closely match the winding arrangement of the detection winding so that (BL)sc is high enough to provide sufficient velocity sensitivity. A quadrupole secondary magnetic field is the preferred embodiment since it is the lowest order secondary magnetic field that eliminates wobble (of the voice coil former as it reciprocates) in the detected signal, but other embodiments are possible if they are geometrically progressive or circumferentially arranged to prevent wobble.

[0050] It should be noted that the first and second magnetic fields do not have to be axially separated, and in other embodiments, there may be an interaction between the two magnetic fields and the two regions may overlap. This does not adversely affect the performance of the detection winding, provided that the winding loops are positioned so that they do not couple with the magnetic field generated when current flows through the voice coil. An example of this is shown in Figures 6c to 6e, where Figure 6c shows the radial flux of the first magnetic field, Figure 6d shows the radial flux of the second magnetic field, and Figure 6e shows the radial flux when the first and second magnetic fields are superimposed at the same axial position. In this quadrupole example, two regions of slightly higher radial flux and two regions of slightly lower radial flux are positioned around the circumference of the magnetic gap. The detection winding is configured to have a circumferential periodicity that matches the second magnetic field.

[0051] FIG. 7 shows the PCB Gerber file for a quadrupole detection winding made of two PCB layers (the diagram is a 2D representation of the winding formed circumferentially around the voice coil former). FIGS. 8 and 9 show more clearly the track arrangement on the individual layers. The arrangement here includes a detection winding with eight series-connected spirals formed as four loops 126 on each of the two PCB layers. The four loops of the layers in FIGS. 8 and 9 are overlapped axially and circumferentially such that adjacent loops 126 in a layer alternate in the direction they turn, with overlapped pairs of loops turning in the same direction. This winding arrangement is optimized in two aspects. First, the lower part of the winding is located in the secondary magnetic field (marked D2 in FIG. 7).

[0052] The individual tracks of the detection winding in this region are spaced further apart than the rest of the tracks and are intended to be oriented in use substantially perpendicular or perpendicular to the axis of reciprocation, and are designed to match the quadrupole magnetic field and provide a nearly constant (BL)sc over a very wide range of coil positions. Secondly, this winding arrangement is optimized to minimize coupling with the magnetic field generated when current flows through the voice coil. In this case, the motor system is nearly axisymmetric, and the magnetic field due to the voice coil is also nearly axisymmetric. The detection winding arrangement has an equal number of aligned / overlapping pairs of clockwise and counterclockwise tracks (around the axis of the voice coil loop), and therefore zero coupling with the magnetic field from the voice coil. It will be appreciated that there are many variations of the same spiral arrangement with slight modifications such as the order and orientation of the spirals, and that the same approach can be used to develop any even-ordered detection winding.

[0053] 10 shows the improvement in sensing winding performance using the present invention compared to a conventional sensing winding wound directly on the voice coil and using the same magnetic gap. The frequency of the notch indicating where the motion EMF and mutual EMF are equal has increased by 1.5 octaves.

[0054] Figure 11 shows a comparison of the output and THD of a prototype loudspeaker using the velocity sensor described above, driven first with a conventional low impedance (voltage output) amplifier, and then with a high impedance (current output) amplifier with negative feedback from the velocity sensor signal. In this case, the amount of negative feedback was adjusted to approximately match the response of the low impedance amplifier. The linear output of both systems is similar, but the system with negative feedback from the velocity sensor has substantially reduced distortion.

[0055] FIG. 12 shows another detection winding arrangement with four coils formed using two PCB track layers to allow the windings in each loop 128 to cross. Compared to the arrangements of FIGS. 7 to 9, this arrangement has the disadvantage that it allows half the number of turns for a given track spacing, which reduces the speed sensitivity by half. However, the advantage is that as the voice coil former moves, a more equal length of the detection winding is immersed in the secondary magnetic field. This winding arrangement also allows the upper part of the coil (the part located away from the two magnetic gaps) to have approximately the same number of turns clockwise and counterclockwise around the axis of the voice coil, which helps to minimize electromagnetic coupling between the detection and voice coils.

[0056] As is evident, there are many possible arrangements for the secondary field and the gap. In the example of FIG. 4, a completely separate set of neodymium magnets is used to generate the secondary field, but it is also possible to use a single magnetic circuit to generate the fields of both the primary and secondary gaps. FIGS. 13 and 14 show two possible alternative geometries (for greater clarity, FIGS. 13a and 14a show a geometry with a voice coil and no former, while FIGS. 13b and 14b show a geometry with a former and a visible detection winding). In the example of FIG. 13, a series of alternating pairs of opposing notches 122 and ridges 122 are formed in the top plate 106′ of the magnet arrangement to generate the secondary field and energize the detection winding 118 (in this case the motor and coil geometries must be carefully designed to minimize mutual inductance). FIG. 14 shows an alternative arrangement to FIG. 4, where a pair of neodymium magnets 120 generate the field for the secondary gap. To increase the magnetic flux applied to the sensing winding and reduce the stray magnetic field, a top plate 106 with notches and ridges can be provided or a piece of steel added as in Figure 13. If there is a large interaction between the primary and secondary gaps and the magnetic circuit, the coil shape needs to be optimized to minimize the mutual inductance.

[0057] It will be seen from Figures 13b and 14b that the detection windings shown diagrammatically in Figures 7 to 9 and 12 are not planar, but rather that while they are "wound" about the voice coil former (which is cylindrical in Figures 13b and 14b) such that the portion of the detection winding that is activated by movement through the second magnetic field (D2 in Figure 7) is axial, circumferential and substantially perpendicular to the axis of reciprocation at all circumferential positions, the loops of each detection winding are not planar, but rather curved in one dimension.

[0058] Of course, it will be appreciated that many variations are possible on the above-described embodiments without departing from the scope of the invention. For example, the invention has been described primarily with reference to cylindrical voice coils and formers.

[0059] However, the invention applies equally to non-circular arrangements such as oval, elliptical or racetrack shaped (figure of eight, or triangular / square / polygonal with rounded corners), planar or hexagonal voice coils and formers, or any shape that lies in a plane generally perpendicular to the voice coil axis and is symmetric in one or two orthogonal directions with a central hole. An array of magnets may be used to energize the voice coil gap, and the invention may be applied to other types of motors or actuators incorporating suitable drive coils, including drivers with multiple coils and / or multiple gaps, voice coil actuators, and dual or multiple voice coil drivers. The second magnetic field may be offset from the first magnetic field, allowing the overhang of the detection winding perpendicular to the axis (D2 in FIG. 7) to be higher than the voice coil. As described, a single magnetic circuit may be used to generate both magnetic fields, separate magnetic circuits may be used to generate each magnetic field, or a combination of multiple magnetic circuits may be used to generate the first and second magnetic fields in combination. Although only embodiments with one second magnetic field are described, there may be one or more second magnetic fields spaced along the axis. The present invention is described herein primarily with respect to the most common former and voice coil arrangement, where the voice coil is wound on the outside of the voice coil former.

[0060] However, the principles of the invention are equally applicable to other former and voice coil arrangements, such as where the voice coil former is around the outside of the voice coil, where there are two voice coils, one on the outside and one on the inside of the voice coil former, or where there are two voice coil formers, one on the outside and one on the inside of the voice coil, etc. The term "around" as used above and in the claims should be interpreted to encompass all of these alternative arrangements and does not imply that an element described as being around another element is only on the outside periphery, but also encompasses arrangements where the element is on the inside periphery.

[0061] Where different modifications or alternative arrangements have been described above, it should be understood that embodiments of the invention may incorporate such modifications and / or alternatives in any suitable combination.

Claims

1. A method for measuring the instantaneous velocity of a loudspeaker driver reciprocating in a magnetic field, wherein the driver has a voice coil helically wound coaxially with a former and a detection winding arranged coaxially with the former, the driver is driven to reciprocate along a reciprocating axis by the application of an electrical signal to the voice coil, and the method is: To provide a first magnetic field that is arranged and adapted to primarily couple with the voice coil, To provide a second magnetic field that is primarily arranged and adapted to be coupled with the detection winding, and To detect the voltage induced in the second detection winding when it reciprocates axially in the second magnetic field. Includes, A method wherein the second magnetic field has an orientation periodicity that is circumferential with respect to the reciprocating axis, and the orientation periodicity extends along the reciprocating axis over at least a portion of the length of the former.

2. The method according to claim 1, comprising arranging the first and second magnetic fields at different positions along the axis.

3. The method according to claim 1, comprising superimposing the first and second magnetic fields.

4. The method according to claim 1, comprising applying the second detection winding to the former in such a pattern that the magnetic drive field generated in the voice coil when the electrical signal is applied to drive the voice coil does not couple with the second detection winding.

5. The method according to claim 1, wherein the first magnetic field is mainly radial with respect to the reciprocating axis.

6. A voice coil wound helically coaxially around a former, which together are adapted to reciprocate along the reciprocating axis within a gap in the magnet arrangement to radiate acoustic energy, such that when an electrical signal is applied to the voice coil, an acoustic diaphragm connected to the former reciprocates along the reciprocating axis, and the voice coil extends a first distance along the axis and the former, A detection winding is coaxially arranged with the former and extends a second distance along the reciprocating axis. Equipped with, The magnet arrangement is adapted and configured to generate two magnetic fields, a first magnetic field primarily driving the voice coil to reciprocate and positioned adjacent to it in the axial direction at a first distance, and a second magnetic field positioned adjacent to it in the axial direction at a second distance and adapted primarily to couple with the sensing winding, the sensing winding being arranged around the former in the form of an even number of loops, the even number of loops being arranged separately around the former but electrically connected to form a single winding, each loop extending around a loop axis substantially perpendicular to the reciprocating axis, in a loudspeaker.

7. The loudspeaker according to claim 6, wherein the first and second magnetic fields and the first and second distances overlap in the direction of the reciprocating axis.

8. The loudspeaker according to claim 6, wherein the first and second magnetic fields and the first and second distances do not overlap in the direction of the reciprocating axis.

9. The loudspeaker according to claim 6, wherein a second detection winding is formed on the outer surface of the former in one or more layers comprising a plurality of separate coils arranged circumferentially around the former, each coil comprising a plurality of adjacent turns extending around a loop axis.

10. The loudspeaker according to claim 9, wherein adjacent coils in the circumferential direction turn in alternating directions.

11. The loudspeaker according to claim 8, comprising two or more printed layers, wherein printed coils in one layer are circumferentially aligned with printed coils in adjacent layers around a reciprocating axis.

12. The loudspeaker according to claim 9, wherein a portion of each coil is aligned perpendicular to the reciprocating axis, and the turns in the portion of each coil are spaced apart from each other with respect to the reciprocating axis by a greater distance than the turns forming the rest of the coil.

13. The loudspeaker according to claim 6, wherein the detection winding is in the form of a printed circuit formed on the inner or outer surface of the former.

14. The loudspeaker according to claim 6, wherein the first distance is smaller than the second distance.

15. The loudspeaker according to claim 6, wherein the magnet arrangement includes separate first and second magnets for generating the first and second magnetic fields.

16. A former for a loudspeaker, the former comprising a voice coil helically wound coaxially with the former, the former and the voice coil being adapted to reciprocate along the reciprocating axis within a gap in a magnet arrangement such that an acoustic diaphragm connected to the former reciprocates along the reciprocating axis to radiate acoustic energy, the former comprising a detection winding formed on the outer and / or inner surface of the former in one or more printed circuit layers comprising a plurality of separate detection coils arranged circumferentially around the former, each coil comprising a plurality of adjacent turns, the detection coils being separately arranged around the former but electrically connected to form a single winding, and each detection coil extending around a loop axis substantially perpendicular to the reciprocating axis.

17. The former according to claim 16, comprising two or more print layers, wherein a printed detection coil in one layer is circumferentially aligned with a printed detection coil in an adjacent layer around the axis.

18. The former according to claim 16, wherein a portion of each detection coil is aligned perpendicular to the reciprocating axis, and the turns of the portion of each detection coil are spaced further apart from each other with respect to the reciprocating axis than the turns forming the rest of the detection coil.

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