WO2001043493A1

WO2001043493A1 - Sound transducer containing dual voice coils

Info

Publication number: WO2001043493A1
Application number: PCT/AU2000/001518
Authority: WO
Inventors: Henricus Johannes Franciscus Gabriel Groothuis
Original assignee: Mass Enterprises Pty Ltd
Priority date: 1999-12-07
Filing date: 2000-12-07
Publication date: 2001-06-14
Also published as: AUPQ449999A0

Abstract

A sound transducer (10) includes a frame (2); a diaphragm (14) resiliently mounted to the frame; a voice coil (28) for actuation of the diaphragm in response to a signal; and a further coil (30) fixedly mounted to the frame and arranged to produce an opposing magnetic field to the voice coil in response to the signal. The frame has an axial post (20), the voice coil being wound on a former (24) provided around the axial post, and the further coil being fixedly mounted on the axial post. The voice coil and the further coil are arranged so that their two magnetic fields are in opposite directions, thereby increasing the efficiency of the transducer. The transducer is also provided with amplification circuitry (32), for example, an integrated circuit (34), which is mounted on a heat sink (38). A filter (36) is also provided. In an alternative embodiment, the transducer can be provided without the amplification circuitry, or the amplification circuitry can be used with known sound transducers. In yet another embodiment, the transducer can be provided with a second further coil, which further increases the efficiency of the transducer. The amplification circuitry can also include phase-shifting means, which enhances the perceived sound quality.

Description

SOUND TRANSDUCER CONTAINING DUAL VOICE COILS

Field of the Invention

The invention relates to sound transducers and to improvements relating thereto.

Background Art

Sound transducers typically consist of a diaphragm resiliently mounted to a frame. The frame supports a permanent magnet positioned in close proximity to a voice coil. The voice coil is connected to the diaphragm by a former so that when a signal is passed through the voice coil, the diaphragm moves relative to the frame.

To increase the power handling of the sound transducer, increasingly powerful permanent magnets are used. However, this creates a magnet field around the sound transducer that can interfere with operation of other devices, such as televisions and other cathode ray tubes.

Further, sound transducers require a separate power source to provide a sufficient amplitude signal to drive the voice coil. In applications where limited space is available, such as in automobiles, finding sufficient space for the amplifier required to drive the sound transducers can be a difficult task.

Disclosure of the Invention

According to a first aspect of the invention, there is provided a sound transducer, comprising:

a frame;

a diaphragm resiliently mounted to the frame; a voice coil for actuation of the diaphragm in response to a signal; and

a further coil fixedly mounted to the frame and arranged to produce an opposing magnetic field to the voice coil in response to the signal.

Preferably, the frame has an axial post, the voice coil being wound on a former provided around the axial post, and the further coil is fixedly mounted on the axial post.

In one arrangement, the axial post includes a circumferential recess in which the further coil is received.

Preferably, the voice coil is provided on an inner surface of the former.

Alternatively, the voice coil is provided on an outer surface of the former.

In one arrangement, the voice coil and the further coil are provided in electrical series and wound in opposite directions.

In an alternative arrangement, the voice coil and the further coil are provided in electrical parallel and wound in opposite directions.

In a further alternative arrangement, the voice coil and the further coil are wound in the same direction.

In a preferred feature of the further alternative arrangement, the voice coil and the further coil are connected to the signal but in opposite polarity.

In an alternative preferred feature of the further alternative arrangement, the voice coil is connected to the signal and the further coil is connected to a further signal that is 180° out of phase relative to the signal.

Preferably, the sound transducer further includes a second further coil arranged substantially co-axially with the voice coil and the further coil and arranged to increase the strength of the opposing magnetic field. Preferably, the frame has an axial post, the voice coil being wound on a voice coil former provided around the axial post, the further coil being fixedly mounted on the axial post, and the second further coil being wound on a coil former provided around the voice coil former.

Preferably, the axial post includes a circumferential recess in which the further coil is received.

Preferably, the voice coil is provided on an outer surface of the voice coil former, and the second further coil is provided on an inner surface of the coil former.

Preferably, the two further coils are wound in series and provided in parallel with the voice coil.

Preferably, the sound transducer further comprises amplifier means provided integrally therewith.

Preferably, the amplifier means is mounted to the frame.

Preferably, the frame forms part of a heat sink for the amplifier means.

Preferably, the amplifier means includes a filter.

Preferably, the sound transducer further includes phase shift means coupled to the amplifier means and arranged to introduce a frequency dependant phase shift to the signal prior to being input to the amplifier means, wherein the magnitude of the phase shift is dependant upon the frequency of the signal.

Preferably, the amplifier means includes two discrete amplifiers, a first amplifier being coupled between the phase-shift means and the voice coil, and a second amplifier being coupled between the phase-shift means and the further coil, and, where the transducer includes a second further coil, the second further coil.

Preferably, the sound transducer further includes gain control means provided between the phase-shift means and the amplifier means. Preferably, the voice coil and the further coil are directly connected to an output of the amplifier means.

Where the voice coil and the further coil are wound in opposite directions, the coils are preferably directly connected to an output of the amplifier means. Where the voice coil and the further coil are wound in the same direction, in one arrangement the coils are preferably directly connected to an output of the amplifier means, with the further coil connected in opposite polarity to the voice coil.

Where the voice coil and the further coil are wound in the same direction, in an alternative arrangement the amplifier means preferably produces an output and an out-of-phase output, one of the voice coil and the further coil being directly connected to the output of the amplifier means, and the other of the voice coil and the further coil being directly connected to the out-of-phase output of the amplifier means. The amplifier means may comprise two discrete amplifiers, for producing the output and the out-of-phase output, respectively.

In accordance with a second aspect of the invention, there is provided a sound transducer, comprising;

a frame;

a diaphragm resiliently mounted to the frame;

a voice coil for actuation of the diaphragm in response to a signal;

a magnet; and

amplifier means mounted to the frame the voice coil being connected directly to an output of the amplifier means such that the amplifier means provides the signal directly to the voice coil.

Preferably, the frame forms part of a heat sink for the amplifier means. Preferably, the amplifier means includes a filter.

Brief Description of the Drawings

The invention and its advantages will be better understood with reference to the following three specific embodiments thereof and the accompanying drawings, in which :

Figure 1 is a cross-sectional view of a sound transducer according to a first embodiment of the invention, showing a detail of the voice coil and further coil wound on a former and post respectively;

Figure 2 is a cross-sectional view of a sound transducer according to a second embodiment of the invention, showing a detail of the voice coil and further coil wound on a former and post respectively;

Figure 3 is a cross-sectional view of a sound transducer according to a third embodiment of the invention, showing detail of the voice coil, and two further coils wound around a voice coil former, post and coil former respectively;

Figure 4 is a schematic circuit diagram of the amplifier circuit of the sound transducer of Figure 3;

Figure 5 is a schematic circuit diagram of the amplifiers for the signal control circuitry of Figure 4;

Figure 6 is a schematic circuit diagram for the signal control circuit of Figure 4;

Figure 7 is a graph of time vs. frequency illustrating the response of the diaphragm of the sound transducer of Figure 3 to frequency;

Figure 8 is a graph of phase-shift as a function of frequency for the sound transducer of Figure 3; Figure 9 is a cross-sectional view of a sound transducer according to a fourth embodiment of the invention; and

Figure 10 is a schematic circuit diagram of a suitable balanced line receiver for use with the transducer of the third embodiment.

Best Mode(s) for Carrying Out the Invention

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

The first embodiment, shown in Figure 1 , is directed towards a sound transducer 10 comprising a frame 12 and a diaphragm 14 resiliently mounted to the frame 12 by a surround 16.

The frame 12 comprises a frusto-conical section 18 and a post 20 provided coaxially with the frusto-conical section 18. The post 20 includes an annular flange 22 upon which the frusto-conical section 18 is mounted.

The diaphragm 14 can be of any desired profile. The surround 16 used in the embodiment has a semicircular profile, however other profiles can be used as desired. The surround 16 is made from a suitable resilient material, for example rubber, a synthetic rubber such as that sold under the trademark Neoprene, or a textile material.

The diaphragm 14 is connected to a former 24 that is provided about the post 20. The former 24 is held in place with respect to the post 20 by a ribbed support 26 made from a resilient material that extends between the former 24 and the frusto- conical section 18.

A voice coil 28 is wound on the inside of the former 24. A further coil 30 is wound onto the post 20 and fixed thereto. The voice coil 28 and the further coil 30 are provided in close physical proximity. For this reason, it is advantageous if the voice coil 28 is wound on the inside of the former 24. However, if necessary the voice coil 28 can be wound on the exterior of the former 24. The detail in Figure 1 shows the voice coil 28 and the further coil 30 wound around their respective former 24 and post 20.

The voice coil 28 can be wound on the former 24 in any convenient manner. For example, to produce a former with a voice coil on the inside thereof, the following process can be used. Firstly, the voice coil is wound onto a spindle (not shown). A former is then provided around the spindle and voice coil and pressed to retain the voice coil to it. The former and voice coil can then be slid off the spindle. The former used in this example may have a cut provided in it to assist in providing the former around the spindle. Further, the former may have a recess formed therein that the voice coil is received in. Advantageously, this method allows close tolerance control over the former and the voice coil according to the size of the spindle, enabling a minimal clearing between the further coil and the voice coil.

In the embodiment, the voice coil 28 and the former further coil 30 are wound in opposite directions. It should be readily appreciated that the voice coil 28 and the further coil 30 could be wound in the same direction and connected to the amplifier described below in opposite polarity with equal efficacy.

The sound transducer 10 further comprises an amplifier denoted generally at 32. The amplifier 32 can be made of discrete components, or a monolithic integrated circuit, such as the LM3886T manufactured by National Semiconductor may be used. In the embodiment, the monolithic integrated circuit LM3886T is shown at 34 with associated bias circuitry and an active filter shown at 36. The integrated circuit 34 is mounted to a heat sink 38. In turn, the heat sink 38 is mounted to the annular flange 22 via a thermally conductive annular spacer 40. A removable cover 42 is provided over the amplifier and filter 32. The removable cover 42 is releasably clipped or screwed onto the heat sink 38.

In the embodiment, the voice coil 28 and the further coil 30 are connected in electrical series and are connected directly to the output of the integrated circuit 34. This direct connection gives the amplifier 32 accurate control over the excursion of the diaphragm 14, resulting in clearer sound with better transient response. The filter 36 acts on a source signal prior to being input to the integrated circuit 34, so that the sound transducer 10 replays only a desired portion of the frequency components present in the source signal. Advantageously, in addition to providing more accurate motion and superior sound quality, the sound transducer 10 of the embodiment does not require an external amplifier, making it well suited for use in application where space is restricted, such as automobile use. However, it should be appreciated that the sound transducer 10 can be used in other applications.

The voice coil 28 and the further coil 30 produce opposing magnetic fields that repel each other. This results in twice the efficiency compared to if a static magnetic field was used, such as a permanent magnet. Further, the close physical proximity of the voice coil 28 and the further coil 30 also increase the efficiency of the sound transducer 10.

Advantageously, the sound transducer 10 does not use a permanent magnet, thereby reducing the interference to surrounding devices. The magnetic field produced by the voice coil 28 and further coil 30 diminishes rapidly away from the coils 28 and 30. Further, because the magnetic field produced by the voice coil 28 and the further coil 30 are opposites, far-field cancellation effects may occur which further reduce the strength of magnetic fields produced by the sound transducer 10 away from the coils 28 and 30, contributing to the reduced magnetic interference produced by the sound transducer 10.

The second embodiment is directed towards a sound transducer 110 shown in Figure 2. Like reference numerals are used to denote like parts to those in the first embodiment, with 100 added thereto.

The sound transducer 110 of the second embodiment is of the same general form as the sound transducer 10 of the first embodiment, however, the sound transducer 110 does not have an amplifier and filter provided integrally therewith. The sound transducer 110 can be used in conventional applications wherever existing sound transducers are used.

The third embodiment is directed towards a sound transducer 310 shown in Figure 3. Like reference numerals are used to denote like parts to those in the first embodiment, with 300 added thereto. The sound transducer 310 of the third embodiment is of the same general form as the sound transducer of the first embodiment. However, in this embodiment, there is provided a second further coil 340, as will be discussed in further detail below.

As in the first embodiment, there is provided a voice coil 328 wound around a former 324, and a further coil 330 wound onto the post 320 and fixed thereto. In this embodiment, the voice coil 328 is wound around the outside of the former 324. This is illustrated in the detail of Figure 3. A second cylindrical former 342, arranged substantially co-axially with the former 324 and the post 320, is mounted within a circular recess 344 provided in the annular flange 322 upon which the frusto-conical section is mounted. Wound around the inner surface of this second former 342 is the second further coil 350, such that all three coils 328, 330, 350 are arranged substantially co-axially with each other as illustrated in Figure 3. As in the first embodiment, all the coils 328, 330, 350 are arranged in close proximity. The further coil 330 is wound around the post 320, in a recess 346, and fixed thereto. The provision of this recess 346 allows the further coil 330 to be arranged to be substantially flush with the outer surface of the post 320, thus allowing the voice coil 328 and the second further coil 350 to be in much closer proximity. The detail of Figure 3 illustrates the coil arrangement. The two further coils 330, 350 are wound in series, and are in parallel with the voice coil 328. As in the first embodiment, the first two further coils 330, 350 are wound oppositely to the voice coil 328, and the provision of two coils, also in close proximity to each other, increases the magnetic flux thereby giving rise to an increased excursion of the diaphragm 314 in response to the incoming signal.

The sound transducer 310 also further comprises an amplifier circuit denoted generally at 348 in Figure 3. This can comprise discrete components, or can be a monolithic integrated circuit. The amplifier circuit 348 is illustrated schematically in Figure 4, and comprises first and second amplifiers 356, 352, and associated signal control circuitry 354 - discussed in further detail below.

The amplifier circuitry 348 is - as in the first embodiment - mounted on a heat sink 338, which is also mounted to an annular flange 322 via a thermally conductive annular spacer 340. There is also a removable cover 340 provided over the amplifier circuitry 348 and associated signal control circuitry 354.

The voice coil 328 and the two further coils 330, 350 are coupled directly to the amplifier circuit 348, as shown in Figure 4, which is a schematic circuit diagram showing the amplifier circuitry 348 and the associated signal control circuitry 354 in more detail. More specifically, the signal control circuitry 354 is coupled to the voice coil 328 via a first amplifier 356, and to the two further coils 330, 350 via a second amplifier 352. Gain control circuitry 358, 360 is provided between the signal control circuitry 354 and the first and second amplifiers 356, 352 respectively to compensate for any offset.

A low level signal - for example, such as that from a compact disc player - is input to the signal control circuitry 354.

Before being input to the signal control circuitry 354, the signal is input to a balanced line receiver 380, an example of which is illustrated schematically in Figure 10, although any suitable receiver could be used. The receiver 380 comprises an NE5534 operational amplifier 381 , and associated circuitry, including capacitors 382, and resistors 383.

The signal control circuit 354 introduces a frequency dependent phase shift, Δφ, to the signal and delivers the signal, with a phase shift applied thereto, to the first and second amplifiers 356, 352. In particular the phase shift, Δφ, is proportional is the exponential of the frequency, F of the applied signal.

The signal output from the signal control circuit 354 is output to the amplifiers 356, 352, which amplifies the signal and delivers the amplified portion directly to the voice coil 328 or to the respective further coils 330, 350. Because the coils 328, 330, 350 are directly connected to the outputs of the amplifiers 356, 352, the amplifiers have accurate control over the motion of the diaphragm 314.

Figure 5 is a circuit diagram for the first amplifier 356. The second amplifier 352 is the same. Each amplifier 356, 352 receives the output from the signal control circuit 354 at amplifier input 360, and comprises an operational amplifier 378 and associated circuitry as illustrated in Figure 6. The input signal is amplified and output to its respective coil 328, 330, 350. The amplifier 356, 352 has a supply voltage Sv, and supply current Sc, determined by the impedance of the respective coil 328, 330, 350. For a coil with an impedance of 8Ω, and a required output of 60 Watt, then a supply voltage of 35V is required. The minimum gain for the amplifier 356, 352 is determined by the output power, coil impedance, and input voltage level. For an input level of 1V and the above output and coil impedance values, the minimum gain required is approximately 21.9. For an input impedance, R₃ of 22KΩ, then, for the above gain, R₂ and Ci are set at 1KΩ and 47μF respectively. No high frequency limiting is required because the bandwidth is set by the signal control circuitry 354. Low frequency limiting - set at about 4Hz is required to limit low-frequency oscillation and is formed as part of the RC network.

The signal control circuitry 354 will now be described in further detail with reference to figure 6, which is a circuit diagram thereof.

The signal control circuitry 354 comprises a fourth order Butterworth filter comprising low pass stages 362 and high pass stages 364.

Each low pass stage 362 takes the same general form, comprising an operational amplifier 366 (in this embodiment, a TL074 Quad operational Amplifier), resistors 368 and capacitors 370. The values of the capacitors 368 and the resistors 370 are chosen in known manner to provide the low frequency cut-off in the signal control circuit 354 according to the desired frequency range to be delivered to the corresponding voice coil 328. For example, with values of the resistors 368 being selected as 22kΩ, then the values of the capacitors 370 are selected in accordance with Table 1 below, for the low-pass threshold frequency required: Table 1

Similarly, each of the high pass stages 364 takes the same general form, comprising an operational amplifier 372 (again, in this embodiment, a TL074 Quad operational amplifier), resistors 374a, 374b and capacitors 376. Again, for values of the resistors 374a, 374b being selected as 22kΩ and 47kΩ respectively the values of the capacitors 38 are chosen from Table 1 above, according to the selected high frequency cut-off frequency to be delivered to the voice coil 328.

The signal control circuitry 354 forms a crossover network, and eliminates the need for any crossover circuit between the output of the amplifiers 356, 352 and the coils 328, 330, 350.

The signal control circuitry 354 also produces a frequency dependent phase shift, Δφ, in the signal as it passes through the circuit. This phase shift, Δφ, introduces the phase shift to the audio signal, which varies depending upon the frequency, F, of the applied audio signal. Figure 7 illustrates how this phase shift, Δφ, varies with the frequency. At around 200Hz, the phase-shift, Δφ, becomes zero. Below that frequency, the phase-shift, Δφ, appears negative because of compression of the signal at the start thereof.. This phase-shift is achieved by using the operational amplifiers 366, 372. The operational amplifier 366, 372 chosen - e.g. the TL074 described above - is not frequency compensated, or is arranged so that frequency compensated pins are not used, i.e. the frequency compensation aspect of the operational amplifier is not used. In the embodiment, the operational amplifiers 366 and 372 each comprise one operational amplifier in the TL074 quad operational amplifier integrated circuit. Because the TL074 quad operational amplifier is not frequency compensated, a phase shift that increases with increasing frequency is introduced to the signal. Whereas such an operational amplifier would ordinarily be rejected for audio applications because of its lack of frequency compensation, it has been found that the phase shift introduced by the operational amplifier 366, 372 - resulting from its lack of frequency compensation - produces a better perceived sound quality from a listeners perspective.

Sound transducers generally have a non-linear response depending upon the frequency of the signal received by the transducer, with the time taken for the diaphragm 314 to return to it's original position during one cycle of sound being inversely proportional to the frequency, F, of the applied signal. This is illustrated in Figure 8. So, for a low frequency audio signal of, say, 100Hz, the diaphragm 314 takes approximately 10ms to oscillate for one cycle, whereas, at 500Hz, the time taken is approximately 1ms. By using a non-frequency compensated operational amplifier, and therefore applying a frequency-dependant phase-shift before the signal is applied to the voice coil 328, compensation is being applied to the signal before amplification, thereby negating the effects of the non-linear response resulting from the oscillation of the diaphragm 314, and resulting in a substantially linear response, and therefore a better perceived resulting audio output from the sound transducer.

By passing the signal through the four stages of the signal control circuitry 354, an additive effect is produced - whereby the signal is rotated four times in- and out- of phase, thereby compounding the effect to produce the curve illustrated in Figure 7. The fourth embodiment is directed towards a sound transducer 210 shown in Figure 9. Like reference numerals are used to denote like parts to those shown in relation to the first embodiment, with the addition of 200 thereto. The sound transducer 210 the third embodiment differs from the sound transducer 10 of the first embodiment in that a conventional voice coil and magnet arrangement is used.

In particular, the frame 212 further comprises an annular plate 250 to which the frusto-conical section 218 is attached. A permanent magnet 252 is provided between the annular plate 250 and the annular flange 222 of the post 220.

The voice coil 228 is provided on the outer surface of the former 224. This is illustrated in the detail of Figure 9.

Although a conventional voice coil and magnet and arrangement are used in this embodiment, the sound transducer 210 still offers advantages compared with conventional sound transducers. By integrating the amplifier onto the frame, the sound transducer 210 can be used in situations where limited space is available. Further, the voice coil 228 is driven directly by the out put of the integrated circuit 234, providing more accurate control over the motion of the diaphragm 214 with the resulting improvement of sound clarity and transient response.

It should be appreciated that the scope of this invention is not limited to the particular embodiment described above.

For example, the source signal and power can be provided to the amplifier and filter 232 in a variety of ways without departing from the spirit of the invention. For example, a balanced line could be used to provide the signal and power, with the signal modulated onto the power lines, or the signal and power could be provided separately, or a digital interface such as the universal serial bus could be provided.

Further, the voice coil and/or further coil could be powered using a 'bridged' signal, depending on the arrangement of the amplifier. It will be obvious to people skilled in the art that various modifications are possible within the scope of the present invention. For example, the phase-shifting and amplification arrangement of the third embodiment could be applied to the sound transducers of the first and second embodiments, and similarly, the amplifier arrangements of the first embodiment could be applied to the third embodiment

Claims

The Claims Defining the Invention are as Follows

1. A sound transducer, comprising:

a frame;

a diaphragm resiliently mounted to the frame;

a voice coil for actuation of the diaphragm in response to a signal; and

2. A sound transducer according to claim 1 , wherein the frame has an axial post, the voice coil being wound on a former provided around the axial post, and the further coil is fixedly mounted on the axial post.

3 A sound transducer according to claim 2, wherein the axial post includes a circumferential recess in which the further coil is received.

4. A sound transducer according to any of claims 1 to 3, wherein the voice coil is provided on an inner surface of the former.

5. A sound transducer according to any of claims 1 to 3, wherein the voice coil is provided on an outer surface of the former.

6. A sound transducer according to any of claims 1 to 3, wherein the voice coil and the further coil are provided in electrical series and wound in opposite directions.

7. A sound transducer according to any of claims 1 to 5, wherein the voice coil and the further coil are provided in electrical parallel and wound in opposite directions.

8. A sound transducer according to any of claims 1 to 5, wherein the voice coil and the further coil are wound in the same direction.

9. A sound transducer according to claimδ, wherein the voice coil and the further coil are connected to the signal but in opposite polarity.

10. A sound transducer according to claim 8, wherein the voice coil is connected to the signal and the further coil is connected to a further signal that is 180° out of phase relative to the signal.

11. A sound transducer according to claim 1 , further including a second further coil arranged substantially co-axially with the voice coil and the further coil and arranged to increase the strength of the opposing magnetic field.

12. A sound transducer according to claim 11 , wherein the frame has an axial post, the voice coil being wound on a voice coil former provided around the axial post, the further coil being fixedly mounted on the axial post, and the second further coil being wound on a coil former provided around the voice coil former.

13. A sound transducer according to claim 12, wherein the axial post includes a circumferential recess in which the further coil is received.

14. A sound transducer according to claims 12 or 13, wherein the voice coil is provided on an outer surface of the voice coil former, and the second further coil is provided on an inner surface of the coil former.

15. A sound transducer according to any of claims 11 to 14, wherein the two further coils are wound in series and provided in parallel with the voice coil.

16. A sound transducer according to any preceding claim, further comprising amplifier means provided integrally therewith.

17. A sound transducer according to claim 16, wherein the amplifier means is mounted to the frame.

18. A sound transducer according to claim 17, wherein the frame forms part of a heat sink for the amplifier means.

19. A sound transducer according to any of claims 16 to 18, wherein the amplifier means includes a filter.

20. A sound transducer according to any of claims 16 to 19, further including phase shift means coupled to the amplifier means and arranged to introduce a frequency dependant phase shift to the signal prior to being input to the amplifier means, wherein the magnitude of the phase shift is dependant upon the frequency of the signal.

21. A sound transducer according to claim 20, wherein the amplifier means includes two discrete amplifiers, a first amplifier being coupled between the phase-shift means and the voice coil, and a second amplifier being coupled between the phase-shift means and the further coil, and when dependant upon claims 11 to 15, and the second further coil.

22. A sound transducer according to claim 20 or claim 21 , further including gain control means provided between the phase-shift means and the amplifier means.

23. A sound transducer according to any of claims 16 to 19, when dependant upon claims 6 or 7, wherein the voice coil and the further coil are directly connected to an output of the amplifier means.

24. A sound transducer according to any of claims 16 to 19, when dependant upon claim 8, wherein, the voice coil and the further coil are directly connected to an output of the amplifier means, with the further coil connected in opposite polarity to the voice coil.

25. A sound transducer according to any of claims 16 to 19, when dependant upon claim 8, wherein, where the voice coil and the further coil are wound in same direction, the amplifier means is operable to produce an output and an out-of- phase output, one of the voice coil and the further coil being directly connected to the output of the amplifier means, and the other of the voice coil and the further coil being directly connected to the out-of-phase output of the amplifier means.

26. A sound transducer according to claim 25, wherein the amplifier means comprises two discrete amplifiers, for producing the output and the out-of-phase output, respectively.

27. A sound transducer, comprising;

a frame;

a diaphragm resiliently mounted to the frame;

a voice coil for actuation of the diaphragm in response to a signal;

a magnet; and

28. A sound transducer according to claim 27, wherein the frame forms part of a heat sink for the amplifier means.

29. A sound transducer according to claim 28, wherein the amplifier means includes a filter.

30. A sound transducer as hereinbefore described with reference to the accompanying Figures.