ASSEMBLY COMPRISING AN ELECTROSTATIC SOUND GENERATOR AND A TRANSFORMER

Abstract

An assembly of a transformer and an electrostatic sound generator is especially efficient if the resonance frequency of the diaphragm is in the frequency range in which the generator is operated, such as in the interval of 1-20 kHz. Then, a smaller transformer with a winding ratio of 5000 or less may be used for feeding the sound generator, making the assembly suitable for hearing aid purposes or in-ear products such as for pro audio use.

Claims

1. A miniature assembly comprising a transformer and a sound generator, the sound generator comprising a housing, a diaphragm and a back plate, wherein the diaphragm divides an inner space of the housing into a first and a second volume, the back plate is positioned in one of the first and second volumes, the diaphragm has a resonance frequency in the interval of 1-14 kHz, the transformer having a first conductor and a second conductor, the second conductor being connected to the signal generator, the first conductor having a first number of windings and the second conductor having a second number of windings, where a ratio of the first number of windings to the second number of windings is lower than 1:5000.

2. An assembly according to claim 1, wherein the back plate is a single back plate.

3. An assembly according to claim 2, the sound generator further comprising a signal input and a conducting area provided on or in one of the back plate and the diaphragm, the conducting area comprising a charge, where the signal input is connected to the other of the back plate and the diaphragm.

4. An assembly according to claim 1, further comprising a second back plate positioned in the other of the first and second volumes, the diaphragm being positioned between the back plate and the other back plate.

5. An assembly according to claim 4, the sound generator further comprising a signal input, a first conducting area comprising a first charge and a second conducting area comprising a second charge, where the first conducting area is provided on or in a first of the diaphragm, the back plate and the second back plate, the signal input is connected to a second of the diaphragm, the back plate and the second back plate, and the second conductive element is provided on or in a third of the diaphragm, the back plate and the second back plate.

6. An assembly according to claim 1, wherein the first conductor comprises no more than 1000 windings and the second conductor has no less than 10000 windings.

7. An assembly according to claim 1, further comprising a low frequency sound generator, the low frequency sound generator being configured to output sound in the frequency interval of 20 Hz-10 kHz.

8. A n assembly according to claim 7, further comprising a medium frequency sound generator, the medium frequency sound generator being configured to output sound in the frequency interval of 200 Hz-12 kHz.

9. An assembly according to claim 1, further comprising a second sound generator comprising a second housing, a second diaphragm and a second back plate, wherein the second diaphragm divides an inner space of the second housing into a third and a fourth volume, the second back plate is positioned in one of the third and fourth volumes, the second diaphragm has a resonance frequency in the interval of 1-14 kHz, the transformer having a third conductor having a third number of windings, a second ratio of the first number of windings to the third number of windings being lower than 1:5000.

10. An assembly according to claim 9, further comprising a third housing comprising therein the housing and the second housing and having a sound outlet.

11. An assembly according to claim 1, further comprising a signal emitter configured to feed a signal with a frequency in the interval of 1-20 kHz to the first conductor.

12. A method of operating the assembly according to claim 1, the method comprising feeding an electrical signal comprising at least a portion within the frequency interval of 1-20 kHz to the first conductor.

13. A method according to claim 12, wherein the sound generator housing has a portion configured to be positioned at or in an ear canal of a person, the sound generator having a sound output in the portion, the feeding step comprising: feeding an AC signal to the first conductor and feeding a sound signal from the sound output into one end of a cylindrical cavity with a diameter of 18.55 mm and a length of 6.6 mm, the sound signal having, at the other end of the cavity, at least 80 dB/V.

14. A personal listening device comprising the assembly according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0096] In the following, preferred embodiments of the invention will be described with reference to the drawing, wherein:

[0097] FIG. 1 illustrates an electrostatic sound generator

[0098] FIG. 2 illustrates a circuit for feeding an electrostatic sound generator and a second sound generator

[0099] FIG. 3 illustrates different set-ups of an electrostatic sound generator, a second sound generator and a transformer in a housing

[0100] FIG. 4 illustrates an embodiment where two sound generators are connected to a mobile telephone for providing audio to a user

[0101] FIG. 5 illustrates multiple electrostatic sound generators fed by the same transformer

[0102] FIG. 6 illustrates a first manner of altering a resonance frequency of a sound generator

[0103] FIG. 7 illustrates a second manner of altering a resonance frequency of a sound generator and

[0104] FIG. 8 illustrates a third manner of altering a resonance frequency of a sound generator.

DETAILED DESCRIPTION OF THE INVENTION

[0105] In FIG. 1, a standard electrostatic sound generator 16 is illustrated having a housing 161, the inner space thereof being divided into two chambers 162 and 164 by a diaphragm 166. The housing 161 has a sound output 169 for outputting sound from the space 162, which usually is denoted a front chamber, where the space 164 then is denoted a back chamber. The back chamber may be completely sealed, may have a so-called vent, or may have a sound output of its own.

[0106] A back plate 168 is illustrated being positioned in the front chamber 162. It may equally well be positioned in the back chamber 164. The back plate is positioned parallel to and rather close to the diaphragm and is usually provided with a number of openings so that air may pass through it and into the remainder of the chamber 162. However, naturally, the back plate may be non-perforated and form a wall or inner surface of the chamber in which the back plate is provided.

[0107] The sound generator operates by a force being applied between the diaphragm and back plate. There are a number of manners of obtaining this.

[0108] In the art, electrostatic generators as that illustrated in FIG. 1 is called “single sided” in that only a single back plate is used.

[0109] In this set-up, the force is generated by adding a charge to one of the back plate and the diaphragm and the input signal, such as input via connection 171 to inputs 165, to the other. The voltage of the signal applied will change the resulting charge of the other of the diaphragm and the back plate and will thus vary the overall force caused by the difference in charge of these two elements.

[0110] The charge added to the back plate or diaphragm may be provided by permanently or semi permanently charging an isolated conducting element of the diaphragm or back plate. This is illustrated in FIG. 1 where the back plate 168 has a non-conducting portion 168′ in the centre of which a conductive portion 168″ is provided. When this conducting element is isolated (electrically and/or galvanically) from other elements of the sound generator (and preferably everything else), it will retain this charge. Alternatively, a DC voltage may be fed to the conducting element of this diaphragm/back plate. The permanent charging of the element has the advantage that no electrical connection is required to that element. A disadvantage is that a so-called collapse, where the back plate and diaphragm touch, so that the charge is removed, may cause the generator to no longer function optimally.

[0111] Alternatively, the input signal may also be fed to the other of the diaphragm/back plate but in an inversed manner, so that when the input signal fed to a first one of the diaphragm/back plate is positive, the signal fed to the other one is negative. Thus, the force will vary over time and will resemble the input signal.

[0112] Naturally, an additional back plate (not illustrated) may be used and positioned in the back chamber 164. This is the usual manner of providing an electrostatic sound generator.

[0113] In this manner, again, multiple manners of operation are possible. In one manner, a permanent or semi-permanent charge is provided to the diaphragm. Alternatively, a DC voltage is applied thereto. Then, the input signal is fed to one back plate and inversed to the other. When one back plate is positive and the other negative, the charge of the diaphragm will move the diaphragm away from one back plate and toward the other.

[0114] Alternatively, a DC voltage may be provided to the back plates (positive to one and negative to the other—or more positive on one and less positive on the other or the like), where the input signal is then fed to the diaphragm. The same overall result is seen.

[0115] Naturally, a combination may be used where charges or DC values are provided to the diaphragm and one back plate and the input signal to the other back plate.

[0116] The resonance frequency of the diaphragm is easily determined either empirically or theoretically. In addition, a resonance frequency is characterized also by a Q value describing how tall and slim the peak is. The higher the Q value, the sharper and taller the resonance peak. The Q-factor is determined as Q=fc/(f2−f1) where fc is the centre frequency of the resonance peak, f2 the upper frequency and f1 the lower frequency at half maximum (FWHM frequencies; −3 dB from the maximum value) of the peak.

[0117] The resonance frequency may be altered or adapted by amending or altering the mass or stiffness of the diaphragm, for example. Usually, a diaphragm is formed by a laminate of different materials, some electrically conducting and others not. More or less layers, thicker or thinner, will alter the resonance frequency. This is known to the skilled person.

[0118] Other manners of altering a resonance frequency are seen in FIGS. 6-8. In FIG. 6, two receivers 202/204 are seen each having a diaphragm of which the diaphragm 206 is pointed to. The upper receiver 202 has an enlarged back volume (upper compartment) whereby the receiver, all other dimensions being equal, has a lower resonance frequency. Another manner of obtaining the same volume increase would be to provide an opening in the chamber to be increased and provide e.g. a tube or the like into which can also travel. This tube may be closed or open at the opposite end.

[0119] The receivers 202/204 have sound outputs outputting sound into tubes 210 and further into an individual lumen of a two-lumen nozzle 208. To the left in the figure a cross section of the nozzle 208 is seen. Alternatively, the tubes 210 may be omitted and the sound emitted directly from the receivers 202/204 into the nozzle lumens.

[0120] In FIG. 7, another set-up is seen with two receivers 202/204 with diaphragms. The two receivers here may have identical dimensions but the tubes 210 now have different dimensions or loads. The upper tube is shorter and fatter whereas the lower one is longer and slimmer. This effectively gives the two receivers different resonance frequencies. Again the sound is fed from the tubes into a nozzle 208. Naturally, the tubes may be omitted and the lumens of the nozzle adapted to give the receivers different resonance frequencies.

[0121] In FIG. 8, a different manner of adapting a resonance frequency is seen. Three receivers 230/232/234 are provided in series so that the receiver 230 outputs sound into the front chamber 233 of the receiver 232 which again outputs the combined sound thereof into the front chamber 235 of the receiver 234 outputting the sound to the nozzle 208 which may be omitted if desired.

[0122] The receiver 230 will have a higher acoustic mass than the other receivers and thus, all other dimensions being equal, have a lower resonance frequency. The receiver 232 will, again all other dimensions equal, have a resonance frequency between those of receivers 230 and 234. Naturally, sound may instead be input into the back chamber of a receiver where it is then fed via the diaphragm to the sound output of that receiver. It is noted that this manner of adapting the resonance frequencies while outputting the resulting sound from multiple receivers from a single output. This technology is not limited to electrostatic receivers as the resonance frequency of any sound generator may be adapted as illustrated in FIGS. 6-8.

[0123] In FIG. 2, a set-up of an electrostatic sound generator 16 is seen which is fed by a signal source 12, such as a source of an audio signal, which outputs a signal for a low frequency sound generator 14 and the sound generator 16, which is intended to output only or primarily high frequencies. The signal output by the source 12 may be a standard audio signal comprising frequencies in the interval of 20 Hz-20 kHz.

[0124] In this context, the sound generator 14 is connected to the source 12 and receives the signal output therefrom. Also, coupled in parallel to the generator 14 is an assembly of a capacitor 20 and a transformer 18 having a primary winding 182 with a first number of windings around a core 186. The transformer has a secondary winding 184 with a second number of windings and which is connected to the high frequency sound generator 16.

[0125] In one embodiment, the transformer has 125 primary windings with a wire diameter of 48 μm and 14250 secondary windings with a wire diameter of 12 μm. The transformer diameter is 10 mm and a width/thickness is 2.6 mm.

[0126] A preferred capacitance of the capacitor is 4.7 μF. The resistance of the primary coil and the capacitance of the capacitor determines the −3 dB point of the high pass filter created thereby.

[0127] The function of the capacitor 20 is to remove or reduce low frequency signals from the signal fed to the transformer 18. When only the higher frequency frequencies reach the transformer 18, this transformer may be made of thinner conductors and thus be made smaller.

[0128] Naturally, a low pass frequency filter may be provided in series with the generator 14, if this generator is not itself either able to output the higher frequency sounds or handle the higher frequency signals. Usually, there is not much power in such higher frequency signals, so ordinary lower frequency generators, such as ones based on balanced armature, moving armature, moving coil technologies or the like, may be fed the full signal frequency spectrum and will output only the lower frequencies thereof. Balanced armature generators, for example, may be suited to output only frequencies below e.g. 6 kHz, independently of whether the signal fed thereto comprises also frequencies above that limit. Another suitable type of low frequency sound generator, or woofer, is a dynamic driver speaker also called a moving coil transducer. Electrostatic sound generators may also be used as low frequency speakers if desired.

[0129] Naturally, additional sound generators may be added to the set-up. Another low frequency generator—or a medium range generator—may be provided in parallel with the generator 14—potentially in series with a filter if desired.

[0130] Suitable midrange speakers may be based on any the moving armature, the moving coil or the electrostatic principle.

[0131] For example, a combination of a balanced armature or moving coil woofer and electrostatic tweeter can be expanded by a balanced armature midrange. The use of two loudspeakers for woofer and midrange allows more control of the frequency response and can therefore provide better sound quality. The woofer and midrange can either be different receivers, or they can be similar receivers tuned differently acoustically. Naturally, a moving coil midrange could be used instead of the balanced armature midrange.

[0132] Alternatively, the combination of a balanced armature or moving coil woofer and electrostatic tweeter can be expanded by a second electrostatic midrange driver. This gives more control of the frequency response, and increases the range of frequencies where the high sound quality of the electrostatic tweeter is used. The two electrostatic loudspeakers can be driven by the same transformer coil, or by two separate coils on the same transformer, or by two separate transformers.

[0133] An additional high frequency generator may also be provided, such as multiple electrostatic drivers with essentially equal frequency response. The advantage is that they can produce the same sound pressure level at lower voltage than a single electrostatic driver, thereby reducing the requirements for the transformer, or even making it obsolete. It also increases the maximum achievable sound pressure level. The electrostatic drivers can either have separate spouts or share a single spout.

[0134] An additional electrostatic receiver may be connected to the second conductor/winding 184, or the transformer 18 may have another secondary winding to which the extra high frequency generator is connected. This is seen in FIG. 5 where the transformer 18, in addition to the primary winding 182 and the secondary winding 184 feeding the electrostatic receiver 16, has another secondary winding 184′ feeding another receiver 16′. Any number of electrostatic receivers may be provided each fed by a separate secondary winding.

[0135] Naturally, this other high frequency generator may alternatively be connected to another transformer having a primary winding connected in parallel with the transformer 18 and thus in series with the capacitor 20. Alternatively, the other transformer may be connected in series with another capacitor and this assembly be connected in parallel with the transformer/capacitor 18/20, if desired.

[0136] It is widely known that electrostatic sound generators, single-sided or not, require a high voltage to operate optimally. The function of the transformer 18 is to provide this high voltage. Using the transformer, lower requirements are put to e.g. an amplifier generating the signal fed to the transformer, so that this amplifier may be operated solely within its linear mode.

[0137] Driving the electrostatic sound generator, however, at or around the resonance frequency, less power and a lower high voltage is required to drive it. This means that less secondary windings may be required and that a thinner wire may be used in the transformer, whereby the transformer may be made much smaller. This enables the use of the assembly also in e.g. hearing aids.

[0138] The efficiency of the sound generator may be quantified by feeding a signal into the transformer (FIG. 2) and correlating the voltage applied with the sound pressure generated under certain circumstances, such as when fed from the output 169 into one end of a cylinder 170, where the sound pressure is then determined at the other end of the cylinder. Preferably, no sound absorption takes place in the cylinder, the inner surface of which preferably is hard, such as made of a hard polymer/plastic/resin material and/or a metal or alloy.

[0139] Different set-ups of a high frequency generator 16, a low frequency generator 14 and a transformer 18 are illustrated in FIG. 3 within a housing 11 for positioning inside an ear canal of a person or partly therein, where the thicker part may be provided in the concha of the person. In FIG. 3A, the transformer 18 is provided in the thicker part and the generators in the thinner part. An element 111 may be used for shielding the generators from magnetic/electric fields of the transformer.

[0140] Connections between the transformer and generator 16 are not illustrated for clarity purposes. The circuit of FIG. 2 may be provided inside or outside the housing 11, if present at all. The generator 14 and the transformer 18 may receive a signal from outside the housing 11 such as via electrical connections (not illustrated) on the housing 11.

[0141] In FIG. 3B, the low frequency generator 14 is positioned behind the transformer 18 and outputs the sound to the output of the housing 11 via a channel 141.

[0142] As an alternative, this channel may be used for a high frequency sound generator, as an example of a tube 201 seen in FIG. 6/7, and thus be configured or dimensioned to adapt the resonance frequency of the sound generator.

[0143] In FIG. 3C, the transformer has another shape and extends into the narrow portion of the housing 11. The generator 16 is provided in the thinner part and the generator 14 is provided in the thicker part from which it outputs its sound via a channel 141.

[0144] In general, the channel 141 may e.g. be a soft tube. The sound channel 141 may not be required. The sound from the generator 14 may find its own way around the generator 16 and out of the housing 11.

[0145] In FIG. 3D, compared to FIG. 3b, the positions of the transformer 18 and the generator 14 are swapped.

[0146] In FIG. 3E, compared to FIG. 3A, an additional high frequency generator 16′ is provided, again in the narrow portion. The transformer 18 may be provided with two secondary windings each feeding a high frequency generator if desired. Alternatively, the same secondary winding may feed both high frequency generators.

[0147] In FIG. 3F, compared to FIG. 3D, an additional high frequency generator 16′ is provided, again in the narrow portion.

[0148] In FIG. 3G, compared to FIG. 3A, the transformer 18 is formed using a portion 112 of the housing 11. In this manner, the coils or conductors of the transformer 18 may be would around this portion 112.

[0149] In FIG. 4, two sound generators 11/16 are illustrated connected by a wire 222 to a mobile telephone 22. The sound generators 11/16 comprise at least electrostatic sound generators 16 but may also comprise low frequency sound generators 14 if desired.

[0150] Naturally, the corresponding transformers may be provided (see FIG. 3) in the housings at 11/16 but may also be provided in the wire 222 as illustrated.

[0151] Using the phone 22 to provide the signal for the transformers and/or the generators 11/16, the signal output by the telephone may be adapted in any desired manner. Also, a microphone (not illustrated) may be provided in or at the generators 11/16 and the signal therefrom fed to the telephone 22 in order for the telephone to monitor the sound output of the generators 11/16 in order to automatically adjust the signals if desired. Such adjustment may be frequency response, for example.