Nested controller for an electrostatic acoustic device

Abstract

Controlling an electrostatic acoustic device. Displacement of a membrane in the electrostatic acoustic device is monitored by producing an audio output signal responsive to time-dependent displacement of the membrane. An input audio voltage is combined with a first portion of the audio output signal to produce a first error signal as negative feedback. The first error signal is input to a control circuit and a control signal is output from the control circuit. The control circuit includes a filter of order N>3. The control circuit is configured to increase a bandwidth for noise cancellation. Ambient noise may contribute to displacement of the membrane and displacement of the membrane due to the ambient noise may be canceled at least in part over the bandwidth.

Description

[0001] NESTED CONTROLLER FOR AN ELECTROSTATIC ACOUSTIC DEVICE

[0002] BACKGROUND

[0003] 1. Technical Field

[0004] The present invention relates to operating electrostatic acoustic devices, including earphones and loudspeakers, and particularly the present invention relates to a control system and method for increasing a bandwidth for noise cancellation in the electrostatic acoustic devices.

[0005] 2. Description of Related Art

[0006] In the art of high-fidelity sound reproduction, the electrostatic loudspeaker has received attention because of inherent excellent sound quality and smooth response over wide frequency ranges. In such devices, a flexible sound producing membrane is positioned near an electrode, or in the case of a push-pull arrangement, a pair of electrodes, one on either side of the membrane. A direct current polarization potential is applied between the membrane and the electrodes, and an audio signal is superimposed on the electrodes, causing the membrane to move in response to the audio signal. Electrodes are acoustically transmissive so that sound produced by the moving membrane radiates outward through the electrode to the listening area.

[0007] Electrostatic devices are highly efficient both electrically and mechanically. Electrical impedance is high and decreases with increasing acoustic frequency. High electrical impedance results in very low operating currents and minimal electrical losses. Mechanically, there are no moving parts other than the moving membrane which is very light in weight. Electrostatic devices are therefore inherently more energy efficient than electrodynamic acoustic devices currently used in battery operated electronic devices.

[0008] BRIEF SUMMARY

[0009] Various circuits and methods are herein disclosed for controlling an electrostatic acoustic device. Displacement of a membrane in the electrostatic acoustic device is monitored by producing an audio output signal responsive to time-dependent displacement of the membrane. An input audio voltage is combined with a first portion of the audio output signal to produce a first error signal as negative feedback. The first portion of the audio output signal may be transformed prior to combining with the audio input signal. The first error signal is input to a control circuit and a control signal is output from the control circuit. The control circuit includes a filter of order N>3. Ambient noise may contribute to displacement of the membrane and displacement of the membrane due to the ambient noise may be canceled at least in part over the bandwidth. The control circuit is configured to increase a bandwidth for noise cancellation. The control circuit may include a multiple series-connected control sub-circuits, including a first sub-circuit and a second sub-circuit. A second portion of the audio output signal may be combined with an output of the first sub-circuit to produce a second error signal. The second error signal may input into the second sub-circuit. The second portion of the audio output signal may be transformed prior to the combining with an output from the first sub-circuit. Each control sub-circuit may include an integrator circuit and a differentiator circuit. The integrator circuit may be a leaky integrator circuit. The differentiator circuit may be a non-ideal differentiator circuit. Alternatively, each sub-circuit may include parallel connected: a low pass filter and a high pass filter.

[0010] The electrostatic acoustic device may include a first electrode and a second electrode. The first electrode may be disposed parallel to the membrane. The membrane may be configured to respond mechanically to a varying first electric field in accordance with respective electric potentials applied between the first electrode and the membrane. The second electrode may be disposed parallel to the membrane opposite from the first electrode. The membrane may be configured to respond mechanically to a varying second electric field in accordance with respective electric potentials applied between the second electrode and the membrane. A probe signal may be produced varying at radio frequency. A first portion of the probe signal may be coupled into the first electrode. A portion of the probe signal may be inverted, and the inverted portion may be coupled into the second electrode. The couplings may be capacitive couplings or inductive couplings. A first DC bias voltage may be first applied to the first electrode. A second DC bias voltage may be applied to the second electrode. The second DC bias voltage may have an opposite polarity from the first DC bias voltage. The first DC bias voltage and the second DC bias voltage may be symmetrically applied with opposite polarities to the first and the second electrodes. Respective portions of the probe signals may be symmetrically applied with opposite polarities to the first and the second electrodes. A voltage signal from the membrane may be sensed by inputting at a high pass filter input the voltage signal from the membrane. The high pass filter may selectively pass at least a portion of the radio frequency of the probe signal and may selectively block at least a portion of audio frequency to produce a filtered signal modulated at the radio frequency. A portion of the filtered signal modulated at the radio frequency may be input at a first multiplier input and at a second multiplier input, at least a portion of the probe signal may be input. A multiplied signal may be output from the multiplier, proportional to a multiplication of the filtered signal and the probe signal. The multiplied signal may be output produce the audio output signal responsive to the time-dependent displacement of the membrane.

[0011] BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

[0013] Figure 1 illustrates schematically a cross-sectional view of an electrostatic device, according to features of the present invention;

[0014] Figure 2 is an electronic block diagram of a feedback control system, according to features of the present invention;

[0015] Figure 2A illustrates an electronic block diagram of a nested controller, according to features of the present invention;

[0016] Figure 3 illustrates an electronic block diagram of a proportional-integral-derivative controller (PID) controller, according to conventional art;

[0017] Figure 4 illustrates an electronic block diagram of an integrator and differentiator block of controller, according to features of the present invention;

[0018] Figure 5 is an electronic block diagram of a circuit including an electrostatic acoustic device, in the forward path of the feedback control system of Figure 2;

[0019] Figure 5A is an alternative electronic block diagram of a circuit including an electrostatic acoustic device, in the forward path of the feedback control system of Figure 2; and

[0020] Figure 6 is a flow diagram of a method, illustrating features of the present invention.

[0021] The foregoing and / or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.

[0022] DETAILED DESCRIPTION

[0023] Reference will now be made in detail to features of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The features are described below to explain the present invention by referring to the figures.

[0024] By way of introduction, different aspects of the present invention are directed to a circuit for in- ear and / or over-ear electrostatic headphones, by way of example, for control of an electrostatic speaker with a smooth monotonically changing frequency response with noise cancellation capability over a wider frequency range. The method, according to features of the present invention may include monitoring time-dependent displacement of the membrane of the electrostatic acoustic device and producing a displacement signal proportional or at least monotonic according to the membrane displacement. The membrane displacement signal and the actual membrane displacement may be proportional to each other for a frequency range below the membrane fundamental frequency in which the membrane motion resembles piston motion. For electrostatic acoustic devices in which the membrane-electrode spacing is greater than 100 microns, for a 5 to 15 millimetre diameter circular membrane, signal -to-noise ratio (SNR) may be comparable or higher than the SNR achieved in a commercial “phantom” 48V electrostatic microphone for frequencies less than the fundamental frequency of the membrane.

[0025] For frequencies above the membrane fundamental frequency, the membrane displacement signal tends to diverge from the actual membrane position averaged at its centre, due to mechanical energy coupling into the higher order eigenmodes. Control of the electrostatic device according to embodiments of the present invention is directed to noise cancellation and specifically increasing a bandwidth over which noise cancellation may be effectively performed. Otherwise, the present invention in different embodiments may be configured to improve detection of membrane displacement, improve signal to noise ratio (SNR) and / or reduce audio distortion. Circuits according to different features of the present invention may be directed to detector circuits and methods for using the acoustic device as an electrostatic microphone and / or in an electrostatic speaker for ambient noise cancellation or adjustment of acoustic transparency. Circuits may be designed for an electrostatic speaker of maximum dimension, e.g. diameter D of 50 millimetres or less, or in some embodiments an electrostatic speaker of dimension D of 25 millimetres or less, or in yet other embodiments an electrostatic speaker of dimension D of 10 millimetres or less. For an earphone application, an electrostatic speaker may have maximum dimension, e.g. diameter D of 5 millimetres or less. Other aspects of the present invention may include optimising dynamic range and protection from over-driving the electrostatic device. According to features of the present invention, mechanical motion of the membrane may be forced to maintain a desired acoustic output including linearising motion of the membrane over at least a portion of a desired frequency range. The displacement signal may be used for feedback to control acoustic transparency and / or to cancel ambient noise. Mechanical response of the membrane due to acoustic ambient noise may be cancelled at least in part, i.e. ambient noise control (ANC) may be performed. Similarly, acoustic transparency of the electrostatic acoustic device may be controlled. Prior art closed-loop controllers, e.g. ANC, generally employ a speaker and multiple microphones. According to embodiments of the present invention a single electro-acoustic device is sufficient to maintain a desired acoustic output.

[0026] Referring now to the drawings, reference is now made to Figure 1, which illustrates schematically an electrostatic acoustic device 10, according to features of the present invention. Vertical axis Z is shown through a centre of acoustic device 10. A tensioned membrane 15 is supported, by edges of electrodes 11, essentially perpendicular to vertical axis Z. Membrane 15 may be impregnated with a conductive, resistive and / or electrostatic material so that membrane 15 responds mechanically to a changing electric field. The central regions of electrodes 11 are mounted proximate to, e.g. in parallel to, membrane 15, nominally equidistant, at a distance d, e.g. 20-500 micrometres from membrane 15. Electrodes 11 are illustrated as perforated or bored with apertures 12 transmissive to sound waves emanating from membrane 15 when electrostatic acoustic device 10 is operating.

[0027] During operation of electrostatic acoustic device 10, constant direct current (DC) bias voltages, e.g. ±VDC=±1 to ±200 volts, may be applied respectively to electrodes 11. A varying bipolar high voltage audio signal F, may be output from a high voltage analogue amplifier 13, and applied to membrane 15. Voltage signal F, may vary at audio frequencies, nominally between 20-20,000 Hertz. High voltage analogue amplifier 13 is shown as being powered by direct current voltage rails ±VEE, which may be in the range of ±10 to ±80 volts.

[0028] Dotted lines illustrate schematically membrane 15 moving in response to a changing electric voltage due to voltage signals F,.

[0029] As distance d decreases, or as DC bias voltage +VDC and / or signal voltage F, increase (in absolute value) then there is an increased chance for a short circuit between membrane 15 and electrode 11 and / or dielectric breakdown of air which is expected nominally at about 3* 106Volt / meter. According to a feature of the present invention, operation of electrostatic speaker 10 may be controlled to avoid over-driving membrane 15.

[0030] Reference is now made to Figure 2, which illustrates a control system 20, according to features of the present invention. System 20 illustrates closed loop operation of electrostatic speaker 10 using lock-in detection signal of time-dependent displacement of membrane 15 and corresponding proportional voltage output Vofrom detection circuit 21, by way of example. In the forward path, G(s) represents open loop gain of the control circuit including system 21, where 5 may be a complex variable representing an alternating voltage signal in the form + (p) where A represents an amplitude, represents an angular frequency, where f represents a frequency in Hertz and (p represents a phase shift in radians. In the feedback path, block 22 represents transform function H(s) of an output voltage signal Vo. The feedback path output from feedback block 22 may output a signal 27, which may be subtracted by comparator 23 from the input signal V, to produce an error signal 25 which is input to controller block 21 so that the output signal Voapproaches a set point. Overall, transfer function of system 20, voltage output Vodivided by voltage input Hof controller 21 may be modelled by equation 1 :

[0031] Stability of control system 20 is contingent upon the denominator! + G(s) - H (s )having sufficiently large absolute value and / or being non-zero. It is well known that in a resonant system 21, including a damped harmonic oscillator with an external drive that the response of an oscillator is in phase (i.e. (p~0) with the external drive for driving frequencies well below the resonant frequency, is in phase quadrature at the resonant frequency, and is anti phase (i.e. (p~7r) for frequencies well above the resonant frequency. If control system 21 includes a resonance and an oscillating energy source, then in order to maintain stability, the oscillating energy source operates either below or above the resonant frequency without ever crossing the resonant frequency. In case of resonance frequency cross-over, a phase shift filter may be added to mitigate the phase response discontinuity.

[0032] In response to input voltage F, and / or ambient noise, distance d between membrane 15 and electrodes 11 changes resulting in a change in capacitance C of electrostatic acoustic device 10. A changing current i(t) due to input voltage V, and / or ambient noise is approximated by:

[0033] Still referring to Figure 2, in response to the voltage output signal Vo„ feedback block 22 may be configured to output signal 27 to comparator 23 which is subtracted from the input signal v, When input signal v, is nominally zero, signal 27 becomes error signal 25. Alternatively, instead of comparator 23, a signal combiner 23 may be used and feedback block 22 appropriately transforms, e.g. inverts voltage output signal Voto signal 27 -Vowhich becomes error signal 25 when input signal v, is nominally zero. Noise cancellation may be based on detection signal Volinear or monotonic with displacement of membrane 15 which may be fed back as signal 27 to comparator 23.

[0034] Reference is now also made to Figure 2A, which illustrates a nested Proportional, Integral and Derivative (PID) block 24E, according to a feature of the present invention. Comparator 23 inputs audio signal input v, at a first input and combines / compares with control signal 27 or portion thereof input at a second input. Comparator 23 outputs error signal 25 to a first PID circuit 24A. PID circuit 24A outputs a first control signal 26A to a first input of a comparator 23A. Feedback signal 27 is input at a second input of comparator 23A. Comparator 23A outputs a second error signal 25A proportional to a difference between the signals at the first and second outputs of comparator 23A. Second error signal 25A is input to a second PID control circuit 24B. Second PID control circuit 24B outputs voltage signal 26 F,

[0035] Reference is now also made to Figure 3, which illustrates a Proportional, Integral and Derivative (PID) block 24, according to conventional art. The feedback loop may include in the forward path G(s) a Proportional, Integral and Derivative (PID) block 24E including two or more nested blocks 24. Block 24 may include relative to error signal 25, a proportional gain, a differential and / or integration in linear combination as well as frequency filtering to output a control signal 26

[0036] Referring back to Figure 2, feedback circuit 20 may be used to tune acoustic transparency of acoustic device 10 when used as an in-ear earphone or over-ear headset. Acoustic transparency is a measure of membrane 15 apparent stiffness, which controls the sound transmission coefficient from the outside space to the inner ear sealed volume through the boundary defined by membrane 15. Acoustic transparency may be controlled via electrostatic feedback actuation and position sensing with a variable gain in block 21 and / or gain adjustments within PID 24E, within the effective frequency bandwidth of the feedback actuation. Controlling the ratio between control signal 26 output V, from PID 24E and input audio signal v, using PID gains may allow a controlled audio noise cancellation and acoustic transparency (AT) adjustment within PID 24E effective bandwidth. Referring again to Figure 3, output voltage of a classic PID controller with input voltage V(t) is in general proportional to:

[0037] The transfer function of a classic PID controller in Laplace domain 5 may be approximated by: where where is the integrator close loop cut-off frequency and where fdis the differentiator closed loop cut-off frequency.

[0038] (Reference: Franklin, G.F., Powell, J.D., Emami-Naeini, A. and Powell, J.D., 2002. Feedback control of dynamic systems (Vol. 4, Chapter 2). Upper Saddle River: Prentice hall.) According to embodiments of the present invention, where amplification is not needed, proportional factor a is set to zero.

[0039] When the integration term approaches infinity and similarly when .s approaches infinity the differentiation term approaches infinity. In order to improve stability, it is suggested to replace the integration term with a leaky integration with Laplace transform: which as the integration term approache which is the closed-loop noise gain at DC. s

[0040] Similarly, the differentiation term —may be replaced with a limited differentiation with Laplace transform: which as infinity, approaches which is the closed-loop signal gain. The

[0041] Laplace transform of a controller according to features of the present invention becomes: (4)

[0042] By combining terms and for simplicity setting frequency cut-offs to be the same, transfer function PID(s) becomes: which is a filter in Laplace domain 5 of second order, N=2. Nesting two or more such identical or similar PID filters yields a filter of order N=4 or greater.

[0043] Reference is now made also to Figure 4 which represents a controller 24, according to features of the present invention including a leaky inverting integrator 42 combined in parallel with a limited inverting differentiator 44 implemented as operational amplifier circuits. In terms of circuit components, the transfer function of integrator 42 (first term of equation 4) (which is negative voltage output divided by voltage input) may be given by:

[0044] The transfer function (which is negative voltage output divided by voltage input)of differentiator 44 (second term of equation 4 )is given by:

[0045] It can be readily seen that leaky integrator 42 appears similar or identical to a low pass filter with cut-off ( i and non-ideal differentiator 44 appears similar or identical with a high pass filter with cut-off (Da A nested filter, according to features of the present invention may include two or more nested subcircuits, each subcircuit including a parallel-connected high pass filter and low pass filter.

[0046] Reference is now made also to Figure 5, which illustrates schematically an alternative 21A for block 21 in closed loop feedback system 20 (Figure 2), according to features of the present invention. In controller 21A, a probe signal RF may be a sinusoid of frequency between 0.1-10 megaHertz, e.g. 1 megaHertz. Probe signal RF may be input to inverter 36 and inverted to output an inverted probe signal RF or a signal equivalently phase shifted 180 degrees. Probe signal RF from local oscillator 51 may be capacitatively coupled onto an electrode 11 and inverted probe signal RF of similar level is capacitively coupled to the other electrode 11. Similarly, bias voltages ±VDC may be symmetrically applied on electrodes 11 with -VDCon a first electrode 11 and +VDC applied on a second electrode 11. Typically, on each electrode 11, RF amplitude (peak-peak) may be a few volts superimposed on a direct current voltage bias of -100 volts with equal and opposite polarities, respectively biasing electrodes 11.

[0047] Audio voltage F,may be applied to membrane 15. The changing current i(t) signal as in equation (2) in electrostatic acoustic device 10 due to membrane 15 displacement in accordance with input audio voltage F, and / or ambient noise is converted to a voltage signal and input to a high pass filter 30. High pass filter 30 as shown in block 21A is configured to pass the RF probe signal, between 0.1-10 Megahertz, e.g. 1 Megahertz and block the input audio voltage signal F, which is typically between 0-20000 Hertz. High pass filter 30 may be a 3-pole Chebyshev design with Salen-Key active filter topology. Other high pass filter designs may be considered by one of ordinary skill, according to different features of the present invention. The high pass filtered output from high pass filter 30 is input to a first input of a multiplier 32. The second input of multiplier 32 is a portion of RF probe signal (or inverse RF thereof). The output of multiplier 32 at least partially includes a multiplication of the signals at the input ports of multiplier 32. The output of multiplier 32 may be demodulated by a low pass filter 34, designed to pass audio frequencies including the audio output signal Voproportional to or in accordance with the displacement of membrane 15 of electro-acoustic device 10.

[0048] Reference is now also made to Figure 5 A which illustrates another alternative 21B for block 21 in closed loop feedback system 20 (Figure 2), according to features of the present invention. Audio voltage F, may be applied to membrane 15. A probe signal from a local oscillator 51 may also be induced onto membrane 15 using a transformer T with primary P connected in parallel with local oscillator 51 and secondary S connected in series between electrodes 11. Similarly, bias voltages ±VDc may be symmetrically applied on electrodes 11 with -VDCon a first electrode 11 and +VDc applied on a second electrode 11. As in block 21A, on each electrode 11, RF amplitude (peak-peak) may be a few volts superimposed on a direct current voltage bias of -100 volts with equal and opposite polarities, RF-VDC, RF+ VDC respectively biasing electrodes 11.

[0049] As in block 21A (Figure 3) high pass filter 30 as shown in block 21B is configured to pass the RF probe signal, between 0.1-10 megahertz, e.g. 1 megahertz and block the input audio voltage signal Vi which is typically between 0-20000 Hertz. The high pass filtered output from high pass filter 30 is input to a first input of a multiplier 32. The second input of multiplier 32 includes a portion of RF probe signal (or inverse RF thereof). The output of multiplier 32 at least partially includes a multiplication of the signals at the input ports of multiplier 32. The output of multiplier 32 may be demodulated by a low pass filter 34, designed to pass audio frequencies including the audio output signal Voproportional, monotonic to or in accordance with the displacement of membrane 15 of electro-acoustic device 10.

[0050] Reference is now made to Figure 6, a flow diagram 60 illustrating a method according to features of the present invention. Displacement of a membrane in the electrostatic acoustic device is monitored (step 61) by producing an audio output signal responsive to time-dependent displacement of the membrane. An input audio voltage is input (step 62) and combined (step 63) with a first portion of the audio output signal to produce a first error signal as negative feedback. The first error signal is input (step 64) to a control circuit and a control signal is output from the control circuit. The control circuit includes a filter of order N>3 implemented with nested or series-connected control sub-circuits. Each control sub-circuit may include an integrator circuit and a differentiator circuit, parallel connected; or a low-pass filter and a high pass filter parallel connected. The control circuit is configured to optimise or increase bandwidth of (step 66) of compensation for ambient noise.

[0051] The term "homodyne" as used herein refers to a method of detection / demodulation of a signal which is phase and / or frequency modulated onto an oscillating signal by combining that signal with a reference oscillation.

[0052] The term "phase sensitive detector circuit" as used herein is an electronic circuit including a multiplier (or mixer) and a loop filter that produces an output signal that is proportional to the product of the amplitudes of an input signal and a reference signal and to the cosine of the phase between them.

[0053] The term "transimpedance amplifier" as used herein converts current to voltage. Transimpedance amplifiers may be used to process current output of a sensor to a voltage signal output.

[0054] The term "charge amplifier" as used herein converts a time varying charge to a voltage output typically by integrated a time varying current signal.

[0055] The term "audio" or "audio frequency" refers to an oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range 0 -20,000 Hertz

[0056] The term "audio signal", "audio output", "audio output signal" as used herein refer to an electrical signal varying essentially at audio frequency.

[0057] The term "radio frequency" (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around twenty thousand times per second (20 kHz) to around three hundred billion times per second (300 GHz).

[0058] The term "transform" or "transforming" refers to phase shifting, inverting, amplifying and / or attenuating.

[0059] The term “order” as used herein referring to an order N of a filter or control circuit, refers to a power of complex variable 5 in Laplace domain of the Laplace transform of the filter or control circuit.

[0060] A “leaky integrator” as used herein refers to component or system that takes the integral of an input, but gradually leaks a small amount of input over time.

[0061] A terms “non-ideal differentiator” or limited differentiator” as used herein outputs a signal proportional to a first derivative of an input signal (or inverse thereof) over a limited frequency range and outside the limited frequency range tends to attenuate the output signal.

[0062] The term “probe signal” refers to a locally produced radio frequency, e.g. sinusoidal signal. The term “coupling” a probe signal as used herein refers to coupling a portion of the probe signal, or an RF signal derived from the probe signal by transforming the probe signal.

[0063] The term “symmetrically” in reference to applying voltages of equal magnitudes, opposite polarities and at opposite sides with respect to membrane 10.

[0064] The term "error signal" as used herein refers to a voltage signal of magnitude proportional to or monotonic with the difference between an actual output signal varying at audio frequencies and a desired audio signal.

[0065] The term "control signal" as used herein refers to a signal input to an acoustic device, responsive to an error signal, to maintain a desired voltage output signal. The transitional term “comprising” as used herein is synonymous with “including”, and is inclusive or open-ended and does not exclude additional element or method steps not explicitly recited. The articles "a", "an" is used herein, such as "a circuit” or "a probe signal" have the meaning of "one or more" that is "one or more circuits", "one or more probe signals". All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another. Although selected features of the present invention have been shown and described, it is to be understood the present invention is not limited to the described features.

Claims

CLAIMSTHE CLAIMED INVENTION IS:

1. A method for controlling an electrostatic acoustic device, the method comprising: monitoring displacement of a membrane in the electrostatic acoustic device by producing an audio output signal responsive to time-dependent displacement of the membrane; inputting an input audio voltage; first combining the input audio voltage with a first portion of the audio output signal to produce a first error signal as negative feedback; inputting the first error signal to a control circuit and outputting from the control circuit a first control signal; wherein the control circuit includes a filter of order 7V>3; and configuring the control circuit to increase a bandwidth for noise cancellation.

2. The method of claim 1, wherein the filter includes a plurality of nested filters each of second order or greater.

3. The method of claim 1, wherein ambient noise contributes to displacement of the membrane, the method further comprising: configuring the control circuit to cancel, at least in part, displacement of the membrane due to the ambient noise.

4. The method of claim 1, further comprising: transforming the audio output signal prior to said first combining with the input audio voltage.

5. The method of claim 1, further comprising: second combining a second portion of the audio output signal with an output from a first sub-circuit to produce a second error signal; inputting the second error signal to a second sub-circuit; and outputting from the second sub-circuit the first control signal 26.

6. The method of claim 5, further comprising: transforming the second portion of the audio output signal prior to said second combining with the output from the first sub-circuit.

7. The method of claim 1, wherein the control circuit includes a plurality of series-connected control sub-circuits.

8. The method of claim 7, wherein each control subcircuit includes an integrator circuit and a differentiator circuit, parallel connected.

9. The method of claim 8, wherein the integrator circuit is a leaky integrator circuit.

10. The method of claim 8, wherein the differentiator circuit is a non-ideal differentiator circuit.

11. The method of claim 7, wherein each control subcircuit includes a low pass filter and a high pass filter, parallel connected.

12. The method according to any of claims 1-11, wherein the electrostatic acoustic device, includes a first electrode and a second electrode, wherein the first electrode is disposed parallel to the membrane, wherein the membrane is configured to respond mechanically to a varying first electric field in accordance with respective electric potentials applied between the first electrode and the membrane, wherein the second electrode is disposed parallel to the membrane opposite from the first electrode, wherein the membrane is configured to respond mechanically to a varying second electric field in accordance with respective electric potentials applied between the second electrode and the membrane, the method further comprising: producing a probe signal varying at radio frequency; first coupling a portion of the probe signal into the first electrode; inverting a portion of the probe signal and second coupling the inverted portion of the probe signal into the second electrode; first applying a first DC bias voltage to the first electrode; second applying a second DC bias voltage to the second electrode, wherein the secondDC bias voltage has an opposite polarity from the first DC bias voltage; sensing a voltage signal from the membrane; inputting at a high pass filter input the voltage signal from the membrane, the high pass filter selectively passing at least a portion of the radio frequency of the probe signal and selectively blocking at least a portion of audio frequency to produce a filtered signal modulated at the radio frequency;inputting at a first multiplier input at least a portion of the filtered signal modulated at the radio frequency; inputting at a second multiplier input, at least a portion of the probe signal; outputting a multiplied signal proportional to a multiplication of the filtered signal and the probe signal at the first multiplier input and second multiplier input; and demodulating the multiplied signal thereby producing the audio output signal responsive to the time-dependent displacement of the membrane.

13. An electronic device comprising: a detector configured to monitor displacement of a membrane in an electrostatic acoustic device by producing an audio output signal responsive to time-dependent displacement of the membrane; an audio input configured to input an input audio voltage; a first comparator configured to combine the input audio voltage with a first portion of the audio output signal as negative feedback to produce a first error signal; a control circuit configured to input the first error signal and to output therefrom a first control signal, wherein the control circuit includes a filter of order 7V>3; wherein the control circuit is configured to increase a bandwidth for noise cancellation.

14. The electronic device of claim 13, wherein the filter includes a plurality of nested filters each of second order or greater.

15. The electronic device of claim 13, wherein ambient noise contributes to displacement of the membrane, the electronic device further configured to cancel, at least in part, displacement of the membrane due to the ambient noise.

16. The electronic device of claim 13, further comprising: a transform block configured to transform the first portion of the audio output signal prior to combining with the audio input signal.

17. The electronic device of claim 13, wherein the control circuit includes a plurality of series- connected control sub-circuits, wherein the control sub-circuits include a first sub-circuit and a second sub-circuit.

18. The electronic device of claim 17, further comprising: a second comparator configured to combine a second portion of the audio output signal with an output of the first sub-circuit to produce a second error signal; wherein the second error signal configured to input into the second sub-circuit.

19. The electronic device of claim 13, further comprising: a transform block configured to transform the second portion of the audio output signal prior to combining with an output from the first sub-circuit.

20. The electronic device of claim 17, wherein each control subcircuit includes an integrator circuit and a differentiator circuit, parallel connected.

21. The electronic device of claim 20, wherein the integrator circuit is a leaky integrator circuit.

22. The electronic device of claim 20, wherein the differentiator circuit is a non-ideal differentiator circuit.

23. The electronic device of claim 17, wherein each control subcircuit includes a low pass filter and a high pass filter, parallel connected.

24. The electronic device according to any of claims 13-23, wherein the electrostatic acoustic device includes the membrane, a first electrode and a second electrode, wherein the first electrode is disposed parallel to the membrane, wherein the membrane is configured to respond mechanically to a varying first electric field in accordance with respective electric potentials applied between the first electrode and the membrane, wherein the second electrode is disposed parallel to the membrane opposite from the first electrode; wherein the membrane is configured to respond mechanically to a varying second electric field in accordance with respective electric potentials between the second electrode and the membrane, wherein the detector configured to monitor displacement of a membrane includes: an oscillator configured to produce a probe signal varying at radio frequency; a first coupler configured to couple a portion of the probe signal into the first electrode; an inverter configured to invert a portion of the probe signal to produce an inverted portion of the probe signal of opposite polarity; a second coupler configured to couple the inverted portion of the probe signal into thesecond electrode; a first direct current (DC) bias supply configured to apply a first DC bias voltage to the first electrode; a second direct current (DC) bias supply configured to apply a second DC bias voltage to the second electrode, wherein the second DC bias voltage has an opposite polarity from the first DC bias voltage; an audio voltage input configured to input the input audio voltage to the membrane; a voltage signal output from the membrane; a high pass filter input connectable to the voltage signal output from the membrane, the high pass filter configured selectively pass at least a portion of radio frequency and to selectively block at least a portion of audio frequency to produce a filtered signal modulated at radio frequency; a multiplier configured to input at a first multiplier input at least a portion of the filtered signal modulated at radio frequency and to input at a second multiplier input at least a portion of either the probe signal or the inverted probe signal, wherein the multiplier is configured to output a multiplied signal proportional to a multiplication of signals at the first multiplier input and second multiplier input; and a demodulator configured to input the multiplied signal output from the multiplier and demodulate therefrom an audio output signal responsive to the time-dependent displacement of the membrane.

Images