Nested controller for electrostatic acoustic devices

Abstract

Control of an electrostatic acoustic device. The displacement of the membrane in the electrostatic acoustic device is monitored by generating an audio output signal in response to the time-dependent displacement of the membrane. The input audio voltage is combined with a first portion of the audio output signal to generate a first error signal as negative feedback. The first error signal is input to a control circuit, from which a control signal is output. The control circuit includes a filter of order N > 3. The control circuit is configured to increase the bandwidth for noise cancellation. Ambient noise may contribute to the membrane displacement, and the membrane displacement due to ambient noise can be at least partially canceled across the bandwidth.

Description

[Technical Field]

[0001] The present invention relates to the operation of electrostatic acoustic devices, including earphones and speakers, and more particularly to a control system and method for increasing the bandwidth for noise cancellation in electrostatic acoustic devices. [Background technology]

[0002] In the field of high-fidelity sound reproduction, electrostatic speakers have attracted attention due to their inherent superior sound quality and smooth response over a wide frequency range. In such devices, a flexible sound-generating membrane is positioned near electrodes, or, in a push-pull configuration, a pair of electrodes are positioned on either side of the membrane. A DC 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. The electrodes are acoustically transparent so that the sound generated by the movable membrane radiates outward through the electrodes towards the listening area.

[0003] Electrostatic speakers are highly efficient devices both electrically and mechanically. Their electrical impedance is high and decreases as the acoustic frequency increases. This high electrical impedance results in very low operating current and minimal electrical loss. Mechanically, there are no moving parts other than the very lightweight moving diaphragm. Therefore, electrostatic devices are inherently more energy-efficient than the electrodynamic acoustic devices currently used in battery-powered electronic devices. [Overview of the project]

[0004] Various circuits and methods for controlling electrostatic acoustic devices are disclosed herein. The displacement of the membrane of the electrostatic acoustic device is monitored by generating an audio output signal in response to the time-dependent displacement of the membrane. An input audio voltage is combined with a first portion of the audio output signal to generate a first error signal as negative feedback. The first portion of the audio output signal may be transformed before being combined with the audio input signal. The first error signal is input to a control circuit, from which a control signal is output. The control circuit includes a filter of order N > 3. Ambient noise may contribute to the membrane displacement, and the membrane displacement due to ambient noise may be at least partially canceled across the bandwidth. The control circuit is configured to increase the bandwidth for noise cancellation. The control circuit may include a plurality of series-connected control subcircuits, including a first subcircuit and a second subcircuit. The second portion of the audio output signal may be combined with the output of the first subcircuit to generate a second error signal. The second error signal may be input to the second subcircuit. The second portion of the audio output signal can be transformed before being combined with the output from the first sub-circuit. Each control sub-circuit may include an integrator and a differentiater. The integrator may be a leakage integrator. The differentiater may be a non-ideal differentiater. Alternatively, each sub-circuit may include a parallel-connected low-pass filter and a high-pass filter.

[0005] An electrostatic acoustic device may include a first electrode and a second electrode. The first electrode may be positioned parallel to a film. The film may be configured to mechanically respond to a first electric field that changes according to the respective potentials applied between the first electrode and the film. The second electrode may be positioned parallel to the film on the opposite side from the first electrode. The film may be configured to mechanically respond to a second electric field that changes according to the respective potentials applied between the second electrode and the film. A probe signal may be generated to vary at radio frequency. A first portion of the probe signal may be coupled to the first electrode. A portion of the probe signal may be inverted, and the inverted portion may be coupled to the second electrode. The coupling may be capacitive or inductive. A first DC bias voltage may be initially applied to the first electrode. A second DC bias voltage may be applied to the second electrode. The second DC bias voltage may have the opposite polarity to the first DC bias voltage. A first DC bias voltage and a second DC bias voltage can be applied symmetrically to the first and second electrodes with opposite polarities. Each portion of the probe signal can be applied symmetrically to the first and second electrodes with opposite polarities. The voltage signal from the film can be sensed by inputting the voltage signal from the film at a high-pass filter input. The high-pass filter can generate a radio-frequency modulated filtered signal by selectively passing at least a portion of the radio frequencies of the probe signal and selectively blocking at least a portion of the audio frequencies. A portion of the radio-frequency modulated filtered signal can be input to a first multiplier input, and at least a portion of the probe signal can be input to a second multiplier input. The multiplied signal is output from the multiplier and is proportional to the multiplication of the filtered signal and the probe signal. The multiplied signal can be output to generate an audio output signal in response to the time-dependent displacement of the film.

[0006] The present invention is described herein, merely as an example, with reference to the accompanying drawings. [Brief explanation of the drawing]

[0007] [Figure 1] This is a schematic cross-sectional view of an electrostatic device according to the features of the present invention. [Figure 2] Fig. 1 is an electronic block diagram of a feedback control system according to features of the present invention. [Figure 2A] Fig. 2 is an electronic block diagram of a nested controller according to features of the present invention. [Figure 3] Fig. 3 is an electronic block diagram of a proportional-integral-derivative controller (PID controller) according to the prior art. [Figure 4] Fig. 4 is an electronic block diagram of an integrator block and a differentiator block of a controller according to features of the present invention. [Figure 5] Fig. 5 is an electronic block diagram of a circuit including an electrostatic acoustic device in the forward path of the feedback control system of Fig. 2. [Figure 5A] Fig. 6 is an alternative electronic block diagram of a circuit including an electrostatic acoustic device in the forward path of the feedback control system of Fig. 2. [Figure 6] Fig. 7 is a flow diagram of a method illustrating features of the present invention. MODE FOR CARRYING OUT THE INVENTION

[0008] The foregoing and / or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawings.

[0009] 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. Hereinafter, features of the present invention will be described with reference to the drawings to explain the invention.

[0010] As a preface, different aspects of the present invention relate, for example, to circuits for in-ear and / or over-ear electrostatic headphones for controlling electrostatic speakers having a smooth, monotonically changing frequency response with noise cancellation capability over a wider frequency range. Methods characterized by the present invention may include monitoring the time-dependent displacement of the membrane of an electrostatic acoustic device and generating a displacement signal that is proportional to, or at least monotonically proportional to, the membrane displacement. The membrane displacement signal and the actual membrane displacement may be proportional to each other for a frequency range lower than the membrane fundamental frequency at which the membrane motion is similar to piston motion. For electrostatic acoustic devices with a membrane-to-electrode spacing greater than 100 microns, for a circular membrane with a diameter of 5 to 15 millimeters, the signal-to-noise ratio (SNR) may be comparable to, or higher than, the SNR achieved with a commercially available "phantom" 48V electrostatic microphone for frequencies lower than the membrane fundamental frequency.

[0011] For frequencies higher than the fundamental frequency of the membrane, the membrane displacement signal tends to diverge from the actual membrane position averaged at its center due to mechanical energy coupling to higher-order eigenmodes. Control of electrostatic devices according to embodiments of the present invention targets noise cancellation, particularly increasing the bandwidth over which noise cancellation can be effectively performed. Otherwise, the present invention in different embodiments may be configured to improve membrane displacement detection, improve the signal-to-noise ratio (SNR), and / or reduce audio distortion. Circuits relating to different features of the present invention may target detection circuits, as well as methods for using acoustic devices in electrostatic microphones and / or electrostatic speakers for ambient noise cancellation or adjustment of acoustic transparency. Circuits may be designed for electrostatic speakers of maximum dimensions (e.g., diameter D of 50 mm or less), or in some embodiments, electrostatic speakers of dimensions D of 25 mm or less, or in yet other embodiments, electrostatic speakers of dimensions D of 10 mm or less. For earphone applications, electrostatic speakers may have maximum dimensions (e.g., diameter D of 5 mm or less). Other aspects of the present invention may include optimization of dynamic range and protection from overdrive of electrostatic devices. According to features of the present invention, the mechanical motion of a membrane can be forced to maintain a desired acoustic output, including linearized motion of the membrane over at least a portion of a desired frequency range. Displacement signals can be used for feedback to control acoustic transparency and / or to cancel ambient noise. The mechanical response of the membrane to acoustic ambient noise can be at least partially canceled, i.e., ambient noise control (ANC) can be performed. Similarly, the acoustic transparency of an electroacoustic device can be controlled. Conventional closed-loop controllers (e.g., ANCs) generally employ a speaker and multiple microphones. According to embodiments of the present invention, a single electroacoustic device is sufficient to maintain a desired acoustic output.

[0012] Referring here to the drawings, we refer to Figure 1, which schematically illustrates an electrostatic acoustic device 10 according to the features of the present invention. A vertical axis Z passing through the center of the electrostatic speaker 10 is shown. A tensioned membrane 15 is supported by the edges of electrodes 11 essentially perpendicular to the vertical axis Z. The membrane 15 may be impregnated with conductive, resistive, and / or electrostatic materials so that the membrane 15 mechanically responds to a changing electric field. The central region of the electrodes 11 is mounted at a distance d (e.g., 20 to 500 micrometers) from the membrane 15, nominally equidistant from the membrane 15, in close proximity to (e.g., parallel to) the membrane 15. The electrodes 11 are shown having an opening 12 through which sound waves emitted from the membrane 15 when the electrostatic acoustic device 10 is operating can pass.

[0013] During operation of the electrostatic acoustic device 10, a constant DC bias voltage (for example, ±V) is maintained. DC A voltage of ±1 volt to ±200 volts can be applied to each electrode 11. A changing bipolar high-voltage audio signal V i The voltage signal V is output from the high-voltage analog amplifier 13 and can be applied to the film 15. i The audio frequency can nominally vary from 20 Hz to 20,000 Hz. The high-voltage analog amplifier 13 has a DC voltage rail ±V which can be in the range of ±10 volts to ±80 volts. EE It is indicated that it will be powered by [this method].

[0014] The dotted line represents the voltage signal V i A schematic diagram shows the film 15 that moves in response to the voltage change caused by [the device / device].

[0015] As the distance d decreases, or the DC bias voltage +V DC and / or signal voltage V i As the (absolute value) increases, a short circuit and / or dielectric breakdown of air occurs between the film 15 and the electrode 11 (nominally about 3 × 10⁻¹⁰). 6 The likelihood of it being volts / meter increases. According to the features of the present invention, the operation of the electrostatic speaker 10 can be controlled to avoid overdrive of the membrane 15.

[0016] Reference is now made to FIG. 2, which schematically illustrates a control system 20 that characterizes the present invention. By way of example, the system 20 provides closed-loop operation of an electrostatic speaker 10 that uses the lock-in detection signal of the time-dependent displacement of the membrane 15 and the corresponding proportional voltage output V from the detection circuit 21 o . In the forward path, G(s) represents the open-loop gain of the control circuit including the system 21, and s may be a complex variable representing an alternating voltage signal in the form of A(e iωt jωt + φ), where A represents amplitude, ω=2πf represents angular frequency, f represents frequency in Hertz, and φ represents phase shift in radians. In the feedback path, block 22 represents the transfer function H(s) of the output voltage signal V o . The output of the feedback path from feedback block 22 may output a signal 27, which is subtracted from the input signal V by the comparator 23 i ref to generate an error signal 25 that is input to the controller block 21 such that the output signal V o approaches a set point. Overall, the transfer function of the system 20, that is, the voltage output V divided by the voltage input V of the controller 21 i out divided by the voltage input V o in, can be modeled by Equation 1. [Mathematical expression] The stability of the control system 20 is conditional on a denominator 1 + G(s) having a sufficiently large absolute value and / or being non-zero. In a resonant system 21 including an attenuated harmonic oscillator with external drive, it is well known that the oscillator response is in phase with the external drive for driving frequencies well below the resonant frequency (i.e., φ≈0), perpendicular to the resonant frequency (i.e., φ≈π / 2), and out of phase with frequencies well above the resonant frequency (i.e., φ≈π). If the control system 21 includes a resonant and oscillation energy source, in order to maintain stability, the oscillation energy source operates at either a frequency lower or higher than the resonant frequency without ever crossing the resonant frequency. In the case of resonant frequency crossing, a phase shift filter can be added to mitigate the discontinuity in the phase response.

[0017] Input voltage V i And / or in response to ambient noise, the distance d between the film 15 and the electrode 11 changes, and as a result, the capacitance C of the electrostatic acoustic device 10 changes. Input voltage V i The change in current i(t) due to and / or ambient noise is approximated by the following equation:

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[0018] Furthermore, referring to Figure 2, the voltage output signal V o In response to the input signal v, the feedback block 22 i The input signal v can be configured to output a signal 27, which is subtracted from the input signal v, to the comparator 23. i When is nominally zero, signal 27 becomes error signal 25. Alternatively, a signal combiner 23 may be used instead of comparator 23, and the feedback block 22 outputs a voltage output signal V o The input signal v i Signal 27-V becomes error signal 25 when nominally zero. o The signal is appropriately converted (e.g., inverted). Noise cancellation is performed using a linear or monotonic detection signal V with respect to the displacement of the film 15, which can be fed back to the comparator 23 as signal 27.o It can be based on this.

[0019] Here, we will refer further to Figure 2A, which shows a nested proportional-integral-derivative (PID) block 24E that is a feature of the present invention. The comparator 23 takes the audio signal input v as the first input. i The input is combined / compared with the control signal 27 or a portion thereof at the second input. Comparator 23 outputs the error signal 25 to the first PID circuit 24A. PID circuit 24A outputs the first control signal 26A to the first input of comparator 23A. The feedback signal 27 is input to the second input of comparator 23A. Comparator 23A outputs a second error signal 25A that is proportional to the difference between the signals at the first and second outputs of comparator 23A. The second error signal 25A is input to the second PID control circuit 24B. The second PID control circuit 24B outputs the voltage signal 26V i Outputs.

[0020] Here, we refer further to Figure 3, which shows a prior art proportional-integral-derivative (PID) block 24. The feedback loop may include a proportional-integral-derivative (PID) block 24E in the forward path G(s), which includes two or more nested blocks 24. The block 24 may include frequency filtering for outputting proportional gain, derivative and / or integral in a linear combination, as well as a control signal 26, to the error signal 25.

[0021] Referring again to Figure 2, the feedback circuit 20 may be used to adjust the acoustic transparency of the acoustic device 10 when used as an in-ear earphone or an over-ear headset. Acoustic transparency is a measure of the apparent stiffness of the membrane 15, which controls the rate of acoustic transmission from the external space to the sealed volume of the inner ear through the boundary defined by the membrane 15. Acoustic transparency may be controlled within the effective frequency bandwidth of the feedback operation via electrostatic feedback operation and position sensing with variable gain in block 21 and / or gain adjustment in PID 24E. Using the PID gain, the output V of the control signal 26 from PID 24E iand input audio signal v i By controlling the ratio of these factors, controlled speech noise cancellation and acoustic transparency (AT) adjustment within the effective bandwidth of PID 24E may be possible.

[0022] Referring again to Figure 3, the output voltage of a typical PID controller with an input voltage V(t) is generally proportional to the following:

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[0023] The transfer function of a typical PID controller in the Laplace domain s can be approximated by the following:

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[0024] (Reference: Franklin, GF, Powell, JD, Emami-Naeini, A. and Powell, JD, 2002, Feedback control of dynamic systems (Volume 4, Chapter 2), Upper Saddle River, Prentice hall.)

[0025] According to embodiments of the present invention, when amplification is not required, the proportionality constant α is set to zero.

[0026] As s→0, the integral term

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[0027] By combining the terms and making the frequency cutoff the same for simplicity, i.e.,

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[0028] Herein, we further refer to Figure 4, which shows a controller 24 that features the present invention, including a restricted inverting differentiator 44 implemented as an operational amplifier circuit and a leakage inverting integrator 42 connected in parallel. With respect to the circuit components, the transfer function of the integrator 42 (the first term of Equation 4) (the negative voltage output divided by the voltage input) can be given by the following equation.

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[0029] The transfer function of differentiator 44 (the second term of equation 4) (the negative voltage output divided by the voltage input) is given by the following equation.

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[0030] The leakage integrator 42 has a cutoff ω i The non-ideal differentiator 44 appears similar to or identical to a low-pass filter having a cutoff ω d It is readily apparent that this is similar to or identical to a high-pass filter having the same characteristics. A nested filter according to the features of the present invention may include two or more nested subcircuits, each subcircuit including a high-pass filter and a low-pass filter connected in parallel.

[0031] Herein, we refer further to Figure 5, which schematically shows an alternative configuration 21A of block 21 in the closed-loop feedback system 20 (Figure 2) according to the features of the present invention. In the controller 21A, the probe signal RF may be a sine wave with a frequency of 0.1 MHz to 10 MHz, for example, 1 MHz. The probe signal RF is input to the inverter 36, inverted, and can output an inverted probe signal RF or an equivalently 180° phase-shifted signal. The probe signal RF from the local oscillator 51 is capacitively coupled to electrode 11, and an inverted probe signal RF of a similar level is capacitively coupled to the other electrode 11. Similarly, the bias voltage ±V DC It is applied symmetrically to electrode 11, and -V is applied to the first electrode 11. DC A voltage is applied, and +V is applied to the second electrode 11. DC A voltage may be applied. Typically, at each electrode 11, the RF amplitude (peak-peak) may be several volts superimposed on a DC voltage bias of approximately 100 volts with equal and opposite polarity, and RF-V DC RF+V DC Each of these applies a bias to electrode 11.

[0032] Audio voltage V i The input audio voltage V can be applied to the film 15. i The changing current i(t) signal in the electrostatic acoustic device 10, as shown in equation (2), due to the displacement of the membrane 15 in accordance with ambient noise, is converted into a voltage signal and input to the high-pass filter 30. The high-pass filter 30, as shown in block 21A, allows the RF probe signal in the range of 0.1 megahertz to 10 megahertz (e.g., 1 megahertz) to pass through the input audio voltage signal V iIt is configured to block frequencies (typically 0 Hz to 20,000 Hz). The high-pass filter 30 may be a three-pole Chebyshev design with a Salen-Key active filter topology. Other high-pass filter designs may be considered by those skilled in the art according to different features of the present invention. The high-pass filter output from the high-pass filter 30 is input to the first input of the multiplier 32. The second input of the multiplier 32 is a portion of the RF probe signal (or its inverse RF). The output of the multiplier 32 includes at least a portion of the multiplication of the signal at the input port of the multiplier 32. The output of the multiplier 32 is an audio output signal V proportional to or following the displacement of the membrane 15 of the electroacoustic device 10. o It can be demodulated by a low-pass filter 34 designed to allow audio frequencies including to pass through.

[0033] Here, we further refer to Figure 5A, which shows another alternative form 21B of block 21 in the closed-loop feedback system 20 (Figure 2) according to the features of the present invention. Audio voltage V i This can be applied to the film 15. The probe signal from the local oscillator 51 may be induced on the film 15 using a transformer T having a primary winding P connected in parallel with the local oscillator 51 and a secondary winding S connected in series between the electrodes 11. Similarly, the bias voltage ±V DC It is applied symmetrically to electrode 11, and -V is applied to the first electrode 11. DC A voltage is applied, and +V is applied to the second electrode 11. DC A voltage may be applied. Similar to block 21A, at each electrode 11, the RF amplitude (peak-peak) may be several volts superimposed on a DC voltage bias of approximately 100 volts with equal and opposite polarity, and RF-V DC RF+V DC Each of these applies a bias to electrode 11.

[0034] Similar to block 21A (Figure 3), the high-pass filter 30 allows the RF probe signal in the range of 0.1 MHz to 10 MHz (e.g., 1 MHz) to pass through, and the input audio voltage signal V iIt is configured to block frequencies (typically 0 Hz to 20,000 Hz). The high-pass filter output from the high-pass filter 30 is input to the first input of the multiplier 32. The second input of the multiplier 32 contains a portion of the RF probe signal (or its inverse RF). The output of the multiplier 32 contains at least a portion of the signal multiplication at the input port of the multiplier 32. The output of the multiplier 32 is an audio output signal V that is proportional to, monotonic, or follows the displacement of the membrane 15 of the electroacoustic device 10. o It can be demodulated by a low-pass filter 34 designed to allow audio frequencies including to pass through.

[0035] Herein, we refer to Figure 6, which is a flowchart 60 illustrating a method characterized by the present invention. The displacement of the membrane of an electrostatic acoustic device is monitored by generating an audio output signal in response to the time-dependent displacement of the membrane (step 61). An input audio voltage is input (step 62) and combined with a first portion of the audio output signal to generate a first error signal as negative feedback (step 63). The first error signal is input to a control circuit, and a control signal is output from the control circuit (step 64). The control circuit includes a filter of order N>3 implemented in nested or series-connected control subcircuits. Each control subcircuit may include a parallel-connected integrator and differentiater, or a parallel-connected low-pass and high-pass filter. The control circuit is configured to optimize or increase the bandwidth of ambient noise compensation (step 66).

[0036] As used herein, the term “homodyne” refers to a method of detecting / demodulating a signal that has been phase- and / or frequency-modulated on an oscillating signal by combining the oscillating signal with a reference oscillator.

[0037] As used herein, the term “phase-sensitive detection circuit” refers to an electronic circuit comprising a multiplier (or mixer) and a loop filter that generates an output signal proportional to the product of the amplitudes of an input signal and a reference signal, and the cosine of the phase between them.

[0038] As used herein, the term “transimpedance amplifier” refers to a device that converts electric current into voltage. A transimpedance amplifier may be used to process the electric current output of a sensor into a voltage signal output.

[0039] As used herein, the term "charge amplifier" typically refers to a device that converts a time-varying charge into a voltage output by integrating a time-varying current signal.

[0040] The terms "sound" or "sound frequency" refer to the oscillation speed of alternating current or voltage in the frequency range of 0 Hz to 20,000 Hz, or the oscillation speed of a magnetic field, electric field, electromagnetic field, or mechanical system.

[0041] As used herein, the terms “audio signal,” “audio output,” and “audio output signal” essentially refer to electrical signals that vary in audio frequency.

[0042] The term "radio frequency" (RF) refers to the oscillation speed of alternating current or voltage, or the oscillation speed of a magnetic field, electric field, electromagnetic field, or mechanical system, in the frequency range of approximately 20,000 times / second (20 kHz) to approximately 300 billion times / second (300 GHz).

[0043] The terms "convert" or "transform" refer to phase shifting, inversion, amplification, and / or attenuation.

[0044] As used herein, the term “order” in reference to the order N of a filter or control circuit refers to the power of the complex variable s in the Laplace domain of the Laplace transform of the filter or control circuit.

[0045] As used herein, "leakage integrator" refers to a component or system that takes the integral of an input but gradually reduces a small amount of input over time.

[0046] As used herein, the terms “non-ideal differentiator” or “restricted differentiator” refer to a device that outputs a signal proportional to the first derivative (or reciprocal thereof) of an input signal over a limited frequency range, and tends to attenuate the output signal outside that limited frequency range.

[0047] The term "probe signal" refers to a locally generated radio frequency (e.g., a sinusoidal signal). As used herein, the term "combine" a probe signal refers to combining a portion of the probe signal, or an RF signal derived from the probe signal by transforming the probe signal.

[0048] The term "symmetrical" refers to applying a voltage that is equal in magnitude, opposite in polarity, and on the opposite side of the film 10.

[0049] As used herein, the term “error signal” refers to a voltage signal whose magnitude is proportional to, or monotonic to, the difference between the actual output signal and the desired audio signal, which varies with the audio frequency.

[0050] As used herein, the term "control signal" refers to a signal input to an acoustic device to maintain a desired voltage output signal in response to an error signal.

[0051] As used herein, the transitional term “equip” is synonymous with “include,” and is comprehensive or non-exclusive, not excluding additional elements or method steps not expressly enumerated. The articles “a” and “an” as used herein, such as “circuit” or “probe signal,” have the meaning of “one or more” meaning “one or more circuits” and “one or more probe signals.”

[0052] All optional preferred features and modifications of the embodiments and dependent claims described herein are applicable to all embodiments of the invention taught herein. Furthermore, the individual features of the dependent claims and all optional preferred features and modifications of the embodiments described herein are combinable and interchangeable with each other.

[0053] While the selected features of the present invention have been shown and described, it should be understood that the present invention is not limited to the features described.

Claims

1. A method for controlling an electrostatic acoustic device, The displacement of the membrane of the electrostatic acoustic device is monitored by generating an audio output signal in response to the time-dependent displacement of the membrane, Inputting an audio voltage, To generate a first error signal as negative feedback, the input audio voltage is first combined with the first portion of the audio output signal, The first error signal is input to a control circuit, and the control circuit outputs a first control signal, wherein the control circuit includes a filter of order N > 3. The control circuit is configured to increase the bandwidth for noise cancellation. A method that includes this.

2. The method according to claim 1, wherein the filter comprises a plurality of nested filters, each of which is of order two or higher.

3. Ambient noise contributes to the displacement of the film, and the method further, The method according to claim 1, further comprising configuring the control circuit to at least partially cancel out the displacement of the membrane due to ambient noise.

4. The method according to claim 1, further comprising converting the audio output signal before first combining it with the input audio voltage.

5. In order to generate a second error signal, the second portion of the audio output signal is secondly combined with the output from the first sub-circuit, The second error signal is input to the second sub-circuit, The second sub-circuit outputs the first control signal 26. The method according to claim 1, further comprising:

6. The method according to claim 5, further comprising the step of converting the second portion of the audio output signal before combining it with the output from the first subcircuit.

7. The method according to claim 1, wherein the control circuit includes a plurality of control subcircuits connected in series.

8. The method according to claim 7, wherein each control subcircuit includes an integrating circuit and a differentiating circuit connected in parallel.

9. The method according to claim 8, wherein the integrating circuit is a leakage integrating circuit.

10. The method according to claim 8, wherein the differentiating circuit is a non-ideal differentiating circuit.

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

12. The electrostatic acoustic device includes a first electrode and a second electrode, the first electrode being positioned parallel to the film and configured to mechanically respond to a first electric field that changes according to the respective potentials applied between the first electrode and the film, the second electrode being positioned parallel to the film on the opposite side from the first electrode and configured to mechanically respond to a second electric field that changes according to the respective potentials applied between the second electrode and the film, and the method further includes, Generating a probe signal that changes with radio frequency, A portion of the probe signal is first coupled to the first electrode, A portion of the probe signal is inverted, and the inverted portion of the probe signal is secondly coupled to the second electrode. The first DC bias voltage is first applied to the first electrode, The method involves secondly applying a second DC bias voltage to the second electrode, wherein the second DC bias voltage has the opposite polarity to the first DC bias voltage. To sense the voltage signal from the aforementioned film, The high-pass filter input is configured to receive the voltage signal from the film, wherein the high-pass filter selectively passes at least a portion of the radio frequency of the probe signal and selectively blocks at least a portion of the audio frequency to generate a filtered signal modulated by the radio frequency. At the first multiplier input, at least a portion of the filtered signal modulated with the radio frequency is input, At the second multiplier input, at least a portion of the probe signal is input, The system outputs a multiplier signal proportional to the multiplication of the filtered signal and the probe signals at the first multiplier input and the second multiplier input. The multiplication signal is demodulated to generate the audio output signal in response to the time-dependent displacement of the film. The method according to any one of claims 1 to 11, including the method described above.

13. A detector configured to monitor the displacement of a membrane in an electrostatic acoustic device by generating an audio output signal in response to the time-dependent displacement of the membrane, An audio input configured to receive an input audio voltage, A first comparator is configured to generate a first error signal by combining the input audio voltage with a first portion of the audio output signal as negative feedback, A control circuit configured to receive the first error signal and output the first control signal, the control circuit including a filter of order N > 3 An electronic device comprising, The control circuit is an electronic device configured to increase bandwidth for noise cancellation.

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

15. The electronic device according to claim 13, wherein ambient noise contributes to the displacement of the film, and the electronic device is further configured to at least partially cancel out the displacement of the film due to the ambient noise.

16. The electronic device according to claim 13, further comprising a conversion block configured to convert the first portion of the audio output signal before combining it with the audio input signal.

17. The electronic device according to claim 13, wherein the control circuit includes a plurality of series-connected control subcircuits, and the control subcircuit includes a first subcircuit and a second subcircuit.

18. The system further includes a second comparator configured to generate a second error signal by combining a second portion of the audio output signal with the output of the first sub-circuit, The electronic device according to claim 17, wherein the second error signal is configured to be input to the second sub-circuit.

19. The electronic device according to claim 13, further comprising a conversion block configured to convert the second portion of the audio output signal before combining it with the output from the first sub-circuit.

20. The electronic device according to claim 17, wherein each control subcircuit includes an integrating circuit and a differentiating circuit connected in parallel.

21. The electronic device according to claim 20, wherein the integrating circuit is a leakage integrating circuit.

22. The electronic device according to claim 20, wherein the differentiating circuit is a non-ideal differentiating circuit.

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

24. The electrostatic acoustic device includes a membrane, a first electrode, and a second electrode, wherein the first electrode is positioned parallel to the membrane and the membrane is configured to mechanically respond to a first electric field that changes according to the respective potentials applied between the first electrode and the membrane, the second electrode is positioned parallel to the membrane on the opposite side from the first electrode and the membrane, the membrane is configured to mechanically respond to a second electric field that changes according to the respective potentials applied between the second electrode and the membrane, and the detector is configured to monitor the displacement of the membrane. A transmitter configured to generate a probe signal that changes with radio frequency, A first coupler configured to couple a portion of the probe signal to the first electrode, An inverter configured to invert a portion of the probe signal and generate the inverted portion of the probe signal with the opposite polarity, A second coupler configured to couple the inverted portion of the probe signal to the second electrode, A first DC bias power supply configured to apply a first direct current (DC) bias voltage to the first electrode, A second DC bias power supply configured to apply a second DC bias voltage to the second electrode, wherein the second DC bias voltage has the opposite polarity to the first DC bias voltage, An audio voltage input configured to input the aforementioned input audio voltage to the aforementioned membrane, The output of the voltage signal from the aforementioned film, A high-pass filter input unit connectable to the voltage signal output unit from the film, wherein the high-pass filter is configured to selectively pass at least a portion of radio frequencies and selectively block at least a portion of audio frequencies to generate a filtered signal modulated by radio frequencies. A multiplier configured to input at least a portion of the filtered signal modulated at a radio frequency to a first multiplier input, and at least a portion of either the probe signal or the inverted probe signal to a second multiplier input, wherein the multiplier is configured to output a multiplier signal proportional to the multiplication of the signals at the first multiplier input and the second multiplier input, A demodulator configured to receive the multiplication signal output from the multiplier and demodulate the audio output signal in response to the time-dependent displacement of the film, An electronic device according to any one of claims 13 to 23, comprising:

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