Control of electrostatic acoustic devices

Abstract

A control circuit and method for maximizing the dynamic range of motion of the membrane, controlling the acoustic transparency of the electrostatic device, and noise cancellation is provided. In the system 20, a control circuit including a system 21 includes a comparator 23 that compares an audio input signal Vi with a feedback signal 27 to generate an error signal 25, and a controller that inputs a control signal 26 to an electrostatic transducer. The control signal controls the acoustic transparency of the electrostatic transducer.

Description

[Technical Field]

[0001] The present invention relates to electrostatic acoustic devices, including earphones and loudspeakers, and more particularly to control circuits for operating electrostatic devices. [Background technology]

[0002] In the field of high-fidelity sound reproduction, electrostatic loudspeakers 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 near a pair of electrodes, one on each side of the membrane in a push-pull configuration. 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. Since the electrodes are acoustically transparent, the sound generated by the movement of the membrane is radiated outward through the electrodes into the listening area.

[0003] Electrostatic devices are highly efficient both electrically and mechanically. They have high electrical impedance, which decreases with increasing acoustic frequency. As a result of this high electrical impedance, the operating current is very low, minimizing electrical losses. Mechanically, there are no moving parts other than the movement of a very lightweight membrane. Therefore, electrostatic devices are inherently more energy-efficient than the electrodynamic acoustic devices currently used in battery-powered electronic devices.

[0004] Therefore, it is advantageous and desirable to have a highly efficient, compact electrostatic device suitable for use in battery-powered electronic devices, which has a control circuit configured to maximize the dynamic range of motion of the membrane, control the acoustic transparency of the electrostatic device, and cancel noise, and to use such an electrostatic device as both a loudspeaker and a microphone. [Overview of the project]

[0005] This specification discloses various control methods for controlling the operation of an electrostatic acoustic device including a film and electrodes positioned near the film. The film is configured to mechanically respond to a variable electric field emitted from the electrodes when a variable audio signal voltage is applied to the electrostatic acoustic device. A radio frequency-varying probe signal is injected into the electrodes. The current or charge signal is detected by converting the current or charge signal into a modulated voltage signal. The current or charge signal includes an audio frequency-varying audio signal that modulates the radio frequency of the probe signal. The modulated voltage signal is demodulated to generate an audio frequency-varying audio output signal. The audio output signal is converted to generate an error signal. A control signal is input to the electrostatic acoustic device in response to the error signal. The control signal is configured to cause the mechanical motion of the film to maintain a desired acoustic output. The audio frequency-varying audio output signal can be obtained by homodyne detection of the modulated voltage signal at the radio frequency. The phase and frequency can be locked between the modulated voltage signal at the radio frequency and the radio frequency carrier signal responding to the probe signal at the radio frequency. A synchronization signal synchronized with the radio frequency carrier of the modulated voltage signal can be generated. The probe signal may be output in response to a synchronization signal. Demodulation of the modulated voltage signal may be performed using a low-pass filter. Alternatively, a sine wave may be locally generated at the radio frequency, and the probe signal may respond to the locally generated sine wave at the radio frequency. Demodulation may be performed by rectification followed by low-pass filtering to generate an audio output signal. The phase and amplitude of the control signal may be configured to at least partially cancel the mechanical response of the film due to ambient noise. The control signal may be configured to limit the mechanical displacement of the film, intended to protect against mechanical collapse of the film to the electrodes due to electrostatic discharge or irreversible electrostatic attraction between the film and the electrodes. The control signal may be further configured to adjust the acoustic transparency of the electrostatic acoustic device.

[0006] This specification discloses various control circuits for controlling the operation of electrostatic acoustic devices. The electrostatic acoustic device includes a membrane and electrodes positioned near the membrane. The membrane is configured to mechanically respond to a variable electric field emitted from the electrodes when a variable audio signal voltage is applied to the electrostatic acoustic device. The control circuit includes an amplifier configured to inject a radio frequency-varying probe signal into the electrodes. A detector is configured to detect a current or charge signal in response to the mechanical motion of the membrane. The current or charge signal includes an audio signal that modulates the radio frequency and varies at an audio frequency. The detector is configured to convert the current or charge signal into a modulated voltage signal. A demodulator is configured to demodulate the modulated voltage signal and generate an audio output signal that varies at an audio frequency. A conversion circuit is configured to convert the audio output signal and generate an error signal. A controller is configured to input a control signal to the electrostatic acoustic device in response to the error signal. The control signal is configured to cause the mechanical motion of the membrane to maintain a desired acoustic output. The audio output signal that varies at an audio frequency can be obtained by homodyne detection of the modulated voltage signal at the radio frequency. The control circuit may include a phase-locked loop configured to lock the phase and frequency of a radio frequency carrier signal in response to a modulated voltage signal and a probe signal at a radio frequency. The phase-locked loop may include a voltage-controlled oscillator configured to generate a signal synchronized with the radio frequency carrier of the modulated voltage signal. The synchronization signal may be input to an amplifier configured to output a probe signal in response to the synchronization signal. A low-pass filter may be configured to filter and demodulate the modulated voltage signal to generate an audio output signal that varies at an audio frequency. Alternatively, a local oscillator may be configured to generate a sine wave at a radio frequency. The amplifier may be configured to take a sine wave at a radio frequency as input and output a probe signal having a frequency corresponding to the sine wave. The demodulator may include a rectifier and a low-pass filter for generating the audio output signal. The phase and amplitude of the control signals may be configured to at least partially cancel out the mechanical response of the film due to ambient noise.The control signal can be configured to limit the mechanical displacement of the membrane, intended to protect against electrostatic discharge between the membrane and the electrode. The control signal can be further configured to adjust the acoustic transparency of the electrostatic acoustic device.

[0007] Here, the present invention will be described merely by way of example with reference to the accompanying drawings.

Brief Description of the Drawings

[0008] [Figure 1] A cross-sectional view of an electrostatic device according to the features of the present invention is schematically shown. [Figure 2] An electronic block diagram of a feedback control system according to the features of the present invention. [Figure 2A] An electronic block diagram of a proportional-integral-derivative (PID) controller according to the prior art is shown. [Figure 3] An electronic block diagram of a control system including an electrostatic acoustic device in the forward path of the feedback control system of FIG. 2. [Figure 3A] An alternative electronic block diagram of a control system including an electrostatic acoustic device in the forward path of the feedback control system of FIG. 2. [Figure 4] Another alternative electronic block diagram of a control system in the forward path of the feedback control system of FIG. 2. [Figure 5] Yet another alternative electronic block diagram of a control system in the forward path of the feedback control system of FIG. 2. [Figure 6] A flowchart of a method showing the features of the present invention. [Figure 7] A flowchart of a method showing the features of the present invention.

Modes for Carrying Out the Invention

[0009] The above aspects and / or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawings.

[0010] The features of the present invention are described below in detail, and examples thereof are shown in the accompanying drawings, where similar reference numerals throughout indicate similar elements. These features are described below by reference to the drawings in order to illustrate the present invention.

[0011] As a preface, various aspects of the present invention are directed toward circuits for in-ear and / or over-ear electrostatic headphones relating to the control of acoustic transmission and / or ambient noise cancellation. Circuits relating to various features of the present invention may be directed toward detector circuits for using an acoustic device as an electrostatic microphone. The circuits may be designed for electrostatic speakers with a maximum dimension of, for example, a diameter D of 50 millimeters or less, or in some embodiments for electrostatic speakers with a dimension D of 25 millimeters or less, and in other embodiments for electrostatic speakers with a dimension D of 10 millimeters or less. For earphone applications, the electrostatic speaker may have a maximum dimension of, for example, a diameter D of 5 millimeters or less.

[0012] Another aspect of the present invention involves using a detector circuit to use an electrostatic device as a loudspeaker and a microphone, and optimizing the dynamic range and protection from overdrive of the electrostatic device.

[0013] According to the features of the present invention, the mechanical motion of the membrane is compelled to maintain a desired acoustic output, including linearized motion of the membrane over at least a portion of the desired frequency range. The mechanical response of the membrane due to acoustic ambient noise is at least partially canceled, i.e., ambient noise control (ANC) can be performed. Similarly, the acoustic transparency of an electrostatic acoustic device can be controlled. Conventional closed-loop controllers, such as ANC, generally use a speaker and multiple microphones. According to embodiments of the present invention, a single electroacoustic device is sufficient to maintain the desired acoustic output.

[0014] Referring now to the drawings, and referring to FIG. 1, an electrostatic acoustic device 10 according to the features of the present invention is schematically shown. A longitudinal axis Z passing through the center of the acoustic device 10 is shown. The tensioned membrane 15 is supported basically perpendicular to the longitudinal axis Z by the edges of the electrodes 11. The membrane 15 may be impregnated with a conductive, resistive, and / or electrostatic material so as to respond mechanically to a changing electric field. The central region of the electrode 11 is mounted in proximity to the membrane 15, for example parallel to the membrane 15, at a distance d, for example 20 to 500 micrometers from the membrane 15, nominally at equal distances. The electrode 11 is shown as being perforated with an aperture 12 that allows sound waves emitted from the membrane 15 during operation of the electrostatic acoustic device 10 to pass through.

[0015] During operation of the electrostatic acoustic device 10, for example +V DC = +100 to +1000 volts, etc., a constant direct current (DC) bias voltage can be applied to the membrane 15 using a conductive contact. Alternatively, a voltage signal V i is applied to the membrane 15, and the electrode 11 can be biased at ±V DC . A voltage signal ±V i can be applied to the electrode 11. The voltage signal ±V i can nominally be an audio frequency of 20 to 20,000 Hertz. A non-inverting voltage signal +V i is applied to one of the electrodes 11, and the same inverted voltage signal -V i can be applied to the other electrode 11. The dotted line schematically shows the membrane 15 that moves in response to the voltage changing due to the voltage signal ±V i .

[0016] As the distance d decreases, or as the DC bias voltage +V DC and / or the signal voltage ±V i increases (in absolute value), there is a high likelihood of a short circuit between the membrane 15 and the electrode 11, and / or dielectric breakdown of the air, which is expected to be about 3×10 6 volts / meter. According to the features of the present invention, the operation of the electrostatic speaker can be controlled to prevent overdrive of the membrane 15.

[0017] Referring to Figure 2, a control system 20 according to the features of the present invention is shown. In the forward path, G(s) represents the open-loop gain of the control circuit including system 21, and s is A(e iωt The AC voltage signal may be represented by a complex variable in the form of +φ), where A represents the amplitude, ω=2πf represents the angular frequency, f represents the frequency in Hertz, and φ represents the phase shift in radians. In the feedback path, block 22 is the output voltage signal V o This represents the transformation function H(s). The feedback path output from the feedback block 22 may output signal 27, which is the output signal V o The input signal V should approach the set value. i This is subtracted by comparator 23 to generate an error signal 25, which is the input to controller block 21. The overall conversion function of system 20 is a voltage output V o Dividing controller 21 voltage input V i This can be modeled by Equation 1.

number

[0018] Referring now to Figure 3, a controller 21A, which is an alternative example to the system 21 in Figure 2 and features of the present invention, is schematically shown. Controller 21A receives a high-voltage audio input +V at the first electrode 11.i Upon receiving the signal, the second electrode 11 inverts the high-voltage audio input -V, which is converted to sound by the electrostatic acoustic device 10 at an audio frequency that is intended to be changed. i The device includes an electrostatic acoustic device 10 which can be configured to receive sounds. In addition, the membrane 15 may be mechanically responsive when the device 10 behaves as a capacitive microphone to unwanted ambient sound waves or noise.

[0019] Here, Figure 6, which is a flowchart 60 of a method illustrating the features of the present invention, is also referred to. It is advantageous to have a control circuit 20 that detects the time-varying displacement of the membrane 15 (step 63) when the input audio signal is below a predetermined threshold (determination block 61), and feeds back a control signal 26 to the acoustic device 10 to reduce the displacement of the membrane 15 due to ambient noise (step 65). Thus, when the electrostatic acoustic device 10 is used as an earphone and sealed in the ear canal, the mechanical displacement of the eardrum is linked to the mechanical displacement of the membrane 15, and tends to actively cancel out ambient noise that would otherwise be perceived by the user.

[0020] 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. The current i(t) that changes due to ambient noise is sensed using the transimpedance amplifier 30.

number

[0021] Alternatively, instead of a transimpedance amplifier, a charge amplifier 30 may be considered, which integrates the current i(t) to sense the charge Q(t) that changes with the change in capacitance of the electrostatic acoustic device 10, and the sensed charge is converted into an output voltage signal.

[0022] The amplifier 30 may be configured as an inverting or non-inverting amplifier and may have a 600-900 Hz bandpass (-3 dB cutoff) centered outside the audio frequency band of 0.1-2 megahertz, preferably far from the resonance of the film 15. The voltage output of the amplifier 30 may be added to a signal coupler or multiplier 32.

[0023] Referring to Figure 3, for example, a probe signal from a local oscillator (LO) 51 at a radio frequency of 0.1 to 2 megahertz can be coupled between the primary windings P of the transformer T. Audio signal +V i and inverted audio signal -V i These are supplied to the electrode 11 through the series-connected secondary windings S1 and S2 of transformer T, respectively. Audio signal ±V i This can be a high-voltage signal, or an audio signal ±V i The probe signal may be a low voltage signal up to approximately ±20V, and a high DC voltage is applied to the film 15 as shown in device 10 (Figure 1). The probe signal generates a current with a magnitude determined by the characteristic reactance of the electrical circuit formed by the film 15 and electrode 11, essentially a variable capacitor. The advantage of using radio frequencies is that radio frequencies do not generate perceptible mechanical motion, but are modulated by electrical changes in capacitance associated with mechanical motion generated when an audio signal is present. The probe signal from the local oscillator (LO) 51 can also be coupled with the voltage output of the amplifier 30 in the signal coupler / multiplier 32. The signal coupler / multiplier 32 receives a voltage output signal V that changes at the audio frequency. o The signal is output to a low-pass filter 34 for demodulation and transmission. System 21A is a homodyne detection circuit that uses a local oscillator 51 as a reference, which is multiplied by the measurement signal output of amplifier 30 at the same frequency. The baseband or DC component of this multiplication includes a signal that is a frequency converted from a narrowband around the LO52 frequency, which is detected with a very high signal-to-noise ratio. The multiplier 32 can be implemented, for example, by an analog circuit AD835 from Analog Devices (Norwood, Massachusetts, USA).

[0024] Referring again to Figure 2, the voltage output signal V converted by the feedback block 22 o The following is shown: Voltage output signal V o In response to the input signal V, the feedback block 22 i It can be configured to output a signal 27 to comparator 23 which is subtracted from the input signal V. i If is nominally zero, signal 27 is added to become error signal 25. Alternatively, a signal coupler 23 may be used instead of comparator 23, and the feedback block 22 outputs a voltage signal V o This signal is appropriately converted, for example, by inverting it, into a signal 27 that becomes an error signal 25.

[0025] Noise cancellation is performed by detecting the position of the film 15, which can be input as a signal 27 to the feedback control mechanisms 23 and 24. o This may be based on the following. The second input is a control or setpoint signal, which is the audio signal v played by device 10. i That's fine.

[0026] System 20 receives a lock-in detection signal V related to the position of the film 15, for example, from the detection circuit 21A. o The closed-loop operation of the electrostatic speaker 10 using this method may be shown.

[0027] Referring again to Figure 2A, a proportional, integral, derivative (PID) block 24 relating to the prior art is shown. The feedback loop may include a proportional, integral, derivative (PID) block 24 in the forward path G(s), and block 24 may include frequency filtering for outputting a proportional gain, a derivative and / or integral in a linear combination, and a control signal 26 for the error signal 25. Null audio signal v i In this regard, system 20 can function as a noise-canceling control system.

[0028] The feedback circuit 20 can 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 criterion for the apparent rigidity of the membrane 15 and controls the coefficient of acoustic transmission from the external space to the ear cavity sealed volume through the boundary defined by the membrane 15. Acoustic transparency can be controlled via electrostatic feedback operation and position sensing, which are sensed by the variable gain shown in block 21A and gain adjustment in PID 24, within the effective frequency bandwidth in which the feedback operates.

[0029] Using the PID gain, the control signal 26 output from PID 24 and the input audio signal v i By controlling the ratio of these factors, the audio noise rejection and acoustic transparency (AT) in PID24 can be controlled to an effective bandwidth.

[0030] Referring to Figure 3A, a controller 21B is shown, which is an alternative example to system 21 (Figure 2) according to the features of the present invention. In controller 21B, the audio voltage V i A voltage can be applied to the film 15. The primary voltage P and audio voltage V are connected in parallel with the local oscillator 51. i Using a transformer T having a secondary S connected in series between the film 15 and the local oscillator 51, a probe signal from the local oscillator 51 can also be induced in the film 15. A bias voltage VDC is applied symmetrically to the electrode 11, and -V DC / 2 is applied to the first electrode 11, +V DC A voltage of / 2 is applied to the second electrode 11. A differential amplifier 31 having inputs capacitively coupled to each electrode 11 may be used. The voltage output of the differential amplifier 31 changes depending on the capacitance of device 10. A probe signal from the local oscillator 51 may also be coupled to the voltage output of the differential amplifier 31 in the signal coupler / multiplier 32. The signal coupler / multiplier 32 receives a voltage output signal V that changes with the audio frequency. oThe signal is output to a low-pass filter 34 that demodulates and transmits it. The differential amplifier 31 can be implemented using Texas Instruments / Burr-Brown® INA105. According to the features of the present invention, the high-voltage audio signal V i When this is used, one high-voltage input amplifier is used instead of two, so controller 21B is more advantageous than controller 21A.

[0031] Referring to Figure 4, an alternative controller 21C (Figure 2, system block 21) according to the features of the present invention is schematically shown. Controller 21C receives an input voltage signal ±V iIf the (absolute value) is less than a predetermined threshold, it may be used to minimize or cancel ambient noise. Amplifier 40 may be a charge amplifier or a transimpedance amplifier. Amplifier 40, as amplifier 30 in circuit 21A, may be inverting or non-inverting and may be configured to have a 600-900 Hz bandpass (-3 dB cutoff) centered out of the audio frequency band, preferably far from the resonance of the film 15, in the 0.1-2 MHz range. The voltage output of amplifier 40 may be an input to a signal coupler or multiplier 42 and may be a component of a phase-locked loop (PLL) 49. The phase-locked loop 49 uses a local oscillator, i.e., a voltage-controlled oscillator (VCO) 48, which is compared with the measurement signal output from amplifier 40. The measurement signal includes small phase / frequency changes compared with the output of the VCO 48, which can be detected with high signal-to-noise using a phase-sensing detector / demodulator, i.e., a mixer 42 and a low-pass filter 44. The second input to the signal coupler or multiplier 42 is the output of the voltage-controlled oscillator (VCO) 48. The multiplier 42 may output to a narrowband loop filter 47 that outputs a DC voltage in response to the input RF carrier frequency. The voltage-controlled oscillator (VCO) 48 outputs a radio frequency that responds monotonically to the DC voltage input from the loop filter 47. The multiplier 42 and the loop filter 47 function as phase detectors. The PLL 49 is configured to lock stably when the inputs to the multiplier 42 are the same frequency with a fixed phase difference. The carrier frequency output from the voltage-controlled oscillator (VCO) 48 is fed back to the amplifier 36, which is coupled to the input of the electrostatic acoustic device 10 by capacitive or inductive coupling 45, injecting a probe voltage signal into the input of the electrostatic acoustic device 10 corresponding to the carrier frequency. The PLL 49 also outputs to a low-pass filter 44, which outputs a voltage output signal V that is sensitive to the relative and constant phase difference between the two inputs to the mixer 42. o This generates the voltage output signal V in the control circuit 21C. o This can then be converted into an error signal 25 for active noise minimization / cancellation (block 22, Figure 2). Alternatively, as in system 21B, detection as shown in Figure 4 is performed by applying a single audio voltage V to the film 15.i The probe signal from the local oscillator 51 is also induced in the film 15, and the bias voltage V DC The voltage is applied symmetrically to electrode 11, and -V DC / 2 is applied to the first electrode 11, +V DC A differential amplifier may be used in which a voltage of / 2 is applied to the second electrode 11 and the inputs are capacitively coupled to each electrode 11. Referring here to Figure 5, an alternative controller circuit 21D (Figure 2, system block 21) characterized by the present invention is schematically shown. The local oscillator (LO) 51 is configured to output a sine wave with a frequency of 0.1 to 2 megahertz, for example, 1 megahertz, as the input to the amplifier 56. During operation, the amplifier 56 injects a sine wave probe voltage corresponding to the input frequency output from the oscillator LO 51 into the input 38 of the device 10 via capacitive or inductive coupling 45. Audio input voltage signal V i If present, the carrier may be modulated near the radio frequency, for example, 1 megahertz. Similarly, noise signals from ambient sound generated internally in the electrostatic acoustic device 10 may modulate the carrier frequency of LO51.

[0032] The amplifier 50 may be a charge amplifier or a transimpedance amplifier, and as amplifier 30 in circuit 21A, it may be inverting or non-inverting and may be configured to have a bandpass (-3dB cutoff) of 600 to 900 Hz centered out of the audio frequency band, preferably far from the resonance of the film 15, in the range of 0.1 to 2 megahertz.

[0033] The voltage output of amplifier 50 may include a rectifier 53 and a low-pass filter 54, and the voltage V can be converted into an error signal 25 (block 22, Figure 2) for active noise minimization / cancellation. o This may be the input to detection block 52, which outputs [the specified output]. Protection against discharge and overdrive

[0034] Controller circuits 20, 21A, 21B, 21C, and 21D may have further utility for protecting the electrostatic acoustic device from unintended air dielectric breakdown or short circuits between the electrode 11 and the film 15. Unintended air dielectric breakdown or short circuits can occur when the electrostatic acoustic device 10 is overdriven and the film 15 is displaced excessively close to the electrode 11. Generally, the displacement of the film 15 is due to the bias voltage VDC and the input voltage signal VDC. i The magnitude and frequency of the voltage output signal V may depend on several factors, including the physical parameters of the electrostatic acoustic device 10. o Or, if that particular frequency component has an amplitude exceeding a predetermined frequency-dependent threshold, the controller circuits 20, 21A, 21B, 21C, or 21D, in particular the feedback path block 22, will react to the input voltage signal v i It may be configured to partially cancel out the electrostatic acoustic device 10 or protect against mechanical collapse of the film to the electrodes due to irreversible electrostatic attraction.

[0035] Referring here to Figure 7, a flowchart 70 of a method relating to a feature of the present invention for controlling the operation of an electrostatic acoustic device including a film 15 and electrodes 11 positioned near the film 15 is shown. The film 15 is configured to mechanically respond to a variable electric field emitted from the electrodes 11 when a variable audio signal voltage is applied to the electrodes 11. A radio frequency-varying probe signal is injected into the electrodes 11 (step 71). The current or charge signal is detected by converting the current or charge signal into a modulated voltage signal (step 73). The current or charge signal includes an audio signal that modulates the radio frequency of the probe signal with an audio frequency. The modulated voltage signal is demodulated to generate an audio output signal that modulates the audio frequency (step 75). The audio output signal is converted to generate an error signal (step 77), and in response to the error signal, a control signal is input to the acoustic device 10 (step 79).

[0036] As used herein, the term “homodyne” refers to a method of detecting / demodulating a signal whose phase and / or frequency are modulated into an oscillating signal by coupling the signal with a reference oscillator.

[0037] As used herein, the term “phase-sensitive detector circuit” essentially refers to an electronic circuit comprising a multiplier and a loop filter that generates a DC output signal proportional to the product of the amplitudes of two AC input signals of the same frequency 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" generally refers to a device that converts a time-varying charge into a voltage output by integrating a time-varying current signal.

[0040] The terms "audio" or "audio frequency" refer to the frequency range of alternating current or voltage, or of magnetic fields, electric fields, or electromagnetic fields or mechanical systems, within the frequency range of 0 to 20,000 Hz.

[0041] As used herein, the terms “audio signal,” “audio output,” and “audio output signal” basically refer to electrical signals that vary with audio frequencies.

[0042] The term "radio frequency" (RF) refers to the frequency range of alternating current or voltage, or magnetic fields, electric fields, electromagnetic fields, or mechanical systems, ranging from approximately 20,000 times per second (20 kHz) to approximately 300 billion times per second (300 GHz).

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

[0044] As used herein, the term "error signal" refers to a voltage signal of a magnitude proportional to, or monotonically, the difference between the actual output signal, which varies with audio frequency, and the desired audio signal.

[0045] As used herein, the term “control signal” refers to a signal input to an acoustic device in response to an error signal in order to maintain a desired voltage output signal.

[0046] As used herein, the transitive term “to include” is synonymous with “to contain,” and is comprehensive or unrestricted, not excluding any additional elements or method steps not explicitly stated. For example, as used herein, the articles “a” and “an,” as in “a circuit” or “an electrode,” mean “one or more,” as in “one or more circuits” or “one or more electrodes.”

[0047] All optional and 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, as well as all optional and preferred features and modifications of the embodiments described herein, are interchangeable and can be combined with each other.

[0048] While selected features of the present invention have been shown and described, it should be understood that the present invention is not limited to the described features.

Claims

1. A control circuit operable for an electrostatic transducer comprising 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 a potential 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 is configured to mechanically respond to a second electric field that changes according to a potential applied between the second electrode and the membrane, the first electrode and the second electrode have through holes configured for acoustic transmission to and from the membrane, and the control circuit is, Audio signal input section, A detector configured to detect a current or charge signal from an electrostatic transducer in response to the motion of the film, wherein the current or charge signal includes an audio signal that varies at an audio frequency, and the detector is configured to generate an audio output signal that varies at an audio frequency. A conversion circuit configured to convert the aforementioned audio output signal and generate a feedback signal, A comparator configured to generate an error signal by comparing the input audio signal at the audio signal input section with the feedback signal, A controller configured to input a control signal to the electrostatic transducer, wherein the control signal responds to the error signal, and the controller Equipped with, The control signal is configured to control the acoustic transparency of the electrostatic transducer, which passes from the external space through the through-hole of the first electrode, through the film, and through the through-hole of the second electrode.

2. The control circuit according to claim 1, wherein the acoustic transparency is controlled according to the ratio between the control signal and the input audio signal at the audio signal input section.

3. The control circuit according to claim 1, wherein a DC bias voltage is applied to the first electrode and the second electrode, and an audio voltage input in response to the control signal is applied to the film.

4. The control circuit according to claim 1, wherein, in response to the control signal, a non-inverting audio voltage input is applied to one of the first electrode and the second electrode, the same but inverted audio voltage input is applied to the other of the first electrode and the second electrode, and the film is biased with a DC bias voltage.

5. The control circuit according to claim 1, wherein the first electrode includes a first conductive layer attached to an electrically insulated substrate, the first conductive layer being assembled in close proximity to the film, and the second electrode includes a second conductive layer attached to an electrically insulated substrate, the second conductive layer being assembled in close proximity to the film.

6. The control circuit according to any one of claims 1 to 5, wherein the control signal is configured to at least partially cancel the mechanical response of the film caused by ambient noise.

7. The control circuit according to any one of claims 1 to 5, wherein the control signal is configured to limit the mechanical displacement of the film.

8. A control circuit according to any one of claims 1 to 7, wherein a probe signal that varies in radio frequency is injected into the electrode, the current or charge signal is detected by converting the current or charge signal into a modulated voltage signal, and the current or charge signal includes an input audio signal that modulates the radio frequency of the probe signal.

9. The control circuit according to claim 8, wherein the audio output signal is obtained by homodyne detection of a modulated voltage signal at a radio frequency.

10. A phase-locked loop configured to lock the phase and frequency of a radio frequency carrier signal in response to a modulated voltage signal at a radio frequency and a probe signal at a radio frequency. The control circuit according to claim 8, further comprising the following:

11. A voltage-controlled oscillator configured to generate a signal synchronized with the radio frequency carrier of the modulated voltage signal. The control circuit according to claim 8, further comprising the following:

12. An amplifier configured to output the probe signal in response to the synchronization signal. The control circuit according to claim 11, further comprising the following:

13. The control circuit further comprises a local oscillator configured to generate a sine wave at a radio frequency, The amplifier is configured to receive a sine wave at a radio frequency and output a probe signal having a frequency corresponding to the sine wave. The control circuit according to claim 12.

14. A low-pass filter configured to filter the modulated voltage signal, demodulate it, and generate the audio output signal. A control circuit according to any one of claims 8 to 13, further comprising the above.

15. A method executable for controlling an electrostatic transducer comprising an audio signal input section, a film, a first electrode, and a second electrode, wherein the first electrode is positioned parallel to the film, and the film is configured to mechanically respond to a first electric field that changes according to a potential applied between the first electrode and the film; the second electrode is positioned parallel to the film on the opposite side from the first electrode, and the film is configured to mechanically respond to a second electric field that changes according to a potential applied between the second electrode and the film; the first electrode and the second electrode have through holes configured for acoustic transmission to and from the film; and the method is: The method involves detecting a current or charge signal from the electrostatic transducer, wherein the current or charge signal includes an audio signal that changes with an audio frequency, thereby generating an audio output signal that changes with an audio frequency. The audio output signal is converted to generate a feedback signal, The input audio signal at the aforementioned audio signal input section is compared with the feedback signal to generate an error signal, In response to the error signal, a control signal is input to the electrostatic transducer, thereby controlling the acoustic transparency of the electrostatic transducer from the external space through the through-hole of the first electrode, through the film, and through the through-hole of the second electrode. A method that includes [a certain feature].

16. The acoustic transparency is controlled according to the ratio between the control signal and the input audio signal at the audio signal input section. The method according to claim 15, further comprising:

17. A DC bias voltage is applied to the first electrode and the second electrode, and an audio voltage input in response to the control signal is applied to the film. The method according to claim 15, further comprising:

18. In response to the control signal, a non-inverting audio voltage input is applied to one of the first and second electrodes, and the same but inverted audio signal input is applied to the other of the first and second electrodes, thereby biasing the film with a DC bias voltage. The method according to claim 15, further comprising:

19. The control signal is configured to at least partially cancel the mechanical response of the film caused by ambient noise. The method according to any one of claims 15 to 18, further comprising:

20. The control signal is configured to limit the mechanical displacement of the film. The method according to any one of claims 15 to 18, further comprising:

21. Injecting a probe signal that changes with radio frequency into the input of an electrostatic acoustic device, The current or charge signal is detected by converting the current or charge signal into a modulated voltage signal, wherein the current or charge signal includes an input audio signal that changes at an audio frequency that modulates the radio frequency of the probe signal. The modulated voltage signal is demodulated to generate the audio output signal, The method according to any one of claims 15 to 20, further comprising:

22. By homodyne detection of a modulated voltage signal at radio frequencies, an audio output signal that changes with audio frequencies can be obtained. The method according to claim 21, further comprising:

23. Locking the phase and frequency of a radio frequency carrier signal in response to a modulated voltage signal at a radio frequency and a probe signal at a radio frequency. The method according to claim 21, further comprising:

24. Configuring a local oscillator to generate a sine wave at a radio frequency, Inputting a sine wave at radio frequencies, Outputting a probe signal having a frequency corresponding to the sine wave, The method according to claim 21, further comprising:

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