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 in particular, the present invention relates to control circuits for operating electrostatic devices. [Background technology]

[0002] In the art of high-fidelity sound reproduction, electrostatic loudspeakers are noted for their inherently superior sound quality and smooth response over a wide frequency range. In such devices, a flexible, sound-generating membrane is placed near an electrode, or, in a push-pull configuration, near a pair of electrodes, one on each 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. Because the electrodes are acoustically transparent, sound generated by the membrane's movement radiates outward through the electrodes to the listening area.

[0003] Electrostatic devices are very efficient, both electrically and mechanically. They have a high electrical impedance that decreases with increasing acoustic frequency. The high electrical impedance results in very low operating currents and minimal electrical losses. Mechanically, there are no moving parts other than a very light-weight membrane. Electrostatic devices are therefore inherently more energy efficient than the electrodynamic acoustic devices currently used in battery-powered electronic devices.

[0004] It would therefore be advantageous and desirable to have a highly efficient, compact electrostatic device suitable for use in battery-powered electronic devices, and for use as a loudspeaker and microphone, that has control circuitry configured for maximizing the dynamic range of motion of the membrane, controlling the acoustic transparency of the electrostatic device, and noise cancellation. Summary of the Invention

[0005] Disclosed herein are various control methods for controlling the operation of an electrostatic acoustic device including a membrane and electrodes disposed near the membrane. The membrane is configured to mechanically respond to a variable electric field emanating from the electrodes when a variable audio signal voltage is applied to the electrostatic acoustic device. A probe signal varying at a radio frequency 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 signal varying at an audio frequency that modulates the radio frequency of the probe signal. The modulated voltage signal is demodulated to generate an audio output signal varying at an audio frequency. 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 membrane to maintain a desired acoustic output. The audio output signal varying at an audio frequency can be obtained by homodyne detection of the modulated voltage signal at a radio frequency. The modulated voltage signal at a radio frequency can be phase- and frequency-locked to a radio frequency carrier signal responsive to the probe signal at a radio frequency. A synchronization signal can be generated that is synchronized with the radio frequency carrier of the modulated voltage signal. 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 radio frequency, and the probe signal may be responsive to the locally generated sine wave at radio frequency. Demodulation may be performed by low-pass filtering after rectification 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 membrane due to ambient noise. The control signal may be configured to limit the mechanical displacement of the membrane to protect against mechanical collapse of the membrane to the electrode due to electrostatic discharge between the membrane and the electrode or irreversible electrostatic attraction. The control signal may further be configured to adjust the acoustic transparency of the electrostatic acoustic device.

[0006] Various control circuits are disclosed herein for controlling the operation of an electrostatic acoustic device. The electrostatic acoustic device includes a membrane and an electrode disposed near the membrane. The membrane is configured to mechanically respond to a variable electric field emanating from the electrode when a variable audio signal voltage is applied to the electrostatic acoustic device. The control circuit includes an amplifier configured to inject a probe signal that varies at a radio frequency into the electrode. A detector is configured to detect a current or charge signal responsive to the mechanical movement of the membrane. The current or charge signal includes an audio signal that varies at an audio frequency that modulates the radio 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 movement 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 a radio frequency. The control circuit may include a phase-locked loop configured to lock the phase and frequency of a radio frequency carrier signal responsive to the modulated voltage signal and the radio frequency probe signal. 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 responsive to the synchronization signal. The 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, the local oscillator may be configured to generate a sine wave at a radio frequency. The amplifier may be configured to input the sine wave at a radio frequency and output a probe signal having a frequency corresponding to the sine wave. The demodulator may include a rectifier and a low-pass filter to generate the audio output signal. The phase and amplitude of the control signal may be configured to at least partially cancel mechanical responses of the membrane due to ambient noise.The control signal may be configured to limit the mechanical displacement of the membrane to protect against electrostatic discharge between the membrane and the electrodes. The control signal may further be configured to adjust the acoustic transparency of the electrostatic acoustic device.

[0007] The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0008] [Figure 1] 1A and 1B show schematic cross-sectional views of electrostatic devices in accordance with aspects of the present invention; [Figure 2] FIG. 1 is an electronic block diagram of a feedback control system in accordance with aspects of the present invention. [Figure 2A] 1 shows an electronic block diagram of a proportional-integral-derivative (PID) controller according to the prior art. [Figure 3] FIG. 3 is 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] FIG. 3 is 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] FIG. 3 is another alternative electronic block diagram of a control system in the forward path of the feedback control system of FIG. 2. [Figure 5] FIG. 3 is yet another alternative electronic block diagram of a control system in the forward path of the feedback control system of FIG. 2. [Figure 6] FIG. 1 is a flow diagram of a method illustrating features of the present invention. [Figure 7] FIG. 1 is a flow diagram of a method illustrating features of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] These and / or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.

[0010] Reference will now be made in detail to the features of the present invention, examples of which are illustrated in the accompanying drawings, in which like reference numerals refer to like elements throughout. This feature will now be described by reference to the drawings for explaining the invention.

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

[0012] Other aspects of the invention include the use of detector circuits for electrostatic devices used as loudspeakers and as microphones to optimize the dynamic range and overdrive protection of the electrostatic devices.

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

[0014] Referring now to the drawings, and with reference to FIG. 1, an electrostatic acoustic device 10 according to aspects of the present invention is shown. A longitudinal axis Z is shown passing through the center of the acoustic device 10. A tensioned membrane 15 is supported by the edges of electrodes 11, essentially perpendicular to the longitudinal axis Z. The membrane 15 may be impregnated with conductive, resistive, and / or electrostatic materials so as to be mechanically responsive to a changing electric field. A central region of the electrode 11 is mounted proximate to the membrane 15, e.g., parallel to the membrane 15, at a distance d, e.g., 20-500 micrometers from the membrane 15, nominally equidistant therefrom. The electrode 11 is shown perforated with an aperture 12 that allows passage of sound waves emanating from the membrane 15 during operation of the electrostatic acoustic device 10.

[0015] During operation of the electrostatic acoustic device 10, for example, +V DC A constant direct current (DC) bias voltage, such as V = +100 to +1000 volts, can be applied to the membrane 15 using conductive contacts. Alternatively, a voltage signal V i is applied to the membrane 15 and the electrodes 11 are DC It can be biased with a voltage signal ±V i can be applied to the electrodes 11. i may be nominally an audio frequency between 20 and 20,000 Hertz. One of the electrodes 11 carries a non-inverted voltage signal +V i is applied to the other electrode 11, and the same inverted voltage signal -V i The dotted lines indicate the voltage signals ±V i Schematically shows a membrane 15 that moves in response to a voltage that varies with

[0016] As the distance d decreases, or the DC bias voltage +V DC and / or signal voltage ±V i (in absolute value), short circuits between the membrane 15 and the electrode 11 and / or 6 The probability of air breakdown is increased by the expected volts / meter. In accordance with a feature of the present invention, the operation of the electrostatic speaker can be controlled to prevent overdriving of the membrane 15.

[0017] 2, a control system 20 according to aspects of the present invention is shown. In the forward path, G(s) represents the open loop gain of the control circuitry including the system 21, and s is the gain of A(e iωt +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 generates an output voltage signal V o The feedback path output from the feedback block 22 may output a signal 27, which represents the transfer function H(s) of the output signal V o The input signal V approaches the set value. i may be subtracted by comparator 23 to generate error signal 25 which is input to controller block 21. The overall transfer function of system 20 is o Divide the voltage input V of the controller 21 i can be modeled by Equation 1.

number

[0018] Referring now to Figure 3, there is shown a schematic representation of a controller 21A that is an alternative to the system 21 of Figure 2, in accordance with an aspect of the present invention. The controller 21A receives a high voltage audio input +V at the first electrode 11.i and at the second electrode 11, an inverted high voltage audio input -V that varies at the intended audio frequency for conversion into sound by the electrostatic acoustic device 10. i In addition, the membrane 15 may be mechanically responsive such that the device 10 behaves as a capacitive microphone to unwanted ambient sound waves or noise.

[0019] Reference is now also made to Figure 6, which is a flow diagram 60 of a method illustrating features of the present invention. If the input audio signal is below a predetermined threshold (decision block 61), it is advantageous to have control circuitry 20 detect the time-varying displacement of membrane 15 (step 63) and feed back a control signal 26 to acoustic device 10 (step 65) to reduce the displacement of membrane 15 due to ambient noise. Thus, when electrostatic acoustic device 10 is used as an earphone and sealed in the ear canal, the mechanical displacement of the eardrum is coupled with the mechanical displacement of membrane 15, tending to actively cancel the ambient noise that would otherwise be perceived by the user.

[0020] In response to ambient noise, the distance d between the membrane 15 and the electrode 11 changes, resulting in a change in the capacitance C of the electrostatic acoustic device 10. The current i(t) that changes with the ambient noise is sensed using a 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] Amplifier 30 may be configured inverting or non-inverting and may have a bandpass of 600-900 Hz (-3 dB cutoff) centered outside the audio frequency band of 0.1-2 MHz, preferably far from the resonance of membrane 15. The voltage output of amplifier 30 may be summed into a signal combiner or multiplier 32.

[0023] Continuing with reference to FIG. 3, a probe signal from a local oscillator (LO) 51, for example at a radio frequency of 0.1 to 2 megahertz, may be coupled across a primary winding P of a transformer T. An audio signal +V i and the inverted audio signal -V i are supplied to the electrodes 11 through the series-connected secondary windings S1 and S2 of the transformer T. The audio signals ±V i can be a high voltage signal or an audio signal ±V i can be a low voltage signal up to about ±20 V, and a high DC voltage is applied to the membrane 15 as shown in device 10 (FIG. 1). The probe signal generates a current whose magnitude is determined by the characteristic reactance of the electrical circuit formed by the membrane 15 and electrode 11, essentially a variable capacitor. The advantage of using radio frequency is that it does not produce perceptible mechanical motion, but is modulated by the electrical change in capacitance associated with the mechanical motion produced when an audio signal is present. The probe signal from local oscillator (LO) 51 may also be combined with the voltage output of amplifier 30 in signal combiner / multiplier 32. Signal combiner / multiplier 32 produces a voltage output signal V that varies at audio frequencies. o The LO52 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 that is multiplied by the measurement signal output by amplifier 30 at the same frequency. The baseband or DC component of this multiplication contains a signal that is frequency converted from a narrow band around the LO52 frequency that is detected with a very high signal-to-noise ratio. Multiplier 32 may be implemented, for example, by an analog circuit AD835 manufactured by Analog Devices, Inc. (Norwood, Massachusetts, USA).

[0024] Referring again to FIG. 2, the voltage output signal V converted by the feedback block 22 o is shown. The voltage output signal V o In response to the input signal V i The comparator 23 may be configured to output a signal 27 that is subtracted from the input signal V i is nominally zero, signal 27 is added to become error signal 25. Alternatively, rather than comparator 23, signal combiner 23 may be used and feedback block 22 may combine the voltage output signal V o is suitably transformed, e.g., inverted, into a signal 27 which becomes the error signal 25.

[0025] Noise cancellation is performed by detecting the position of the membrane 15, V, which can be input as a signal 27 to the feedback control mechanisms 23, 24. o The second input is a control or setpoint signal, which may be based on the audio signal v played by device 10. i It may be.

[0026] The system 20 may, for example, detect a lock-in detection signal V related to the position of the membrane 15 output from the detection circuit 21A. o 1 may illustrate the closed-loop operation of the electrostatic loudspeaker 10 using

[0027] 2A, a prior art proportional, integral, derivative (PID) block 24 is shown. A feedback loop may include a proportional, integral, derivative (PID) block 24 in the forward path G(s), which may include proportional gain, derivative and / or integral in a linear combination with an error signal 25, and frequency filtering to output a control signal 26. A null audio signal v i In this regard, the system 20 may function as a noise canceling control system.

[0028] Feedback circuit 20 can be used to adjust the acoustic transparency of 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 membrane 15, which controls the acoustic transmission coefficient from the external space through the boundary defined by membrane 15 into the in-ear seal volume. Acoustic transparency can be controlled via electrostatic feedback actuation and position sensing with variable gain and gain adjustment in PID 24 shown in block 21A over the effective frequency bandwidth over which the feedback is actuated.

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

[0030] 3A, an alternative controller 21B for system 21 (FIG. 2) is shown in accordance with an aspect of the present invention. In controller 21B, audio voltage V i can be applied to the membrane 15. A primary P and an audio voltage V connected in parallel with the local oscillator 51 i A probe signal from a local oscillator 51 can also be induced in the membrane 15 using a transformer T with a secondary S connected in series between it and the membrane 15. A bias voltage VDC is applied symmetrically to the electrodes 11, and -V DC / 2 to the first electrode 11, and +V DC / 2 is applied to the second electrode 11. A differential amplifier 31 may be used with inputs each capacitively coupled to the electrodes 11. The voltage output of the differential amplifier 31 varies with the capacitance of the device 10. A probe signal from a local oscillator 51 may also be combined with the voltage output of the differential amplifier 31 in a signal combiner / multiplier 32. The signal combiner / multiplier 32 produces a voltage output signal V that varies at audio frequencies. oThe differential amplifier 31 may be implemented using a Texas Instruments / Burr-Brown® INA105. In accordance with a feature of the present invention, the high voltage audio signal V i is used, controller 21B has an advantage over controller 21A because one high voltage input amplifier is used instead of two.

[0031] 4, an alternative controller 21C (FIG. 2, system block 21) in accordance with an aspect of the present invention is shown schematically. Controller 21C receives input voltage signals ±V iIf the absolute value of the amplifier 40 is below a predetermined threshold, it can be used to minimize or cancel ambient noise. Amplifier 40 may be a charge amplifier or a transimpedance amplifier. Amplifier 40, like amplifier 30 in circuit 21A, can be inverting or non-inverting and configured with a 600-900 Hz bandpass (-3 dB cutoff) centered outside the audio frequency band, between 0.1 and 2 MHz, preferably far from the resonance of membrane 15. The voltage output of amplifier 40 may be input to a signal combiner or multiplier 42, which may be a component of a phase-locked loop (PLL) 49. Phase-locked loop 49 uses a local oscillator, i.e., voltage-controlled oscillator (VCO) 48, that is compared to the measurement signal output from amplifier 40. The measurement signal contains small phase / frequency variations compared to the output of VCO 48 that can be detected with high signal-to-noise ratios using a phase-sensitive detector / demodulator, i.e., mixer 42 and low-pass filter 44. The second input to the signal combiner or multiplier 42 is the output of a voltage-controlled oscillator (VCO) 48. The multiplier 42 may output to a narrowband loop filter 47, which 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 a phase detector. The PLL 49 is configured to stably lock 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 and injects 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 generates a voltage output signal V that is sensitive to the relative and constant phase difference between the two inputs to the mixer 42. o The control circuit 21C generates a voltage output signal V o can then be converted (block 22, FIG. 2) into an error signal 25 for active noise minimization / cancellation. Alternatively, as in system 21B, detection as shown in FIG. 4 can be performed by detecting a single audio voltage V applied to membrane 15.i The probe signal from the local oscillator 51 is also induced in the film 15, and the bias voltage V DC is applied symmetrically to the electrodes 11, and -V DC / 2 to the first electrode 11, and +V DC / 2 may be applied to the second electrode 11, and a differential amplifier may be used with its inputs capacitively coupled to each electrode 11. Referring now to FIG. 5, an alternative controller circuit 21D (FIG. 2, system block 21) in accordance with aspects of the present invention is shown schematically. A local oscillator (LO) 51 is configured to output a sine wave at a frequency between 0.1 and 2 megahertz, e.g., 1 megahertz, as an input to an amplifier 56. In 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. The audio input voltage signal V i If present, this may modulate the carrier at near radio frequencies, such as 1 megahertz. Similarly, noise signals due to internally generated ambient sounds in electrostatic acoustic device 10 may modulate the carrier frequency of LO 51.

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

[0033] The voltage output of amplifier 50 may include rectifier 53 and low pass filter 54, which may be converted (block 22, FIG. 2) into error signal 25 for active noise minimization / cancellation. o , which may be input to a detection block 52 that outputs: Discharge and overdrive protection

[0034] Controller circuits 20, 21A, 21B, 21C, and 21D may have further utility in protecting electrostatic acoustic devices from unintentional air breakdown or shorting between electrode 11 and membrane 15. Unintentional air breakdown or shorting can occur if electrostatic acoustic device 10 is overdriven and membrane 15 is displaced too close to electrode 11. In general, membrane 15 displacement is controlled by a bias voltage VDC, an input voltage signal V i may depend on several factors, including the magnitude and frequency of the voltage output signal V, and the physical parameters of the electrostatic acoustic device 10. o or that particular frequency component has an amplitude that exceeds a predetermined frequency-dependent threshold, the controller circuit 20, 21A, 21B, 21C, or 21D, particularly the feedback path block 22, controls the input voltage signal v i , and can be configured to protect against mechanical collapse of the membrane to the electrodes due to overdriving of the electrostatic acoustic device 10 or irreversible electrostatic attraction.

[0035] Referring now to FIG. 7, a flow diagram 70 of a method according to an aspect of the present invention for controlling the operation of an electrostatic acoustic device including a membrane 15 and an electrode 11 disposed near the membrane 15 is shown. The membrane 15 is configured to mechanically respond to a variable electric field emanating from the electrode 11 when a variable audio signal voltage is applied to the electrode 11. A probe signal varying at a radio frequency is injected into the electrode 11 (step 71). The current or charge signal is detected by converting the current or charge signal to a modulated voltage signal (step 73). The current or charge signal includes an audio signal varying at an audio frequency that modulates the radio frequency of the probe signal. The modulated voltage signal is demodulated to generate an audio output signal varying at an audio frequency (step 75). The audio output signal is converted to generate an error signal (step 77), and a control signal is input to the acoustic device 10 in response to the error signal (step 79).

[0036] As used herein, the term "homodyne" refers to a method of detection / demodulation of a signal that is phase and / or frequency modulated onto an oscillator signal by combining the signal with a reference oscillation.

[0037] As used herein, the term "phase sensitive detector circuit" is essentially an electronic circuit that includes a multiplier (or mixer) and a loop filter that produces 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" converts current to voltage. A transimpedance amplifier may be used to process the current output of a sensor into a voltage signal output.

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

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

[0041] As used herein, the terms "audio signal," "audio output," and "audio output signal" refer to electrical signals that vary primarily at audio frequencies.

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

[0043] The term "transform" or "transforming" refers to phase shifting, inversion, amplification, and / or attenuation.

[0044] As used herein, the term "error signal" refers to a voltage signal whose magnitude is proportional or monotonic to the difference between the actual output signal and the desired audio signal that varies at audio frequencies.

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

[0046] As used herein, the transitional term "comprising" is synonymous with "including" and is inclusive or open-ended, not excluding additional, unspecified elements or method steps. For example, the articles "a" and "an" used herein, such as "a circuit" or "an electrode," mean "one or more," as in "one or more circuits" and "one or more electrodes."

[0047] All optional and preferred features and modifications of the described embodiments and dependent claims can be used in all aspects of the invention taught herein, and 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.

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

Claims

1. A control circuit operable for an electrostatic transducer including a membrane, a first electrode, and a second electrode, wherein the first electrode is disposed parallel to the membrane, the membrane being configured to mechanically respond to a first electric field that varies according to an electric potential applied between the first electrode and the membrane, the second electrode is disposed parallel to the membrane on an opposite side from the first electrode, the membrane being configured to mechanically respond to a second electric field that varies according to an electric potential applied between the second electrode and the membrane, the first electrode and the second electrode having through holes configured for acoustic transmission to and from the membrane, the control circuit comprising: an audio signal input unit; a detector configured to detect a current or charge signal from the capacitive transducer in response to movement of the membrane, the current or charge signal comprising an audio signal varying at audio frequencies, the detector configured to generate an audio output signal varying at audio frequencies; a conversion circuit configured to convert the audio output signal to generate a feedback signal; a comparator configured to compare an input audio signal at the audio signal input with the feedback signal to generate an error signal; a controller configured to input a control signal to the capacitive transducer, the control signal being responsive to the error signal; Equipped with A control circuit configured to control the acoustic transparency of the electrostatic transducer, wherein the control signal is transmitted from an external space through the through-hole of the first electrode, through the membrane, and through the through-hole of the second electrode.

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

3. 2. The control circuit of claim 1, wherein a DC bias voltage is applied to the first electrode and the second electrode, and an audio voltage input responsive to the control signal is applied to the membrane.

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

5. 10. The control circuit of claim 1, wherein the first electrode comprises a first conductive layer deposited on an electrically insulating substrate, the first conductive layer assembled proximate to the membrane, and the second electrode comprises a second conductive layer deposited on an electrically insulating substrate, the second conductive layer assembled proximate to the membrane.

6. The control circuit of any one of claims 1 to 5, wherein the control signal is configured to at least partially cancel mechanical responses of the membrane due to ambient noise.

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

8. 8. The control circuit of claim 1, wherein a probe signal varying at a radio frequency is injected into the electrode and the current or charge signal is detected by converting the current or charge signal into a modulated voltage signal, the current or charge signal comprising an input audio signal that modulates the radio frequency of the probe signal.

9. 9. The control circuit of 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 of claim 8 further comprising:

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

12. an amplifier configured to output the probe signal in response to the synchronization signal; The control circuit of claim 11 further comprising:

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

14. a low pass filter configured to filter and thereby demodulate the modulated voltage signal to generate the audio output signal; The control circuit according to any one of claims 8 to 13, further comprising:

15. 1. A method executable to control an electrostatic transducer including an audio signal input, a membrane, a first electrode, and a second electrode, wherein the first electrode is disposed parallel to the membrane, the membrane being configured to mechanically respond to a first electric field that varies according to a potential applied between the first electrode and the membrane, the second electrode is disposed parallel to the membrane on an opposite side from the first electrode, the membrane being configured to mechanically respond to a second electric field that varies according to a potential applied between the second electrode and the membrane, the first electrode and the second electrode having through holes configured for acoustic transmission to and from the membrane, and the control circuitry comprising: detecting a current or charge signal from the capacitive transducer, the current or charge signal comprising an audio signal varying at audio frequencies, the detector configured to generate an audio output signal varying at audio frequencies; converting the audio output signal to generate a feedback signal; comparing an input audio signal at the audio signal input with the feedback signal to generate an error signal; inputting a control signal to the electrostatic transducer in response to the error signal, thereby controlling acoustic transparency of the electrostatic transducer from an external space through the through-hole of the first electrode, through the membrane, and through the through-hole of the second electrode; A method comprising:

16. controlling the acoustic transparency in accordance with a ratio between the control signal and an input audio signal at the audio signal input. The method of claim 15 further comprising:

17. applying a DC bias voltage to the first electrode and the second electrode and an audio voltage input to the membrane responsive to the control signal; The method of claim 15 further comprising:

18. applying a non-inverted audio voltage input to one of the first electrode and the second electrode in response to the control signal, applying an identical but inverted audio signal input to the other of the first electrode and the second electrode, and biasing the membrane with a DC bias voltage. The method of claim 15 further comprising:

19. Configuring the control signal to at least partially cancel mechanical responses of the membrane due to ambient noise. The method of any one of claims 15 to 18, further comprising:

20. configuring the control signal to limit mechanical displacement of the membrane; The method of any one of claims 15 to 18, further comprising:

21. injecting a radio frequency varying probe signal into an input of an electrostatic acoustic device; detecting the current or charge signal by converting the current or charge signal into a modulated voltage signal, the current or charge signal comprising an input audio signal varying at an audio frequency that modulates the radio frequency of the probe signal; demodulating the modulated voltage signal to generate the audio output signal; The method of any one of claims 15 to 20, further comprising:

22. Homodyne detection of a modulated voltage signal at radio frequencies to obtain an audio output signal that varies at audio frequencies 22. The method of claim 21 further comprising:

23. Locking the phase and frequency of a radio frequency carrier signal in response to a radio frequency modulated voltage signal and a radio frequency probe signal 22. The method of claim 21 further comprising:

24. configuring a local oscillator to generate a sine wave at a radio frequency; inputting a sine wave at radio frequency; outputting a probe signal having a frequency corresponding to the sine wave; 22. The method of claim 21 further comprising:

Images