Electrostatic electroacoustic transducer

[57] ABSTRACT

An electroacoustic transducer apparatus for electrostatically transducing audio frequencies from electrical signals to sound or from sound to electrical signals. One embodiment includes a headset having a pair of insulated electrodes positionable on opposite sides of and in contact with a wearer’s head near the auditory centers. A single-channel audio signal is coupled through an isolation transformer and a fixed or automatic d.c. bias circuit to the electrodes. Each electrode and the adjacent head tissue form plates of a capacitor and the insulation there between constitutes the dielectric. On application of an electric audio signal, the tissue is caused to vibrate and propagate pressure waves in accordance with the frequency and amplitude of the signal and thereby produce monaural sound reproduction. In another embodiment, two stereo channels of electric audio signals are similarly coupled to the headset electrodes with their common output coupled to a third uninsulated electrode in contact with the wearer’s body for producing threedimensional sound reproduction. In still another embodiment, an insulated electrode constitutes a sound pickup having its output connected through a d.c. bias circuit directly to the input of an audio signal responsive device such as a loud speaker system or recorder. When held against the wearer’s head near the vocal center, the electrode and the tissue adjacent thereto act as the plates of a capacitor and the insulation constitutes the dielectric. Vibration of the tissue due to sound propogation from the vocal center produces an electric signal in accordance with the frequency and amplitude of the vibration.

Claims, 8 Drawing Figures

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PATENTED SEP 5 1972

SHEET 1 OF 2

3,689,709

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INVENTORS

Max A. Salmansohn Daniel A. LaMonica BY

ATTORNEY

PATENTED SEP 51972

3,689,709

SHEET 2 OF 2

RELATIVE DB FORCE, DYNES VOLTS

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10000

INVENTORS

Max A. Salmansohn Daniel A. LaMonica

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ATTORNEY

3,689,709

ELECTROSTATIC ELECTROACOUSTIC TRANSDUCER

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

The present invention relates to electroacoustic transducers utilizing capacitatively coupled electrodes, and more particularly to novel improvements in such transducers as applied to headsets and pickups used in sound detection and reproduction.

Electrical signals are normally converted to acoustic waves in air by means of transducers held near to the ears or near other facial areas that are acoustically coupled through the bones of the head to the normal audi- 20 tory centers where the sensation of sound is produced. Conversely, acoustic waves are normally converted to electrical signals by means of transducers mechanically responsive to acoustic pressure waves propagated in air.

Such transducers are usually complex electrical and mechanical devices which involve relatively high production costs. At relatively low atmospheric pressure such as experience by pilots at high altitudes and at high ambient pressures such as experienced by scuba ³⁰ the apparatus of FIG. 1; divers in deep sea, the normal acoustic coupling via air efficiency is substantially degraded. Such devices are also susceptible to ambient noise. This last factor has often led to combining the transducer with bulky and discomforting structure for excluding ambient noise.

To avoid these disadvantages and extend the application of acoustic transducers to use in high altitude, deep water and high noise environments, electrostatic acoustic transducers have been more recently investigated as valid substitutes. In one configuration of the prior art, an audio amplitude modulated radio frequency signal is applied through insulated electrodes to the wearer’s head on or near the ears. This device, however, utilizes an r.f. carrier signal and drive circuitry for maintaining audibility of very weak input signals. Nevertheless, the output levels are severely limited by the demand to avoid any form of diathermy (r.f. heating) in the wearer’s head.

SUMMARY OF THE INVENTION pair of insulated electrodes positionable on opposite sides of and in contact with a wearer’s head near the auditory centers. A single-channel audio signal is coupled preferably through an isolation transformer and a fixed or automatic d.c. bias circuit to the electrodes for producing monaural sound reproduction. In another embodiment, two stereo channels of audio signals are similarly coupled to the headset electrodes with their common output coupled to a third uninsulated electrode in contact with the wearer’s body for producing three-dimensional sound reproduction. In a third embodiment, an electrostatic pickup adapted to be placed against the wearer’s skin near the vocal center has its output coupled directly to the input of an audio signal response device.

It is postulated that the present invention operates on the same principles as the well-known condenser microphone except that the tissue of the skin replaces the diaphragm as one of the plates of the condenser and is maintained in continuous direct acoustic coupling contract with an electrostatically charged plastic dielectric element.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of one embodiment of a transducer apparatus constructed according to the invention for monaural sound reproduction;

FIG. 2 is a cross-sectional view of one earphone of

FIG. 3 is a schematic diagram of a d.c. bias circuit as applied in the apparatus of FIG. 1;

FIG. 4 represents typical audio signals in terms of applied voltage and resulting force of prior art devices;

FIG. 5 represents a typical audio signal in terms of applied voltage and resulting force of the apparatus of FIG. 1;

FIG. 6 represents a typical frequency response curve for the apparatus of FIG. 1;

FIG. 8 is a schematic block diagram of another embodiment of a transducer apparatus constructed according to the invention for stereophonic sound reproduction; and

FIG. 9 is a cross-sectional view of a microphonic pickup in combination with a schematic diagram of another transducer apparatus constructed according to the invention for detecting acoustical signals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Accordingly, among the many purposes and objects of the invention, it is a general purpose to provide a novel and improved electroacoustic transducer apparatus which is not adversely affected by ambient pressures, which has virtually distortion-free signal reproduction at acoustic levels up to the pain threshold, and which is relatively simple in construction and inexpensive to manufacture.

It is another object of the invention to provide a new electroacoustic transducer apparatus which is particularly suitable for use by high performance aircraft pilots operating in severe noise environments and at high altitudes, and by scuba divers operating in the high pressure environment of deep water.

Briefly, these purposes and objects and others are accomplished in one embodiment by a headset having a

Referring now to FIG. 1 of the drawing, there is illustrated a monaural audio source 10 such as a radio receiver or tape recorder-player which produces an 55 audio signal characterized by a frequency and amplitude varying voltage across two output terminals. The signal is connected through an isolating transformer 11 to two inputs of a d.c. bias circuit 12. The transformer 11 essentially steps up the voltage for matching the source output impedance to the load impedance. This is desired to ensure the maximum signal level at the point of electrical coupling to the body. The d.c. bias 12, as shown in more detail in FIG. 3 includes ₆₅ input terminals respectively connected through seriesconnected resistor R1 and capacitor Cl, and resistor R2 and capacitor C2 to a regulated d.c. voltage. The resistors R1 and R2 act as current limiters in event of an

3,689,709 electrical short across the dielectric of the earphones described below, and the capacitors Cl and C2 as couplers and isolators. The respective junctions of the capacitors and resistors are connected through current limiting resistors R3 and R4 to two output terminals electrically connected to a headset indicated generally by the numeral 13.

The headset 13 includes a pair of earphones 14 which receive the signal from the respective output terminals of the bias circuit 12. As shown in more detail in FIG. 2, each earphone 14 comprises a cup-shaped rigid housing 16 formed with a peripheral flange 18 about its open end and filled with a sound absorbing material such as plastic foam. The earphone 14 also includes an annular cushion, generally indicated as 20, secured to the flange 18 for providing a comfortable, contiguous contact with the wearer’s head around each ear. The cushion 20 comprises an outer flexible dielectric cover 21 with a slightly elastic lip 21α formed to be stretched over the outer flange 18 for securing the cushion 20 firmly thereto. Within the cover 21 adjacent to the surface that contacts the wearer’s head, is contained a pliable electrode 23 connected through an electrical conductor 24 to a respective output terminal of the bias circuit 12. The cover 21 also forms a cavity therein between the electrode 23 and the flange 18 for receiving a soft resilient material 25. This enables the covering 21 and electrode 23 to deflect and conform to the shape of the head around the ears. It is contemplated that the cover 21, particularly where it contacts the wearer’s skin, be a polyvinyl plastic or similar material having a relatively high dielectric constant and within good design practice be as thin as possible in order to maintain optimum coupling for audio signal transmission efficiency . The electrode 23 may be of any electrically conductive material such as lead, silver or brass, taking the form of thin sheets, metalized fabrics, metal coatings, liquid conductors and the like.

The graphs of FIGS. 4 and 5 are included to visually illustrate the effect of applying a fixed d.c. bias to the audio signal. As shown in FIG. 4 in which the audio signal is unbiased, the force generated by the audio signal varies with the magnitude of voltage deviation from zero regardless of polarity. Accordingly, all negative voltages appear as positive forces indistinguishable from positive voltages. This results in distortion of the audio output signal. As shown in FIG. 5 where a d.c. bias voltage is added to the audio signal at least equal to the maximum anticipated negative voltage in the audio signal, all voltages become positive so that the variation in force becomes a replica of the voltage signal. In this manner, distortion is substantially reduced. The d.c. bias also has the same effect as using a relatively highpolarized dielectric material for the cover 21.

FIG. 6 is an actual frequency versus db threshold hearing curve for a system constructed according to the invention of FIG. 1. It is a measure of the hearing threshold voltage (in terms of decibels) of the system relative to the threshold voltage (in db) of the system operating at 1,000 Hz and serves to illustrated that it closely conforms to the normal hearing curve of the human ear and is therefore useful in both voice and music systems.

Limiting resistors R7 and R8 are also provided respectively from the cathodes of DI and D2 to the junction of capacitor C5 and R5 and junction of capacitor C6 and resistor R6.

Referring now to FIG. 8, there is illustrated another embodiment of the invention for producing three⁵ dimensional sound. A stereo audio source 27, such as a multiplexing radio receiver or recorder-player, has three output terminals; two of independent audio signals and a common ground. The audio signal outputs ^are connected to one terminal of respective primary windings of an isolating transformer 28, and the ground to the other terminals of each primary winding. One terminal of each secondary winding is connected to respective inputs of d.c. bias circuit 12 or 12' (FIG. 3 j5 ^Or 7); the other ends of the secondary windings are connected to ground. The output terminals of the bias circuit 12 are connected to respective electrodes 23 of earphones 14. A separate uninsulated electrode 29, adapted to be placed in direct contact with the wearer’s 20 body, is connected directly to ground.

The two systems of FIGS. 1 and 8 can now be compared. In FIG. 1, the earphones 14 and the adjacent tissue T at each ear constitute two series-connected capacitors in which the tissue at each ear vibrates in 25 unison to effect monaural sound reproduction. In FIG.

8, the earphones 14 and the adjacent tissues T constitute two parallel-connected capacitors and the tissues vibrate independently to effect stereophonic sound reproduction.

Referring to FIG. 9, there is shown another embodiment of the invention in which audio signals in a body are converted to electrical signals. A microphone indicated generally by the numeral 30 includes a cylindrical base 32 closed at one end but for an opening for ³⁵ receiving a coaxial cable 33. The open end of the base receives a nonconductive bushing 34 having embedded therein an electrode 36 with an exposed planar surface. The electrode 36 and base 32 are electrically con₄₀ nected respectively to the center conductor and shield conductor of cable 33. A dielectric 37, such as vinyl plastic, is retained against the exposed surface of the electrode 36, and a retaining nut 38 is threadingly engaged over the open end of the base 32 for securing the 45 bushing 34, electrode 36 and dielectric 37 to the base 32.

The cable 33 is electrically connected to a d.c. bias supply 40 with its input connected to the center conductor of the cable 33 and the shield to ground. A d.c. 50 voltage is applied between ground and a limiting resistor R9 to the input terminal. The bias circuit input also connects through a coupling capacitor C7 to an amplifier 41 the output of which comprises two terminals; one the audio output signal and the other a 55 ground reference. When the microphone 30 is placed against the tissue T of the body with the dielectric 37 and nut 38 in contiguous contact, as shown in FIG. 9, audio vibrations of the tissue directly induce electrical ₆θ signals in the charged dielectric and at the output of the d.c. bias circuit 40 useful for sound reproduction in a loud speaker or recording system.

Some of the many advantages of the present invention should now be readily apparent. The use of an 65 ^e^^ecI^rosI^aIi^c coupling for sound reproduction directly from the tissues of the skin affords a very simple and inexpensive audio apparatus. Ambient pressure variations in the surrounding atmosphere do not degrade

3,689,709 audio transmission efficiency, surrounding noise is excluded, and there are no requirements for radio frequency carrier signals requiring additional circuitry or introducing r.f. heating (diathermy) in the body.

It will be understood of course that various changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended ¹⁰claims.