Electrostatic Transducer (1964)

U.S. Patent No. 3,118,979, granted on January 21, 1964, to Gerhard M. Sessler and James E. West, describes an advanced electrostatic transducer (such as a condenser earphone or microphone) designed to eliminate mechanical contact distortion while maintaining high acoustic sensitivity. Assigned to Bell Telephone Laboratories, Incorporated, this invention tackled a major obstacle in high-fidelity acoustic reproduction: physical diaphragm collapse under high signal levels.

Prior condenser earphones relied on an extremely thin air gap between a vibratile diaphragm and a rigid back plate to maximize efficiency. However, large signal excursions inevitably forced the diaphragm into direct contact with the back plate. This physical contact cut off normal vibration modes, sparked unwanted harmonic overtones, and introduced severe distortion. Sessler and West solved this by inserting intermediate dielectric films to construct a self-spacing, multilayer diaphragm structure.

The Innovation: The Multilayer Dielectric Cushion

Conventional designs tried to avoid contact by widening the air gap (which destroyed acoustic efficiency) or inserting spongy rubber layers (which required substantial thickness).

Sessler and West introduced a simple mechanical intervention: sandwiching one or more auxiliary sheets of nonmetallized, thin-film dielectric material between the active outer conductive diaphragm and the metallic back plate.

Why the Multilayer Design?

• Contact Cushioning: If a high-amplitude signal drives the outer conductive film inward, it encounters an intermediate dielectric layer rather than crashing directly into the solid metal back plate.
• Randomized Air Pockets: Microscopic surface irregularities naturally trap microscopic, irregularly shaped air gaps between adjacent layers. Along any given normal vector, the probability of simultaneous contact across all layers is virtually zero.
• Homogeneous Surface Vibration: When the outer layer touches an intermediate layer, the compliant dielectric yields and moves along with it. Rather than arresting vibration at a rigid point, the system preserves uniform, free movement across the entire diaphragm surface.
• Simplified Tolerances: Instead of requiring ultra-precise machining to maintain sub-mil clearances, standard manufacturing surface variations automatically create the required micro-cavities.

Technical Components

Component	Patent Reference	Function & Specification
Back Plate	10	Rigid conductive disk (e.g., 35 mm brass disc) featuring ~200 drilled wells (12) and low ridges (11) (~0.025 mm high) that define the acoustic back-cavity volume and set system resonance.
Auxiliary Dielectric Layer	14	Nonmetallized plastic film, such as 0.15 to 0.25 mil Mylar (polyethylene terephthalate), that divides the air gap into nonuniform micro-layers.
Conductive Diaphragm Foil	15	Thin flexible film (0.25 mil Mylar) coated on one side with vacuum-deposited aluminum, serving as the movable capacitor electrode.
Insulating & Metal Frames	13, 16	Rigid Lucite inner frame (13) and flanged metal outer frame (16) providing peripheral structural mounting.
Clamping Ring	17	Annular ring with a convex protrusion mating into a groove on frame 16 to secure and maintain uniform radial tension on all foil layers.
Spherical Alignment Mechanism	20, 21	Threaded member (20) and spherical bearing member (21) that permit fine axial advancement and self-aligning tilt adjustment of the back plate against the diaphragm.
Front Cap & Cavity	24, 25	Protective cap engineered with an acoustic port forming a Helmholtz resonator tuned to ~12 kHz, packed with porous paper damping material (25).

How the Apparatus Functions

The assembly operates through electrostatic force balance and controlled acoustic damping:

Step	Action	Acoustic & Mechanical Result
1. Polarization & Bias	A DC polarizing bias is applied across terminals 18 (foil) and 22 (back plate).	Sets up an electrostatic field across the composite dielectric and distributed micro-air gaps without risking short circuits.
2. Signal Excitation	Incoming alternating audio voltages modulate the net potential between the plates.	Varies the electrostatic attraction, driving the outer conductive foil (15) inward and outward in direct proportion to the audio signal.
3. High-Excursion Yielding	Under heavy audio peaks, the conductive layer deflects into the intermediate film (14).	The intermediate layer moves compliantly; because the total gap S(r) = S1 + S2 is distributed unevenly, rigid contact is avoided and harmonic distortion is suppressed.
4. Cavity Tuning	Emitted sound pressure waves interact with the acoustic cavity and damping pack (25).	Produces a flat acoustic pressure response up to the ~14 kHz mechanical resonance, smoothly broadened by the ~12 kHz Helmholtz front chamber.

Performance: Distortion & Frequency Response

The patent provides comparative laboratory measurements contrasting the multilayer condenser transducer against conventional condenser units and dynamic earphones:

• Harmonic Distortion: At 80 dB sound pressure level (SPL), the multifoil transducer exhibited substantially lower distortion across all frequencies compared to a conventional single-gap condenser unit. Even pushed to 100 dB SPL, total harmonic distortion remained below 1.0% across the operating audio band.
• Frequency Response: Produced an exceptionally flat sound pressure curve across the audible band, outperforming dynamic earphones that suffer from pronounced high-frequency peaks and roll-offs.
• Transient Fidelity: In square-wave impulse tests, the condenser earphone reproduced an almost true replica of the applied step pulse, whereas conventional dynamic units exhibited severe differentiation spikes, ringing, and resonant overshoot.
• Reciprocity: Operating reversibly under electroacoustic reciprocity, the assembly functions equally as a wide-band, low-distortion condenser microphone.

About the Inventors: James E. West & Gerhard M. Sessler

James Edward West and Gerhard M. Sessler formed one of the most productive research collaborations in acoustic history during their tenure at Bell Laboratories in Murray Hill, New Jersey.

• Pioneering Innovations: West and Sessler co-invented the foil electret microphone in 1962, followed by foundational structural patents like U.S. Patent No. 3,118,979. Their electret technology permanently eliminated the need for bulky external high-voltage polarizing power supplies.
• Global Impact: Foil electret and electrostatic transducer principles developed by West and Sessler serve as the foundation for modern acoustic sensors, powering over 90% of all manufactured microphones worldwide—found in smartphones, hearing aids, studio equipment, laptops, and aerospace communications.
• Honors & Legacy: James E. West holds over 250 U.S. and foreign patents, was inducted into the National Inventors Hall of Fame in 1999, received the National Medal of Technology and Innovation, and served as a professor at the Whiting School of Engineering at Johns Hopkins University.

Summary of Claims

The patent explicitly claims:

1. A compliant multilayer vibratile member comprising a conductive flexible outer layer, a perforated rigid metallic back plate, and an intimate sandwich of multiple noncompressible thin dielectric films trapping highly irregular configurations of minute static air layers.
2. An acoustic front cover configured with a large-diameter port forming a Helmholtz resonator tuned to approximately 12 kilocycles per second.
3. An electrostatic air-gap dividing structure where the sum of intermediate gaps S(r) maintains a substantially constant total separation normal to the plate while remaining irregular and discontinuous in parallel planar coordinates.
4. Specific material construction utilizing a thin aluminum coating deposited on plastic film, intermediate dielectric plastic sheets approximately 0.25 mil thick, and a back plate formed of a perforated copper-zinc alloy (brass).