Diaphragm Design
Part of Telephony
Engineering the thin flexible disk that converts sound to mechanical motion (and back) in telephone handsets.
Why This Matters
The diaphragm is the mechanical heart of both microphone and earphone. In the microphone, it converts sound pressure into mechanical motion that drives a transduction element. In the earphone, it converts mechanical motion from an electromagnetic driver into sound pressure waves. Every telephone conversation passes through two diaphragms — one at each end — and the acoustic quality of the communication is fundamentally limited by the quality of these diaphragms.
A poorly designed diaphragm resonates at specific frequencies, creating peaks and valleys in the frequency response. It may flex non-uniformly, causing distortion. It may be too stiff to respond to weak sounds or too compliant to accurately track strong ones. Conversely, a well-designed diaphragm is essentially transparent — it transmits the acoustic signal faithfully across the voice frequency range.
Diaphragm design is a sub-discipline within acoustical engineering, but the core principles are accessible and directly applicable to telephone instrument construction and repair.
Physical Requirements
A telephone diaphragm must satisfy several competing requirements simultaneously:
Frequency response: The diaphragm must move uniformly in response to all frequencies from 300 to 3,400 Hz — the telephone audio bandwidth. Its fundamental resonant frequency should be at or below 300 Hz so that the entire voice band is above resonance, where the diaphragm moves as a controlled piston rather than in its resonant mode. A resonance within the voice band creates a peak in response at that frequency and a rolloff above it.
Mass and compliance: Low mass and appropriate compliance (flexibility) push the resonant frequency lower. The resonant frequency of a circular diaphragm is proportional to the square root of stiffness divided by mass. For a given material, increasing diameter reduces stiffness faster than it increases mass, lowering resonant frequency. Typical telephone diaphragms are 30-60 mm in diameter.
Damping: Undamped diaphragms ring at their resonant frequency long after the driving sound has stopped, smearing transients. Adequate damping requires either material damping (rubber, composite materials) or controlled acoustic loading — the air cavity behind the diaphragm provides both a restoring force and damping.
Material Properties
Different materials suit different applications within telephone acoustics:
Mica: Natural mica sheets split to 0.1-0.2 mm thickness have an exceptionally high ratio of stiffness to density, producing a high resonant frequency for a given diameter — which means mica diaphragms must be larger to bring resonance into the correct range. Mica has inherent internal damping from its layered crystal structure. It is dimensionally stable and immune to humidity.
Aluminum alloy: 0.05-0.1 mm thick aluminum diaphragms are lightweight and easily formed. Resonant frequency is lower than mica for the same diameter and thickness, making smaller diaphragms feasible. Aluminum has low intrinsic damping and must be damped by a rim gasket or acoustic loading. Modern speaker cones use treated aluminum for similar reasons.
Stainless steel: Thin cold-rolled stainless (0.05-0.08 mm) is durable, non-corroding, and has consistent properties. Slightly higher density than aluminum requires careful thickness control. Used in high-durability telephone applications.
Mylar/polyester film: 0.025-0.05 mm polyester film is extremely lightweight and uniform. Higher internal damping than metals. Susceptible to humidity changes in thickness and compliance, but for indoor telephone instruments this is manageable. Easiest material to obtain and work with for improvised construction.
Mounting and Boundary Conditions
The boundary condition at the diaphragm rim critically affects its behavior. Clamped edges (rigidly fixed around the entire circumference) produce a higher fundamental frequency and more controlled piston-like behavior than a simply-supported edge (free to rotate but not translate).
Classic telephone transmitter diaphragms are clamped between two rings, with a thin gasket material (cork, felt, or rubber) preventing metal-to-metal contact that would over-constrain the rim and create ringing. The gasket also provides acoustic sealing around the rim, preventing sound from bypassing the diaphragm face and entering the rear cavity directly.
The gap between the diaphragm and the rear electrode (in the case of a carbon microphone) or the magnet assembly (for electromagnetic types) determines how much the diaphragm can travel and the compliance of the air cushion behind it. A gap of 0.5-1.5 mm for carbon transmitters and 1-3 mm for electromagnetic receivers represents typical values.
Testing Acoustic Response
Without specialized measurement equipment, diaphragm quality can be assessed by ear using comparison testing. Build two identical microphone capsules with different diaphragm variations (material, thickness, or diameter). Speak identical sentences into each at the same distance with the same DC bias current. Connect each to the same earphone and compare the audio quality. Prefer the capsule that reproduces your voice with more clarity and less boxiness (resonance coloration).
A more quantitative approach uses a reference speaker generating a swept tone at a fixed level (measure SPL with a calibrated meter or smartphone app) while measuring the output voltage of each microphone at each frequency. Plot output versus frequency to visualize the response curve. A flat response from 300-3,400 Hz is ideal; peaks greater than 6 dB indicate resonances worth damping.
The human voice fundamental for males is approximately 85-180 Hz; for females 165-255 Hz. The telephone audio band begins at 300 Hz and includes the first several harmonics of the voice. A diaphragm with a resonance at 800 Hz will make the caller’s voice sound “honky” or nasal to the listener at the far end — the resonance emphasizes mid-voice frequencies unnaturally. Identify and suppress such resonances before finalizing a diaphragm design.