Sound to Electricity

Part of Telephony

The physics and mechanisms by which sound pressure waves are converted into electrical signals in telephone transmitters.

Why This Matters

The conversion of sound to electricity is the fundamental enabling technology of all voice communication systems. Without a reliable way to convert the mechanical energy of acoustic waves into electrical signals, telephone, radio, recording, and broadcasting are all impossible. Understanding this conversion in depth — not just the fact that it happens but the physics of how it happens — lets you select the right transducer for each application, diagnose problems intelligently, and if necessary design or build new transducers from principles.

Every microphone technology involves a different physical coupling between acoustic waves and electrical circuits. The carbon microphone modulates resistance. The electromagnetic microphone generates voltage through induction. The piezoelectric microphone generates voltage through crystal deformation. The condenser microphone changes capacitance. Each approach has tradeoffs in sensitivity, frequency response, power requirements, and environmental robustness. Knowing the underlying physics of each lets you reason about these tradeoffs rather than memorize them.

Acoustic Wave Pressure

Sound is a longitudinal pressure wave propagating through air. At the source (a vibrating vocal cord or speaker cone), molecules are pushed outward, creating a zone of compression (slightly above atmospheric pressure). As the source reverses, it creates a zone of rarefaction (slightly below atmospheric pressure). These pressure zones radiate outward in all directions at the speed of sound (343 m/s in air at 20°C).

At a microphone diaphragm, the arriving pressure variations flex the diaphragm in and out. The amplitude of diaphragm motion is tiny: at 60 dB SPL (ordinary conversation at one meter), the pressure variation is about 0.02 Pascal (0.00002 atmospheric pressure). This moves a typical telephone diaphragm less than one micrometer peak-to-peak. The challenge of microphone design is coupling this microscopic motion to a transduction mechanism with enough sensitivity to produce a usable electrical signal.

Frequency content matters too. Voice spans approximately 100-8,000 Hz for full intelligibility, but telephone systems band-limit to 300-3,400 Hz because this range is sufficient for speech recognition and word identification. The microphone must respond faithfully to all frequencies in this band with minimal variation in sensitivity.

Variable Resistance Transduction (Carbon)

The carbon microphone (described in detail in the carbon microphone article) is the simplest acoustic-to-electrical transducer from a circuit perspective. The diaphragm bears against a chamber of carbon granules. Diaphragm motion compresses and relaxes the granules, changing their resistance. With an external battery and line circuit in series, the varying resistance produces varying current — a directly modulated signal.

The signal power is relatively large because the carbon acts as a modulator of an already-powered circuit rather than a generator. The output is the battery power modulated by the audio signal. This makes carbon microphones the highest-output acoustic transducers — they drive telephone lines directly without amplification.

The tradeoff is noise. Carbon granules do not move smoothly — they shift discontinuously, creating random resistance fluctuations (frying noise) that add a characteristic background noise to carbon microphone audio. This noise is low-level enough for telephone intelligibility but audibly higher than dynamic or condenser microphones.

Electromagnetic Induction (Dynamic)

The dynamic microphone (also called moving-coil microphone) attaches a coil of fine wire to the diaphragm, positioning the coil in the narrow gap of a strong permanent magnet. When sound moves the diaphragm, the coil moves with it through the constant magnetic field.

By Faraday’s Law, a conductor moving through a magnetic field develops an EMF (electromotive force, or voltage) proportional to the velocity of motion and the strength of the magnetic field. The coil moves at voice-frequency velocities — fractions of a millimeter per second — but in a strong field (0.1-1 Tesla in a good design), generates millivolts of output.

The output voltage of a dynamic microphone is proportional to the velocity of diaphragm motion, not its displacement. This velocity-sensitive characteristic gives dynamic microphones a naturally flat response to sound pressure over a wide frequency range, because pressure and velocity have a specific relationship determined by the acoustic impedance of the air.

Piezoelectric Effect

In certain crystals and ceramics, mechanical stress causes charge separation at the crystal faces. This is piezoelectricity (from Greek piezo-, to press). When a piezoelectric disk bends in response to sound pressure on one face, positive charges accumulate on one face and negative charges on the other — a voltage appears across the element.

The voltage output is proportional to the strain (deformation) of the crystal, not the rate of strain. This displacement-sensitive response gives piezoelectric microphones a rising frequency response at low frequencies (less displacement per unit pressure at low frequencies) that must be compensated by circuit or mechanical design.

Output voltages are high (100-500 mV at normal voice levels) but the source impedance is capacitive and very high — typical element capacitance of 5-50 nF means source impedances of tens of thousands of ohms at voice frequencies. The following amplifier circuit must have input impedance much higher than the source impedance to avoid signal loading.

Capacitance Change (Condenser)

The condenser (electrostatic) microphone uses a thin conductive diaphragm suspended near a fixed backplate, forming a capacitor. With a bias voltage applied (or in electret designs, a permanently charged dielectric), the charge on the capacitor is fixed. As the diaphragm moves in response to sound, the capacitance changes, and the voltage across the fixed charge changes in proportion.

Condenser microphones offer the most accurate flat frequency response of any microphone type, because the capacitance-change mechanism is purely geometric with no mass or spring resonances in the transduction path itself. The limitations are the requirement for a polarizing voltage (typically 48V phantom power in professional audio) or an electret layer, and high sensitivity to humidity, which changes the dielectric properties of the air gap.

For telephone construction, condenser microphones are the most demanding to build from scratch due to the precision required in the diaphragm-to-backplate spacing. However, salvaged electret condenser capsules (from discarded consumer electronics, headsets, or intercom systems) can be recovered and used with only simple impedance-matching circuitry.