Electricity to Sound

Part of Telephony

How varying electrical current in a telephone receiver is converted back into audible sound at the listener’s ear.

Why This Matters

The telephone receiver completes the communication chain. After your voice has been converted to varying electrical current by the microphone, transmitted through kilometers of wire, and arrived at the distant telephone, something must convert that electrical signal back into sound pressure waves that vibrate the listener’s eardrum. This is the receiver’s function, and the physics governing it are among the most important in applied electromagnetics.

Every audio reproduction device — earphone, loudspeaker, magnetic tape playback, hard drive head — operates on the same fundamental principle: varying electrical current creates a varying magnetic field, which exerts a varying mechanical force on a movable element, which creates varying pressure in air. Understanding this conversion clearly lets you design better telephone receivers, diagnose earphone failures, and appreciate the elegant simplicity of electromagnetic transduction.

For practical telephone construction, the receiver is typically harder to fabricate from scratch than the microphone because it requires a precisely gapped magnet and a tightly wound coil. However, the principles are teachable and the construction achievable with basic metalworking and winding skills.

Electromagnetic Force Fundamentals

A magnetic field exerts force on iron and other ferromagnetic materials. The force is attractive and proportional to the square of the magnetic flux density, divided by twice the magnetic permeability. This force increases sharply as the gap between magnet and iron decreases — it follows an inverse square law with gap distance.

In a telephone receiver, a permanent magnet provides a steady bias field. The coil wound around the magnet core carries the voice-frequency current from the telephone line. This AC current adds and subtracts from the permanent field alternately, creating a field that varies at audio frequencies. The varying field exerts varying force on the iron diaphragm suspended in front of the magnet pole face.

The permanent magnet bias is crucial. Without it, the diaphragm would be attracted twice per cycle of the audio signal (force is always attractive regardless of current direction) — the output sound would be at twice the input frequency, creating gross distortion. The bias polarizes the transducer: with bias, positive line current increases the total field and attracts the diaphragm strongly; negative line current reduces the total field and the diaphragm retreats, pulled back by its own stiffness. The result is single-frequency output matching the input.

Receiver Construction

The classic telephone receiver consists of a U-shaped (horseshoe) or bar-shaped permanent magnet, a coil wound on the magnet poles, and an iron diaphragm clamped across the open ends of the magnet’s field.

Magnet selection: Alnico or ceramic magnets are ideal for modern construction. For improvised construction, case-hardened steel magnetized by stroking with a known magnet provides adequate field strength. The magnet must maintain its magnetization despite the alternating fields from the coil — high coercivity (resistance to demagnetization) is essential.

Coil winding: For a telephone receiver, the coil typically has 3,000-8,000 turns of 40-44 AWG (0.08-0.16 mm) enameled copper wire. Finer wire allows more turns in the same space (higher impedance, better current sensitivity) but breaks more easily during winding. Wind evenly, layer by layer, using a winding fixture. The finished coil should measure 500-2,000 ohms DC resistance for typical telephone-grade receivers.

Diaphragm placement: The iron diaphragm sits 0.5-1.5 mm from the pole faces. The gap distance determines sensitivity and frequency response. Smaller gap increases sensitivity (the force law intensifies as gap decreases) but reduces the available travel before the diaphragm hits the poles. The diaphragm must not touch the poles at any signal level.

Acoustic Loading and Ear Coupling

A telephone receiver by itself, held at arm’s length, sounds thin and faint. The same receiver pressed against the ear with the earphone cup forming a sealed cavity produces much louder and fuller sound. The coupling cavity between the receiver diaphragm and the eardrum is acoustically critical.

When the receiver is held against the ear, the trapped air volume couples the diaphragm motion directly to the eardrum. No energy is wasted radiating sound into the room. The small volume concentrates the pressure variations — the same diaphragm movement creates larger pressure swings in a small cavity than in open air.

The earphone cup geometry and volume affect the frequency response of the received sound. A larger cavity favors lower frequencies (lower resonant frequency of the cavity-diaphragm system). A smaller cavity favors higher frequencies. Standard telephone earpiece volumes of 1-3 cc are optimized for voice intelligibility rather than music reproduction.

Matching Receiver to Circuit

The electrical impedance of the receiver must match the source impedance of the telephone circuit to maximize power transfer. Early receivers were high-impedance devices (600-2,000 ohms) because telephone lines themselves have significant impedance and high-impedance receivers draw less current from the line.

Mismatch reduces volume. A 600-ohm receiver driven from a 50-ohm circuit loses signal. An impedance-matching transformer solves this: it transforms the 50-ohm source to 600 ohms at the receiver terminals, or vice versa, presenting each device with its optimum load.

For receivers built to match the specific telephone circuit, calculate the required coil impedance from the circuit source impedance and wind accordingly. The coil impedance at 1,000 Hz (roughly mid-voice frequency) should equal the circuit source impedance for maximum power transfer to the acoustic output.

Testing a receiver is straightforward: connect it to a battery through a 1,000-ohm resistor and tap the resistor with a fingernail to inject a pulse. The receiver should click audibly. For audio testing, connect to a low-power audio oscillator (or a simple Wien bridge oscillator) and verify that the receiver produces audible sound at voice frequencies with no harsh buzzing or mechanical scraping from the diaphragm.