Sidetone Control

Part of Telephony

Managing the amount of the speaker’s own voice heard in the earpiece — balancing feedback for natural conversation.

Why This Matters

Sidetone is a double-edged characteristic of telephone design. Too much, and the speaker reflexively lowers their voice until the distant party can barely hear them. Too little, and the telephone feels “dead” — users speak uncomfortably loudly because they receive no acoustic feedback that their voice is being transmitted. The correct amount of sidetone makes conversation feel natural and effortless, which is the entire goal of telephone communication.

Sidetone control is therefore not merely a technical parameter but a user experience design decision that affects how well people can actually use the telephone. It is one of the reasons that telephone instruments from different manufacturers sounded and felt distinctly different, even though they ostensibly connected to the same network. A telephone that produces the right sidetone level feels “right” in a way that users notice immediately even if they cannot articulate why.

The anti-sidetone network (a separate topic) handles the circuit design of sidetone reduction. This article addresses the broader question of how to set, adjust, and verify sidetone levels for a complete telephone instrument.

What Level of Sidetone Is Correct

Bell System research in the mid-twentieth century established that the optimal sidetone level places the speaker’s own voice in the earpiece approximately 18-22 dB below the level at which it would be heard if speaking without the telephone in a quiet room (that is, 18-22 dB below the direct bone and air conduction path the speaker has to their own voice).

This corresponds roughly to what you hear if you cup your hand over your ear while speaking — a faint reminder of your own voice, not the dominant sound. The phone doesn’t try to suppress your voice entirely (that creates an unnerving sensation of speaking into a void) or amplify it (which causes the voice-lowering reflex).

In practical terms, with a standard telephone handset held normally to the ear and speaking at a conversational voice level 1 cm from the mouthpiece, the sidetone should be audible but noticeably quieter than the received voice of the far party during conversation.

Setting Sidetone in Circuit Design

In a two-wire telephone circuit (no hybrid transformer), sidetone is completely uncontrolled — all the transmitter signal appears in the receiver. The only way to reduce it is through the anti-sidetone bridge or hybrid transformer network.

The degree of sidetone cancellation achieved by the bridge depends on how well the balance network matches the line impedance. Perfect balance cancels sidetone completely. Practical bridges achieve 20-35 dB of cancellation across the voice band. At 25 dB cancellation, the sidetone level in the earpiece is 25 dB below the transmitter output level — typically placing sidetone right in the correct perceptual range.

For a telephone instrument where the sidetone is too loud (bridge balance is poor or absent), add a series resistor in the earphone circuit to reduce received level. This reduces both sidetone and received voice proportionally — it is a blunt instrument but effective as a last resort.

For a telephone where sidetone is completely absent (well-balanced bridge, or modern active cancellation), add a small controlled sidetone path by intentionally unbalancing the bridge slightly, or by adding a small coupling capacitor from the transmitter circuit to the receiver circuit, calibrated to produce the correct sidetone level.

Adjusting for Different Line Lengths

A telephone instrument adjusted for correct sidetone on a 2 km loop will have different sidetone on a 10 km loop. The longer line has different impedance, and the bridge that balanced well at 2 km is no longer balanced at 10 km. Sidetone increases on longer loops in a typical design because the line impedance shifts away from the balance network impedance.

Network operators historically specified balance networks for “average line conditions” — a compromise that worked acceptably for lines within a range of lengths. Subscribers far from the exchange on long loops typically experienced more sidetone than subscribers close to the exchange.

For a community telephone network with a known range of loop lengths (say, all subscribers within 5 km), design the balance network for the midpoint of the range (2.5 km average) and accept the variation at the extremes.

Measurement and Verification

Measuring sidetone level requires an audio signal generator, an electrical reference, and a sound level meter or a calibrated microphone. The standard test method:

1. Connect the telephone under test to the exchange through a line simulator (a resistive network that mimics the actual loop impedance).
2. Inject a 1,000 Hz test tone of known level into the transmitter circuit (representing the signal a real carbon microphone would produce at a given SPL).
3. Measure the voltage at the receiver terminals due to the sidetone path.
4. The sidetone coupling ratio (in dB) = 20 × log₁₀(sidetone voltage / transmitter signal voltage).
5. Target: sidetone coupling ratio of -18 to -25 dB for typical telephone use.

Without calibrated equipment, conduct subjective testing: have a representative user make calls over the test telephone and ask them to compare the natural feel of the sidetone. Compare against a reference telephone known to have correct sidetone behavior. User-panel testing of telephone instruments for sidetone acceptability has a long history in telephony research and remains the most practical validation method for small-scale networks.