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An electroacoustic transducer converts between an electrical signal and acoustic motion. A loudspeaker or a pair of headphones converts electrical input into sound; a microphone converts acoustic input into an electrical output. The name describes the conversion role, not one universal physical mechanism.

In an idealized moving-coil device, a current-carrying conductor in a magnetic field experiences a force. If the active conductor length is $\ell$ and the relevant field strength is $B$, the perpendicular-component model gives $F=B\ell I$. With $B\ell =2N/A$ and current $I=0.020\mathrm{A}$, the force is $0.040\mathrm{N}$. A diaphragm and its mechanical load turn that force into motion and sound pressure; force alone does not determine the complete acoustic output.

Conversion therefore has several linked stages: electrical drive, mechanical motion and acoustic coupling. A pressure transfer function of the complete device can depend on frequency and its surroundings. Headphone measurements on an acoustic mannequin, for example, varied when the cushions were repositioned, despite using the same nominal transducer. This demonstrates a change in the coupled system rather than requiring a change in the electrical recording itself.^[ref]

The distinction is useful in calibration. A quoted electrical sensitivity, mechanical displacement or acoustic pressure is meaningful only with its input, measurement point and loading conditions specified. An electroacoustic transducer transfers energy and information through a physical interface; its useful behavior belongs to the device together with that interface.

Headphones are electroacoustic transducers worn at or near the ears that convert an electrical signal into local acoustic pressure variations. A two-channel pair can deliver different signals to the two ears. The relevant output is the sound pressure reaching the ear, which depends on the transducer, its placement and the surrounding acoustic geometry, not just the electrical input.

For a linear small-signal description, let $V(f)$ be the input electric potential difference and $p(f)$ the complex pressure amplitude at a specified ear location. A headphone transfer function $H(f)=p(f)/V(f)$ describes both amplitude and phase. Two headphones can receive the same $V(f)$ yet produce different $p(f)$; even the same pair can produce different responses after being repositioned.^[ref]

Abhijit Kulkarni and H. Steven Colburn measured responses on a KEMAR acoustic mannequin with repeated placements of the headphone cushions. Placement-dependent variation complicated compensation by a fixed inverse filter. This experiment shows why a measurement needs a defined coupling arrangement: moving the device can change the acoustic system being calibrated.^[ref]

For an illustrative amplitude example, if pressure doubles at a particular frequency while the reference pressure is unchanged, sound pressure level increases by $20{\log }_{10}2\approx 6.02$ decibels. That is a statement about pressure at the measurement point; it is not automatically a doubling of perceived loudness or a uniform change across frequencies. Headphone reproduction is therefore an electrical-to-mechanical-to-acoustic chain that must be assessed at its intended output location.