= Electroacoustic transducer
An \b[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 $B\ell=2\,\mathrm{N/A}$ 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.<Variability in the characterization of the headphone transfer-function by Abhijit Kulkarni and H. Steven Colburn>[Repeated-position measurements and ear-coupling model.]{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.
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