= Headphones
\b[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.<Variability in the characterization of the headphone transfer-function by Abhijit Kulkarni and H. Steven Colburn>[Section I: transfer-function description from headphone input to a point in the ear canal.]{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.<Variability in the characterization of the headphone transfer-function by Abhijit Kulkarni and H. Steven Colburn>[Abstract and introduction: tested supra-aural headphones, repeated placement and compensation problem.]{ref}
For an illustrative amplitude example, if pressure doubles at a particular frequency while the reference pressure is unchanged, <sound pressure level> increases by $20\log_{10}2\approx6.02$ 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.
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