The sound wave that reaches the eardrum is quite different than the sound wave traveling through the air towards the ear. The incident sound is modified by reflections from the head, pinna, and within the ear canal. These effects are mathematically represented by the head-related transfer function (HRTF). An impulsive sound will reach the eardrum spread out over time, and this function of time is the HRTF. An example is shown here [44.6 kb]. The frequency-domain representation of the same HRTF is here [47.5]. It is different for sounds arriving from different angles. The magnitude dependence on angle is shown here [35.7 kb]. This is the HRTF for the right ear, located at 0-degrees elevation and 90-degrees azimuth. An angle of 0-degrees elevation and -90-degrees azimuth represents a source on the opposite (left ear) side of the head. 90-degrees elevation is directly overhead. The HRTF is a little different for every person, but one can expect the general characteristics to be similar. All examples shown here are drawn from the diffuse-field equalized data posted on the web by Bill Gardner and Keith Martin (sorry to say this link seems to not exist anymore).

This definition is in context to Audio Technology. See more contextual defintions for Head-related Transfer Function.