To find the sound pressure that an arbitrary
source x(t) produces at the ear drum, all we need is the impulse response
h(t) from the source to the ear drum. This is called the Head-Related Impulse
Response (HRIR), and its Fourier transform H(f) is called the Head Related
Transfer Function (HRTF). The HRTF captures all of the physical cues to
source localization. Once you know the HRTF for the left ear and the right
ear, you can synthesize accurate binaural signals from a monaural source.*
The HRTF is a surprisingly complicated function of four variables: three
space coordinates and frequency. In spherical coordinates, for distances
greater than about one meter, the source is said to be in the far field,
and the HRTF falls off inversely with range. Most HRTF measurements are
made in the far field, which essentially reduces the HRTF to a function
of azimuth, elevation and frequency.
We made a series of HRIR measurements on an acoustic
manikin known as KEMAR, which stands for Knowles Electronics
Manikin for Auditory Research. To get an idea of how KEMAR's response varies
with azimuth and elevation, take a look at the following graphical representations
of the HRIR and the HRTF:
Back
to Systems 
On
to HRTF-Based Systems 
Up to 3-D Audio