Here is an image representation of KEMAR's experimentally measured head-related
impulse response. The picture shows the response of the right ear to an
impulsive source in the horizontal plane. The strength of the response is
represented by brightness. Thus, we see at once that the sound is strongest
and arrives soonest when it is coming from the right side (azimuth = 90°).
Similarly, it is weakest and arrives latest when it is coming from the left
side (azimuth = 270°). Note that the arrival time varies with azimuth
in a more or less sinusoidal fashion. In fact, the arrival time conforms
quite well to the ITD equation.
In particular, notice that the difference between the shortest and the longest
arrival times is about 0.7 ms, just as the theory predicts.
It is also possible to explain some of the features seen in this image by
thinking about the physics involved. For example, the initial sequence of
rapid changes (bright and dark bands) is due to pinna reflections. The peak
that arrives about 0.4 ms after the initial peak is due to a shoulder reflection.
Finaly, note that the response when the source is in front is quite similar
to the response when the source is in back. The differences that do exist
show up as a lack of perfect symmetry about a horizontal line at 90°,
e.g., in the dark "trough" following the initial pulse that is
prominent in front but not in back. People also have trouble distinguishing
front from back, and often resolve this problem by head motion.
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