Page 70 - Master Handbook of Acoustics

differentiation of sound from the front as compared to sound from the rear as well as allowing
differentiation of sound from all around the listener. Both ears working together provide additional
spatial cues. At least in a free field, it is easy to close your eyes, and accurately point to sound
sources.

A Demonstration of Directional Cues

A simple psychoacoustics demonstration can show how changes in sounds falling on the ear can yield
subjective directional impressions. Listen with a headphone on one ear to an octave bandwidth of
random noise with an adjustable notch filter in the signal path. Adjusting the filter to attenuate at 7.2
kHz will cause the noise to seem to come from a source on the level of the observer. With the notch
adjusted to 8 kHz, the sound seems to come from above. With the notch at 6.3 kHz, the sound seems to
come from below. This experiment demonstrates that the human hearing system extracts directional

information from the shape of the sound spectra at the eardrum.

The Outer Ear—Auditory Canal

The auditory canal, commonly called the ear canal, also increases the loudness of the sounds
traversing it. In Fig. 4-2, the ear canal, with an average diameter of about 0.7 cm and length of about
2.5 cm, is idealized by straightening and giving it a uniform diameter throughout its length.
Acoustically, this is a reasonable approximation; the ear canal acts like a pipelike duct, open at one

end and closed at the inner end by the eardrum.