Page 91 - Master Handbook of Acoustics

Binaural Localization

Two ears function together in binaural hearing to allow localization of sound sources in the horizontal
plane. Signals from both ears are combined in the brain; thus localization largely takes place in the
brain, and not in the individual ears. Two factors are involved, the difference in intensity and the
difference in time of arrival (phase) of the sound falling on the two ears. In Fig. 4-17, the ear nearest
the source receives a greater intensity than the far ear because it is closer, and because the head casts
an acoustical shadow. Because of diffraction, acoustical shadows are much weaker at lower
frequencies. However, at higher frequencies, the acoustical shadow, combined with the path-length
difference, results in a higher intensity at the nearest ear.

FIGURE 4-17 Our binaural directional sense depends in part on the difference in intensity and time
of arrival of the sound falling on two ears.

Because of the difference in distance from the source, the near ear receives sound somewhat
earlier than the far ear. Below 1 kHz, the phase (time) effect dominates while above 1 kHz the
intensity effect dominates. There is one localization blind spot. A listener cannot tell whether sounds
are coming from directly in front or from directly behind because the intensity of sound arriving at

each ear is the same and in the same phase. Using these cues, the ear can localize sound sources in the
horizontal plane to within 1° or 2°.

Law of the First Wavefront

The sound that arrives first creates in the listener the perception of direction; this is sometimes called
the law of the first wavefront. Imagine two people in a small room, one person speaking and the other
listening. The first sound to reach the listener is that traveling a direct path because it travels the
shortest distance. This direct sound establishes the perception of the direction from which the sound
came. Even though it is immediately followed by a multitude of reflections from the various surfaces
of the room, this directional perception persists and tends to diminish the effects of later reflections