Sound Localization

Many animals use sound cues to determine the location of significant sound sources, such as prey, hazards, predators, and mates. This is called sound localization.

Many vertebrates and all mammals can localize sounds by analyzing several acoustic features of signals received at each ear. If the sound is on the midline of the head, which means directly in front of, behind, above, or below the center of head, the sound cues received at each ear are identical. If the sound source is not on the midline, some component of these cues will be different in each ear. The auditory centers in the brain can compare these cue differences to determine distance and direction of the source. In particular, they may analyze the interaural difference in the times of arrival, received intensity, and phase.

Time of arrival

If a sound source is located towards one side of the head, it is heard at the ear on that side (ipsilateral) before being heard by the opposite (contralateral) ear. As the angle of the source relative to the midline increases, the times-of-arrival difference of the sound between the two ears increases from microseconds to milliseconds. [include link to section on terrestrial ears] The time differences that the brain can process to determine sound localization may be as small as milliseconds or even microseconds.

Intensity

The sound will also be less intense at the contralateral ear, in part as a result of “head shadow.” Head shadow means that some frequencies of the sound will be blocked or filtered by the head and will thus be attenuated, or less intense. As frequencies increase, their wavelengths decrease. Shorter wavelengths are more readily blocked or impeded by objects than longer wavelengths. Therefore, the higher the frequency, the greater the head shadowing effect.