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12.4 Interaural intensity differences

The brain has another process for localizing high-frequency sounds (above 1500 Hz): interaural intensity differences.


Where does processing of interaural intensity differences take place?


In the lateral superior olivary nucleus.

For any sound, there is a direct relationship between the direction that the sound comes from and the extent to which the intensity of the sound at the two ears differs. If the sound comes directly from the right, the sound will be lower in intensity in the left ear, if it comes from directly in front, the intensity at the two ears is the same and with sound coming from intermediate directions, there are intermediate intensity differences. Intensity differences between the ears can result from two factors: differences in the distance the sound must travel to the two ears and differences in the degree to which the head casts a sound shadow. The greater the sound shadow cast by the head, the greater the level difference between the ears. The extent of the sound shadow cast by the head depends on the frequency of the sound. Low-frequency sounds have a wavelength that is long compared to the size of the head. The sound therefore bends very well around the head and there is very little sound shadow cast. In contrast, high-frequency sounds have a wavelength that is short compared to the dimensions of the head. This means that the head casts a significant sound shadow.


What would be the consequences of the difference in sound shadow cast by the head in response to low-frequency and high-frequency sounds?


For low frequencies, the sound shadow cast by the head would cause a minimal difference in the intensity of sound at the two ears and so there would be little scope for using interaural intensity differences in order to localise the sound. For high frequencies, the sound shadow cast by the head would cause a significant difference in the intensity of the sound at the two ears and so facilitate the use of interaural intensity differences.

In fact, interaural differences in intensity are negligible at low frequencies, but may be as large as 20 dB at high frequencies (Figure 45).

Figure 45
Figure 45 Low-frequency tones are not affected by the listener's head, so the intensity of a 200 Hz tone is the same at both ears. High-frequency tones (e.g. 6000 Hz) are affected by the presence of the listener's head and result in an acoustic shadow that decreases the intensity of the tone reaching the listener's far ear

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