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Wednesday, December 15, 2010

Deducing sound direction

Although sound is less directional than light, information about direction can be deduced from sound. The role of the pinna seems to be to route the sound towards the eardrum, but they also produce small echoes, which allow us to distinguish something that is high up above us from something that is below us. Horizontal information is given by comparing signals from the two ears. For low frequencies (up to about 1000 Hz) the time of arrival of each peak of the sound vibration contains this information. A sound wave coming from the right will reach our right ear about 1 ms before it reaches our left ear, and this 1 ms difference is detected by the auditory system. For higher frequencies, where there are too many wave crests per second for a 1 ms difference to be meaningful, it is the amplitude of the sound that matters. A source to our right will project a higher amplitude to the right ear than to the left ear, since the head attenuates sound (in other words, sound reduces in amplitude as it passes through the ead by being partially absorbed). This, too, provides directional information.

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