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perception of direction of sounds

susan moreland smorelan at MAGNUS.ACS.OHIO-STATE.EDU
Thu Aug 31 10:20:55 EST 1995

>Hi, I am a physicist with no knowledge of audiology but..
>some understanding of acoustics. I am involved in a discussion..
>about the perception ( by humans ) of the direction of..
>incoming sounds. I feel certain that some group of people..
>(maybe audiologists) know all about this, so come on folks..
>save me the trouble of thumbing through innumerable irrelevant..
>textbooks and put me straight. Any help would be appreciated.

well, here's my best, highly abbreviated, answer re: localization.  i'm sure
that others will either correct me if i'm wrong or alaborate on my answer.

localization is based on interaural time differences (ITD) and interaural
intensity differences (IID)

ITD- due to the head shadow effect the head blocks the arrival of sound to
the ear on the opposite side of the head from where the sound source is
located, so sound arrives sooner to the nearer ear. This results in a time
of arrival shift and phase shift of the signal from the near side vs. the
far side. At low freqs, this provides clear cues as to the location of the
sound source, however at high freqs the delay becomes more than one cycle
because the cycles are so short.  When the signal arrives from the far ear
we have no way of knowing how many cycles of the signal from the near ear
have already passed.  Therefore, ITD's are most useful for low freq info 

IID - again due to head shadow, low freq sounds have long enough wave
lengths that they can bend around the head to fill in the space on the far
side of the head so no shadowing occurs.  High freq sounds have wavelengths
that are too short to bend around the head, and so there is a shadowing
effect which can result in as much as a 20 dB attenuation of the sound.
So, IID's are mlst useful for high freq. info.

For complex sounds either time or intensity can be used for localization as
long as there is no ambiguity.

highly simplified, but that's the general concept as i was taught it.


we stroll together silently,
just the two of us -- my wolf and me...

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