--- In "Mike Rooke" <> wrote:
>
> > Now someone tell me how you can hear the direction of a 200Hz sound,
> > with wavelength 1.7met, with your ears at 170mm apart, and you will
> > know how our own ears hear stereo. :-)
>
> ILD
Mike, at such a low frequency of 200 Hz interaural level difference (ILD), will
not contribute much to space perception. Low frequencies (i.e. a wave length
that is long compared to the size of the head) simply bend around the head
(diffraction), and no 'sound shadow' is cast. At high frequencies though a
sound shadow is cast by the head, which is detected and used for space
perception. Roughly, ILD is negligible below 500 Hz but it may be up to 20 dB
for high frequencies.
At lower frequencies, the interaural time differences (ITD) play a prominent
role in space perception. The ITD for a sound coming from one side (90 deg
azimuth) is about 690 microseconds, which corresponds to 13.8% of a complete
wave cycle at 200 Hz. This phase difference is detected and used to determine
the angle of the sound. For high frequencies, say 10 kHz, there are many cycles
of phase difference between the ears so the phase difference cannot be
unambiguously related to a time difference, and ITD is not used for space
perception (at least not in the case of pure tones). This ambiguity starts at
around 750 Hz for pure tones.
Thus, the 'duplex theory' of sound localization states that ITD is used for low
frequencies and IID is used for high frequencies. There is much more to space
perception than this, but, fundamentally, ITD and IID together play an
important role.
Best, Gabriel
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