> 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 wav=
e 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 s=
pace perception. Roughly, ILD is negligible below 500 Hz but it may be up t=
o 20 dB for high frequencies.
>
> At lower frequencies, the interaural time differences (ITD) play a promin=
ent 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 com=
plete wave cycle at 200 Hz. This phase difference is detected and used to d=
etermine the angle of the sound. For high frequencies, say 10 kHz, there ar=
e 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 f=
or space perception (at least not in the case of pure tones). This ambiguit=
y starts at around 750 Hz for pure tones.
>
> Thus, the 'duplex theory' of sound localization states that ITD is used f=
or low frequencies and IID is used for high frequencies. There is much more=
to space perception than this, but, fundamentally, ITD and IID together pl=
ay an important role.
Gabriel, thanks for bringing us back to science. The Wikipedia article (whi=
ch has footnoted sources) says:
> For determining the lateral input direction (left, front, right) the audi=
tory system analyzes the following ear signal information:
> =95 Interaural time differences
> Sound from the right side reaches the right ear earlier than the left ear=
. The auditory system evaluates interaural time differences from
> =95 Phase delays at low frequencies
> =95 group delays at high frequencies
> =95 Interaural level differences
> Sound from the right side has a higher level at the right ear than at the=
left ear, because the head shadows the left ear. These level differences a=
re highly frequency dependent and they increase with increasing frequency.
> For frequencies below 800 Hz, mainly interaural time differences are eval=
uated (phase delays), for frequencies above 1600 Hz mainly interaural level=
differences are evaluated. Between 800 Hz and 1600 Hz there is a transitio=
n zone, where both mechanisms play a role.
> Localization accuracy is 1 degree for sources in front of the listener an=
d 15 degrees for sources to the sides. Humans can discern interaural time d=
ifferences of 10 microseconds or less.
-Dan
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