Re: perception of circling horn

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Thu, 20 Apr 2000, Tim Folkerts wrote:

> For low frequencies, timing differences are used.  The crest of a wave will
> reach one ear at a different time than it will reach the other.  The closer
> ear will obviously hear it first, by an amount depending on the location of
> the source and the distance between the ears.  Simultaneous arrival means
> straight ahead (or behind).

To be precise, it isn't a matter of "hearing it first" and this is why it
is important to distinguish between phase sensitivity and time
sensitivity.  The hair cells along the basilar membrane emit nerve
impulses that tend to be phase-synchronized with the stimulus.  At low
enough frequencies (i.e., for lambda > 2*inter ear displacement) the *time
delay* between the nerve impulses carries information about the *phase
difference* between the waves being detected that can be used to provide
information about the direction from which the sound is arriving.

> I believe that essentially all location is done by phase difference at low
> frequency and by intensity at high frequency,  with a middle ground where
> neither works well.

That's not quite true as we do a very good job of distinguishing back from
front, high from low, and more generally, the azimuthal angle about an
axis through the two ears.  This kind of directional determination
requires spectral cues that arise from the directionally dependent
filtering done primarily by the pinnae.

John
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