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From: jbm@aurora.UUCP (Jeffrey Mulligan)
Newsgroups: sci.electronics
Subject: Re: Comb filters
Message-ID: <802@aurora.UUCP>
Date: Fri, 17-Jul-87 17:09:43 EDT
Article-I.D.: aurora.802
Posted: Fri Jul 17 17:09:43 1987
Date-Received: Sat, 18-Jul-87 16:58:24 EDT
References: <883@omepd>
Organization: NASA Ames Research Center, Mt. View, Ca.
Lines: 51

in article <883@omepd>, randys@mipon3.intel.com (Randy Steck) says:
> Posted: Wed Jul 15 12:07:49 1987
> 
> In article <19698@ucbvax.BERKELEY.EDU> kavaler@zion (Robert Kavaler) writes:
>>
>>
>>The human ear is sensative to the relative phase difference between sounds
>>entering both ears.  This is how we (and animals) can tell where sounds
                              ^
                          one way

>>originate.  By shifting the phase difference a sound can "appear" to 
>>come from elsewhere.  Phase-shifting techniques are used extensively in
>>all sorts of stereo equipment, from cheap Boom-boxes to very expensive
>>mixers.
> 
> I've sometimes wondered about this......
> 
> It seems to me that the phase of the signal does not have as much to do
> with apparent direction as the delay and intensity of the sound reaching
> the ears does.  Given the wavelength of the sound, phase-shifting would
> seem to have a similar effect as changing this delay.  A quick calculation
                                                             ^
                                                     too quick, I think

> shows that the delay apparent (worst case) from one ear to the other is
> about .2 usec.  Is this sufficient to resolve directions?  
> 

Let    v       represent the speed of sound.
Let    h       represent the interaural distance (width of head)

v =~ 1000 feet/second    (within a factor of 2, anyway)
h =~ 0.5 feet		 (also within a factor of 2)

The delay (d) for a sound source at some angle theta from the observer
(theta=0 == straight ahead) is  ( h sin( theta ) ) / v .
"Worst case" occurs when theta = +- 90 degrees, so
d = h / v = 0.5 MILLIseconds.

Interaural phase differences are only useful for wavelengths less than
2h, i.e. frequencies less that 1kHz.  For higher frequencies, delays
of transients as well as intensity differences ("acoustical shadow"
effects) are probably important.


-- 

	Jeff Mulligan (jbm@ames-aurora.arpa)
	NASA/Ames Research Ctr., Mail Stop 239-3, Moffet Field CA, 94035
	(415) 694-5150