[sdiy] The eardrum and brownian motion

[Chris Stecker]

That's very interesting and could relate to another phenomenon I've observed 
in the anechoic chamber but wasn't going to mention.  Actually, we replicated 
it later with headphones, so the anechoic chamber is probably a red-herring.  

After listening to a constant-level click-train stimulus (the clicks are 
band-limited impulses of around 0.1-1 ms total duration, the trains have 
clicks repeating every 2-10 ms (100-500 Hz), depending on condition...I 
forget the most effective parameters), we exited the chamber and my colleague 
(who is blind) noted that the ceiling appeared to be hung lower (or maybe it 
was higher) than before.  I could hear a profound flanging or phasing effect 
on some of the broadband noise sources in the room (a hissing thermostat in 
particular).  The effects persisted for several minutes.  

We argued for some time about possible mechanism for this phenomenon, finally 
deciding on comb-filtering as the best (though only poorly supported) 
explanation.  The spectrum of the click-train is a band-limited comb pattern. 
We suppose that mechanisms of spectral representation in the auditory system 
(perhaps in the cochlea, but maybe more central) adapted to the constant comb 
pattern, "turning down" frequency channels tuned to the harmonics of the 
pulse rate.  Exiting the room (or taking off the headphones) leaves the 
system in an adapted state, whereby flat-spectrum sounds are percieved as 
comb-filtered.  This apparently comb-filtered sound interacts with the 
filtering of the head-related transfer function to produce spatial illusions 
(low ceiling?) and with the steady-state acoustics of the room (which would 
normally be subject to proper adaption and thus unheard) to produce spectral 
illusions (phasing sounds) when the listener moves about.  Neat.

-Chris 


On Friday 04 October 2002 03:03 pm, Barry Klein wrote:
> Along these lines, there is a "weird science" site I'm sure a lot of you
> have come across, here:
> http://www.amasci.com/freenrg/audhole.html
> You take a box of stirring straws and cap the end and hold it near your ear
> and you go wacko.  Mabye a clue to your theory.
>
> I've been wanting to make this thing for weeks now.....
>
> Barry
>
>
> -----Original Message-----
> From: Tim Ressel [mailto:madhun2001 at yahoo.com]
> Sent: Friday, October 04, 2002 10:04 AM
> To: Grant Richter; synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] The eardrum and brownian motion
>
>
> Yo,
>
> Fascinating. Brings up something I discovered a while
> back: if one stays in a completely dead room i.e.
> anechoic chamber, after a few minutes it starts to
> freak you out. Then I think about all these noise
> generators you can buy that creates white noise to
> sooth you. Perhaps there is something here.
>
> Then there is the topic of the qualities of the noise.
> Level, spectrum, distribution...
>
> I recall the lunches I'd spend in the hills above my
> place of work. Get on the Rice Burner, grab a burger,
> and eat it along side a burbling brook while looking
> at the trees. Very nice.
>
> --TR
>
>
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