[sdiy] The eardrum and brownian motion

[Chris Stecker]

As someone who has spent a considerable amount of time in anechoic chambers 
(I'm a psychoacoustician and auditory neuroscientist), I had to chime in on 
this topic.

"electrical noise from your nervous system" probably == tinnitus.  (this is 
nervous-system related, but it's not exactly the same as "hearing" your 
neurons firing)

"low pitched cycle" probably == blood flow 

I imagine the latter becomes noticable just because other sounds (which 
normally mask it) are absent, but the former additionally involves the inner 
ear becoming more non-linear / high-Q at low stimulation levels.  Tinnitus is 
not completely understood, but I believe it partly reflects some 
high-frequency regions of the basilar membrane (in the cochlea) 
self-oscillating.  There is disagreement as to whether there is amplification 
in the cochlea however, and it's not my area of expertise, so I won't say my 
explanation is definitive.  Whatever the case, when listening at very low 
levels, these physiological noises can become very distracting (subjectively 
loud).

Tim's post, however, seemed to refer to acoustically "dead" rooms "freaking 
[him] out," not just quiet rooms causing him to be aware of physiological 
noise.  Care to elaborate?   I've noticed several effects in anechoic rooms.  
One involves sound localization (an area I'm quite interested in) : sounds 
can become ambiguous as to whether they come from in front of or behind you, 
and distance becomes very unclear.  I often have people stand at one end, 
facing away from me.  Then I move to the other end and whisper.  Often they 
think I'm right behind them (or possibly in front, if they disbelieve their 
eyes).  Strangely (?) I don't have this perception anymore.  The chamber 
sounds totally normal like any other room, just quieter.  I still hear the 
physiological noise, and have some visual experiences (see below), but the 
lack of reverb doesn't freak me out.

It's true that both the auditory system and visual system are adaptive to 
reduced sensation levels.  In both, there are multiple mechanisms of adaption 
at different levels from peripheral (cochlea, iris, retinal processes) to 
central (cortical).  Adaptation occurs along different time courses: some 
mechanisms are fast (pupillary dilation) and others are slow.  The 
interaction of these different mechanisms may be responsible for the changing 
character of the experience over time.  The weirdest visual thing, for me, is 
a sense that just outside of my field of vision there is bright light, like 
an open window, the sky or the sea.  I don't actually see anything, but it 
feels as if I could just glance over and see it if I wanted to. 


-Chris


On Friday 04 October 2002 02:16 pm, you wrote:
> Just plug your finger tips in your ears,
> you  will be able to hear your heart beat and the flow of blood through
> your fingers.
> Seems the ear it self has a "low noise" blood supply to keep the noise
> floor low.
> At least that is what they told us at high school biology class.
>
> I too heard the high pitched sound in a sound dead/proof chamber.
> Never knew it came from the nervous system,
> somehow it feels a bit scary to hear your own nerves.
> Mmm, would the noise change character depending on ones mood?
>
> Theo
>
>
> From: Mitchell Hudson <mitch at sirius.com>
>
> > speaking of this I remember reading an interview with John Cage. He
> > was talking about going into a room at some university that sound
> > proof. He goes in and come out a while later and talks to the tech.
> >
> > He says I'm in a sound proof room but I am hearing two noises, first
> > is a high pitched hum and the second is a very low pitched cycle. The
> > tach says the first sound is the sound of the electrical noise from
> > your nervous system and the other is your heart beat.
> >
> > This is just what I remember feel free to correct add to this, I'm
> > sure I'm missing something. I too hear that high pitched sound and had
> > often wondered where it was coming from!
> >
> > --M
> >
> > On Friday, October 4, 2002, at 10:03 AM, Tim Ressel wrote:
> > > 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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