ear buzz update

[sarah at graphic-fusion.com]

Hi all,

I keep a pretty sleepy eye on the list these days, catching one of every
hundred or so posts.  Anyway, being an auditory physiologist, this thread
just now caught my eye.  I'll offer what few insights I have on the matter:

To understand tinnitus, you have to understand something about hair cells,
which are the sensory cells of the inner ear (functioning both for auditory
and vestibular purposes).  In the main auditory organ, the cochlea, hair
cells are lined up along the basilar membrane.  The basilar membrane is
long and is shaped to be roughly tuned to different frequencies/pitches in
much the same way a soundboard is, with big, fat, floppy regions
corresponding to low pitches on one end, and with narrow, stiff regions
corresponding to high pitches on the other end.  Overlying the basilar
membrane is the tectorial membrane, into which the cilia of the hair cells
are embedded.  As the basilar membrane and tectorial membrane vibrate up
and down, there is a shearing motion between them, resulting in a flexing
of the hair cell cilia back and forth.  These motions result in stimulation
of the cells, causing nerve stimulation, etc.  The hair cells seem to be a
bit more sharply tuned, somewhat like piano strings.

Here's where it gets tricky:  There are two rows of hair cells.  The inner
hair cells seem to be the sensory units.  Vibrate them, and you hear tones.
The outer hair cells (all three rows of them) are the "muscle" of the
organ.  They respond to nervous stimulation and are driven to vibrate their
cilia, shaking the tectorial membrane, and therefore stimulating the inner
hair cells.  Why do they do this?  They seem to provide feedback, much like
a microphone's feedback.  When sound shakes a patch of the basilar membrane
(remember, matched for frequency), the outer hair cells get all excited and
start shaking too, which accentuates and refines the response.  If the
feedback is adjusted low enough (through neural input), then the patch will
merely ring a bit longer, more intensely, and more sharply than it would
without the outer hair cells.  However, if the gain of the system is set
too high, then the patch will just keep on ringing on its own.  That's
where tinnitus comes from.

To understand the system better, just imagine making an "electric piano"
with inductance coils independently on each string of a trichord.  Two of
the inductance coils are used to detect vibrations and actively INDUCE
vibrations in their respective strings, providing positive feedback
(similar to microphone feedback).  The inductance coil on the third string
is used entirely as a pickup and feeds to an amp, so that we can hear the
piano.  Now, instead of hitting the strings with a hammer, we're going to
let soundboard vibrations drive the system.  The task of our trichord is to
detect when THAT FREQUENCY (the frequency of their tuning) is present in a
signal.  So the soundboard is vibrated, and the strings start vibrating
too.  If the frequency is present, the active two strings in the trichord
will ring/whine for a short time, inducing an accentuated and prolonged
vibration of the third (pickup) string.  (Again, the task of the system is
to determine when that frequency is present!)  But of the feedback gain on
the active two strings is set too high, then the strings will keep ringing
indefinitely.

So tinnitus is caused by "hyperactive" outer hair cells.  It is not
understood whether it's because they're irritated, hypersensitive to
stimulation, misregulated, or whatever.  However, the important thing is
that this is a very physical thing.  Thinking the system backwards, the
outer hair cells shake the tectorial membrane back and forth rhythmically,
inducing a shearing motion between the tectorial membrane and basilar
membrane.  The shearing motion, in turn, causes the basilar membrane and
tectorial membranes to vibrate up and down.  The resulting waves travel
down the length of the basilar membrane, creating waves in the inner ear
fluids.  These waves impinge on the oval window, causing it to vibrate. 
>From there, the vibrations travel through the middle ear bones to the
tympanic membrane (eardrum), inducing it to vibrate too. The end result is
that the tympanum DOES vibrate and indeed DOES create a tiny sound in the
ear canal that can be detected with a very sensitive microphone.  The
ringing in your ears can actually be recorded!

All this leaves open the possibility of noise cancelling technology.  One
approach might be to introduce a microphone into the ear canal (and
microphones can indeed be that tiny) and to selectively null out the
selected frequency(ies) with sound introduced 180 deg out of phase. 
However, I suspect the difference in acoustic impedance between the air in
the canal and the tympanum would make that a technically problematic
approach.

A better approach might be laser interferometry, whereby a tiny laser beam
can monitor the vibrations of the tympanum.  The interferometer output can
be digitally filtered to select out the target frequency(ies), and then
sound can be introduced 180 deg out of phase to cancel the measured
vibrations of the tympanum at that(those) frequency(ies).  Of course it's
not as easy as all that, but that would be one promising approach.

So I guess the up-shot is that it probably CAN be done, if someone cares
enough to pay the money to develop the system.  Beyond making tuning much
easier for piano technicians, I'm sure there would be a lot of tinnitus
sufferers who would enjoy the relief.  Of course that would require wearing
a hearing aid-like device constantly, and I'm not sure who would be willing
to do that.  I think most tinnitus sufferers simply "tune out" the ringing
they hear.  In fact I suspect MOST people have some degree of tinnitus but
simply don't think about it.  Me?  Oh yeah, now that the subject has come
up, I hear a few frequencies going in the backdrop.  But if you were to ask
me on a given day whether I suffer from tinnitus, I'd probably say "no,"
without really pausing to listen first.

Hope that helps.

Personal OT update, for those of you who might write off list:  I'm still
doing fine.  I'm still just as busy as ever -- even more so now.  However,
I'm happy and healthy.  I haven't had any more time to work on my piano. 
I'm lucky these days even to have time to practice.  But I still manage
enough miscellaneous moments of down time to keep my sanity -- mostly with
boating these days, since that seems to be the most common stress-relief
denominator in this family.  ;-)

Take care, y'all!

Peace,
Sarah





Original Message:
-----------------
From: John Ross jrpiano at win.eastlink.ca
Date: Thu, 05 Jul 2007 21:45:49 -0300
To: pianotech at ptg.org
Subject: Re: ear buzz update


I realize that.
My tone is constant in pitch.
I had a signal generator producing a tone, years ago when it first started, 
and I worked on electronic organs.
I recollect, that I could zero beat it, at a frequency, about A5.
John M. Ross
Windsor, Nova Scotia, Canada
jrpiano at win.eastlink.ca
----- Original Message ----- 
From: "Mark Purney" <mark.purney at mesapiano.com>
To: "Pianotech List" <pianotech at ptg.org>
Sent: Thursday, July 05, 2007 8:07 PM
Subject: Re: ear buzz update


> Such a device would only work if we had a way for it to "hear" the 
> tinnitus noise exactly as you are hearing it. This would be the only way 
> to produce a cancellation signal that is perfectly matched in frequency, 
> but out of phase with, the original noise. Otherwise, you would get 
> phasing or beats, and/or end up doubling the noise.
>
> John Ross wrote:
>> *How about a noise cancelling device, that is manually controlled.*
>> *I am lucky enough, that with my tinnitus, I just 'tune it out'.*
>> *Brings to mind the way someone doesn't listen to ones mate, but just 
>> says ugh, ugh.*
>> *Selective hearing, does work, with practise.*
>> *We train our ears, to listen for certain partials, so the opposite 
>> should be possible.*
>
>
> -- 
> This message has been scanned for viruses and
> dangerous content by MailScanner, and is
> believed to be clean.
> 



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