Re: underwater listening - bone-conduction

[Mike Ravicz <Mike_Ravicz@xxxxxxxxxxxxxxxx>]

Hello Andre,

Sorry to see that no one has answered your inquiry yet...  There are  
other folks out there who will probably have more to say on this, but  
here's some info:

> One thing that has been fascinating me is the fact that our  
> perception of underwater sound is realized through bone conduction.

I  don't know that for sure, but it seems plausible - what are your  
sources for this?

> For what I could gather by relating different sources is that when  
> inside water the impedance matching/communication from the ear drum  
> (air) to the inner ear (fluid) through the ossicles is stopped. As  
> a result our air conducted listening is also stopped.

The middle ear has apparently evolved to reduce the impedance  
mismatch between the air in the ear canal and the liquid-filled inner  
ear.  When the head is immersed, there's another water-air interface  
in the ear canal (I think that the ear canal doesn't fill with water,  
so there's a bubble near the eardrum), so there's another impedance  
mismatch there without any mechanism to compensate for it, and sound  
transmission through the ear canal is reduced.

On the other hand, the impedance mismatch between the surface of the  
skull and the surrounding fluid (air or water) is reduced when the  
head is immersed, since water has a much higher specific impedance  
than air.  In air, sound transmission through the head is 40-60 dB  
lower than through the ear canal; in water, sound transmission  
through the head is likely to be considerably higher and may be  
higher than through the ear canal.

Another factor is that, at least at low frequencies, sound perceived  
by "bone conduction" is conducted through the head to the ear canal  
and (apparently) causes the ear canal wall to vibrate, setting up a  
sound signal in the ear canal near the eardrum that is conducted  to  
the cochlea in the usual fashion.  At higher frequencies, other  
mechanisms seem to be dominant.  (See Shyam Khanna's work for this -  
ref. below.)

And of course sound can be conducted to the head through the body.  A  
paper we put out a few years ago suggests that the body-conduction  
mechanism is less efficient than conduction through the head, but not  
by a lot.  The old "Bone-Fone" loudspeakers worked this way - you'd  
wear them draped over your shoulders.  (I never tried them out though.)

David Mountain and Darlene Ketten have done a lot of work, especially  
recently, on hearing and sound conduction in marine mammals - you  
might look up some of their recent papers (and maybe this post will  
spur them to weigh in).

And it raises the question: How well do people with a conductive  
hearing loss hear underwater?  Is their hearing underwater comparable  
to that of normal-hearing people?

Please let us know what you find out.
	- Mike Ravicz

Khanna et al., "Mechanical parameters of hearing by bone conduction,"  
J. Acoust. Soc. Am. 60: 139-154 (1976).
Ravicz et al., "Isolating the auditory system from acoustic   
noise...," J. Acoust. Soc. Am. 109: 216-231 (2001) - has references  
to a lot of the early work.