From: Mike Ravicz <Mike_Ravicz@xxxxxxxxxxxxxxxx>
To: Andre Castro <andre_castro83@xxxxxxxxxxx>
CC: AUDITORY@xxxxxxxxxxxxxxx
Subject: Re: underwater listening - bone-conduction
Date: Wed, 5 Apr 2006 11:11:11 -0400
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.