Subject: Sound underwater vs. sound in air

Jennifer D. Philips (jphilips@soest.hawaii.edu)
Sun, 03 May 1998 13:22:07 -1000

>Has there been any research done with cetacean communication/whale song 
>to account for the difference in air conduction of sound waves versus 
>water conduction of sound waves?  There has been much recording of 
>humpback whale songs, for example, but sound that's being sent and what 
>humans hear through recordings are two very different things.  I'm 
>wondering if research/computer simulation could be done to simulate what 
>the human ear, if evolved to process sound through a water medium, would 
>hear and if that would be significantly different than what we are 
>interpreting now.  Also, what sounds are occuring in registers the human 
>ear cannont hear?  It would make a decent research project.
>
>I am an elementary school teacher with specialty in science education 
>and would not pursue this research myself. If you know of any research 
>or additional web sites, I'd be interested in pursuing this info.
>
>Thanks! P.E. Wright
>

You raise an interesting issue - one that is often not thought of.  You are
very right in your observation that recordings of whale sounds must surely
'sound' different to us when we hear them projected from speakers in air
than they must sound to the whales emitting them.  Even when SCUBA divers
hear whales singing underwater, the sound must travel from the water medium
it is propogating through into the air medium that is contained in our
ears, and so the sound must change somehow.  Anytime a sound must travel
across mediums that are very different, much of it reflects off that
boundary, and some makes it across, resulting in change to the original
sound wave.  Even whales must deal with this physical relationship somehow,
though for them the sound travels from the water into their flesh and inner
ear, which are apparantly evolved to be as acoustically similar to water as
possible.  So, the result is that the sound that reaches the ears of whales
could and probably does sound different to whales than it does when we hear
it.  When acousticians study sound, however, we study it in ways that does
not involve listening to it from speakers.  We analyze the physical
characteristics of the sound itself as it is acquired directly from the
water medium it is propogating through.  We analyze properties such as the
frequency of the sound (or the number of times the sound wave cycles per
second.  This is similar to our perceived pitch of a sound), the amplitude
of the sound (or its loudness) and its duration.  Looking at these
properties as they are then imaged on a computer screen gives us some idea
of what the sound 'looks' like, though not necessarily what the sound
really 'sounds' like to a whale.  So, back to your idea of a computer
simulation to attempt to create the sound as it might be perceived by a
whale:  I think it is a good idea, one that I am not sure has been truly
examined.  A 60Hz sound, as emitted by a humback whale, might change after
it is acquired and retransmitted by a speaker, or it might only change a
bit.  I am not sure!  A 2 kHz sound might change more ,or less than the 60
Hz sound, I am still not sure!  The issue is intriguing, and I know it has
been thought about, though I am not sure what papers might have been
produced on the topic.  But you've spurred me to look into it!   Another
thing to keep in mind is that to truly understand how an animal 'perceives'
a sound is very very difficult.  We can document the peak frequency of a
sound as much as we want, we still remain unaware of what the sound 'sounds
like' to a whale.

Thanks for your thoughts - 

Aloha - 
Jen
__________________________________

Jennifer D. Philips		
jphilips@soest.hawaii.edu

Marine Mammal Research Program
HIMB, University of Hawaii at Manoa
PO Box 1106			
Kailua, HI  96734
voice:  (808) 247-5063
fax: (808) 247-5831
__________________________________