On Wed, 21 Nov 2001, S Cooney wrote:
> Don't believe anything you hear on TV. For example who else has been
> suffering through the ridiculous noises edited onto the beautiful Blue
> Planet footage? Anyone who has ever swum underwater knows that its pretty
> quiet under there (apart from the incessant hum of some fisherman trying to
> run you over in his boat) and those stupid noises added to the coral
> spawning last week were just dumb. In fact when they later played the call
> of the male Sperm Whale, I wasn't sure if the echo I could hear was what
> they actually sound like, or what they sound like if they are singing in St
> Paul's Cathedral!
Human's hearing is inefficient underwater. Your eardrum works well
in air, but transmits little sound underwater. Much of what you hear
underwater is thought to be via conduction through your skull - a less
efficient mechanism, and divers often worsen the situation by wearing
neoprene hoods.
The underwater sounds on the Blue Planet soundtrack will have been
recorded using a hydrophone, which as its a microphone designed for
underwater use, naturally works efficiently underwater.
Even with inefficient human hearing, if you put your head underwater in
rocky areas anywhere near Sydney you should hear at least one animal sound
source - snapping shrimps. They are even present even in the grotty
polluted reaches of Sydney Harbour where I live in inner Sydney.
Their sounds are ubiquitous in warmer water and were on the soundtrack
of one piece of the Blue Planet that I caught. Their "snaps" are very
intense with most of the energy well above human hearing ranges.
Their sounds caused great concern in the American navy during 1941,
suspicous that they were a Japanese weapon. Although Snapping Shrimps
were observed making their sounds in the lab in 1843 it wasn't until
last year that how they make the sound was discovered. It involves,
on a small scale, temperatures as hot as the surface of the sun!
See http://stilton.tn.utwente.nl/shrimp/
Snapping shrimps produce so much noise that there are attempts to
visualize underwater objects by detecting the reflections of snapping
shrimp sounds from the objects. This is effectively using the snapping
shrimp sounds to illuminate the underwater environment and is hence termed
"acoustic daylight" by the researchers.
Snapping shrimps certainly aren't the only marine sound source and because
sound propagates better in water, in some senses the marine environment
might be noiser than above water, although probably less diverse.
Admittedly, I haven't seen quantitative comparisons.
Freshwater environments aren't quiet either. The most conspicuous
sound source is probably Water Boatmen (corixids). They are present in
many water bodies and make frequent species-specific "calls". With a
hydrophone, you can easily hear choruses of several species in ponds in
Sydney parks.
My apologies for the lengthy non-avian posting - at this time of year
any excuse to take a break from exam marking is welcome.
Andrew Taylor
Birding-Aus is on the Web at
www.shc.melb.catholic.edu.au/home/birding/index.html
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