MadSci Network: Zoology |
Greetings:
References:
1.Sounding Out the Ocean' Secrets
National Academy of Sciences
Beyond Discovery
Office of News and Public Information
500 Fifth Street, NW
Washington, DC 2000
http://www.beyonddiscovery.org/con
tent/view.txt.asp?a=219#Propagation_of_Sound_in_the_Ocean
2. Blue whale calling in the Rottnest Trench-2000, Western
Australia
RD McCauley, C Jenner, JL Bannister, CLK Burton, et al
Curtin University Perth Project cmst, 2001 - Australia
http://www.scholar.google.com/
scholar?hl=en&lr=&ie=UTF-
8&q=cache:rmBPLDdySCsJ:www.celaf.ibama.gov.br/sismica/sismica_r6/biblio/McC
auley2001B.PDF+blue+whale+sound+propagation
The questions that you have asked about long distance sound
propagation in sea
water and the sounds of the Blue whale have been studied extensively
and more
that 200 references can be obtained by using Google searches for the
terms:
BLUE WHALE SOUND PROPAGATION
and: TUTORIAL OCEAN SOUND PROPAGATION
I'll present the basics to answer your questions by referring to the
two
references above.
The following quotations are extracted from Reference 1. The
parenthetic
references in the text are sub headings in the referenced work:
"Scientists have been interested in the properties of sound waves for
a long
time. Early 19th century investigators realized that the speed of
sound in
water is different from its speed in air (see Good Vibrations).
Building on
fishermen's and other seafarers' knowledge of the way sound travels
through
water, researchers developed devices that used sound and echoes to
measure
distances. These devices were adapted for military use by the United
States
Navy, but naval officers encountered problems they couldn't explain
with their
limited understanding of how sound traveled in water (see Navigation
by Sound).
Using a new device called a bathythermograph, scientists mapped out
the thermal
layers in the ocean and studied how the speed of sound varied with the
temperature and pressure of the water. They realized that there were
certain
"shadow zones" in the water where objects were invisible to detection
using
sound waves (see A Sound-free Shadow Zone). Several factors including
salinity,
temperature, and pressure influence how far sound travels underwater.
Research
on these issues has yielded great insight into the structure of the
ocean
(see Propagation of Sound in the Ocean). Scientists then conducted
experiments
to test earlier proposed theories about the bending of sound waves in
seawater.
In analyzing the results of these tests, they discovered a channel
across
which sound waves could travel thousands of miles with minimal loss of
signal.
The U.S. Navy exploited this to devise a system called SOSUS which
allowed
them to detect and get information about submarines (see A Sound
Pipeline).
After the cold war ended, the Navy permitted civilian use of this
device and
scientists were able to learn a great deal about the geology and
biology of
the ocean, including the movement of whales around the world (see
Listening to
the Ocean). SOSUS also has enabled scientists to make ocean
temperature
measurements on a global scale, which has provided some evidence for
global
warming (see Probing the Ocean Interior with Sound).
A SOUND PIPELINE
In 1943, Maurice Ewing and J. L. Worzel at Columbia University
conducted an
experiment to test a theory Ewing had proposed a few years earlier.
Ewing
theorized that low-frequency waves, which are less vulnerable than
higher
frequencies to scattering and absorption, should be able to travel
great
distances, if the sound source is placed correctly. The researchers
set off
an underwater explosion of 1 pound of TNT in the Bahamas--and learned
that
it was detected easily by receivers 3,200 kilometers (2,000 miles)
away on
the coast of West Africa. In analyzing the results of this test, they
discovered a kind of sound pipeline, which they called the sound
fixing and
ranging, or SOFAR, channel. Also known as the "deep sound channel,"
this
pipeline was discovered independently by Russian acoustician Leonid
Brekhovskikh
of the Lebedev Physics Institute, who analyzed the signals received
from
underwater explosions in the Sea of Japan.
The scientists had found that, because of the laws of refraction,
sound waves
can be trapped effectively in a narrow channel that straddled a region
of
minimum speed where the bottom of the thermocline met the top of the
deep
isothermic layer. As shown in the illustration, a sound wave traveling
obliquely through the thermocline will bend downward as the speed of
sound
decreases, and then bend upward when increasing pressure causes sound
to speed
up--only to bend downward again toward the depth of minimum speed as
warming
temperatures cause sound velocity to increase. Sound introduced into
this
channel thus could travel thousands of miles horizontally with minimal
loss of signal. The deep sound channel occurs at a depth that varies
with
ocean temperature; in the polar regions, for example, where the colder
surface
temperature brings the thermocline nearer to the surface, the deep
sound
channel approaches the surface as well. "
Reference 2. discusses the studies of a large group of Blue
whales in a long
undersea trench west of Perth Australia. These studies found that
Blue Whales
do indeed use low frequency sound to communicate over hundreds of
kilometers.
The following information I extracted from the 60 page report.
A typical Blue whale call has three components:
1.A 19 Hz (Hz means cycles per second) tone for 21 seconds
2.followed by a 21 Hz tone for 22 seconds
3.After a 5 to 10 second pause the whale produces an up sweep tone
from
20 Hz to 26 Hz over 23 seconds.
The whale will them wait about 78 seconds before repeating the process.
It was noted that the whales change depth during the calling process
and it is
suggested by the authors that they are searching for the optimum depth
for long
distance signaling and communication. It is well known in RADAR and
SONAR
technology that swept frequency waveforms, often called chirps, are
used for
distance measuring. Thus the whale can determine direction and
distance from
cooperative whales that answer the whales calling signals. The whales
also
simultaneously produce clicks at a 65 Hz rate, which probably are
used for
short distance SONAR ranging similar to the techniques used by bats
and
dolphins.
Thank you for your interesting question and I'm sure you will find the
references very interesting reading and they also reference a number
of
related papers.
Best regards, Your Mad Scientist
Adrian Popa
Try the links in the MadSci Library for more information on Zoology.