MadSci Network: Zoology
Query:

Re: Blue whales call across vast areas.How do they do it?

Date: Sun May 15 18:34:56 2005
Posted By: Adrian E. Popa, Laboratory Director Emeritus
Area of science: Zoology
ID: 1115569244.Zo
Message:




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


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