Subject: Sound,NMFS, LFA Sonar (fwd)

Mike Williamson (pita@www1.wheelock.edu)
Sat, 6 Nov 1999 08:13:11 -0500 (EST)

Concerned colleagues,
    I too am concerned about the environmental effects of sound on many
marine mammals. The LFA has not been of enough concern to me to allocate much
time to studying it. As a former Navy acoustician, I am vaguely familiar with
it and have some degree of trust in people I know who are involved in the
program and who are capable acousticians. Even capable people need some
oversight from experts from other fields associated with impact of their work.
    I write to express concern about some of the "information" being quoted
by the MARMAM community that impairs their effectiveness on having an impact.
I have not yet decided how serious the threat to marine mammals really is.
The "information" being quoted by some shows their misunderstanding of
underwater acoustics and sonar. If the MARMAM community is to be taken
seriously by the U. S. Navy, concerns must be expressed in accurate terms
rather than have a flood of unsupportable claims flooding Navy offices. That
tactic may work with politicians but should not work with sonar and
bioacoustic experts. It did not  work with me when I was a Navy acoustician.
    I do not mean to pick on Dr. Green, as I do not find much to disagree
with in her accurate statements. The problem arises in areas that I do not
necessarily expect her to understand. Not everyone can be proficient in every
phase of the LFA problem areas. The next piece of documentation below
illustrates my point.

"US Navy Testing of Low Frequency Active Sonar
On Humpback Whales: Scientific Issues
Marsha L. Green, Ph.D.
Ocean Mammal Institute
Albright College*
http://www.oceanmammalinst.com/mgpaper.html
Documented Effects of Sonar on Whales and Humans

The U.S. Navy's new low frequency active sonar (LFA sonar) broadcasts loud,
low frequency sound (up to 240 dB) to locate quiet submarines. From 1980 to
1995 the Navy developed and tested this system without obeying any of the
applicable environmental laws. While the Navy was illegally developing and
testing LFA sonar, they were also building a ship (TAGOS-23, estimated cost
$60 million) to deploy the sonar. The Navy finally agreed to prepare an
Environmental Impact Statement (EIS) after being challenged by environmental
groups. They hired Christopher Clark (Cornell U.) and Peter Tyack (Woods Hole
Oceanographic Institute) to test the effects of the sonar on blue, fin, gray
and humpback whales in 1997 and 1998. It is important to note that the LFA
sonar test levels were always much lower than the actual sonar which is about
240 dB."


Let us examine this piece of documentation to show why testing at 240 dB
source levels is not important.
The reason relates to the near field and far field of sonar projectors to
show that 240 dB sound fields do not exist for a 240 dB LFA sonar. Actually
the 240-dB sonar is an idealization of a distributed source back to a point
source that is entirely fictional. It is one that makes calculations with the
sonar equation easy for neophytes to use. The LFA uses a "line" source of
discrete projectors. I do not know the length of the line but for my purpose
here I will considr it to be 10 wavelengths at 300 Hz. At 300 Hz the
wavelength is about 5 m so the line is 50 m. Let us assume that the sound
field at the end of the near field is 186 dB. The far field point is the
point where the sound field begins to undergo spherical spreading or
(elementary from textbooks on sound) the length-squared divided by the
wavelength (500 m for this case). These same textbooks might tell you that
within the near field the acoustic field is highly collimated with the sound
field fluctuating at a level of about 170 dB. Now to determine the source
level of the sonar we take 170 + 20 log 500 = 240 dB. I do not know the exact
levels that LFA has proposed  to use but am sure that Christopher Clark
(Cornell U.) and Peter Tyack (Woods Hole Oceanographic Institute) must have
been given correct numbers to use. I also do not know the resonance frequency
of the projectors but, as someone well acquainted with sound projector, I do
know that in the 100 to 1000 Hz range the peak level only applies to the
resonance frequency.
    My advice to you, who wish serious consideration from the Navy, is: be
sure you have your information correct before you express your concern. You
may avoid being placed with the lunatic fringe.


Now to pick on a group that should know better I find the following excerpt
from the Federation of American Scientists

http://www.fas.org/irp/program/collect/lfa.htm
FAS (Federation of American Scientists)

Low-Frequency Active (LFA)

The U.S. Navy plans to deploy a new submarine detection system, known as Low
Frequency Active Sonar (LFA), throughout 80% of the world's oceans. LFA is
based on the fact that very low frequency sound [100-1000 Hz] can travel
great distances and detect quiet submarines. The LFA system uses intense
sound, reportedly [the Navy has given a figure of 160 dB at about 2 km from
the LFA] at levels in the range of 235 decibels or greater [the noise level
of a jet engine is about 120 dB] generated by massive sound transmitters
towed behind TAGOS-class ships.


I would hope all bioacousticians would have learned by this time that 235 dB
in water represents much less power than 235 dB in air. In water 1 watt of
power is ~171 dB. In air 171 dB is ~100,000 watts. A 240 dB level in water
would be more than
10 exp(12) watts. Don't make the mistake of casually comparing sound levels
in air and water as the excerpt above appears to do.

Joe Blue




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