low frequency underwater sound is known to have an effect
on humans, the Navy sponsored two controlled studies
to determine the physical and behavioral effects to divers
exposed to the low frequency sound.
The first study was conducted by the Applied Research Laboratory
at the University of Texas from 1993 to 1995. Eighty-seven
subjects participated in 437 tests under the control of the
Naval Submarine Medical Research Laboratory.
During these tests divers were exposed to nine 100-second
signals, with a 100-second break in between, at 160-dB sound
pressure level (SPL) at various frequencies down to 160 Hz.
This is a duty cycle of 50 percent, which is more than twice
the maximum duty cycle (20 percent) of the SURTASS LFA sonar.
This study did not result in any long-term effects on major
organ systems and concluded that exposure to low frequency
sound levels below 160 dB would not be expected to cause
physiological damage to a diver.
During Low Frequency Sound Exposure Tests
second study was conducted in 1997 and 1998 by the Office
of Naval Research (ONR) and the Naval Submarine Medical Research
Laboratory (NSMRL) in conjunction with a consortium of university
and military laboratories. The purpose of this study was to
develop guidance for safe exposure limits for recreational
and commercial divers to low frequency sound.
This study concluded that the maximum intensity used during
the tests (received level of 157 dB) did not produce physiological
evidence of damage in human subjects. Furthermore, there was
only a two percent “very severe” aversion reaction by divers
at a level of 148 dB. NSMRL, therefore, determined that scaling
back the intensity by 3 dB (3-dB reduction equals a 50 percent
reduction in signal strength) would provide a suitable margin
of safety for divers. In June 1999 NSMRL set interim guidance
for the operation of low frequency underwater sound sources
in the presence of recreational divers at 145 dB. This guidance
has been endorsed by both the Navy's Bureau of Medicine and
Surgery and the Naval Sea Systems Command.
Based on this guidance, the operation of the SURTASS LFA sonar
will be restricted in the vicinity of known recreational
and commercial diving so that sound levels will not exceed
Because of this conservative 145 dB criterion and the related
operational restrictions, deployment of the SURTASS LFA sonar
should have no physiological effects and minimal aversion
reactions on divers.
representative attaching monitoring element on diver subject