Effects on Divers
Because 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
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.
Divers During Low Frequency Sound Exposure Tests
The 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 145 dB. 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