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 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.
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 divers.
Diver Low Frequency Sound Research Program