Effects on Divers
Since 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 (NSMRL). During these tests, divers were exposed to nine 100-second low frequency sound signals with sound pressure levels (SPL) of 160-dB and frequencies varying down to 160 Hz, with a 100-second break between signals. This represented a duty cycle of 50 percent, which is more than twice the maximum duty cycle (20 percent) at which SURTASS LFA sonar transmits. The results of the first diver study showed no 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.
The second study was conducted in 1997 and 1998 by the Office of Naval Research (ONR) and the NSMRL in conjunction with a consortium of university and military laboratories. The purpose of this second diver study was to develop guidance for safe exposure limits for recreational and commercial divers to low frequency sound.
The second study concluded that the maximum intensity used during the tests (received level of 157 dB) did not produce physiological evidence of injury 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) to 145 dB would provide a suitable margin of safety for human divers. In June 1999, NSMRL set interim guidance for the operation of low frequency underwater sound sources in the presence of recreational divers as 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 dive sites such that received sound levels will not exceed 145 dB. Due to this conservative 145-dB criterion and the associated geographic restrictions on power level, transmission of SURTASS LFA sonar should have no physiological effects and minimal aversion reactions on divers.