Noise Exposure in Hyperbaric Environments

New developments in diving systems and underwater tools are increasing the extent of noise exposure of divers. The need for effective, but appropriate hearing-conservation regulations has been recognized. This paper presents a review of the physical and physiological factors which determine auditory sensitivity in hyperbaric environments and the effects of noise on divers. Experimental evidence on hearing and noise exposure in hyperbaric environments is also reviewed. In general, it is essential that noise measurements made in diving chambers and helmets take into account the characteristic impedance of the breathing gas used in specific operations. Failure to do so results in a gross overestimation of the noise intensity to which divers are exposed. Further, changes which occur in the functioning of the external ear tend to reduce overall auditory sensitivity at frequencies below 6000 Hertz thus providing some protection from the effects of noise. However, auditory sensitivity may be enhanced at the higher frequencies (6000 to 12000 Hertz) with consequences that are as yet unknown. Such changes in auditory function are not due to physiological changes but are attributable wholly to the environment in which the diver is functioning. Theory and existing evidence clearly indicate that the application of current hearing-conservation standards for normobaric conditions to hyperbaric environments is inappropriate.