Why measure underwater hearing?
Jacques Cousteau popularised the idea of the sea as a silent underwater world, but those who dive for a living or for sport know that actually it can be very noisy. Commercial divers often have to operate underwater tools powered by compressed air which can generate high levels of underwater noise. Even sports divers are used to being startled by everything from sports boats to underwater blasting.
The role of noise as a pollutant and a source of industrial injury (deafness) is well recognised in air, and there are well defined standards which are used to set the maximum acceptable level of noise in the workplace. However, there are no equivalent standards for underwater noise exposure, despite considerable evidence that divers suffer from deafness. Subacoustech have been working to make this information available.
The range of sources by which exposure occurs include transmissions, explosive sources, compressed air tools, the noise of divers life support systems, hyperbaic chamber compression etc. A recent study by Subacoustech has shown that a diver operating an explosive bolt gun is exposed to an instantaneous noise level in excess of 200 dB re 1µPa (1 bar acoustic wave).
Subacoustech have been involved with defining the efficiency of human hearing underwater, so that the existing standards for noise exposure in air may be corrected and applied to underwater noise. This allows employers of divers to ensure that their diver’s hearing is not being damaged, allows companies who are using explosives underwater to define standoff zones within which there is a significant risk of injury to divers, and enables companies shooting offshore seismic surveys to ensure that the public are safeguarded.
The research has shown that human hearing underwater is less efficient than in air, so that there is some protection of hearing from noise. The protection is least at low frequencies, and increases with increasing frequency, accounting for the “muffled” sound that divers tend to hear. Hearing thresholds for a human in air and water are shown below.
Where the cause of noise exposure is known and understood, control of noise usually only involves modest changes in equipment or improvement in work practices. The changes also have beneficial effects in other respects, for instance the improvement of communications if diving bell noise levels are decreased or improvement of diver efficiency and safety if an improved working environment results.
For further information regarding underwater hearing, the following publications may be of interest:
Parvin S J, Nedwell J R (1995) ‘Underwater sound perception by divers and the development of an underwater noise weighting scale‘. Journal of the Society for Underwater Technology Vol 21, No 1, 1995
Parvin S J (1998) ‘The effects of low frequency underwater sound on divers‘. Proceedings of Undersea Defence Technology 1998, pp227-232, Nexus Media Ltd., Nice, France, June 1998
Parvin S J, Heathershaw A D, David A, Rogers R, Ward P D (1999) ‘The environmental effects of low frequency underwater sound‘. Proceedings of Undersea Defence Technology Europe 1999, pp221-227, Nexus Media Ltd., Nice, France, June 1999