Manmade underwater noise is a form of pollution that can have adverse effects on marine life, ranging from disturbance and behavioural modification to injury and death. The information gained from a full study of aquatic animal hearing will make it possible to establish evidence-based guidelines on the distance (dependant on the sound frequency) an animal should be from a noise source before a seismic survey or sonar test is allowed to commence. Current guidelines recommend that a cetacean must be over 500 meters from the source or after a 20 minute delay if the animal can no longer be seen prior to commencing a survey or sonar sweep, even though the potential for intense low frequency sounds to impact on these animals may extend over many square kilometres.   The oceans are virtually transparent to sound and opaque to light and radio waves. At a wavelength of 1 m (1,500 Hz), water is nearly 1,000,000 times more transparent to sound than to radio signals (Pilgrim and Lovell, 2002). This fact underlies the current intense interest in acoustic exploration of the oceans and the need to mitigate it. Naturally produced sounds arise from a number of sources, such as breaking waves, heavy rain, volcanic activity, or from marine animals (bio-acoustic sources). Vocalisations such as whale song, along with the grunts and whistles from sonic fish are especially relevant for communication purposes and during predator prey interactions (Myrberg, 1981). There are several types of anthropogenic sources used routinely that produce intense levels of low frequency noise, from commercial shipping and powered leisure craft, to deliberately produced signals such as the Low Frequency Active Sonar (LFA) used by the military in anti-submarine warfare, or from the airgun arrays used during a seismic survey of the substrate beneath the seafloor by the petroleum industry. These activities can generate noise levels in excess of 253 dB (re 1 uPa at 1 m) (Engås et al., 1996) and are comparable to the noise levels generated by a seafloor volcanic eruption, which can produce a source level of in excess of 255 dB (re 1 uPa) (Northrop, 1974). 

Growing concern by environmental organisations regarding the generation of Low Frequency anthropogenic noise in the marine environment by the military and oil industry, is stimulating considerable interest in the diagnosis of the existence and extent of hearing loss in marine mammals. Concise morphological and physiological information on the hearing systems is critical to the assessment of the potential effect of anthropogenic noise pollution, being especially relevant where a cetacean or other marine mammal is thought to have died as a consequence of intense noise exposure. Trauma to the auditory system can result in lesions developing along the VIII nerve pathway, or ruptures in the blood vessels surrounding the inner ear. A number of techniques have been developed to study gross physiological damage to the inner ear (e.g., Costa et al., 2003; Ketten, 1995; Richardson et al., 1995; Todd et al., 1996; Whitlow et al., 1997), though these investigations do not necessarily verify the impairment of hearing and balance. In this study, samples of tissue from the inner ear and brain are analysed using Transmission Electron Microscopy (TEM), a methodology that can be applied to look for evidence of trauma to the ultrastructure of the inner ear and central nervous system. Hearing damage can be caused by intense sources of man made noise, induced chemically by antibiotics such as gentamicin sulphate (Lombarte et al., 1993), or in response to reactive oxygen species (ROS) from environmental pollutants. The production or chemical activation of free radicals may lead to oxidative stress and ultimately to permanent cellular damage (Nicholls & Budd, 2000). 

A number of techniques have been developed to investigate gross physiological damage, though concise evidence of raised hearing thresholds from odontocetiforms exposed to loud noise has as yet to be presented. In addition, a number of audiograms produced for the smaller cetaceans show that they cannot hear low frequency sounds well; however, these findings could be indicative of the inefficiency of ceramic transducers (used in many of the experiments) at frequencies below 2 or 3 kHz. Low frequency sounds propagate over great distances in the ocean and are produced by a variety of both natural and anthropogenic sources. Due to this, evidence showing the sensitivity of dolphins to sound frequencies of below 1000 Hz is of significant importance in the assessment of the effect of noise in the natural environment.

Link To Inner Ear Physiology

In order for an accurate diagnosis of raised hearing thresholds as a result of exposure to intense noise, the audiogram for a normally hearing animal must first be established. Until recently, very little has been documented regarding the hearing abilities of marine animals, with a number of authors purporting that fish and invertebrates are only responsive to strong vibrations and near field disturbances (e.g. Cohen and Dijkgraaf 1961; Larsel, 1967; Wever, 1976). This, however, is contrary to the findings of Parker (1903) and von Frisch (1938) on fish species, and Lovell et al. (2005) on the hearing abilities of crustaceans. Hearing thresholds from any animal possessing the appropriate receptor mechanism are illustrated in an audiogram (Myerberg, 1981), which presents the lowest level of sound that a species can hear as a function of frequency. Both the sensitivity of hearing and the frequency range over which sound can be heard varies greatly from species to species. For man, sound is ultrasonic above 18 to 20 kHz, whilst for many fish species, sounds above 1 kHz are ultrasonic and for a number of odontocetiforms, sounds above 150 kHz are ultrasonic. This diversity in hearing ability between organisms indicates the importance of being able to accurately define hearing thresholds, especially when evaluating the influence of intense underwater sounds on both the physiology and ecology of marine animals.

The intensity of a sound in air is not the same as the intensity in water, primarily because of differences in the way the two measurements are referenced (Urick, 1983). In air, the lowest sound pressure level audible to humans is around 20 micro Pascal, which, on the dB scale is termed 0 dB (re. 20 µPa). However, the sound pressure level in water is referenced to 1 micro Pascal (re.1 µPa); thus the factor for converting 0 dB (re. 20 µPa) in air, into dB water is 20 log (pwater/1 µPa) = 20 log (20) = 26 dB (re.1 µPa). The characteristic impedance of water is about 3600 times greater than that of air, thus an equivalent sound intensity between air and water is 10 log (3600) = + 36 dB. By adding together the converted reference intensity (26 dB) with the impedance matching factor (36 dB), an intensity of 0 dB (re. 20 µPa) in air becomes 62 dB (re. 1 µPa) in water.