The Effect of Sonar on Human Hearing

Abstract

Human use of the Earth’s oceans has steadily increased over the last century resulting in an increase in anthropogenically produced noise. This noise stems from a variety of sources including commercial shipping, oil drilling and exploration, scientific research and naval sonar. Sonar (for sound navigation and ranging) is a technique that uses sound propagation (usually underwater) to navigate, communicate or to localize: sonar may be used as a means of acoustic location (acoustic location in air was used before the introduction of radar). The term sonar is also used for the equipment used to generate and receive the sound. The range of frequencies used in sonar systems vary from infrasonic to ultrasonic. Sonar uses frequencies which are too much high-pitched (up to 120,000 cycles per second) for human ears to hear. Although there is a growing concern among the public that human generated sounds in the marine environment could have deleterious impacts on aquatic organisms, only few studies address this concern on the effects of these sounds on the human auditory system. The effects of sound on the human auditory system have been the subject of several studies, but one question needs to be resolved yet: the effects caused by the naval sonar. For these reasons this chapter wants to show the effects of active middle frequency sonar on human. Published data from humans under water in literature are scarce and sometimes use different terminology with regard to sound levels. For example sound pressure levels measured in air are normally reported with a reference pressure of 20μPa whereas levels measured in water are normally reported with a reference pressure of 1μPa. Therefore, in the diving environment it is recommended to use SPL (sound pressure level) threshold with reference pressure of one micropascal (1 μPa) for both water and air measurements in order to compare values from different sources. The non-intuitive nature of decibels and the different reference values of air and water have led to a plethora of misconceptions concerning the magnitude and potential effects of noise levels in air and water. The magnitude of sound pressure levels in water is normally described by sound pressure on a dB scale relative to a reference root-meansquare (rms) pressure of 1 μPa (dB re 1 μPa). For these misconceptions concerning the measurement of noise in the marine environment, studies on cetaceans have highlighted hearing damage and behavioural change at levels of sounds exposure lower than those that would cause physiological damage to the auditory system. Continued emission of noise can increase the damage, due to the “habituation” to a familiar sound to which it is difficult to react more strongly. The “habituation” is known as being provoked by continued acoustical stimuli, reducing the hearing sensitivity to high-level