Underwater sound has probably been used by marine animals for millions of years. The field of underwater acoustics is closely related to a number of other fields of acoustic study, including sonar, transduction, acoustic signal processing, acoustical oceanography, bioacoustics, and physical acoustics. The water may be in the ocean, a lake or a tank. Underwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water and its boundaries. Although the true correlation between sonar and hearing damage is difficult to show, (absence of technical information, level of sound exposure and other environmental variables) this study wants to show the effects of sonar on human auditory system. For this reason, some of the previous considerations can be applied on humans. Studies on marine animals have demonstrated that changes in hair bundle density paralleled changes in hair cell nucleus density, indicating that entire hair cells disappeared following noise exposure the inner ear damage is characterised by a permanent threshold elevation after an exposure to white noise ranging in intensity from 130 to 170 dB re 1 μPa for 24 h.Īlthough there are differences among the ears of different species, the basic processes of hearing are the same between marine and terrestrial mammals. For these reasons, the introduction of new types of military sonar, such as low-frequency system, should proceed with caution the low-frequency sounds produced by the systems will travel much farther than the mid-frequency sonar sounds currently causing concern. The “habituation” is known as being provoked by continued acoustical stimuli, reducing the hearing sensitivity to high-level sounds the hearing sensitivity may be regulated at both conductive (stapedial reflex) and sensorineural levels (adaptation). 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).įor 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.Ĭontinued 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 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. 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. 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. Published data from humans under water in literature are scarce and sometimes use different terminology with regard to sound levels. For these reasons this chapter wants to show the effects of active middle frequency sonar on human. 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. Sonar uses frequencies which are too much high-pitched (up to 120,000 cycles per second) for human ears to hear.Īlthough 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 range of frequencies used in sonar systems vary from infrasonic to ultrasonic. The term sonar is also used for the equipment used to generate and receive the sound. 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). This noise stems from a variety of sources including commercial shipping, oil drilling and exploration, scientific research and naval sonar. Human use of the Earth’s oceans has steadily increased over the last century resulting in an increase in anthropogenically produced noise.
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