Creatures such as whales, and specialized machines, can use reflected waves to locate distant objects and sense their shape and movement. Dolphins and whales can tell the difference between objects as small as a shirt button 15 meters away, and they use SONAR much more than sight to find their food, families, and direction. Today, we can name tens of significant applications being applied in various fields of life based on the SONAR principle.
Petroleum companies use what is called Depth Sounding, a technique that applies SONAR to determine the depth and the layout of land surface at sea depth during underwater operations.
SONAR is essentially used during inspection of dams and underwater pipelines and in marine archaeology and reef monitoring. Researchers at Scientific Solutions Inc. Supporters say a reliable high-frequency sonar could help protect whales from a variety of ocean hazards: long-range military sonar; collisions with ships; underwater demolitions; Navy battle simulations involving live explosives; and seismic mapping by oil and gas companies.
The Navy has been criticized in recent years for its low- and mid-frequency sonars, which can travel long distances to detect enemy submarines. These sonars have been blamed for injuring or killing whales, whose hearing can be severely damaged by the sound. The tests were delayed for a year after a lawsuit prompted an environmental assessment, which found that the research would not significantly impact marine life. A five-year permit from the National Marine Fisheries Service was upheld last month, allowing the tests to proceed.
Vision is the best way to sense distant objects in air, but sound is the best way to sense objects that are far away under the sea. Low frequency sounds can travel hundreds of miles in the right conditions. When mammals entered the ocean tens of millions of years ago, they evolved mechanisms to sense objects by listening for echoes from their own sounds, and to use sound to communicate over long distances.
Modern ships generate enough noise from their engines and propellers to have reduced the range over which whales can communicate. The low frequency noise from ships travels so well in the ocean that it has raised the noise levels ten to one hundred times compared to a century ago Stocker, Marine mammals are of particular concern regarding the effects of noise as they typically have sensitive underwater hearing and they use sound for important activities such as communicating, orienting and finding prey.
It has been suggested that overexposure to noise could induce permanent physiological damage and deleterious behavioural alterations. For these reasons: there has been growing concern that the noise humans have introduced into the sea might disrupt the behaviour of marine mammals Salami et al.
Some marine animals, such as whales and dolphins, use echolocation systems similar to active sonar to locate predators and prey. It is feared that sonar transmitters could confuse these animals and cause them to lose their way, perhaps preventing them from feeding and mating. Recent articles report findings to the effect that military sonar may be inducing some whales to experience decompression sickness and resultant beachings Parsons et al.
These temporally and spatially overlapping events seem to indicate that high-intensity sonar may instigate some marine mammal strandings. Recent work has suggested that sonar exposure could induce a variety of effects in marine mammals including changes in dive profile, acoustically induced bubble formation or decompression sickness Salami et al.
High-powered sonar transmitters can kill marine animals. In the Bahamas in , a trial by the US Navy of a decibel transmitter in the frequency range 3 to 7 kHz resulted in the beaching of sixteen whales, seven of which were found dead.
However, these hypotheses typically lack controlled experimental conditions to best evaluate potentially deleterious noise effects.
Thus, the actual mechanisms that may be initiated by sonar exposure, which could actually result in multi-species strandings, have yet to be empirically supported.
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 Salami et al.
However, at low powers, sonar can protect marine mammals against collisions with ships. Disruption of feeding, breeding, nursing, acoustic communication and sensing, or other vital behavior and, if the disruption is severe, frequent, or long lasting, possible decreases in individual survival and productivity and corresponding decreases in population size and productivity;.
Psychological and physiological stress, making animals more vulnerable to disease, parasites and predation;. Changes in the distribution, abundance, or productivity of important marine mammal prey species and subsequent decreases in both individual marine mammal survival and productivity and in population size and productivity. These changes in prey species possibly could be caused both directly and indirectly by the low-frequency sonar transmissions: for example, transmissions conceivably could kill or impair development of the eggs and larval forms of one or more important marine mammal prey species; they might also disrupt feeding, spawning, and other vital functions or cause shifts in distribution patterns of certain important prey species and make some prey species more vulnerable to disease, parasites, and being eaten by other predators.
Although these evidences, recent studies showed the absence of side effects on marine animals: the sensory tissue of the inner ears did not show morphological damage even several days post-sound exposure; similarly, gross- and histopathology observations demonstrated no effects on nonauditory tissues Popper et al.
The animal would then have to maintain at most that distance for the approximate 2—2. Exceptions may be if the sonar signals are rapidly repeated which is unlikely due to overlap of returning echoes or if oceanographic conditions are such that sound levels do not attenuate regularly over short distances i. Perhaps such a situation could occur with multiple sonar sources over steep bathymetric conditions Mooney et al.
These data show as repeated exposures are necessary to generate effects. In the limited existing research on the effects of sound on marine animals hearing and behavior, different scientists have discovered that exposure to some very loud sounds, such as seismic air guns, can produce no effect, or result in a range of effects from temporary hearing loss to more lasting damage to the haircells of marine animal' inner ears.
But it is hard to say that effects on one species indicate that another species will be affected in the same way by the same signal. Furthermore, subtle behavioural changes are also associated with sonar exposure. Animals that prolong apnea must optimize the size and use of their oxygen stores, and must deal with the accumulation of lactic acid if they rely upon anaerobic metabolism Popper et al.
Pathologies related to effects of pressure are well known among human divers, but marine mammals appear to have developed adaptations to avoid most mechanical and physiological effect. The hazard of bubble formation during decompression is best known for humans breathing compressed gases, but empirical studies and theoretical considerations have shown that breath-hold divers can develop supersaturation and possible decompression-related problems when they return to the surface.
Supersaturation has not been measured during normal diving behaviour of wild marine mammals but rather in specially designed experiments performed by trained subjects Tyack, Recent reports show the presence of gas and fat emboli in marine animals during exposure to naval sonar Tyack, These reports suggest that exposure to sonar sounds may cause a decompression-like syndrome in deep-diving whales either by changing their normal diving behaviour or by a direct acoustic effect that triggers bubble growth Tyack, Nonetheless, the geographical pattern of strandings suggests that animals are impacted at ranges significantly greater than those required for acoustically driven bubble growth, implying that the observed pathologies may follow from a behavioural response that has adverse physiological consequences Tyack, In order to further understand these pathophysiological mechanisms, recent experiences examined post-mortem and studied histopathologically different marine animals Ziphius cavirostris, Mesoplodon densirostris and Mesoplodon europaeus after exposure to midfrequency sonar activity: no inflammatory or neoplastic processes were noted, and no pathogens were identified.
Macroscopically, whales had severe, diffuse congestion and haemorrhage, especially around the acoustic jaw fat, ears, brain, and kidneys. Gas bubble-associated lesions and fat embolism were observed in the vessels and parenchyma of vital organs. In vivo bubble formation associated with sonar exposure that may have been exacerbated by modified diving behaviour caused nitrogen supersaturation above a threshold value normally tolerated by the tissues as occurs in decompression sickness.
Alternatively, the effect that sonar has on tissues that have been supersaturated with nitrogen gas could be such that it lowers the threshold for the expansion of in vivo bubble precursors gas nuclei.
Exclusively or in combination, these mechanisms may enhance and maintain bubble growth or initiate embolism. Severely injured whales died or became stranded and died due to cardiovascular collapse during beaching.
Because of the high speed of sound under water, it is perceived by both ears virtually simultaneously and the orientation error may be possible. Bad orientation under water is also due to the prevalent bone conductivity. Sufficient audial orientation is possible to be acquired only after systematic training. The diving suit isolates the human ear from the surrounding water medium.
That is why sound waves penetrate the helmet and the layer of air but reach the eardrum partly absorbed and scattered. In this case, sound perception through air conductivity is insignificant. However, while diving without a helmet, which is possible in warm water, sound is perceived just like in the air. If the rubber helmet fits tightly, sound is well perceived because of bone conductivity — sound waves are transmitted through the bones of the human skull.
With no helmet, a diver can hear very well, with a rubber helmet — fairly well, and with a metal one — very bad. Also, the increasing use of active low-frequency sonar by submarines and ships raises the risk of accidental exposure to low frequency underwater sounds.
While hearing conservation programs based on recognized risks from measurable sound pressure levels exist to prevent occupational hearing loss for most normal working environments, there are no equivalent guidelines for noise exposure underwater. The Threshold Limit Values TLVs represent conditions under which it is believed that nearly all workers may be repeatedly exposed without adverse effect on their ability to hear and understand normal speech.
These recommended limits set at the middle frequencies of the one-third octave bands from 10 kHz to 50 kHz are designed to prevent possible hearing loss caused by the subharmonics of the set frequencies, rather than the ultrasonic sound itself.
These TLVs represent conditions under which it is believed that nearly all workers may be repeatedly exposed without adverse effect on their ability to hear and understand normal speech.
Previous TLVs for frequencies in the 10 kHz to 20 kHz range, set to prevent subjective effects, are referenced in a cautionary note below. All instrumentation should have adequate frequency response and should meet the specifications of ANSI S1. Measuring any source suspected of producing sound at levels exceeding the ACGIH recommended limits requires the use of a precision sound level meter, equipped with a suitable microphone of adequate frequency response, and a portable third-octave filter set.
Different studies highlighted as behavioural and memory disturbances, intellectual impairment, depression, and other long-term neuropsychiatric changes are well known in professional divers: these symptoms are probably caused by repeated focal ischemia due to intravascular gas bubbles and hyalinosis of the walls of small blood vessels Reul et al. The lesions were predominantly in the subcortical white matter and basal ganglia, suggesting a vascular pathogenesis Reul et al.
Other studies highlight as diving puts the inner ear at risk. Inner ear barotrauma and inner ear decompression can lead to permanent sensorineural hearing loss, tinnitus and vertigo Klingmann et al. Inner ear barotrauma is related to pressure changes in the middle and inner ear. Barotrauma refers to tissue damage that occurs when a gas-filled body space e. During descent, as ambient pressure increases, the volume of gas-filled spaces decreases unless internal pressure is equalized.
If the pressure is not equalized by a larger volume of gas, the space will be filled by tissue engorged with fluid and blood. Barotrauma of the inner ear during descent develops when middle ear clearing fails and the eustachian tube is blocked and locked Klingmann et al. Under these conditions, the raised intracranial pressure brought about by forceful efforts to equalize pressure might be transmitted to the inner ear through a patent cochlear aqueduct.
Symptoms often occur during ascent when expanding air in the middle ear is forced through a round window membrane fistula into the inner ear. The resulting gas bubble in the labyrinth expands during ascent and replaces the perilymph fluids.
Barotrauma of the inner ear during ascent is a result of a blocked eustachian tube with air expanding in the middle ear forcing the tympanic membrane into the auditory canal. As a result, the oval window membrane is dislocated into the middle ear and the round window membrane is forced into the inner ear with increasing tension on both membranes Klingmann et al. When there is an abrupt pressure equalization, either because of a tympanic membrane rupture or because the blocked eustachian tube releases the increased middle ear pressure, the oval and round window membranes snap back to their original position causing a pressure wave running through the inner ear.
Whether uneventful scuba diving in the absence of a decompression incident is a risk factor for cochlear disorders is a matter of debate. Most studies of diving associated hearing loss reveal an association with occupational noise exposure. Different reports showed as divers exposed to high levels of underwater sound can suffer from dizziness, hearing damage, somnolence, lightheadedness inability to concentrate or other injuries to other sensitive organs, depending on the frequency and intensity of the sound.
This may include neurological symptoms such as blurred vision, lightheadedness, vibratory sensations in hands, arms and legs, and tremors in upper extremities Fothergill et al. Most reports of diving injury have concentrated on acute injuries rather than chronic disability e. Hence, while many divers reported aural symptoms, few attributed them to diving. It is possible that repeated hyperbaric exposure among very experienced divers may be responsible for their aural symptoms, despite the lack of an obvious acute injury for many.
The cause s of the aural disorders described above are unknown. Different authors have reported that hearing loss in divers may be due to external ear canal obstruction, tympanic membrane perforation, middle ear disorders and sensorineural hearing damage Taylor et al.
However, aural barotrauma is the most likely cause as it is a relatively common occurrence. It is known that the strain exerted upon the tympanic membrane TM and middle ear from minor barotrauma results in reversible impairment of the recoiling capacity of the TM elastic fibrils. It has been postulated that, if this barotrauma is repeated over lengthy periods, the TM changes could become irreversible Taylor et al.
Hence, hearing loss is a possible outcome Taylor et al. Sub-clinical brain and inner ear injury may offer an alternative explanation. Different authors found that divers had significantly more hyper-intense lesions of the sub-cortical cerebral white matter on MRI compared to controls Taylor et al.
The exact mechanism of this degeneration remains unclear although paradoxical gas embolism, through a patent foramen ovale, has been postulated Taylor et al. However, the association between diving and hearing loss, in the absence of clinically apparent diving injury, may not be as clear cut.
Therefore, the effect of acoustic trauma or potential harmful effects of increased pressure and partial pressures of breathing gases cannot be differentiated.
In fact the following well-recognized factors can affect the inner ear in divers: inner ear decompression sickness, noise, and potentially chronic effects of the breathing gases. A number of studies have compared the hearing threshold in professional divers Klingmann et al. In in a group of 62 Royal Navy divers and submarine escape training instructors, a high-frequency hearing loss was found in most of the divers. However, these divers had been exposed to gunfire and machinery noise during their naval careers, and noise could not be excluded as the causative mechanism.
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