Why is sonar bad

For over a decade, the Navy has been trying to convince the courts that they can use an ultra-loud sonar array in a way that is safe for marine life.

The ruling came down to a Navy-friendly interpretation of the National Marine Protection Act, which prohibits any US citizen, agency, or organization from harming creatures like whales, dolphins, and seals. During the s, however, the Soviet Union was developing quieter submarines.

At the same time, the ocean itself was getting noisier from activities like oil drilling and marine shipping. So the Navy started working on a special long-range sonar tool. But sonar it is. The system deploys from the aft ends of special sub-hunting surface ships.

The problem is those frequencies—from around to hz—also happen to be the sweet spot for a lot of marine life. Whales, dolphins, and porpoises use sound to find food, meet mates, avoid predators, maintain social groups, or simply navigate the wide seas.

In the area of underwater acoustics, the primary interest is in ratios of power levels and signal levels rather than absolute numerical values. In the decibel system, the bel is the fundamental division of a logarithmic scale for expressing the ratio of two amounts of power.

The number of bels to express such a ratio is the logarithm to the base 10 of the ratio. The conversion factors in table can in themselves be cumbersome to use, but when expressed in dB, only addition or subtraction is required. When converting from a pressure referenced to 1 bar to one referenced to 1 Pa, simply add dB. When converting from 0.

If converting from 1 Pa to the others, merely subtract the appropriate values. With such a profusion of reference standards and measurement systems, there were ample opportunities for misunderstandings as an operator or planner consulted different sources of acoustic information.

The ocean is not a homogeneous medium, and the speed of sound varies from point to point in the ocean. This variation in sound speed is one of the most important characteristics affecting the transmission of sound. Sonar is an acronym for Sound Navigation and Ranging. There are two broad types of sonar passive and active in use Salami et al. The main types of active sonar are commercial, civilian and military sonars.

The sonars used by military forces are Gong et al. Low-frequency LF : low frequency sonars have been defined as those that emit sound below Hz. These sonars are designed to provide theatre level protection, such as for an Aircraft Carrier Task Group out to many miles up to miles from the ships. This is possible because of the extended propagation possible at low frequencies. Outputs are similar to medium frequency sonars described below but the sound travels further because of the significantly enhanced seawater propagation.

Medium frequency MF : medium frequency active sonars emit sounds at frequencies between and 10, Hz ; these sonars represent a sliding scale of compromise between possible detection range and size of the transmission array; at the lower end of the frequency range Hz the systems are capable of extended detection ranges using high output power, but the size of the transducer limits applications to large warships. These systems are designed to provide area protection for a small Task Group out to a few tens of miles.

High frequency HF : high frequency active sonars operate between approximately 30, and , Hz 30 kHz and kHz ; these systems allow increasingly greater resolution as the frequency increases but at the expense of range. The highest frequencies are only effective over short distances because of the rapid attenuation of high frequency sounds in seawater. Typically high power military active sonars are operated infrequently during voyages and the sounds are not emitted continuously but as short bursts 'pings' during operation Gong et al.

Commercial and civilian sonars are generally designed to detect the sea floor echo sounders , map the sea floor and search for sunken objects sidescan sonars and to locate fish fish finders. Sonars of at least one of these types are fitted to nearly all vessels. Even some small boats have fish finding and echo sounders. The characteristics of these sonars are broadly similar to the high frequency military sonars described above Gong et al.

Dolphins and other toothed whales are renowned for their acute hearing sensitivity, especially in the frequency range 5 to 50 kHz Mooney et al. For example the hearing threshold of the killer whale occurs at an rms acoustic pressure of 0.

By comparison the most sensitive fish is the soldier fish, whose threshold is 0. The potential for active sonar to impact on a species is dependent on the ability of the species to hear the sound. Species hear sounds over different frequencies ranges, and the efficiency of sound detection varies markedly with frequency. Additionally, species behavioural responses to a detected sound may vary according to the sensitivity of the species to disturbance and what activities the animals are engaged in at the time.

Determination of potential impact on a species must therefore include estimation of the ability of the species to detect the sound, and the likelihood of disturbance to critical activities such as feeding or parental protection of juveniles.

In terrestrial habitats, increasing sound levels have been shown to induce various effects across taxa including behavioural changes, temporary physiological alterations and permanent anatomical damage.

While it is apparent that anthropogenic noise may affect marine animals, we know relatively less about the actual causes or mechanisms of these effects. We can usually see things that are miles away, but if you have ever snorkelled, you know that vision is limited to a few tens of meters underwater. 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. Beaked whales have been the primary species of interest for mid-frequency sonar because, at least under certain conditions, they appear to be most sensitive. By contrast, large whales have received the most attention in the low frequency context because their hearing abilities and vocalizations tend to be concentrated in the low frequency range.

The Navy and NMFS have developed an extensive set of mitigation measures to reduce potential impacts to marine mammals. Recent Litigation. Litigation over low frequency sonar has touched on a wide variety of legal topics under the MMPA, including:.