The term
mid-frequency sonar is usually used to refer to sonars that project sound in the 3 to 4
kilohertz (kHz) range. Ever since the launch of the on 17 January 1955 the US Navy knew it was only a matter of time until the other naval powers had their own
nuclear submarines. The mid-frequency sonar was developed for
anti-submarine warfare against these future boats. The standard post-WWII
active sonars (which were usually above 7 kHz) had an insufficient range against this new threat. Active sonar went from a piece of equipment attached to a ship, to a piece of equipment that was central to the design of a ship. They are described in the same 1961
Time magazine article by the quote "the latest shipboard sonar weighs 30 tons and consumes 1,600 times as much power as the standard postwar sonar". On June 13, 2001, Lockheed Martin announced that it had delivered its 100th AN/SQQ-89
undersea warfare system to the
U.S. Navy. There was
anecdotal evidence that mid-frequency sonar could have adverse effects on whales dating back to the days of whaling. The following story is recounted in a book published in 1995: Another innovation by the whalers was the use of sonar to track whales they were pursuing underwater. But there was a problem; as the boat gained on the whale, the whale started exhaling while still submerged. This produced a cloud of bubbles in the water that reflected sound better than the whale did and made a false target (akin to what a pilot does when releasing metal chaff to create a false radar echo). I suspect that this behavior by whales was simply fortuitous since exhaling while still submerged is simply a means by which a whale can reduce the time it has to remain at the surface, where surface drag will slow it down. Whalers quickly discovered that a frequency of three thousand hertz seemed to panic the whales, causing them to surface much more often for air, This was a "better" use for sonar because it afforded the whalers more chances to shoot the whales. So they equipped their catcher boats with sonar at that frequency. Of course the sonar also allows the whalers to follow the whale underwater, but that is its secondary use. Its primary use is for scaring whales so that they start "panting" at the surface. In 1996 twelve
Cuvier's beaked whales beached themselves alive along the coast of
Greece while
NATO (North Atlantic Treaty Organisation) was testing an active sonar with combined low and mid-range frequency transducers, according to a paper published in the journal
Nature in 1998. The author established for the first time the link between atypical mass
strandings of whales and the use of military sonar by concluding that
although pure coincidence cannot be excluded there was better than a 99.3% likelihood that sonar testing caused that stranding. He noted that the whales were spread along 38.2 kilometres of coast and were separated by a mean distance of 3.5 km (
sd=2.8,
n=11). This spread in time and location was atypical, as usually whales mass strand at the same place and at the same time. At the time that Dr. Frantzis wrote the article he was unaware of several important factors. • The time correlation was much tighter than he knew. He knew about the test from a notice to mariners which only published that the test would occur over a five-day period within a large area of the ocean. In fact the first time the sonar was turned on was the morning of 12 May 1996, and six whales stranded that afternoon. The next day the sonar was turned on again and another six whales stranded that afternoon. Without knowing the coordinates of the ships he would not have realized that the ship was only about 10–15 miles offshore. • The sonar being used in the test was an experimental research and development sonar, which was considerably smaller and less powerful than an operational sonar on board a deployed naval vessel. Dr Frantzis believed that wide distribution of the stranded whales indicated that the cause has a large synchronous spatial extent and a sudden onset. Knowing that the sound source level was fairly low (it was only 226 dB
(decibels) at 3 kHz which is low compared to an operational sonar) would have made the damage mechanism even more puzzling. • The experimental sonar used in the test, Towed Vertically Directive Source (TVDS) which had the dual 600 Hz and 3 kHz transducers, had been used for the first time in the Mediterranean Sea south of Sicily the year before in June 1995. Previous activated towed array sonar research using different sources on board the same ship included participation in NATO exercises "Dragon Hammer '92" and "Resolute Response '94". Since the source level of this experimental sonar was only 226 dB at 3 kHz re 1 uPa m, at only 100 meters the received level would drop by 40 dB (to 186 dB). A NATO panel investigated the above stranding and concluded the whales were exposed to 150-160 dB re 1 μPa of low and mid-range frequency sonar. This level is about 55-65 dB less (about a million times lower intensity) than the threshold for hearing damage specified at 215 dB by a panel of marine mammal experts. The idea that a relatively low power sonar could cause a mass stranding of such a large number of whales was unexpected by the scientific community. Most research had been focused on the possibility of masking signals, interference with mating calls, and similar biological functions. Deep diving marine mammals were species of concern, but little definitive information was known. In 1995 a comprehensive book on the relation between marine mammals and noise had been published, and it did not even mention strandings. In 2013, research showed
beaked whales were highly sensitive to mid-frequency active sonar.
Blue whales have also been shown to flee from the source of mid-frequency sonar, while naval use of mid- and high- frequency side-scan sonar was deemed "the most probable cause" of a
mass stranding of around 50
short-beaked common dolphin (
Delphinus delphis) on 9 June 2008 in
Falmouth Bay,
Cornwall, UK. A review of evidence on the mass strandings of beaked whale linked to naval exercises where sonar was used was published in 2019. It concluded that the effects of mid-frequency active sonar are strongest on Cuvier's beaked whales but vary among individuals or populations, which may depend on whether the individuals had prior exposure to sonar, and that symptoms of decompression sickness have been found in stranded whales that may be due to their response to sonar. It noted that no more mass strandings had occurred in the Canary Islands once naval exercises where sonar was used were banned there, and recommended that the ban be extended to other areas where mass strandings continue to occur. ==Acoustically induced bubble formation==