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  • AutorenbildMichael Mutter

How much sonar can divers tolerate?

In an incident in international waters between the Australian and Chinese navies, tensions escalated in November last year when Australian naval divers were submerged. While they were removing fishing nets tangled in the propeller of the frigate Toowoomba, a Chinese destroyer approached and activated its sonar, causing minor injuries to the divers. This raises the question of how dangerous sonar can be for divers.

The most famous "ping" in movie history. "Hunt for Red October"

Sonar and its intensity

Sonar, an acronym for Sound Navigation and Ranging, uses sound waves to navigate, communicate and detect objects underwater. Sonar plays a crucial role in naval operations, with the systems emitting sound waves in a specific frequency range (active sonar), which are reflected by objects under water. The reflected sound waves are detected, processed further and displayed on screens. Typical marine sonar can reach intensities between 185 and 215 decibels or more.

Decibels (dB) measure the relative pressure of sound using a logarithmic scale based on human auditory perception, which is not linear. This means that a small change in the decibel level corresponds to a large change in the actual sound pressure.

The scale must be adjusted for underwater conditions, as sound propagates differently in water than in air. A sound pressure level of 200 dB under water corresponds to approx. 10000 Pa or 0.1 bar. This means that water pressure fluctuations of 0.1 bar occur at this level. Depending on the area of application, low-frequency (50 Hz to 3 kHz), medium-frequency (3 kHz to 15 kHz) or high-frequency active sonar is used. Low-frequency sonar is used to locate submarines, for example, with transmission distances of over 20 km. Passive sonar, on the other hand, does not emit active sound pulses, it only listens for sounds.

Harmful effects on divers

The main health risk for divers exposed to strong sonar is barotrauma, in other words damage caused by pressure. The sound waves from active sonar cause high-frequency pressure fluctuations. For example, a sonar pulse of 200 dB at a low frequency of 50 Hz means that the pressure fluctuates by 0.1 bar 50 times within one second. The higher the sound intensity and the higher the frequency, the stronger and more frequent these pressure fluctuations are.

A typical US Navy sonar profile for submarine hunting covers a frequency range of 100 to 500 Hz, a sound level of about 215 dB consisting of different waveforms in a sequence of about 60 seconds with transmission pauses of 6 to 15 minutes during which the sonar listens.

It is not surprising that such exposure causes injuries to typical pressure-sensitive anatomical structures such as the ears, sinuses and lungs. Divers can suffer pain and, in severe cases, permanent damage to sensitive structures. Presumably due to direct stimulation of the semicircular canals, dizziness, visual field shifts and nausea can occur.

In addition, exposure to high-intensity sonar has been linked to symptoms similar to decompression sickness. Even if divers adhere to standard diving protocols, there is a risk of inert gas bubbles (nitrogen or helium) forming in the tissues and circulation due to the rapid pressure fluctuations that pass through the body. This is likely due to the mobilization of microbubbles from the tissues, which grow more or less rapidly depending on the extent of the prevailing supersaturation caused by the inert gases, thus triggering decompression sickness.

Furthermore, long sonar pulses (one second or longer) can disturb depth gauges and cause regulators to free flow.

For these reasons, warships at anchor can also use sonar pulses to deter enemy combat divers from attacking.

Guidelines and exposure limits

Navies generally prohibit the use of sonar when divers are submerged. If sonar must still be used for operational reasons, strict safety measures apply, which take into account factors such as the intensity and duration of sonar exposure and the depth of the dive in order to protect divers from sonar damage. For example, the US Navy's diving manual allows a maximum exposure of 215 dB for a total of 13 minutes within 24 hours, whereby a minimum distance of 13 to 56 yards (12 to 51 m) from the sonar source must be maintained and it is clearly stated that pressure levels above 205 dB should be avoided whenever possible.

If you consider what this means in terms of pressure fluctuations (215 dB = 0.56 bar or a change of 5.6 m water column), these specifications are not surprising.

Neoprene offers a certain degree of protection as it absorbs pressure waves, but only up to a maximum of 215 dB. The probability of physiological damage increases significantly when the sound pressure rises above 200 dB. Under these circumstances, divers may only be exposed to sonar if they are wearing full-body suits and hoods.

Effects on marine life

Marine mammals, especially whales, are very sensitive to underwater noise. The disruptive effect of sonar can disturb their orientation and cause them to change their migration or diving depths. Mass strandings of cetacean groups are repeatedly associated with marine sonar. In addition, there has for some time been a debate as to whether sonar can also cause bubble formation in the tissue of whales and dolphins and thus trigger decompression sickness, leading to the death of these marine mammals.

Commercial sonar vs. marine sonar

Commercial fish finders or echo sounders operate at much lower intensities than marine sonars. These devices, which generally emit sound waves in the range of a few tens of decibels, are designed for a limited range and pose no risk to divers. So if a fishing boat in Lake Zurich picks you up, there is no danger. But if you are pinged by the USS "Dallas" during the "Hunt for Red October", you have a problem.

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