Sonar | Bangkok Post: learning

THE CHINGCHOK Hunter

I have a question for you, the answer to which I will give you at the end of this article. You are on a boat looking for a school of fish when your sonar finally finds a school. It emits a "ping" that travels at the speed of sound, which is about 1,500 metres per second in water, and the reflected sound is received half a second later. Using the formula distance equals to speed multiplied by time, work out how deep the fish are, but be careful - there is a catch (pun intended).

What is sonar?

The word sonar is an acronym for SOund NAvigation and Radar. Sonar was developed into a widely-used tool during the Second World War to detect enemy ships and submarines.

It is also used by submarines to navigate as well as by fishermen looking for fish. It is a very useful tool and, as you will find out, sonar has been around for millions of years, but let me first explain how it works.

Science of sonar

Sonar is the use of sound to navigate or find things where sight is not optimal, which is especially true in the case of trying to see great distances through water. By using sound in the absence of sight, you can build a virtual image of your surroundings.

Sonar relies on the fact that sound travels at a constant speed through a known medium such as air or water. Certain factors affect the speed of sound, such as air temperature, and sound travels marginally slower in fresh water than it does in salt water, but the difference is marginal.

Different objects emit different sounds that can usually be differentiated by whomever may be listening. For example, the sound given out by an engine of a submarine is vastly different from the sound of a singing whale, so whoever is listening can usually tell what is in the vicinity.

In water, sound travels at a relatively constant 1,484 metres per second, which is over four times faster than it does in air. By recording the time and using the formula stated at the beginning of the article (distance = speed x time), the distance of an object and the material it is made of can be determined.

Sound is a result of vibrations causing pressure waves which travel by the transmission of energy from particles to neighbouring particles. As the particles in water are much closer together than those in air, the transmission of energy is easier and quicker, which is why sound travels faster in water than in air. Based on this knowledge, you can deduct that sound travels faster in solids than in liquids.

Passive and active

Passive sonar receives or reflects signals rather than generates its own signal. This term is used when listening devices simply listen to the noises in the ocean and interpret what they hear. Scientists use passive sonar to study marine animal behaviour.

Ships and submarines also use passive sonar to find out what is in the water around them. Many ships have passive sonar devices in order to determine who and what they are sharing the water with and where it is coming from or going to (based on Doppler shifts) without giving their own position away, providing other neighbouring vessels aren't listening to them!

Active sonar is the more familiar version of sonar. An electrical impulse from a transmitter is converted to a sound wave via a transducer that is known as a ping, which is emitted from sea vessels. The sound wave travels through the water until it hits something and it is then reflected, just like when you hear your echo in a cave.

Reflections make the sound wave change direction without changing the speed of the wave. The reflected waves are then detected by a receiver and converted into an electrical signal that can be "read" on a display.

This image tells the person where the objects are, the sizes of the objects and the direction in which they are travelling. Active sonar can be dangerous to use in warfare, however, because emitted pulses can be detected by potential enemy vessels, thus giving away the emitter's position.

Stolen from the animals

As I said earlier, sonar has been around for millions of years but has obviously not been used by humans for that long. Bats, cetaceans (a marine mammal of the order Cetacea, e.g., a whale, dolphin, wholphin or porpoise), shrews and even some birds use sonar, which makes the human use of sonar seems extremely underdeveloped and unsophisticated.

Dolphin sonar is so sophisticated that it could easily tell the difference between a 1-baht and 10-baht coin in the water from 20 metres away! But in fairness to humans, our sonar has only developed over the last century, whereas dolphins have been developing it over millions of years.

Swimming with dolphins is not only an awe-inspiring experience, it is also a noisy one! Clicks and squeaks are being constantly emitted, with many more at frequencies well above our hearing range.

These clicks are emitted from their nasal passage and travel through a fatty organ in their forehead known as a melon. This acts as a kind of focusing lens that allows the dolphin to direct the sound wave into a beam, thereby allowing greater accuracy.

The sound waves reflect off surrounding features under the sea, and the reflected energy waves are picked up by the jawbone of the dolphin, which then sends signals to the brain via nerves where it is interpreted. If you consider that a dolphin can emit and receive about 600 clicks per second, the amount of information that it processes is phenomenal.

In the depths of the sea where visibility may be only a few metres, the dolphin can create a very accurate virtual image by using echolocation and can navigate amongst objects safely. Consequently, it can hunt very effectively, even if some of the fish it hunts are under the sand!

Sadly for dolphins, fishing nets do not reflect the dolphins' sonar, and this can often result in dolphins being inadvertently caught in fishermen's nets, where they drown. Other problems with sonar occur when human sonar interferes with dolphins' and other cetaceans' sonar.

There is very strong evidence to suggest that whales and dolphins beach themselves, that is, they swim onto land, become stranded and die, as a result of interference and confusion with sonar emitted from ships and submarines. Thankfully, efforts are being made to make dolphin-friendly nets, and sonar technology is being developed to not interfere with marine mammal navigation. And let's be honest, it's the least we can do as we stole the technology from them in the first place!

In answer to the question at the beginning of the article, distance equals to speed (1500m/s) multiplied by time (0.5s), thus giving a result of 750 metres. But remember, the sound wave has to travel to the fish/object and back, so halving the answer will give you a distance of 375 metres. What a useful tool!

Dave Canavan has an MSc in Behavioural Ecology and is the Head of Secondary at Garden International School. Dave is fascinated by science and loves animals, especially the dangerous kind! You

may contact Dave at davidc@gardenbangkok.com .

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