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Dolphins are highly intelligent, curious and playful animals. They live in large areas of the ocean and travel thousands of miles. There have been many documented cases going back thousands of years that tell of dolphins befriending humans and saving the lives of stranded swimmers in troubled waters. There are also cases of dolphins bravely sacrificing themselves to save swimmers and divers from predatory sharks.

So why does this brave protector and friend of humans sometimes strand themselves on beaches or in estuaries for no apparent reason? Fist we need to find out a little bit about them.

How do dolphins find their way around in murky waters?

In the coasts around Europe the sea is often so murky that that it is impossible to see for more than a few centimetres. Divers who work in British waters know this well and spend a lot of time working by touch. So how do dolphins ‘see’ in conditions where their eyes are not much use?

To investigate how they do this, scientists blindfolded a number of dolphins and released them into large underwater obstacle courses. What they discovered was truly amazing. Dolphins can detect the size, shape and speed of objects hundreds of metres away. They can ‘see’ obstacles only a few millimetres across and know whether they are soft or hard, alive or dead.

Underwater Detection

Do dolphins have super-human powers?

Powers like this seem almost super-human so how do they do it? They use echolocation. This is the location of objects by their echos. Water is an excellent sound conductor, much better than air. Sound in water travels at about 1500 m/s compared to only 330 m/s in air, this is over four times as fast. It will also travel over very long distances.

What about the whistles and clicks?

The whistles and clicking sounds dolphins make have nothing to do with echolocation because the frequency is far too low. Dolphins produce other sounds much too high for humans to hear. The human range of hearing is 20-20,000 Hertz (Hz). One hertz is equal to one vibration per second. A sound with a low frequency will have a low pitch, such as a human's heartbeat. A sound with a high frequency will have a high pitch, such as a dog whistle.

Humans cannot hear sounds of every frequency. Any sound above 20,000 Hz is called ultra sound. The 'other' sounds dolphins produce are extremely high, around 150,000 Hz. Bats are the only creatures on land that can hear these sounds.

Can dolphins see sound?

How Dolphins see

It is thought that dolphins can actually ‘see’ sound, much the same way humans can see images. We can work out how much detail they can see by working out the wavelength of the ultra sound. Dolphins can ‘see’ objects no smaller than their wavelength. To work this out, we can use:

Wavelength = wave speed/frequency, to calculate a wavelength of about 3.6mm.

So they can see pretty small objects, but the bigger the frequency, the more they can see. If we were talking about light waves we’d say dolphins can see in colour!

The picture shows a face in the kind of detail that a dolphin could ‘see’ with ultrasound. About as good as a typical ultrasound scan of an unborn baby!

So, how do they do it?

When the high-pitched clicks dolphins make hit objects, some of the sound echos back. The dolphin listens to the echo and interprets the time it took before the echo came back, that way it can estimate the distance of the object.

Dolphin Perception

The clicks pass out through a big wax acoustic lens just behind their bulbous forehead. The lens focuses the sound into a tight beam. The sound waves are reflected back off any object the creature is studying. The dolphin receives the reflected signals in fat-filled chambers in both sides of its lower jaw.

Nerves carry the signal to the middle ear and back to the brain. By looking at the time between sending the click and hearing the echo, the distance can be found, also, as there are two receiving chambers they should be able to ‘see’ in stereo, locating the direction with great accuracy.

If the object is moving, the wavelength of the sound that reflects back is changed slightly. This is called the Doppler Effect. This allows the dolphin to estimate how fast the object is moving. If you’ve heard a fire engine go past and heard the note of the siren change, then you’ve heard the Doppler Effect.

 Select the Doppler Effect button to hear some examples.

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This is how police radar guns work to catch speeding motorists. They use radio waves instead of sound waves though.

What has SONAR and sand got to do with dolphin strandings?

Modern war ships use ultrasound sonar (SOundNavigationAndRanging) to find submarines. In fact, a number of dolphin and whale strandings have occurred when naval ships have been in the area. Coincidence? Probably not. No one is quite sure how it disrupts their navigation, but it seems to. It may be as simple as having a very bright light shone in the eyes. The ship's sonar may be so loud it will temporarily ‘blind’ the dolphin’s ultrasound system.

Warships using sonar to find submarines

Another thing that seems too much of a coincidence to be just chance is the fact that most strandings happen on shallow, sloping, flat, sandy beaches. It may be that the sound beam is totally internally reflected between the surface of the water and the beach, just like light being reflected up an optical fibre. Each time the sound is reflected some of its energy is absorbed, so no sound bounces back to the dolphin. To them the beach looks just like open sea.

The beach to a dolphin

It would be like a person walking in a pitch dark desert with only a torch. If no light bounces back when the torch shines in front, you would think the way is clear, so do the dolphins.

This is the sort of problem that anyone with a good knowledge of GCSE science could think about and just maybe come up with a valuable answer. There must be many factors involved.

Big questions:

Do you think you could use some the ideas in this article to help a blind person find their way around? Try and think of the possible problems and solutions, there are many!

What do the terms Infra and Ultra actually mean? How do they relate to light and the electromagnetic spectrum?