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1. Reflection of sound waves

If a source of sound is directed at a vertical surface some distance away, an ECHO may be heard. Sound waves "bounce" off the vertical surface, and are REFLECTED back towards the source. Reflection of sound waves obey the laws of reflection.

The human ear cannot distinguish between two sound bursts if they are less than about 0.1 s apart. For that reason, in order to hear an echo, the reflecting object must be more than 17 m away from the hearer (assuming sound travels at 340 m·s-1).

The principle of reflection of sound waves finds application in the detection of underwater objects: if a pulse of sound, emitted by the sonar equipment of a warship, encounters an underwater obstacle, such as a submarine, an echo of the pulse may be picked up by the emitting vessel, thereby enabling the position of the submarine to be determined quite accurately, even through the submarine might be several kilometers away (). (Animation by courtesy of the Institute for Maritime Technology), (See also Echolocation.)

2. Refraction of sound waves

Sound waves travel more slowly when they pass from a lighter to a heavier gas. Thus a balloon filled with carbon dioxide acts as a converging lens, and is able to focus sound waves:

This occurs because sound travels more slowly inside a heavier gas, and the sound waves are refracted towards the normal. If the gas is lighter than air, the sound waves are refacted away from the normal. A balloon filled, for example, with hydrogen, will act as a diverging lens. See also Echolocation.)

3. Absorption of sound waves

When a sound wave impacts on an object, part of the energy of the wave is transferred to the particles making up the object. We say that the sound has been ABSORBED by the material. It is found that high frequency sound waves are absorbed more readily than low frequency sounds, and so it is more accurate to talk about SELECTIVE ABSORPTION. Absorption takes place best with soft, porous materials.

One makes use of such materials in improving the ACOUSTICS of large rooms, such as theaters and concert halls. In these places, it is undesirable for sounds to echo, as the short delay between the emission of the sound and its reflection results in undesirable effects as far as the listeners are concerned. By lining the walls of the room with cork or plastic foam tiles, the reflection of the sounds is suppressed, and the acoustics of the room are thereby improved.

4. Resonance

Sound waves from a source may, under certain circumstances, increase or initiate vibrations in other sources. This occurs when the frequency of the two sources are equal.

This can easily be demonstrated by placing a vibrating tuning fork above a straight-sided jug. By adjusting the level of water in the jug, a point is reached when a sound is emitted by the jug when the fork is brought over its opening.

5. Additional questions

Echolocation in animals

ECHOLOCATION is a process whereby certain animals can locate their prey. For example, bats emit very high pitched sounds that are reflected by insects. The very sensitive ears of the bats pick up the reflected sound, and the bats then homes in on the source of the reflection.

Dolphins have a very complicated echolocation apparatus. Clicking sounds emitted by the nasal sacks of the dolphin are refracted by a fatty organ called the "melon", which allows a narrow sound beam to be emitted. The sound waves from the beam, upon striking a fish, are reflected back and picked up by organs in the skull of the dolphin, thus enabling it to locate the fish.