Children can often identify the source of a sound and they know that their ear is required to detect it but they have difficulty in understanding what happens in between.
The vibrating sound source moves in such a way that the air around it is also made to move. Imagine a drum skin being hit hard with a beater. The skin moves to one side and squashes or compresses the air next to it. This compressed air "pocket" in turn pushes and compresses the air next to it while the "pocket" itself bounces back towards the position it came from. The compressing effect and stretching effect therefore moves outwards from the sound source. The movement of the air back and forth is itself a rapid vibration and the movement of the effect outwards is in a wave form. Eventually the effect reaches the ear and is made into signals which are sent to the brain.
Fig 1 Sound vibrations travelling through the air
Sound vibrations, then, travel outwards in all directions in waves from a sound source. As they travel outwards the energy they contain becomes dissipated and therefore the sound becomes weaker the further it is from the source. The shape of a sound wave with no obstacles in its way would be approximately spherical.
Figure 1 shows the air as particles or molecules. Where the molecules are pushed closer together is an area of compression and when they spring back (even further apart than before) there is an area of rarefaction. It can be seen that while the wave of compressed molecules moves away from the source, the molecules themselves only move a very small distance to and fro. Thus, the air does not flow from the source to the ear - an idea often held by children.
A very good way of demonstrating how the molecules of a substance behave when transmitting a sound is to use a "slinky" spring extended on a desk surface. Push one end rapidly and a wave is sent along the length of the spring. It is possible to see the wave bounce back (an echo) and of course to see that the parts of the spring (representing the molecules) do not move along with the wave but merely "vibrate" back and forth.
Fig 2 Use a slinky to demonstrate sound waves.
Sound waves are called longitudinal waves because the particles move back and forth in the direction of the wave movement. A transverse wave is like a wave on the sea in which the particles of water move vertically and not in the direction of the wave itself. for this reason it is a good idea to avoid likening a sound wave to the ripples on a pond or the wave produced by a skipping rope attached to a wall.
Sound needs a medium in which to travel. Sound waves cannot form unless there are molecules to bump into each other to pass the wave form along. Sounds will therefore not travel in space where only a vacuum exists. You may have seen a classic demonstration in which an electric bell is enclosed in a glass bell jar. As the air is slowly pumped out of the jar the ringing bell is seen to be still moving but the sound gradually diminishes until it cannot be heard at all. Astronauts working in space or on the surface of the moon can therefore only talk to each other by using radio communication.
ContentsWhat is sound, Vibration