Sound for Sound Engineers

Chapter 1 : The Nature of Sound : Part 4 of 5

   Part 4 - More About Waves
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The Water Analogy

Sound waves are often compared to the ripples you get after dropping a stone into the middle of a still pond. You see the waves (ripples) spread from the impact point to the edge of the pond - but it's easy to forget that the particles of water themselves don't move far away from their usual rest positions.

Think of a boat sitting out well beyond the shore break in a big swell. Your eyes can follow a wave coming in past the boat all the way to the beach, but it's an illusion - the water itself (the medium which carries the wave) is just moving up and down, causing our boat to move up and down with it. Pick a point on the line and watch it carefully - it only moves vertically, but the wave itself travels from left to right.

In some ways, sound waves are very similar to water waves, but there is one big difference. Water particles move at right angles to the direction of travel of the wave, whereas air particles move backwards and forwards in the same direction that a sound wave is travelling.

In air, the particles have a certain natural spacing between them - they repel other particles that come too close. If a vibrating solid (like a guitar string) moves an air particle closer to its neighbour than it should be, the neighbouring particle is forced away a split-second later - but then it gets too close to the next particle and that one moves away too.

When the guitar string moves back, the air particles move back too, each one allowing its neighbour to move back in the opposite direction. The particles themselves just move backwards and forwards, but the compression zones formed by their crowding together move continuously from the source of the sound outwards.

Still confused about waves? Click here for an explanatory window.

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