A tuning fork which is set into vibration gives out a sound which we hear if we are reasonably near to it, so it is clear that the effects of the movement of the prongs travel outwards from the fork in some way. In this chapter we shall look at the principles which govern the travelling of sound waves and at the various ways in which wave-motions may be propagated or transmitted.

The waves with which we are most familiar are those which can be seen on the surface of water. If a stone were dropped neatly into the middle of a perfectly still pond, we should see ripples begin to spread outwards in all directions towards the edge of the pond. Seen from above these ripples would appear as concentric circles. After some time the disturbance which took place in the middle of the pond would affect the surface of the water some distance away, in other words the wave-motion would have travelled from the middle towards the edge of the pond. Without thinking about the matter very closely, we may well be inclined to believe that this means that drops of water which started out in the middle of the pond have progressed towards the edge, but this is not of course the case. Suppose that something very light, such as a small cork, were floating on the water before the stone was dropped; when the ripples reached it, it would simply move rhythmically up and down as successive waves passed through the spot, without getting any nearer to the edge of the pond.