CHAPTER VIII

SOUND. ITS CAUSES. ECHO

             NOW let us start by treating of sound and hearing. Sound is twofold: the sort that is actual, and the sort that is potential. For certain things, we say, have no sound, such as sponges, wool and fur; while others, such as bronze and all other smooth and hard things, have sound, because they are able to produce it, i.e. to cause actual sound in the medium and in the hearing.§§ 439-41

             Sound in act is always of something, on something, in something: for it is caused by percussion. Hence it is impossible for anything by itself, as a single thing, to produce sound. For there must be one thing that strikes and another that is struck; hence whatever emits sound does so 'on something', i.e. by contact with something--which, when touched with a blow, sounds. And the blow necessarily implies movement.§ 442

             As we said before, it is not a blow upon anything whatever that gives sound: wool makes no sound, although it be struck; but bronze, or anything smooth and hollow, is such. Bronze because it is smooth; whilst hollow things by repercussion produce many 'blows' after the first, since what is set in motion cannot find an outlet.§§ 443-4

             Further: sound is heard in air and water, but less in water. It is, however, neither air nor water that, properly, sounds; there must be a percussion of solid objects on each other, and on air. This happens if air is confined when struck, and cannot disperse. Hence if the striking is rapid and violent sound results. For the movement of what strikes must be sudden, if it is to out-run the breaking-up of the air; just as if one were to strike a rapidly-moving heap of sand or pile of stones.§§ 445-6

             Echo arises when air rebounds like a ball against air rendered a compact unity by a restraining vessel that prevents its dispersion.§§ 447-9

             It seems there is always some echo, but not always a clear one. For the same occurs with sound as with light; which also is always reflected: otherwise it would not spread to every part, but beyond the area illuminated by the sun there would be darkness. Still, it is not [always] reflected as it is reflected by water or bronze or other smooth things; hence it makes the shadow by which we discern the boundaries of light. § 450