The explanation of this too will be given later.
8
Now let us, to begin with, make certain distinctions about sound and
hearing.
Sound may mean either of two things (a) actual, and (b) potential,
sound. There are certain things which, as we say, 'have no sound',
e.g. sponges or wool, others which have, e.g. bronze and in general
all things which are smooth and solid-the latter are said to have a
sound because they can make a sound, i.e. can generate actual sound
between themselves and the organ of hearing.
Actual sound requires for its occurrence (i, ii) two such bodies and
(iii) a space between them; for it is generated by an impact. Hence it
is impossible for one body only to generate a sound-there must be a
body impinging and a body impinged upon; what sounds does so by
striking against something else, and this is impossible without a
movement from place to place.
As we have said, not all bodies can by impact on one another produce
sound; impact on wool makes no sound, while the impact on bronze or
any body which is smooth and hollow does. Bronze gives out a sound
when struck because it is smooth; bodies which are hollow owing to
reflection repeat the original impact over and over again, the body
originally set in movement being unable to escape from the concavity.
Further, we must remark that sound is heard both in air and in
water, though less distinctly in the latter. Yet neither air nor water
is the principal cause of sound.
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