Book II, Chapter 8

Now let us first determine the facts about sound and hearing. Sound is of two kinds: the one is a sort of actuality, the other a potentiality. For we say that some things have no sound, such as a sponge or wool, while others have sound, such as bronze and all bodies that are solid and smooth, because they are able to make a sound; that is, they are able to produce a sound in actuality in the space between themselves and the organ of hearing.

Sound in actuality always arises from something, against something, and in something; for it is a stroke that produces it. Hence it is impossible that a sound should be produced where there is only one thing, since that which strikes and that which is struck are different; so that what makes a sound makes it against something. And a stroke does not occur without locomotion. But, as we said, sound is not the striking together of any chance bodies; for wool, if struck, produces no sound at all, whereas bronze does, and so do all bodies that are smooth and hollow: bronze because it is smooth, while hollow bodies, by reverberation, produce many strokes after the first, since that which has been set in motion is unable to escape.

Again, sound is heard in air, and also in water, though less well. But neither air nor water is that upon which sound chiefly depends; rather, there must be a stroke of solid bodies against one another and against the air. And this occurs when the air, on being struck, stands its ground and is not dispersed. Hence, if the air is struck quickly and violently, it gives out a sound; for the movement of the striker must outstrip the crumbling away of the air, just as one might strike a heap or a column of sand while it is being swept rapidly along.

An echo occurs when the air, having been made one mass by a vessel which bounds it off and prevents it from being scattered, is driven back again, like a ball that rebounds. And it seems that an echo is always produced, though it is not always distinct, since what happens in the case of sound is just what happens in the case of light. For light too is always reflected — otherwise there would not be light everywhere, but darkness outside the region lit by the sun — yet it is not reflected in the way in which it is reflected from water or bronze or any other smooth surface, so as to cast a shadow, by which we define light.

The void is rightly said to be that upon which hearing chiefly depends. For air is thought to be void, and it is air that produces hearing, when it is set in motion as something continuous and one. But because it is friable, it does not carry a sound unless the body struck is smooth. In that case the air becomes one all at once because of the surface; for the surface of a smooth body is one.

That, then, is capable of producing sound which is capable of moving a single continuous mass of air right up to the organ of hearing; and the organ of hearing has air naturally united with it. And because the organ is in air, when the air outside is moved the air within is moved also. Hence the animal does not hear with every part of itself, nor does the air pass through every part; for the part which is to be moved and which is ensouled does not everywhere contain air.

The air, then, taken by itself, is soundless, because it is so easily broken up; but when it is prevented from being broken up, its movement is sound. And the air within the ears has been walled in so as to be immovable, in order that the animal may perceive accurately all the varieties of the movement. For this reason we hear in water too, because the water does not make its way in to the connate air itself; indeed it does not even enter the ear, owing to the convolutions. But when this does happen, the animal does not hear; nor does it hear if the membrane is diseased, just as one does not see when the skin over the pupil is diseased.

Moreover, it is a sign of hearing or of its absence that the ear rings continuously, as a horn does; for the air in the ears is always moved with a certain movement of its own, but the sound heard is something foreign and not its own. And it is for this reason that men say we hear with what is empty and resonant, namely because we hear with that which contains air bounded off.

But is it the body struck that sounds, or the body striking? Or is it both, though in different ways? For sound is a movement of that which is capable of being moved in the way in which things rebound from smooth surfaces when someone strikes them. Not everything, as has been said, gives out a sound when it is struck or strikes — for instance, if one needle is struck against another — but the body struck must be even, so that the air may rebound and be shaken in a single mass.

The differences between sounding bodies are made manifest in sound in actuality; for just as colours are not seen without light, so neither is the high note nor the low perceived apart from sound. These terms are used metaphorically, being transferred from the objects of touch; for the sharp, or high, moves the sense much in a short time, while the heavy, or low, moves it little in a long time. It is not, then, that the high note is quick and the low slow, but the movement of the one becomes such as it is on account of its quickness, and that of the other on account of its slowness. And there seems to be an analogy here with the sharp and the blunt in the province of touch; for the sharp, as it were, stabs, while the blunt, as it were, pushes, because the one moves in a short time and the other in a long, so that it turns out that the one is quick and the other slow. Concerning sound, then, let this be our determination.

Voice is a kind of sound belonging to an ensouled thing; for none of the soulless things has voice, though they are said to have voice by a likeness, as, for instance, the flute and the lyre and all other soulless things which have extension of pitch, melody, and articulation. For the likeness seems to consist in this, that voice too has these attributes. But many animals have no voice, such as the bloodless animals, and among those that have blood, the fishes. And this is reasonable, if indeed sound is a certain movement of air. But those which are said to have voice, such as those in the Achelous, make their sound with their gills or with some other such part. Voice is a sound made by an animal, and not made with any chance part of the body.

But since everything makes a sound when something strikes something else and in something, and this last is air, it is reasonable that only those animals should have voice which take in air. For nature already uses the breath that is drawn in for two functions, just as it uses the tongue both for tasting and for speech, of which tasting is necessary (and hence belongs to more animals), while expression is for the sake of well-being; so likewise it uses the breath both for the internal heat, as something necessary (the reason for this will be stated elsewhere), and for voice, in order that well-being may be attained. The organ of respiration is the throat, and the part for the sake of which this member exists is the lung; for it is by this part that land animals have more heat than the rest. But the region about the heart is the first that stands in need of respiration. Hence the air must necessarily pass inward when the breath is drawn in.

So that voice is the striking of the inhaled air, by the soul that resides in these parts, against what is called the windpipe. For not every sound made by an animal is voice, as we have said (since it is possible to make a sound with the tongue as well, or in the way that men do when they cough), but that which strikes must be ensouled and must act with some imagination; for voice is indeed a sound with a meaning, and not merely a sound of the inhaled air, as a cough is. Rather, with this air the animal strikes the air in the windpipe against the windpipe itself. An indication of this is that one cannot speak while breathing in or breathing out, but only while holding the breath; for he who holds his breath uses it to produce this movement. It is clear too why fishes are voiceless: it is because they have no throat. And they lack this part because they do not take in air and do not breathe. But the reason for this belongs to another discussion.