De Anima

Let us now begin by determining the nature of sound and hearing. There are two sorts of sound, one a sound which is operant, the other potential sound. For some things we say have no sound, as sponge, wool; others, for example, bronze and all things solid and smooth, we say have sound, because they can emit sound, that is, they can produce actual sound between the sonorous body and the organ of hearing. When actual sound occurs it is always of something on something and in something, for it is a blow which produces it. Hence it is impossible that a sound should be produced by a single thing, for, as that which strikes is distinct from that which is struck, that which sounds sounds upon something. And a blow implies spatial motion. As we stated above, it is not concussion of any two things taken at random which constitutes sound. Wool, when struck, emits no sound at all, but bronze does, and so do all smooth and hollow things; bronze emits sound because it is smooth, while hollow things by reverberation produce a series of concussions after the first, that which is set in motion being unable to escape. Further, sound is heard in air and, though more faintly, in water. It is not the air or the water, however, which chiefly determine the production of sound: on the contrary, there must be solid bodies colliding with one another and with the air: and this happens when the air after being struck resists the impact and is not dispersed. Hence the air must be struck quickly and forcibly if it is to give forth sound; for the movement of the striker must be too rapid to allow the air time to disperse: just as would be necessary if one aimed a blow at a heap of sand or a sand-whirl, while it was in rapid motion onwards.

Echo is produced when the air is made to rebound backwards like a ball from some other air which has become a single mass owing to its being within a cavity which confines

it and prevents its dispersion. It seems likely that echo is always produced, but is not always distinctly audible: since surely the same thing happens with sound as with light. For light is always being reflected; else light would not be everywhere, but outside the spot where the sun's rays fall there would be darkness. But it is not always reflected in the same way as it is from water or bronze or any other smooth surface; I mean, it does not always produce the shadow, by which we define light.

Void is rightly stated to be the indispensable condition of hearing. For the air is commonly believed to be a void, and it is the air which causes hearing, when being one and continuous it is set in motion. But, owing to its tendency to disperse, it gives out no sound unless that which is struck is smooth. In that case the air when struck is simultaneously reunited because of the unity of the surface; for a smooth body presents a single surface.

That, then, is resonant which is capable of exciting motion in a mass of air continuously one as far as the ear. There is air naturally attached to the ear. And because the ear is in air, when the external air is set in motion, the air within the ear moves. Hence it is not at every point that the animal hears, nor that the air passes through: for it is not at every point that the part which is to set itself in motion and to be animate has a supply of air. Of itself, then, the air is a soundless thing because it is easily broken up. But, whenever it is prevented from breaking up, its movement is sound. But the air within the ears has been lodged fast within walls to make it immoveable, in order that it may perceive exactly all the varieties of auditory movement. This is why we hear in water also, because the water does not pass right up to the air attached to the ear, nor even into the ear at all, because of its convolutions. Should this happen, hearing is destroyed, as it is by an injury to the membrane of the tympanum, and as sight is by an injury to the cornea. Further, we have evidence whether we hear or not, according as there is or is not always a ringing sound in

the ears, as in a horn: for the air imprisoned there is always moving with a proper motion of its own. But sound is something of external origin and is not native to the ear. And this is why it is said that we hear by means of what is empty and resonant, because that by which we hear has air confined within it.

Does that which is struck emit the sound or that which strikes? Is it not rather both, but each in a different way? For sound is motion of that which is capable of being moved in the same Ianner as things rebound from smooth surfaces when struck sharply against them. Thus, as above remarked, it is not everything which, when struck or striking, emits sound: supposing, for instance, a pin were to strike against a pin, there would be no sound. The thing struck must be of even surface, so that the air may rebound and vibrate in one mass.

The varieties of resonant bodies are clearly distinguished by the sound they actually emit. For, as without light colours are not seen, so without sound we cannot distinguish high and low or acute and grave in pitch. These latter terms are used by analogy from tangible objects. For the acute, that is, the high, note moves the sense much in a little time, while the grave or low note moves it little in much time. Not that what is shrill is identically rapid, nor what is low is slow, but it is in the one case the rapidity, in the other the slowness, which makes the motion or sensation such as has been described. And it would seem that there is a certain analogy between the acute and grave to the ear and the acute and blunt to the touch. For that which is acute or pointed, as it were, stabs, while the blunt, as it were, thrusts, because the one excites motion in a short, the other in a long time, so that per accidens the one is quick, the other slow. Let this account of sound suffice.

Voice is a sound made by an animate being. No inanimate thing is vocal, though it may by analogy be said to be vocal, as in the case of the pipe, the lyre and all other inanimate things that have pitch and tune and articulation: for these qualities, it would seem, the voice also possesses. But many animals have no voice: that is to say, all bloodless animals and, among animals that have blood, fishes. And this is what we might expect, since sound is a movement of air. Those fishes which

are said to possess voice, such as those in the Achelous, merely make a noise with their gills or some other such part. Voice is sound made by an animal, and not by any part of its body indifferently. But, as in every case of sound there is something that strikes, something struck and a medium, which is air, it is reasonable that only creatures which inhale air should have voice. For here nature uses the air that is inhaled for two purposes, just as it uses the tongue for tasting and for speech, the former use, for tasting, being indispensable and therefore more widely found, while expression of thought is a means to well-being. Similarly nature uses the breath first as a necessary means to the maintenance of internal warmth (the reason for which shall be explained elsewhere) and, further, as a means of producing voice and so promoting well-being. The organ of respiration is the larynx, and the part to which this part is subservient is the lung: for it is this organ, namely, the lung, which enables land animals to maintain a higher temperature than others. Respiration is also needed primarily for the region about the heart. Hence, as we draw breath, the air enters: and so the impact upon the windpipe, as it is called, of the air breathed is voice, the cause of the impact being the soul which animates the vocal organs. For, as we said before, it is not every sound made by an animal that is voice. Noise can be produced even with the tongue or as in coughing: but it is necessary for voice that the part which strikes should be animate and that some mental image should be present. For voice is certainly a sound which has significance and is not like a cough, the noise of air respired: rather with this air the animal makes the air in the windpipe strike against the windpipe. A proof of this is the fact that we cannot speak while inhaling or exhaling breath, but only while we hold it in: for anyone who holds his breath uses the breath so held to cause motion. And it is evident why fishes are voiceless. It is because they have no larynx. And they are without this part because they do not take in the air nor breathe. Why this is so does not concern us here.