LECTIO SIXTEEN

             § 439. Having disposed of the visible, the Philosopher now deals with the audible, that is, with sound. This section divides into two parts. In the first he deals with sound in general. In the second he deals with one kind of sound, namely voice, at 'Voice' etc. The first part is again divided into two parts. First, he deals with sound itself; then with difference of sound, at 'Differences in sonorous things'. The former part again subdivides. First, he reaches a conclusion on sound; then he raises a doubt about this conclusion, at 'Is it that which strikes'. The former of these sections again divides into two conclusions, the first on the origin of sound, and the second on the way that sound affects the hearing, at 'Empty space'.

             Should it be asked why he now raises the question of the origin of sound, whereas he did not deal with the production of colour, but only with colour's effects on sense-organ and medium, the answer is that colour and odour and taste and the tangible qualities have a fixed and permanent existence in their subjects. The consideration therefore of these qualities in themselves is one thing, and that of the way they affect the senses is another. The two questions are quite distinct. So the Philosopher deals with the origin of colour, taste and smell in his work De Sensu et Sensato; and with that of tangible qualities in the De Generatione, and in part also in the Meteorologica; whilst in the present book he is only considering the objects of sensation in so far as they affect the sense-organs. But as sound is caused by change and has no fixed and stable existence in a subject, but actually consists in a movement or change, therefore it can be considered at one and the same time in its objective origin and in its effect on the senses. Its origin, then, is regarded under two aspects: first he deals with the primary origin of sound; and then, at 'Echo arises', with its secondary origin, produced by reverberation. As to the primary origin, he first explains that sound is sometimes in act and sometimes in potency; after which he shows how sound comes to be in act, at 'Sound in act'.

             § 440. He says, then, first that, before dealing with touch and taste, we must consider sound and smell; but first sound, because it is more spiritual, as has been shown. We speak of sound in two ways: as in act and as in potency. We say a thing has a sound when it is actually sounding, and also when it is only able to make sounds; as when we say, 'this bell sounds well', though it may not be actually ringing. In the same way we say of some things that they have no sound, meaning that they have no power to produce sound, e.g. sponges and similar soft objects; whereas other things, on the contrary, are said to sound because they are especially apt to produce sound, such as bronze, and other smooth and even materials. So it is clear that sound is sometimes spoken of as potential, and sometimes as actual.

             § 441. But the actuality of sound involves the medium and the faculty of hearing. For we can speak of a sense-object as actual in two ways: (1) So far as the object is actually being sensed, i.e. when its likeness is affecting the sense-organ. In this way a sound is actual when it is heard. (2) So far as the object actually is such that it can be sensed, but is such simply in its own objective being, outside the senses. And in this way the other sense-objects, colour, odour, savour, etc., exist actually in coloured or odorous or savourable bodies. But not so sound; for in a sound-productive body there is sound only potentially: actual sound exists only when the medium is affected by a disturbance from that body. Therefore the act of sound exists, he says, in the medium and in the hearing, but not in the audible body.

             § 442. Then at 'Sound in act', Aristotle shows how sound actually comes about: first with regard to the number of factors required for sound to come into act; and then, at 'As we said before', with regard to what these factors are. First then, there are three concurrent factors in sound: Sound is of something, on something, in something. Therefore no single thing by itself can produce sound--the reason (or sign) of this being that the cause of sound is percussion, which implies a thing struck and a striker. Hence his saying, 'of something, on something', i.e. of the thing striking and on the thing struck. What produces sound must touch something as with a sudden blow. Now a blow implies local movement, which implies a medium. Hence the need for a medium, if sound is to be produced actually. Hence his saying, 'in something'.

             § 443. When he says 'As we said before', he shows what sort of factors produce sound. And first he shows of what sort must the striking and the struck objects be; and then, at 'Further etc.', what sort of thing the medium must be. First, then, as we have already noted, sound is not produced by striking any material body whatever. Wool and sponges and suchlike soft things cannot give out sound even when struck; the reason being that soft things yield to the impact, so that no air is expelled by it--no air in which a sound might be formed by the blow of a striker and the resistance of something struck. Yet when such soft things are compressed firmly enough to resist an impact, then a sound results, though only a dull one. But bronze objects, and bodies that are smooth and concave, give out sound when struck. Hardness is needed; else air will not be driven out by the blow, and consequently no sound be caused. And the body struck should also be smooth, so that the air expelled may hold compactly together in one, as will be explained below.

             § 444. Hollow bodies give out sound very well, when struck, because they hold the air in, and, as this air first set in motion cannot at once come out, it disturbs the rest of the air, and thus by repercussion the impact and the sounding are multiplied. For this reason also materials with a suitable disposition of air in their composition are especially sonorous, such as bronze and silver; but materials in which air is less well disposed are less sonorous, like lead and such things as are more earthy and sedimentary.

             § 445. Then, at 'Further: sound is heard', he explains the medium in which sound is produced: observing that the medium of sound is air or water, but water less than air. The most suitable medium for both the production and the reception of sound is air. And since the medium of any sense must lack all the sensible qualities perceived by that sense, if it is to serve as the receiver of them, it is evident that neither air nor water has a sound of its own; but if sound is to be produced in air or water some firm and solid and hard objects must strike upon one another, and so cause an impact on the air.

             § 446. Now if the impact of solid bodies upon one another is to be simultaneous with the production of sound in the air, it is necessary that the air remain a whole and continuous, that it be not broke up before receiving the effect of the impact. Hence it is, as experience shows, that when two bodies are brought slowly together no sound occurs: for the air recedes and melts away before the contact is made. But if the impact is swift and violent, then a sound results; for if sound is to occur, the movement of what strikes must precede the division of the air, so that the air can be struck while still en bloc, or collected, and so a sound be produced in it. It is as when one strikes swiftly with a whip, hitting a whole heap of stones all at once, before they fall apart. For the same reason, when anything passes rapidly through the air, it makes a noise as it goes, because the air itself, being compact, serves as the object struck, not merely as the medium.

             § 447. Next, at 'Echo arises', he deals with the secondary production of sound, which is by reverberation. This sound is called echo. He settles, first, how echo itself is produced; and then, at 'It seems', how its varieties occur. With regard to the former point, we may note that the production of sound in air follows the movement of air, as has been said. What takes place in the air when sound is produced is like what happens in water when something is thrown into it. Obviously, circular undulations form in the water where it is hit; and these are small around the point of striking, but with a strong movement; whilst further away the undulations are large and the movement is weak. Ultimately, the movement disappears altogether and the undulations cease. But if the undulations meet an obstacle before they cease moving a contrary wave-movement is set up, so much the more violent as it is closer to the original impact.

             § 448. Now, in a similar way, when sounding bodies strike together the air is moved in a circular motion and the sound diffused in all directions. Near to the impact the air-circles are small, but are moving swiftly. Hence, there the sound will be very distinctly heard. Further away the circles are larger and the sound is less distinct; and at length it ceases altogether. But if, before the circles vanish, the air so moving and so carrying sound, is repulsed from some body, the undulations return on their tracks and a new sound comes in the reverse direction. And this is called 'echo'.

             § 449. This effect is realised most perfectly when the obstacle in question is concave, for then it acts like a vessel that holds the air together in its own unity, preventing its dispersal. For then the moving air, thus held together and unable to move further because of the obstacle, is thrown back on the air behind and a reverse movement begins--just as when a ball thrown against an obstacle rebounds.

             § 450. Then, where he says 'It seems', he explains how echo occurs in various ways, saying that there would seem to be always some echo produced, but it is not always definite, i.e. clearly perceptible. This he shows by a simile drawn from light. Light, he says, is always reflected; but sometimes the reflection is perceptible, sometimes not. The reflection of light is visible when it comes from a shining body; for then the reflection is accompanied by some brightness, as was the original emission of light. The reflection is not visible when it comes from an opaque body, for then it takes place with no brightness or radiance. Yet unless the sun's rays were reflected by opaque bodies there would be no light at all in the air of any part of the upper hemisphere, but instead only darkness everywhere away from the sun, i.e. outside the places reached by the solar rays directly. But light is not reflected by opaque bodies in the same way as by water or air or any of the smooth and polished bodies which throw back light brightly and radiantly. The reflection from opaque bodies is 'dark', that is to say, it forms the shadow extending outside the limit of the clear light of the solar rays. So also, then, when the repercussion of sound takes place in a concave body, wherein sound cannot but be multiplied, a clear and distinctly perceptible echo results. But when the repercussion of sound is from other bodies which do not naturally redouble it, there is no perceptible echo.

419b 33-420b 5

HEARING. ITS MEDIUM HIGH AND LOW SOUNDS

             EMPTY space is rightly said to be necessary for hearing,--for by that is meant air which, when moved as a simple continuum, causes hearing. On account of its instability, however, it gives no sound unless what is struck be smooth; then it has the required unity, holding together on account of the even surface, for the surface of a smooth thing is uniform. The sonorous, therefore, is that which moves a compact mass of air continuous as far as the organ of hearing.§§ 451-452

             Hearing is naturally conjoined with air; and because it is in air, therefore by a movement in the air outside is caused an interior movement also. Hence an animal does not hear all over its body, nor does air pass through every member. For the animate body and the part to be set in motion have not air throughout (as [also liquid is only in the eyeball]). § 453

             Of itself air is soundless; for, being mobile, it easily yields. But when its motion cannot be diffused a sound arises. There is air, built into the ears, so as to be immobile; and it accordingly registers every variety of motion with exactitude.§ 454

             On this account therefore we hear also in water, for this does not penetrate to that inner air, nor, by reason of its many convolutions, into the ear. Should it do so, one would not hear; nor if the eardrum were ailing--just as we do not see if the cornea of the pupil is diseased.§§ 455-7

             A test of good hearing on the contrary is whether there is a continual ringing in the ear, like a horn. For then the air in the ear is perpetually moving by a motion of its own; whereas sound is from without, and is not the ear's own. And for this reason they say that we hear by a "resounding vacuum", because we hear by what holds air in constraint.§ 458

             Is it that which strikes, or that which is struck, which sounds? Or both, but in different ways? For sound is a movement of something that can move in the way that a bouncing body flies off a smooth surface one flings it at. As has been said, not everything that strikes or is struck gives a sound, for instance one needle struck against another. But what is struck must have a plane surface smooth and regular so that the air rebound and be set in motion instantaneously. §§ 459-60

             Differences in sonorous things are apparent in the actual sounding. For just as colours are not seen without light, so there is no high or low note apart from sounding.§ 461

             These terms are used by metaphor from things perceived by touch.  § 462

             A high note moves the sense-organ much in a brief space of time; the low note, little, in a longer time.§ 463

             But this does not mean that the fast is the high and the slow the low; rather, the former arises because of swiftness of motion, the latter because of slowness.§ 464

             So there seems to be an analogy with the tangible, as sharp and blunt. For the 'sharp' pierces, while the 'blunt' thuds; and the reason is that the one moves in a brief period, the other in a greater. Hence it comes about that the former is swift, the latter slow. Let this serve to define sound.§ 465