Lecture 16 — The causes of sound and echo

Now let us start by treating of sound and smell. Sound is twofold: the sort that is a certain act, 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.

Sound in act is always of something, on something, and 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.

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, does—bronze because it is smooth, while hollow things by repercussion produce many blows after the first because what is set in motion cannot find an outlet.

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 outrun the breaking up of the air, just as if one were to strike a rapidly moving heap of sand or pile of stones.

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

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 thus reflected as it is reflected by water or air or other smooth things; hence, it makes the shadow by which we discern the boundaries of light.

439. Now let us start by treating of sound (419b4). Having defined 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 (420b5; [466]).

The first part is again divided into two parts.

First, he deals with sound itself;

second, he deals with the differences of sound, at differences in sonorous things (420a26; [461]).

The first part again subdivides.

In the first part, he reaches a determination about sound.

In the second, he raises a doubt about this determination, at is it that which strikes (420a19; [459]).

The first of these sections again divides into two conclusions.

In the first, he reaches a conclusion about the generation of sound.

And in the second, he determines the way that sound affects the hearing, at but a vacuum (419b33; [451]).

Should it be asked why he now raises the question of the generation of sound, whereas he did not deal with the generation of color, but only with color’s effects on the sense organ and the medium, the answer is that color and odor and taste and the tangible qualities have a fixed and lasting existence in their subjects. (Lect. 14, 15.)

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 generation of color, taste, and smell in his work On Sense and What is Sensed; (Aristotle, On Sense and What is Sensed, ch. 3, 439b18–440b28; ch. 4, 441a3–442b26; ch. 5, 442b27–443b16; cf. Aquinas, Commentary on Aristotle’s On Sense and What is Sensed, cc. 6–11.) and he deals with the generation of tangible qualities in On Generation (Aristotle, On Generation and Corruption 2.2, 329b7–330a29; cf. Aquinas, Commentary on Aristotle’s On Generation and Corruption, bk. 2, lect. 2.) and, in part, also in the Meteorology; (Aristotle, Meteorology 1.2, 339a11–32; 4.4–11, 381b23–389b22; cf. Aquinas, Commentary on Meteorology, bk. 1, lect. 2.) while in the present book, he is only considering the objects of sensation insofar as they affect the sense organs. But as sound is caused by change and has no fixed and resting existence in a subject, but consists in a certain change, it can be considered at one and the same time insofar as it is generated in its species and insofar as it affects the sense. Its origin, then, is regarded under two aspects.

First, he deals with the primary origin of sound.

Second, at echo arises (419b25; [447]), he deals with its secondary origin, produced by reverberation.

About the first, he does two things.

First, he explains that sound is sometimes in act and sometimes in potency.

Second, he shows how sound comes to be in act, at sound in act (419b9; [442]).

440. He says first, then, that before dealing with touch and taste, we must consider sound and smell (419b4); but first we must consider sound, because it is more spiritual, as has been shown. (Lect. 14, nn. 417–18.) 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, for example, sponges and wool and similar soft objects; whereas other things, on the contrary, are said to have a sound because they are especially able to produce sound, such as bronze, and other smooth and hard things. So it is clear that sound is sometimes spoken of as potential.

441. But sound in act involves the medium and the power of hearing. For we can speak of a sense-object as being in act in two ways: in one way, insofar as the sensible object is actually being sensed, that is, when its species is in the sense. In this way, a sound is actual when it is heard. In another way, insofar as the object has the proper species through which it is able to be sensed, but is such in its own subject. And in this way the other sense-objects, color, odor, savor, etc., exist in act in colored or odorous or savorable bodies. But not so with 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 body that is able to sound.

442. Then, at sound in act (419b9), Aristotle shows how sound actually comes about. And about this, he does two things.

First, he shows how sound comes into act with regard to the number of factors required;

and second, at as we said before (419b13; [443]), he shows this with regard to what these factors are.

First, then, there are three concurrent factors in sound: sound is of something, on something, and in something. Therefore, no single thing by itself can produce sound—the reason of this being that the cause of sound is percussion, which implies a thing struck and a striker.

Hence, his saying that sound is of something, on something, that is, of the thing striking and on the thing struck. For what produces sound must touch something as with a sudden blow, thereby generating the sound. 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 that there must be sound in act not only of something and on something, but also in something.

443. When he says, as we said before (419b13), he shows what sort of factors are required for the generation of sound.

And first, he shows of what sort must the striking and the struck objects be;

and second, at further (419b18; [445]), 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) makes no 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 hollow 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. (Lect. 17, n. 451.)

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 find an outlet, 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 (419b18), 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 on each other is to be simultaneous with the production of sound on the air, it is necessary that the air remain in its integrity, that it be not broken 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 united or collected and so a sound be produced in it. It is as when one strikes swiftly, hitting a whole heap of stones all at once before they fall apart, which does not happen if they are slowly moved. 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 (419b25), 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 second, at it seems (419b27; [450]), 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, while, 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 undulation 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 undulations are smaller, but the motion is stronger. Hence, there the sound will be very distinctly perceived. Further away, the undulations are larger and the motion weaker, and the sound heard less clearly; and at length it ceases altogether. But if, before such undulations fail, the air thus moving and 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 an “echo.”

449. This effect is realized most perfectly when the obstacle in question is hollow, for then it acts like a vessel (419b25) 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 (419b27), he explains how echo occurs in various ways, saying that there would seem to be always some echo produced, but it is not always a clear one, that is, 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 the reflection takes place with no brightness or emission of radiance.

For 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, beyond the area illuminated by the sun there would be darkness, that is, 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.

Therefore, the reflection from opaque bodies is not like that of a shining body, for the reflection from opaque bodies makes darkness, that is, it forms a shadow, which is outside that part where the manifest light is limited. So also, then, when the repercussion of sound takes place in a hollow 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.