Let us now proceed to determine the nature of sound and hearing. Sound is double—one actual and another potential; for we say that some substances, such as sponge and wool, are without sound; and that others, as brass, and bodies which are hard and smooth, have sound, because such objects are able to sound; are able, that is, to create actual sound by the action of the medium between the object and the hearing. Sound of the actual kind is the invariable result of something in relation to something and in something; for its producing cause is percussion. It is impossible, therefore, that sound should be produced when there is only one substance, as that which percusses must be distinct from that which is percussed; so that the sonorous object sounds by its relation to another object. But there can be no percussion without movement, and sound is not produced by the percussion of any kind of substance, as we have said, (since wool, however percussed, does not, while brass and smooth and hollow bodies—brass because it is smooth—do give out sound,) and hollow bodies create, by reflexion, many percussions after the first, owing to the medium within them having been set in motion and being unable to make its escape. Sound is audible in air, and so it is in water, although less distinctly; but neither air nor water is the efficient cause of sound, as for it there must be percussion of solid bodies against each other and against the air, and this is effected whenever the air, having been percussed, remains, is not, that is, dispersed. Thus, if the air be struck sharply and forcibly it gives out sound; for the motion of that which percusses should anticipate the dispersion of the air, as if any one were striking a rapidly moving heap or cloud of sand.
An echo is produced whenever the external air has been more than once repelled by the air contained within a vessel, by the sides of which that air is precluded from being dispersed, just as a ball rebounds. It seems as though an echo ought to be a constant occurrence, although it may not be audible, since that happens to sound which happens to light, and light is continually undergoing reflexion (for, otherwise, as light could not be everywhere, darkness would prevail beyond the spot illumined by the sun), but yet it is not everywhere reflected, as it is from water or brass or any other smooth body, so as to form a shadow whereby we are able to distinguish the light itself.
A Void is rightly said to be the sovereign cause of hearing;—for the air seems to be a void, and the air, when moving continuously and as one body, is creative of hearing. But, owing to its being very diffluent, it gives out no sound, unless that which is percussed be smooth; when this, however, is the case, the air becomes simultaneously one over the surface, as the surface of every smooth body is one. Every sonorous body is so constituted as to set in motion the air which, by continuity, is one up to the hearing, and the hearing is naturally connected with the air; and owing to sound being in the air, the air which is without sets in motion that which is within. An animal, therefore, does not hear everywhere, neither does the air penetrate everywhere; for the part to be set in motion is a living part, and does not everywhere contain air. The air itself, owing to its ready diffusibility, is without sound; but, when precluded from being dispersed, its motion is productive of sound. The air which is within the ears has been so immured as to be immovable; and this in order that the sense may perceive accurately all variations of its movement. It is for these reasons that we are able to hear when in the water, as the water cannot gain access to the congenital air, or pass into the ear through the convolutions; when, however, this does happen, there is no hearing, any more than there is when the membrane of the ear, which is to it what the skin over the pupil is to the eye, is diseased. But proof is afforded whether the hearing is perfect or not, in that the ear is constantly giving out sound, just as a horn does; for the air within the ears is continually moving in some peculiar manner, and yet sound is foreign to that air and forms no part of its properties. It is on this account, however, that we speak of hearing by a void and something resonant, because we hear by the part which contains the air confined within it.
But is it that which percusses, or that which is percussed, which gives out sound ? Or do both contribute to its production, each in its own way? Now, sound is the motion of something which admits of being moved after the manner of bodies rebounding from smooth surfaces, whereon they may have been impelled. But every kind of body, whether percussing or percussed, does not, as has been said, give out sound ; as when a sharp point, for example, strikes a sharp point, there is no sound; but in order to produce sound, that which is percussed must be so smooth, that the mass of air upon its surface may rebound from, and be agitated over it. The distinctions among sonorous bodies are revealed in the actual sounds which they give forth; for as without light colours are not visible, so without sound the acute and grave are not audible. These terms (acute and grave) are derived from tangible properties, and employed, in a metaphorical sense, for sounds; for the acute moves the hearing quickly and sharply, the grave moves it slowly and dully; not, however, that the acute is quick or the grave slow; but that such is the motion of the one from the celerity, and such the motion of the other from the tardiness of its operation upon the sense. And there does seem to be an analogy between those sounds and the sharp and blunt, as perceived by the Touch; for the sharp pricks, and the blunt pushes, as it were, because the motion exerted by the one is rapid, by the other tardy; and it is in this manner that the terms in question have originated. Let us here, however, close our observations upon the nature of sound.
The voice is a sound produced by a living creature; for nothing inanimate speaks, although there are objects, such as the flute, lyre, and others, which, having range of note, harmony, and expression, are said, from a resemblance between their tones and the voice, to do so; and the voice does seem to have all the variations of note possessed by those instruments. Many creatures have no voice (as all the insanguineous, for instance, and some of the sanguineous, as fishes), which is very understandable, seeing that sound is a certain motion of the air; and with respect to those fishes which are found in the Achelous and said to speak, they produce sound by their gills, or other such part. But although the voice is a sound emanating from a living creature, it does not imply any kind of sound, or a sound produced by any kind of part; and as all sound is produced by the conditions of something which percusses, something percussed, and a something, that is the air, in which percussion can be made, it might reasonably be assumed, that such creatures only as take in air can have a voice. Now, nature employs simultaneously the air respired for two functions, just as she employs the tongue for taste and for speech; and of these the former is necessary (and therefore imparted to most creatures), and the latter, as an organ for interpretation, is for their higher good; so too does she employ the breath both as necessary for tempering the heat within (as shall be explained elsewhere), and for the production of voice, which is for the welfare of the individual. The pharynx is the organ of respiration, for the sake of which is another part, the lung, and it is owing to this part that quadrupeds have more heat than other creatures.
It is the place about the heart which first needs respiration; and, therefore, it is necessary that the air, during inspiration, should pass inwards; and thus the percussion of the air respired by the living principle in those parts, against the so-called trachea, constitutes the voice. But every sound produced by an animal is not voice, as we have said (for it is possible to produce sound by the tongue, as in coughing), but in order to constitute the voice, there must be a percussing living force, and the sound produced must be expressive of something. The voice is, in fact, a sound expressive of something—it is not, that is, as in coughing, a mere sound of the air inspired; and speech is the percussion, by the living principle, of the air in the trachea, against the trachea itself. As proof of which, we are unable to speak when holding the breath, that is, when we neither inspire nor expire; for the act of holding the breath sets in motion the air which is inspired. It is now manifest why fishes, having no pharynx, are without a voice; and they have no pharynx, because they neither admit the air nor breathe. It is foreign to our present purpose, however, to inquire into the cause of their having been thus constituted.