The difference between sounds acute and grave.
3. That bodies when they are stricken do yield some a more grave, others a more acute sound, the cause may consist in the difference of the times in which the parts stricken and forced out of their places return to the same places again. For in some bodies, the restitution of the moved parts is quick, in others slow. And this also may be the cause, why the parts of the organ, which are moved by the medium, return to their rest again, sometimes sooner, sometimes later. Now, by how much the vibrations or the reciprocal motions of the parts are more frequent, by so much doth the whole sound made at the same time by one stroke consist of more, and consequently of smaller parts. For what is acute in sound, the same is subtle in matter; and both of them, namely acute sound and subtle matter, consist of very small parts, that of time, and this of the matter itself.
The third distinction of sounds cannot be conceived clearly enough by the names I have used of clear and hoarse, nor by any other that I know; and therefore it is needful to explain them by examples. When I say hoarse, I understand whispering and hissing, and whatsoever is like to these, by what appellation soever it be expressed. And sounds of this kind seem to be made by the force of some strong wind, raking rather than striking such hard bodies as it falls upon. On the contrary, when I use the word clear, I do not understand such a sound as may be easily and distinctly heard; for so whispers would be clear; but such as is made by somewhat that is broken, and such as is clamour, tinkling, the sound of a trumpet, &c. and to express it significantly in one word, noise. And seeing no sound is made but by the concourse of two bodies at the least, by which concourse it is necessary that there be as well reaction as action, that is to say, one motion opposite to another; it follows that according as the proportion between those two opposite motions is diversified, so the sounds which are made will be different from one another. And whensoever the proportion between them is so great, as that the motion of one of the bodies be insensible if compared with the motion of the other, then the sound will not be of the same kind; as when the wind falls very obliquely upon a hard body, or when a hard body is carried swiftly through the air; for then there is made that sound which I call a hoarse sound, in Greek συριγμος. Therefore the breath blown with violence from the mouth makes a hissing, because in going out it rakes the superficies of the lips, whose reaction against the force of the breath is not sensible. And this is the cause why the winds have that hoarse sound. Also if two bodies, how hard soever, be rubbed together with no great pressure, they make a hoarse sound. And this hoarse sound, when it is made, as I have said, by the air raking the superficies of a hard body, seemeth to be nothing but the dividing of the air into innumerable and very small files. For the asperity of the superficies doth, by the eminences of its innumerable parts, divide or cut in pieces the air that slides upon it.
The difference between clear & hoarse sounds, whence.
4. Noise, or that which I call clear sound, is made two ways; one, by two hoarse sounds made by opposite motions; the other, by collision, or by the sudden pulling asunder of two bodies, whereby their small particles are put into commotion, or being already in commotion suddenly restore themselves again; which motion, making impression upon the medium, is propagated to the organ of hearing. And seeing there is in this collision or divulsion an endeavour in the particles of one body, opposite to the endeavour of the particles of the other body, there will also be made in the organ of hearing a like opposition of endeavours, that is to say, of motions; and consequently the sound arising from thence will be made by two opposite motions, that is to say, by two opposite hoarse sounds in one and the same part of the organ. For, as I have already said, a hoarse sound supposeth the sensible motion of but one of the bodies. And this opposition of motions in the organ is the cause why two bodies make a noise, when they are either suddenly stricken against one another, or suddenly broken asunder.
The sound of thunder and of a gun, whence it proceeds.
5. This being granted, and seeing withal that thunder is made by the vehement eruption of the air out of the cavities of congealed clouds, the cause of the great noise or clap may be the sudden breaking asunder of the ice. For in this action it is necessary that there be not only a great concussion of the small particles of the broken parts, but also that this concussion, by being communicated to the air, be carried to the organ of hearing, and make impression upon it. And then, from the first reaction of the organ proceeds that first and greatest sound, which is made by the collision of the parts whilst they restore themselves. And seeing there is in all concussion a reciprocation of motion forwards and backwards in the parts stricken; for opposite motions cannot extinguish one another in an instant, as I have shown in the [11th article] of chapter VIII; it follows necessarily that the sound will both continue, and grow weaker and weaker, till at last the action of the reciprocating air grow so weak, as to be imperceptible. Wherefore a possible cause is given both of the first fierce noise of the thunder, and also of the murmur that follows it.
The cause of the great sound from a discharged piece of ordnance is like that of a clap of thunder. For the gunpowder being fired doth, in its endeavour to go out, attempt every way the sides of the metal in such manner, as that it enlargeth the circumference all along, and withal shorteneth the axis; so that whilst the piece of ordnance is in discharging, it is made both wider and shorter than it was before; and therefore also presently after it is discharged its wideness will be diminished, and its length increased again by the restitution of all the particles of the matter, of which it consisteth, to their former position. And this is done with such motions of the parts, as are not only very vehement, but also opposite to one another; which motions, being communicated to the air, make impression upon the organ, and by the reaction of the organ create a sound, which lasteth for some time; as I have already shown in this article.
I note by the way, as not belonging to this place, that the possible cause why a gun recoils when it is shot off, may be this; that being first swollen by the force of the fire, and afterwards restoring itself, from this restitution there proceeds an endeavour from all the sides towards the cavity; and consequently this endeavour is in those parts which are next the breech; which being not hollow, but solid, the effect of the restitution is by it hindered and diverted into the length; and by this means both the breech and the whole gun is thrust backwards; and the more forcibly by how much the force is greater, by which the part next the breech is restored to its former posture, that is to say, by how much the thinner is that part. The cause, therefore, why guns recoil, some more some less, is the difference of their thickness towards the breech; and the greater that thickness is, the less they recoil; and contrarily.
Whence it is that pipes, by blowing into them, have a clear sound.