Acoustics
THIS term embraces all that science can teach us regarding the phenomena of sound and hearing. If the question be asked, what is a sound? no doubt most of my readers will at once think that it is a very easy question to answer. But such is not the case. It is by no means easy to explain the nature of sound in a simple and satisfactory manner, but nevertheless I will endeavour to do so. Suppose that we are in a room full of people, that we take a paper bag, blow it full of air, and suddenly explode it by clapping our hands upon it. Every one in the room will hear the explosion, provided of course that none of them are deaf. Now what causes them to hear this noise? We know quite well that no portions of the bag can strike them a blow, and yet the noise is so sudden that they start as if they had really been struck by something. Let us see what this something is.
The room is full of air, the air which we breathe, and without which we could not live. This air, although regarded by the ancients as an elementary body, is really of a compound character. It is composed of two gases called nitrogen and oxygen, the former comprising four-fifths and the latter one-fifth of its bulk. Now we may imagine that this air is composed of a multitude of small bodies called molecules, a word signifying little masses, of matter; that is to say, supposing that we had a microscope powerful enough to see such minute objects, and were to magnify a layer of air, we should see these molecules crowding together and running about in constant movement. It is supposed that everything in nature is made up of molecules. In solids they are tightly packed together; in liquids they are more loosely combined, so that they can run and slide over one another; but in vapours and gases, such as common air, they are still more widely separated. Let us, then, consider air, as being made up of these tiny masses. They are so tiny that a philosopher has calculated that, if a raindrop the size of a pea were magnified to the size of this earth, each molecule being magnified to the same extent, they would appear to be less in size than ordinary cricket balls. Having glanced at what is known as molecular structure, we shall now be better able to understand the meaning of the word sound.
Returning once more to the exploded bag, we may imagine that the particles of air in its immediate neighbourhood were driven away from it at the moment that it popped. These would give motion to other particles further away, these again would push on succeeding particles, until the molecules close by the hearers were sent hammering against the drums of their ears. The drum of the ear is a little oval membrane placed within the ear, hardly half an inch across. It is quite elastic, and moves in and out as the particles of air touch it. This movement is called vibration, and can very well be seen in the following way. Strew some fine sand upon the parchment of a toy drum. Now tap the lower parchment, and the sand will jump about as the upper surface vibrates. By this simple experiment we learn that sound is caused by vibration, and that such vibrations can be communicated by means of the air to other bodies. For although only one drumhead is struck, the other one is also thrown into motion. In this way the particles of air give motion to the drum of the ear.
We have thus traced a sound from its source to the drum of the ear. This little membrane is connected with some small bones, which are alternately pushed and pulled as the drum vibrates. The vibrations are in this way communicated to the inner ear, where they affect in some unexplained way the fibres of what is called the auditory nerve. This nerve gives up to the brain a particular sensation which we call sound.
If a bell worked by a clockwork hammer be placed beneath the exhausted receiver of an air-pump, we can see that the hammer is vigorously at work, but no sound is heard, for the air has been taken away, and there are no particles to convey it, but directly the air is restored, the sound of the bell is made evident to our senses.
To show further that sounds are dependent upon vibration, we may strike a glass tumbler with the knuckle, when it will give forth a ringing sound, but directly it is touched with the finger, the vibrations are stopped, and the sound ceases.
Sounds travel through solids as well as through air. We may test this by placing the ear to the end of a long wooden rod. The other end may be placed against a watch, when the ticking can be plainly heard. We may vary this last experiment by stopping the ears with the fingers or with cotton wool, and placing the end of the rod between the teeth, when the sounds will be heard just the same. In this case the vibrations are conveyed through the bones of the head to the auditory nerve, and the sensation of sound is conveyed to the brain.
An instrument called the audiphone, for the use of people who are partially deaf, has lately been invented; and it owes its effectiveness to the principle just explained. It consists of a kind of fan of vulcanised india-rubber, which is made to assume a curved form by strings placed at its back. This fan acts like a soundboard, and collects the sounds which are conveyed to the hearer's brain through the teeth, as shown in the cut below.
Sound, like light, can be reflected. It is the reflection of a sound that constitutes what is called an echo. When there is only one reflecting surface, the echo is single, but when there are many, the sound is repeated over and over again. In large buildings, such as St. Paul's Cathedral, where there are many reflecting surfaces, the sound of a speaker's voice is much confused by the echoes which follow it up and crowd upon it. The velocity of sound in air depends upon the temperature. It is generally something over 11,000 feet per second. By noticing the puff of smoke from a distant cannon, and by noting the number of seconds which elapse before we hear the sound, its distance can be readily calculated. In the same way we can estimate the actual distance of a thunder-storm, by counting the number of seconds which elapse between the lightning flash and the thunder, which in reality take place simultaneously. The velocity of sound in water is more than four times greater than that of sound in air. This was determined many years ago by some experiments carried out on the Lake of Geneva. Two boats were stationed on the lake at a measured distance from one another. From one boat a bell was sounded under water, and at the instant that the sound was made a charge of gunpowder was fired in the boat. The observers in the other vessel were furnished with a long ear trumpet which dipped into the lake, and by this means they were able to hear the sound of the distant submerged bell. By noting the time which elapsed between the flash of the gunpowder and the reception of the sound of the bell they were able to determine the velocity of sound in water.
The velocity of sound in iron is seventeen times its velocity in air. This can be roughly tested in the following manner. Choose some spot where there is a continuous iron railing, such as skirt the grass plots in some of the public parks. Let a friend go to some distance and violently strike the iron with a hoop stick. Placing your ear to the railing you will first hear the sound conveyed through the iron, and by raising your head you will again hear it through the air.
All solids convey sounds to a greater or less degree. Savages place their ears to the ground when they suspect that horsemen are approaching, and they are by this method able to hear them long before they are in sight. If we watch a number of soldiers marching with their band playing before them, we shall notice that those at the rear of the company are not keeping step with those in front; the explanation being that they are all keeping time with the music as they each hear it, but as it reaches them at different times according to their distance from the instruments, they step accordingly. Sounds in which the vibrations are irregular, and reach us as rough impulses, are known as noise; but if the vibrations are perfectly regular, and follow one another periodically, we have a musical note. We may test this in a ready manner with a shilling and a piece of writing paper. Scrape the milled edge of the coin slowly with the paper, and a rasping sound will result; do the same quickly and regularly, and the sound will give a definite note. The quicker we move the paper the higher will be the pitch of the note. This experiment shows that vibrations must follow one another at a certain speed before a musical sound can result, and also that the greater number of vibrations in a given time, the higher the pitch of the note produced.
Of late years some very remarkable instruments have been invented which depend upon the union of acoustical science with electricity. I may specially name the microphone and the telephone. These will be fully considered in their proper places, and directions will be given which will enable those wishful to do so to construct instruments for themselves.