This term is used to designate the Science of the phenomena known as Sound. In other words, by the term Acoustics we mean the body of facts, laws and rules which has been brought together by those who have systematically observed Sound and have collected their observations in some intelligible form.

Piano Tuning itself, as an Art, is merely one of the branches of Practical Acoustics; and in order that the Branch should be understood it is necessary to understand also the Trunk, and even the Root.

But I might as well begin by saying that nobody need be frightened by the above paragraph.

I am not proposing to make any excursions into realms of thought too rarefied for the capacity of the man who is likely to read this book. I simply ask that man to take my word for it that I am going to be perfectly practical and intelligible, and in fact shall probably make him conclude that he has all along been a theorist without knowing it; just as Moliere's M. Jourdain discovered that he had been speaking prose all his life without knowing it. The only difference has been that my reader has not called it "theory." He has called it "knowing the business."

Anyhow, we are going to begin by discovering something about Sound. We are in fact to make a little excursion into the delectable kingdom of Acoustics.

What is Sound?

When a street-car runs over a crossing where another line intersects, we are conscious of a series of grinding crashes exceedingly unpleasant to hear, which we attribute perhaps to flat tires on the wheels or to uneven laying of the intersecting trackage. The most prominent feature of such a series of noises is their peculiarly grating and peculiarly spasmodic character. They are on the one hand discontinuous, choppy and fragmentary, and on the other hand, grating, unpleasant to the hearing, and totally lacking in any but an irritant effect. These are the sort of sounds we speak of as ''noise."

In fact, lack of continuity, grating effect and general fragmentariness are the distinguishing features of noises, as distinguished from other sounds.

If now we listen to a orchestra tuning up roughly off-stage, the extraordinary medley of sounds which results, may and frequently does have the effect of one great noise; although we know that each of the single sounds in the uproar is, by itself, musical. So it appears that noises may be the result of the chance mixture of many sounds not in themselves noises, but which may happen to be thrown together without system or order. Lack of order, in fact, marks the first great distinction between noises and other sounds.

If now we listen to the deep tone of a steamer's siren, or of a locomotive whistle, we are conscious of a different kind of sound. Here is the immediate impression of something definite and continuous, something that has a form and shape of its own, as it were, and that holds the same form so long as its manifestation persists. If, in fact, we continue to seek such sounds, we shall find that what are called Musical Sounds are simply more perfect examples of the continuity, the order and the definite character which we noticed in the locomotive whistle's sound. The more highly perfected the musical instrument, the more perfectly will the sounds evoked by it possess the qualities of continuity, order and definite form.

Continuity, persistence and definiteness, then, are the features which distinguish Musical Sounds from Noises. And there are therefore only two kinds of sounds: musical sounds and noises.

Now, what is Sound?

The one way in which we can know it, plainly, is by becoming conscious of what we call the Sensation of Sound; that is, by hearing it. If one considers the matter it becomes plain that without the ability to hear there would be no Sound in the world. Sound cannot exist except in so far as there previously exist capacities for hearing it. The conditions that produce Sound are obviously possible, as we shall soon see, to an interminable extent in all directions; yet what we may call the range of audible Sound is very small indeed. We can hear so very little of the conceivably bearable material; if I may use so rough an expression.

So it becomes quite plain that Sound cannot be considered as something in itself, existing in the sounding body apart from us, but must rather be thought of as the form in which we perceive something the form, in fact, in which we perceive the behavior of certain bodies, which behavior could not be perceived in any other way. Sound then can be considered only from the view-point of the physical laws which govern the behavior of the bodies in question.

The laws which govern that sort of behavior which we perceive as Sound, alone form the subject of Acoustics. Why we should experience these perceptions as Sound rather than as Light or Heat is not a, question to be decided by Acoustics; is not a problem of the natural sciences, but of Metaphysics.

Limited therefore to a strictly mechanical investigation, let us consider the production of Sound from this viewpoint. Suppose that I strike a tuning-fork against the knee and hold it to the ear. I am conscious of a sound only moderate in intensity but of persistent and quite definite character, agreeable, and what we call "musical"

No one has any hesitation in calling this a "musical sound." But what produces it, physically speaking? We can discover this for ourselves by making a simple experiment.

By lightly touching the prongs of the fork while it is sounding I discover them to be in a state of vibration. If I examine them under a microscope I shall perhaps be able to detect an exceedingly rapid vibratory motion.