HANDBOOK OF ACOUSTICS - online book

A complete view of Acoustical Science & its bearings on music, for musicians & music students.

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2                        HAND-BOOK OF ACOUSTICS.
bowed, it will emit a sound, and the little suspended weight will be violently dashed away; the rattle of the moving glass against the projecting point will also be plainly heard. Even where the move­ment cannot be seen, as in most wind instruments, it may easily be felt.
Although all sounds are thus produced by motion, movements do not always give rise to the sensation, sound. We have therefore to ascertain, what particular kind of motion is capable of producing the sensation, and the conditions necessary for its production. Sounds may be roughly classified as musical or unmusical. As we are only concerned here with the former, it will be as well first to distinguish as far as possible between the two classes. For acoustical purposes, we may define a musical sound to be that, which, whether it lasts for a long or short period of time, does not vary in pitch. In other words, a musical sound is a steady sound. In an ordinary way, we say that a sound is musical or unmusical, accord­ing as it is pleasant or otherwise, and on examination, this will be found to agree fairly well with the more rigid definition above, especially if we bear in mind the fact, that most sounds consist of musical and unmusical elements, and that the resulting sound is agreeable or the reverse, according as the former or the latter pre­dominate. For example, the sound produced by an organ pipe consists of the steady sound proper to the pipe, and of the unsteady fluttering or hissing sound, caused by the current of air striking the thin edge of the embouchure; but, as the former predominates greatly over the latter, the resulting sound is termed musical. Again, in the roar of a waterfall we have the same two elements, but in this case, the unsteady predominates over the steady, and an unmusical sound, or noise, is the result.
We have just seen that the external cause of a musical sound is motion; we shall further find on examination, that this motion is a periodic one. A periodic motion is one that repeats itself at equal intervals of time; as, for example, the motion of a common pendulum. In order to satisfy ourselves that a musical sound is caused by a periodic motion, we will examine into the origin of the sounds produced by strings, reeds, and flue pipes.
A very simple experiment will suffice in the case of the first named. Stretch a yard of common elastic somewhat loosely between two pegs. On plucking it in the middle, it begins vibrating, and although its motion is somewhat rapid, yet we have no difficulty in counting the vibrations; or at any