ACOUSTICS, i-koo'sfiks, (from to hear), is the science of the production, propa gation and audition of sound. The term sound is sometimes by definition restricted to the sen sation involved in hearing, but is never consist ently so used. Both by derivation and by com mon and best usage it should be applied to those aerial or other vibrations which, were they to reach the ear, would produce audition. The term being thus used, sound consists of waves of longitudinal vibration, that is to say of waves of to and fro motion perpendicular to the wave front. Such motion, propagated through an elastic medium with a finite velocity, results in alternate rarefaction and condensation.
A moment's consideration of any source of sound will show it to be of such a nature as to give either a single impulsive blow or re peated blows, usually systematically repeated, to the surrounding medium. In the great majority of cases, and those the more interest ing both theoretically and practically the source of sound consists of an elastic boc(y distorted from its normal shape, and, released, vibrating more or less symmetrically about this normal shape or position. It results from this vibra tory motion that a series of impulses is given to the surrounding medium which are periodic, nearly similar in character, and nearly equally timed. These impulses, propagated through the surrounding medium all with the same velocity, follow each other in the form of a train of waves. The distance from a point in one impulse to the corresponding point in the next impulse is called the wave length of the sound. The frequency of these waves as they strike the ear determines the pitch of the sound; the character of the wave in respect to form de termines the quality of the sound; while both of these together with the amplitude of vibra tion and the density of the medium determine the loudness or strength of the sensation.
In respect to pitch sounds audible to the human ear range in frequency from about 24 vibrations per second to 40,000 vibrations. Sounds very much higher in pitch are audible to some animals, the cat for example, while for some animals it is probable that the upper limit is not so high, although in regard to the latter point no reliable data have been secured, the interest of the biologists apparently being to extend the range. In regard to the lower
limit in other auditors than man no reliable ex periments have been made, and if attempted would be extremely difficult because of the diffi culty of distinguishing the reactions due to the mechanical disturbance from the reaction due to true audition,— a difficulty which indeed affects all such experiments, but which is enhanced in the case of the lower limit.
The quality of a sound is determined by the wave form. A pure musical tone is due to sim ple harmonic motion, a type of periodic motion best described as the projection on a diameter of uniform circular motion, and most famil iarly illustrated by the motion of the pendulum of a clock. Perfectly pure tones are rare, the nearest approach being that of a tuning fork re enforced by a resonator. Most musical sounds are far from being pure tones. They may, how ever, be regarded as a complex of a number of pure tones, each pure tone being then called a partial tone. Of these partial tones the lowest, which is generally though not always predom inant, is called the first partial, and the other partial tones in order of their pitch are called the second, third, etc., partials. In many of the more interesting cases such as the tones of the organ pipe, or of a bowed, struck or plucked string, the upper partials are harmonics of the fundamental. The pitch and the relative in tensities of the partial tones determine what is called the quality of the sound, the pitch of the whole being usually rated as that of the lowest partial. When a sound is incapable of analysis into pure tones it is called a noise. In many, indeed it is safe to say that in most, sounds that are classed as noise there is some trace of a predominant note, and of a definite musical pitch which a trained ear can detect.