Physicists divide sounds into two classes—musical sound and noise. They both have the same origin, the vibrations of a body transmitted to the air. The short duration of a noise, and the lack of isochronism in its vibrations, do not permit us to appreciate its musical value, and this distinguishes it from musical sound. Thus the explosion of gas or powder, the crack of a whip, or the breaking of a branch make a noise, but give no musical sound. The limit between sound and noise is otherwise insensible, and varies according to the individual. A noise as well as a sound may be grave or acute, feeble or intense. The difference in the duration of the sensation does not permit us to compare noise to sound, but yet the ear seizes the relation between two noises as well as between two musical sounds.
A sound is called musical when its pitch can be estimated absolutely and relatively to other sounds grave or acute; or, in other words, when the number of vibrations follows a constant law and can be determined.
Whatever may be the difference, however, between a noise and a musical sound, the one is only a variety or degree of the other, and both, proceeding from the same source, may be studied under the generic denomination of sound.
Sound has four fundamental properties—duration, pitch, intensity, and timbre or distinctive quality. The three first named are defined by the words which express them: as for the timbre, it is the resonance peculiar to each instrument, to each voice, which enables us to distinguish without difficulty the notes of a violin, a clarionette, or a flute, and to recognize individuals by hearing them speak or sing.
The duration of a sound is measured by the time that the body vibrates from which it proceeds; it is high and acute according to the number of vibrations, and its intensity is measured by the amplitude or range of the vibrations which cause it, and this amplitude is in proportion to the force acting on the sonorous body.
The "timbre" of sounds was long an insoluble enigma to the physicist and the physiologist J. Muller had suspected its origin in attributing it either to the isochronism of sonorous waves of different velocity, or to waves of different length, producing a compound wave of a peculiar form, or else to a longitudinal vibration in the sonorous body taking place at the same time as the transverse vibration. M. Longet states, with greater precision, that the timbre of the human voice, and of wind-instruments, results from the co-existence of several sonorous waves of different tone and intensity, which modify the general form of the principal wave. But at last the beautiful experiments of M. Helmholtz have demonstrated, that the timbre of a sound depends upon the number of the harmonic notes which are produced at the same time as the fundamental note, and upon their relative intensity.
When a cord of a piano giving the C, for example, is struck, that note is heard, but a little attention enables the ear to hear other simultaneous and weaker sounds; they are the result of partial vibrations which take place in the length of the cord, according to certain laws which cannot be explained here. The C given by the shock impressed on the cord is the fundamental note, the other notes which are superposed upon it are the harmonics. From their fusion with the fundamental note, there results to the ear a complex sound which it decomposes instinctively into simple sounds, but they cause only a single sensation in the brain, that of a C having a special timbre. Whether the fundamental note be given by an instrument or by the human voice, the same phenomena are produced, and the timbre proves equally characteristic to the ear. The timbre is therefore the distinctive quality of the sonorous body—the form, in a certain sense, of sounds.
Sound moves more rapidly in warm air than in cold; its velocity in the atmosphere is 1118.45 feet in a second at 160 C. (6o.8° F.) or 1086.37 feet at zero, according to experiments made by the Bureau of Longitudes in 1822; and according to those of Bravais and Martins made in 1844, it is 1092.89 feet at zero (Cent.) This velocity is not modified by the variations in the pressure of the atmosphere, and it is the same whether in a horizontal, vertical, or oblique direction. It is increased or diminished by the wind, according as it blows in the direction of the sound or contrary to it, though the velocity is not changed if the wind blows perpendicularly to this direction. Sound cannot be produced in a vacuum, and it is therefore less intense in propordon as the air is more rarefied. It is weaker, for instance, on the tops of high mountains than in the lower strata of the atmosphere, although the profound silence which reigns at times in these elevated regions permits even very feeble sounds to be heard at great distances. We were enabled to prove this with M. Martins in 1844. Near St. Cheron (Seine-et-Oise), at an elevation of 459 feet, a diapason placed on a drum could be heard in the daytime 277 yards off; while on the great plateau of Mont Blanc, at a height of 13,123 feet, the sound of the same instrument could be heard at a distance of 368 yards. On the top of Mont Blanc, we could hear our guides talk at a distance of 437 yards, and they could hear us speak also.
Humboldt observes that sound is more intense and is propagated farther in the night than in the daytime, in spite of the noises and of the wind which in tropical countries increase after sunset This diminution in sounds during the day is attributed by the illustrious observer to the unequal temperature of the strata of the atmosphere, under the influence of the sun and the radiation from the earth.
Sound moves much more quickly in water and in solid bodies than in the air. Colladon and Sturm found its velocity to be 4708 feet in a second in the waters of the Lake of Geneva at 8° C. (46.4° F.) of temperature; according to the experiments of Biot, its average velocity is 10,663 feet in cast-iron pipes. This is about five times greater in water than in air, and nine times greater in the pipe.
Humboldt records that sometimes volcanic detonations have been transmitted through the earth a distance of 500 to 745 miles.
It is stated that the gravest sound which can be perceived by the ear is 32 vibrations in a second (16 according to Savart), and the most acute, according to Despretz, is 73,700 vibrations. A sound of 60,000 vibrations is, according to M. Martins, very feeble, difficult to hear, and of such sharpness as to cause a painful impression on the ear. The sounds which are easily perceived and appreciated by the ear vary from 100 to 2000 vibrations. The gravest C of a piano of six octaves and a half counts 128, and the most acute 8192.