acoustics, or acoustics

[In Fr. acoustique.] [ACOUSTIC. A term introduced by Saveur. The science which treats of Two particles which are in the same state of vibration-i.e., are equally displaced from the positions which they occupied in equilibrio , and are moving in the same direction, and with equal velocities are said to be in the same phase; whilst those which are proceeding in a contrary direction are said to be in opposite phases. If the vibration of particles takes place in the same direction as that in which the disturbance is moving from particle to particle, it is called longitudinal ; if at right angles to it, transverse . So analogous are the sound-producing vibrations of particles to those of waves in the ocean, that the terms waves and undulations are used in Acoustics as well as in Hydrology. The distance which separates two particles in the same phase is called the length of a wave . As in Optics, so in Acoustics, there are refraction and reflection, the laws in both cases being the same. Refraction of sound: The change of direction which is produced when a wave of sound, travelling through one medium, meets a second one not of the same kind, and excites in it a wave of a different velocity and direction from the first. Reflection of sound : The change of direction which is produced when a wave of sound, travelling through one medium, meets a second one diverse from the first, and in addition to transmitting to it a refracted wave, excites in it an undulation travelling in a different direction, but with the same velocity as the other. A sound may be frequently repeated, as from an echo-producing cliff, and in a whispering gallery or a tunnel. wave. Two or more sonorous waves travelling through the same medium, and acting on the same particles, are said mutually to interfere with each other. If they move towards such an interference from exactly opposite directions, they produce between them a stationary This expre ion does not imply that every particle of the wave thus produced is motionle . Some particles are so, whilst others vibrate longitudinally or transversely. The points at which the particles are stationary are called nodes, and the vibratory portions ventral segments . A vibrating musical string, a tuning-fork, or other stiff rod vibrating longitudinally, make stationary waves. These are generated also inside wind-instruments when the latter are blown. The vibrations of a solid are best communicated to another solid hence a tuning-fork being struck is applied to a table, and violin-strings are placed in contact with a hollow wooden box, which imparts to their sound a greater intensity than if its transmi ion to the ear were entrusted to the air alone. Noise is a single blow given to the ear, whilst Music is caused by a series of feeble blows following one another at regular intervals. [MUSIC, HARMONY, SOUND.] Some writers have divided Acoustics into Diacoustics , which treats of those sounds which pa directly from the sonorous body to the ear; and Catacoustics , which investigates the phenomena of reflected sounds. Another division is into Acoustics proper , or the science of hearing, and Phonetics, or the science of sound; the latter word being from Gr. φωνή ( phōnē ) = sound.