[Fr., from L. sonare.] That which can be heard. In physics, it is applied to the external cause which produces the sensation. In this sense the word sound stands either for the vibrations of the sounding body or for the impulses it has communicated to the air, and which immediately affect the ear. It can be shown by experiment that sound is the result of a vibratory move-ment which when sufficiently rapid produces a sound. A bell, a glass plate, a tuningfork, a piano string, if put into a state of vibration, will produce a sound if the vibrations take place in a suitable medium. It has been found that sound is not transmitted in a vacuum. A bell struck in the exhausted receiver of an air-pump is nearly inaudible. In water, sound travels nearly four times as fast as in air, in which its speed is about 1,093 feet per second. In solids the velocity varies widely. In inelastic sub-stances like lead or wax it is small, while in those like wood and steel it is large. Musical sounds differ from one another in respect to intensity, pitch, and character or timbre. The intensity depends upon the amplitude of the vibrations. Pitch is the quality which distinguishes an acute sound from a grave one. It depends upon the frequency of vibration. As with other forms of wave motion, sound may be reflected and refracted ; and if not in accord with each other their interference gives rise to beats.