Sound

as usually understood, a vibration or jar of the atmosphere which sets in motion an inner membrane of the ear and causes a sensation to pass along the auditory nerves to the brain. All sound originates in a quick, short movement of some sounding body, as a piano string, the lip of a bell, the vibrations of the edges of one's throat, that is to say, the vocal chords, the fluttering of a leaf, in short of any material substance capable of conveying a jar to the air. In conveying sound the air strikes the membrane of the ear as a pulse or jar, not as a current. Sound may be conveyed by any elastic substance, like wood. For instance, a watch may be laid on a table and its sound smothered by a pillow until it cannot be heard in a room. If the experimenter will then place his ear at the wood at the farther end of the table, he can hear the ticking of the watch distinctly.

Some substances convey sounds very much more rapidly than others. Sound travels through free air, not freezing air, at about 1,091 feet per second. Moist air carries sound faster than dry air; warm air faster than cold air. The loudness of sound varies inversely as the square of the distance. Doubling the distance gives one-fourth as great intensity. Trebling the distance gives an intensity one-ninth as great. Sound travels in all directions from a sounding body, and is heard naturally in a space the shape of a globe. If, however, the air be confined, as by a speaking tube, so that the energy travels chiefly in one direction, one's voice may be heard much more distinctly.

If sound pulses reach the ear with sufficient frequency and with regularity, the sound is changed from a mere noise into a musical note.

Two hundred and sixty-four beats per second, for instance, give a note called middle C. Twice as many, or 528 beats per second, give an octave above, and twice as many again, or 1,056, give a note called upper C. To say that one note is lower than another is another way of saying that it is made by fewer beats per second. A note is in harmony with another note produced by twice as many or but half as many vibrations per second, or with any other note whose number of beats bears a simple arithmetical relation to its own number, so that the beats come together at frequent and regular intervals. A note taken as a basis is called a fundamental. Any note is in harmony with the fundamental whose vibrations are made by two, three, or any number of times the number of vibrations made by its fundamental. The greater the number of vibrations per second, the higher the pitch. Sound has not only intensity and pitch, but quality or timbre. The same notes struck by different bells differ in quality. It is an indescribable richness or the want of it to which the term timbre is applied.

See Sonometer; Pitch; Piano; Speaking Trumpet; Echo; Violin; Orchestra