In the so-called wind instruments, sound is produced by vibrating columns of air inclosed in tubes or pipes of different lengths. The air column is thrown into vibration either directly, by blowing across a narrow opening at one end of a pipe as in the case of the whistle, or indirectly, by exciting vibrations in a thin strip of wood or metal, called a reed, which in turn communicates its vibrations to the air column within.
The shorter the air column, the higher the pitch. This agrees with the law of vibrating strings which gives high pitches for short lengths.
The pitch of the sound emitted by a column of air vibrating within a pipe varies according to the following laws:
1. The shorter the pipe, the higher the pitch.
2. The pitch of a note emitted by an open pipe is one octave higher than that of a closed pipe of equal length.
3. Air columns vibrate in segments just as do strings, and the tone emitted by a pipe of given length is complex, consisting of the fundamental and one or more overtones. The greater the number of overtones present, the richer the tone produced.
FIG. - Open organ pipes of different pitch.