The Principle of Superposition

How are standing sound waves generated in musical instruments?

Many instruments depend on standing waves to produce their sound. Standing waves are created on the strings of a guitar, piano, or violin, and in the air columns of a trumpet, flute, or organ pipe. The string is caused to oscillate either by plucking it (pulling it aside and then letting go), or by bowing it, where the stickiness of the horsehair on the bow also pulls the string aside and then releases it. In a piano a felt-covered hammer strikes the string, starting it vibrating. In a trumpet or other brass instrument the player’s vibrating lips create the traveling sound wave that is reflected when it reaches the open end of the instrument. In a flute or organ pipe that mimics a flute, the player blows air over a hole. The moving air interacting with the hole produces a periodic change in the pressure of the air inside the tube, which creates the traveling sound wave. In a clarinet or saxophone the player blows through a narrow gap between a flexible piece of bamboo called the reed and the mouthpiece. Oboes and bassoons have two reeds separated by a thin gap. The stream of air causes the reed to vibrate, periodically stopping the air flow and causing the sound wave.

In order to change the pitch produced by an instrument the standing wave inside the instrument must be altered. By changing the length of a wind instrument, or the tension and length of the strings for a string instrument, a different frequency standing wave is produced, which creates a different musical pitch. Pressing a key on a trumpet inserts an additional length of tubing into the instrument. On a flute, clarinet, or saxophone a hole on the instrument is covered or uncovered, changing the effective length of the instrument. On a piano each note uses a string of a particular length. On a violin or guitar the player’s finger is used to change the length. Thicker strings have lower pitches than thinner strings of the same length. Increasing the tension on a string increases the frequency of the standing waves, and thus the pitch.

Many musical instruments, such as clarinets and violins, depend on standing waves to produce sound.

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