Jim Reese, age 13, of Bennington, Vt., for his question:
IS IT A MUSICAL SOUND OR NOISE?
Sound can be classified as either noisy or musical. A noise is produced by a vibrating object, such as a rattling window, that sends out irregular vibrations at irregular intervals. Music, on the other hand, is made by a vibrating body that sends out regular vibrations at regular intervals, such as a piano played by a musician.
Some other kinds of sound include the sounds made by people and animals, and ultrasound and stereophonic sound.
Musical sounds are made by three types of instruments: stringed, wind and percussion.
Stringed instruments have strings stretched over some kind of resonance box or board. Violins and guitars have resonance boxes while pianos have resonance boards.
The tension or tightness of a string controls the frequency at which it vibrates and, therefore, the pitch of the sound it makes. An increase in the tension of a string increases its frequency. However, the longer the string, the lower the frequency.
With a wind instrument, a musician blows a stream of air across the opening of a tube or a pipe. The stream of air makes the column of air inside the tube or pipe vibrate.
The frequency of the vibration of this air column depends on the length of the tube or pipe. If the tube is closed at the bottom, the note produced has a wave length four times the length of the tube. If the tube is open at both ends, the note has a wave length twice the length of the tube.
A reed makes the air columns vibrate in wind instruments such as saxophones, clarinets, oboes and bassoons. In brass instruments, such as trumpets and French horns, the vibrating lips of the musician make the air columns vibrate.
Percussion instruments include drums, bells, gongs and cymbals. Musicians use sticks or their hands to make the percussion instruments vibrate.
Noise can harm people in two ways. Intense noise may actually produce deafness while continuous or periodic noise may cause people to become tired or irritable, even if it is not extremely loud.
The steady whine of a saw or the periodic ringing of a telephone can cut a worker's production in half. Builders often cover the inside walls of offices and factories with felt, cork or other sound absorbing materials. This reduces the noise and improves the efficiency of the workers.
The human voice is also an example of sound produced by air columns vibrated by membranes. A pair of membranes called vocal cords are located on each side of the larynx, or voice box, in the throat. You make these cords vibrate and produce the sounds by forcing air past them. Changing the muscular tension of the cords produces sounds of different pitches.
Musical instruments and the human voice can produce either noise or musical sounds.