Sound waves move through the air and other materials and have the common wave characteristics of wavelength, frequency, amplitude and speed (velocity). Musical instruments create sound by creating vibrations that result in sound waves.
Strings, woodwinds, brass, percussion, keyboards and electronic instruments are instrument groups each of which produces sounds in its own way. Stringed instruments include the violin, the guitar and the harp. Wind instruments include the flute, clarinet and saxophone, while brass instruments include the trumpet, trombone and tuba. The drum is a familiar percussion instrument, and this group also includes the tambourine, xylophone and cymbals. Besides the piano, keyboard instruments include the harpsichord, pipe organ and accordion. Synthesizers are the most familiar modern electronic instrument.
The strings of stringed instruments can be made to vibrate by a variety of methods, including the plucking of the guitarist and harpist and the bowing of the violinist. With wind and brass instruments, the player creates a vibrating column of air inside a tube by blowing across an edge, as with the flute or even a bottle; between a reed and a fixed surface, as with the saxophone; or between two reeds, as with the bassoon. The name “percussion” gives away the usual method, hitting, by which the membranes or solid material of a percussion instrument is made to vibrate. Keyboards produce vibration when a key is struck, which causes hammers to hit strings in the piano and air from a pump to be directed across an edge into a pipe in the pipe organ. With electronic instruments, electronic signals are turned into vibrations by a speaker.
The pitch of a sound refers to the sensation of a frequency, with a high pitch corresponding to a high frequency sound wave and a low pitch corresponding to a low frequency sound wave. How to change the pitch of an instrument depends on the instrument. With a stringed instrument, length, thickness, tension and density affect the pitch of a string. Different pitches are achieved on a harp by having strings of different length, and on a violin or guitar by stopping the vibrating strings at different points.
Volume is increased by applying more force or blowing harder; by resonance, supplied by the body of an instrument; or electronically. The large wooden body of the acoustic guitar, also called the “sound box,” is made to vibrate by the vibrating strings, which in turn makes the air inside the hollow body vibrate. In an electric guitar, tiny magnets generate electricity when the strings move, and the current is fed into an electronic amplifier that increases the current many times.