Sound waves combine and the waveforms show that the first tuning fork [A] has twice the frequency of the second one [B]. The combination [C] produces a sound equal to the differences in frequencies and the altered shaped of the waveform [green curve] shows that it has changed its tone. Two waves that are only slightly different in frequency combine to give a slow beating (pulsing) sound.
The waveforms of a flute [A], oboe [B], and clarinet [C] show the differences in tone of the instruments. The flute's rounded waveform indicates a gentle, fluid sound. The clarinet wave has a similar shape with 'jinks' reflecting the instrument's more reedy sound. The jagged waveform of the oboe shows that its sound is even more reedy. Every note of an instrument has a fundamental frequency but also vibrates at frequencies that are simple multiples of this.
Musical instruments, from the deepest to the highest members of each family, cover almost the entire range of human hearing. The woodwind family has a particularly wide compass, the lowest note of the contrabassoon and the harmonics [dotted line] of the piccolo nearing the limits of audibility.
Why should one musical instrument sound so different from another? Instruments are played in various ways: some are struck, some are blown, while others are bowed or plucked to produce many kinds of sounds. But what is different about the sound itself?
Frequency and pitch
Every instrument produces a sound by making something vibrate and the frequency of the vibration is related to the pitch of the note produced. If the vibration is more rapid, the number of vibrations in the sound wave that reaches the ear (the frequency of the wave) is greater and the pitch is higher or more to the treble. If the frequency is less, the pitch will be lower or more to the bass.
The frequency of a sound wave (number of vibrations per second) is measured in hertz (Hz). The audible range for most human beings lies between 20 and 20,000 Hz. But some animals, such as bats and dogs, hear over a far wider range.
Every instrument produces a certain set of notes within a particular range of pitch. But each note is in fact a combination of many more notes. The pitch of the main note heard by the ear is called the fundamental, and above it every instrument also produces a group of higher-pitched notes called harmonics. The harmonics are produced because the vibrating object making the sound vibrates at several frequencies at once and the extra frequencies are simple multiples of the fundamental frequency.
These higher notes can sometimes be produced deliberately on certain instruments – on brass instruments by blowing harder and on string instruments by a particular method of fingering – but normally they are not heard individually. If they were, each note on an instrument would sound like a vast chord. Instead, all the harmonics combine with the fundamental note to produce a complex waveform. Each instrument produces its own particular waveform because the relative intensity of the harmonics is different. The modern music synthesizer works by producing several waveforms of basic shapes – a sine-wave, a saw-toothed wave, and a square wave – and then combining them to make all kinds of sounds.
Not all instruments produce a note of definite pitch. Several such as (some) drums and cymbals, produce noise, which consists of a wide range of frequencies without any particular dominant frequency.
The effect of volume
Volume, or the degree of loudness, is another quality of musical sound. Music employs contrasts of volume on a large time scale for dramatic effect, but on a small time scale the change of volume at the beginning of a note is essential to the quality of a sound. The starting characteristics, called transients, determine whether a note begins quickly or takes some time to build up; transients are complex and involve changes in the waveform as well as in volume as the instrument begins to sound. Transients are vital to recognition; if the transients are removed from a recording of an oboe, for example, the character of its sound changes until it sounds more like a mouth organ.
Two other qualities present in a musical sound are echo and vibrato. Echo is often believed to improve music, giving it a more rounded sound, and it is produced by the reflections of sound from the walls of a concert hall or added artificially to recordings. Vibrato is a slight wobble in pitch that many musicians like to use; a violinist moves his left wrist to an fro to produce vibrato.
The nature of the vibrating object is the basis of family groupings of instruments. In string instruments – the violin, viola, cello, double bass, guitar, piano, harpsichord, and harp – a aut string is vibrated by stroking it with a bow, plucking it with the fingers or a plectrum, or striking it wit a soft hammer. A longer string produces a lower note, and the pitch of notes from an instrument is altered either by pressing the string against a fingerboard to change its length, or by playing a string of a different length. The tension and thickness of the string also affect the note, a tauter or thinner string giving a higher note.
Types of musical instruments
Wind instruments work by making a column of air vibrate. In brass instruments – the trumpet, trombone, and horn – the player's lips vibrate in the mouthpiece. In some woodwind instruments, such as the bassoon, oboe, and clarinet, the mouthpiece contains one or two vibrating reeds, and in the flute the the player blows across a hole to set the air column in the instrument vibrating. When a player presses down keys or valves, he alters the length of the air column and produces notes of different pitch. Also, he can obtain some harmonics instead of the fundamental.
Some precision instruments are played by striking either a taut skin, as in a drum, or a solid object of some kind – a disk of metal in a cymbal, for instance. Tuned percussion instruments give definite pitches. They include the vibraphone and xylophone, in which metal or wooden bars of different lengths are struck to sound various notes.
Electric instruments pick up the vibration of a string, as in an electric guitar, or a rod in an electric piano, and convert the vibration into an electrical signal that passes to an amplifier and loudspeaker to produce the sound. Electronic instruments include the electronic organ and synthesizer, in which oscillator circuits produce electric signals.