Most instruments do not produce a pure sine wave when played, as a computer does. They produce a loud basic tone, along with other tones, indistinguishable to the human ear, forming a type of chord. These extra tones are called overtones or harmonics, and they are what make a clarinet sound different from a flute, an oboe, or a guitar. These harmonics are at integral multiples of the base tone. For example, the first harmonic of the A at 440 Hz will be at 880 Hz, the second at 1320 Hz, the third at 1760 Hz, etc.
Depending on which overtones are present and how loud they are, the sound of different instruments can vary greatly. For example, a flute's, overtones are very quiet, producing a very pure sound. This sound is the most like a sine wave out of any normal instrument sound.
The clarinet, however, only produces the odd harmonics, and their amplitudes are in inverse proportion to them. For example, it contains the first harmonic (the base tone), the third harmonic at a volume of one third of the base tone, the fifth harmonic at a volume of one fifth the base tone, and so on. The sum of these waves produces a square wave, rather than a sine wave.
The oboe produces both odd and even harmonics, but still in inverse proportion to their amplitudes. The result of this is a sawtooth wave, which is also the type of wave produced by most string instruments. This is why the oboe's sound is very similar to the sound of a violin.
The last major type of wave is the triangle wave, which consists of only odd harmonics, and amplitudes which are inversely proportional to the squares of the harmonics. It sounds similar to a sine wave, but slightly more metallic.
Sources:
- http://www.teachnet.ie/amhiggins/squaresaw.html
- http://www.csm.astate.edu/music.html
- http://www.wikipedia.org/wiki/Triangle_wave
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