There is no such thing in music as a “pure” tone, in the sense of their being only one frequency produced. Instead, the sounds made by musical instruments are in reality a complex blend of the fundamental note and various combinations of upper partials. This difference in blend produces different timbres or tone colors, enabling us to distinguish, for example, between the sound of a flute, oboe and a clarinet, even when they are playing at the same pitch. The flute produces only a few, weak harmonics, which is why it has a simple, mellow sound. The oboe, on the other hand, is rich in higher harmonics. The clarinet sounds reedy because it produces mostly odd harmonics. Its higher partials are also fairly strong.
Modern electronic equipment is able to measure both the number and relative strength of these upper partials or harmonics fairly accurately. This is how electronic synthesizers are able to imitate orchestral sounds.
