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There are two categories of sound covered in this section: simple and complex. Simple sounds are also called pure or simple tones because they consist of a single frequency, though the intensity of that frequency can vary. An example of a simple tone is a tuning fork because the sound it produces is almost entirely only one tone for musical instruments to match. Simple tones do not characterize normal real-world sounds because real-world sounds are more complicated. In everyday life, we hear sounds that consist of the second type of sound: complex sounds (or complex tones). A complex tone comprises two or more simple tones. Complex tones make up the vast majority of sounds we hear day-to-day. Some examples of complex sounds include voices and music.

The tone that has the lowest frequency in the complex tone is called the fundamental frequency, and the other tones are known as overtones or harmonics. The fundamental frequency is what we perceive as being the pitch of a sound. Because the fundamental frequency is the perceived pitch we hear in a complex tone, we say the complex tone is characterized by its fundamental frequency. For example, if a complex tone had three overtones of varying frequencies that were a multiple of 100 Hz, and a fundamental frequency of 100 Hz, the perceived pitch of this sound would be 100 Hz.

Timbre is the quality of complex tones produced. The timbre of a sound depends on its waveform, which varies with the number, frequency, and relative intensity of the overtones that are present. Different waveforms are produced by synthesizing (combining) different pure tones of various frequencies and intensities. For this reason, the timbre of a sound varies with the characteristics of the overtones. Two different musical instruments can be playing the same note but have different timbres because they have different complex tones despite having the same fundamental frequency.