When two or more sound waves meet, they add to and subtract from each other. If their peaks and troughs are perfectly in phase, they reinforce each other, resulting in a waveform that has higher amplitude than either individual waveform.

In‑phase waves reinforce each other.

If the peaks and troughs of two waveforms are perfectly out of phase, they cancel each other out, resulting in no waveform at all.

Out‑of‑phase waves cancel each other out.

In most cases, however, waves are out of phase in varying amounts, resulting in a combined waveform that is more complex than individual waveforms. A complex waveform that represents music, voice, noise, and other sounds, for example, combines the waveforms from each sound together.

Because of its unique physical structure, a single instrument can create extremely complex waves. That’s why a violin and a trumpet sound different even when playing the same note.

Two simple waves combine to create a complex wave.