Editing Musical acoustics (section)

==Physical aspects==

[[File:Sound spectrography of infrasound recording 30301.webm|thumb|Sound spectrography of infrasound recording 30301]]

[[Image:Spectrogram showing shared partials.png|frame|A spectrogram of a violin playing a note and then a perfect fifth above it. The shared partials are highlighted by the white dashes.]]
Whenever two different pitches are played at the same time, their sound waves interact with each other – the highs and lows in the air pressure reinforce each other to produce a different sound wave. Any repeating sound wave that is not a sine wave can be modeled by many different sine waves of the appropriate frequencies and amplitudes (a [[frequency spectrum]]). In [[human]]s the [[hearing (sense)|hearing]] apparatus (composed of the [[ear]]s and [[brain]]) can usually isolate these tones and hear them distinctly. When two or more tones are played at once, a variation of air pressure at the ear "contains" the pitches of each, and the ear and/or brain isolate and decode them into distinct tones.

When the original sound sources are perfectly periodic, the [[Musical note|note]] consists of several related sine waves (which mathematically add to each other) called the [[fundamental frequency|fundamental]] and the [[harmonic]]s, [[Harmonic series (music)#Partial|partial]]s, or [[overtone]]s. The sounds have [[harmonic]] [[frequency spectrum|frequency spectra]]. The lowest frequency present is the fundamental, and is the frequency at which the entire wave vibrates. The overtones vibrate faster than the fundamental, but must vibrate at integer multiples of the fundamental frequency for the total wave to be exactly the same each cycle. Real instruments are close to periodic, but the frequencies of the overtones are slightly imperfect, so the shape of the wave changes slightly over time.{{Citation needed|date=January 2009}}