Chapter One: An Acoustics Primer
7. What are wave shapes and spectral content? | page 4
Composers have been able to get inside these analyses and manipulate the pitch, timing and evolving amplitudes of individual partials with very interesting results — made easy by programs such as Audiosculpt (IRCAM) and Spektral Delay (Native Instruments). A related process called phase vocoding allows these successive analysis snapshots to be resynthesized by creating a sine wave for each partial (or band), which follows the original analysis. The beauty is that a sound can be reconstituted at any desired speed, forward or backward, without altering its pitch, as would be the case if a tape were sped up or slowed down. This technique exemplifies a type of synthesis by analysis.
It is beyond the scope of this article to discuss the specific spectra created by synthetic waveforms (such as triangle waves, square waves, etc.), but please visit Basic Synthesis Facts for further information).
In the real world, many different frequencies dynamically combine, either from the same source or from different sources, to create our impression of timbre. It should be noted that the summation of waveforms creates a series of complex interactions over time. However, at any instantaneous moment in time, only one sound pressure level is acting upon our eardrum and only one voltage is being applied to a loudspeaker, even if you are listening to or reproducing an orchestra of 80 instruments. We perceive the mixture of pitches through the complexities of a varying pressure stream and also a bit of psychoacoustic extrapolation on our part (the 19th Century father of acoustics Helmhotz believed that our perceiving the musical pitch of a sound was dependent on the presence of its fundamental sine wave this has now been proven to not always be the case).
For further study, see Hyperphysics->Waveforms