Terminology Review of Terminology Time signal or waveform The description of a sound in the time domain as fluctuations in some physical property like pressure over time. Often, because the pressure fluctuations have been transduced by a microphone or other measurement instrument, we have converted pressure fluctuations to voltage fluctuations over time. Fourier transform or spectrum The description of a sound in the frequency-domain as the amplitude or extent of fluctuation occurring at different frequencies. The kind of spectrum that is found for sounds that are purely periodic, that is, for sounds that repeat the same pattern infinitely. Each line in a line spectrum is a harmonic of the fundamental period of the waveform and represents a sinusoid at a particular frequency and amplitude.
Line spectra are the ideal case of harmonic spectra.
Harmonic Spectrum Similar to a line spectrum except that sounds giving rise to harmonic spectra are not purely periodic, but only approximately so. Such sounds produce an harmonic spectrum in which the lines have some discernible width. As sounds deviate increasingly from true periodicity, their spectra deviate increasingly from line spectra to approach a continuous spectrum. For example, any sound that has finite duration is not strictly periodic. Many natural sounds, like the human voice, are quasi-periodic in that the sound deviates in a variety of ways from one period to the next.