Formant: an amplitude peak in the spectral envelope of a sound that remains relatively constant even when the fundamental frequency of the sound changes.

The formants of a sound can give you clues about structure of the sound-producing object. For example, imagine a resonator (like a room or the sound box of a musical instrument) being hit by a periodic pulse. The sound waves bounce off the walls of the enclosure and reflect (with attenuation), and all the reflections are 'mixed' or added together. At some wavelengths, the waves reinforce each other; for other wavelengths, the waves cancel each other out. This produces peaks and notches in the spectrum. The rate at which the pulses hit the resonator affects the fundamental frequency but has little effect on where the notches and peaks occur.

The position of the formants also tells you something about the size and shape (and material) of the resonating chamber. For example, a small object tends to reinforce frequencies that have shorter wavelengths (higher frequencies), and a larger object tends to reinforce longer wavelengths (lower frequencies). Due to our experiences in the physical world, we tend to associate high formants with tiny nonthreatening objects (e.g. chipmunks and small children) and low frequency formants with large and dangerous objects (e.g. giants and cephlaphages).

If you take a bandpass filter and feed a pulse generator into it, you have created a physical model that has one formant.

-- CarlaScaletti - 25 Nov 2003

It is possible to derive the shape of a formant from a single cycle regardless of the period. Unlike FFT that requires many cycles to give you a spectrum shape, and will normally have the fundamental period giving the highest point in the spectrum and dwarfing out the little formants.

-- PeteJohnston - 25 Nov 2003