Spectral glide

A spectral glide is a "modification of the vowel quality of a tone" (Erickson 1975, p.72). Since the vowel quality of a tone is determined by the overtones, spectrum, or timbre (all three terms being near equal) of that tone, a spectral glide is a move from one spectrum characteristic of a vowel to another. A spectral glide may be accomplished through a wah-wah mute or pedal or through the modiciation of ones vocal tract while speaking, singing, or playing an instrument such as the didgeridoo. Buzzed-lip instruments with large mouthpieces such as tuba and trombone allow extensive modification of vowel quality while woodwinds have a smaller range with the exception of the flute in air-sound mode. Strings have the smallest range. (ibid)

Important in the compositional use of spectral glides is the glide rate and the vowel contrasts used. Karlheinz Stockhausen specifies the use of a wah-wah pedal in his "Punkte" (1952/1962) through open and closed circles connected by a line. A. Wayne Slawson's computer generated "Wishful Thinking about Winter" (Decca DL 710180) uses speechlike sounds featuring a large range of spectral glide rates. Loren Rush began investigating in 1967 the computer-generated modeling of timbres "in between" familiar instruments such as a bassoon and bass clarinet and devised a program to provide a smooth transition between timbres. (ibid, p.73)

ource

*Erickson, Robert (1975). "Sound Structure in Music". University of California Press. ISBN 0-520-02376-5.