A spectral glide is a music-composition concept, consisting of a "modification of the vowel quality of a tone" (Erickson 1975, p. 72).
A spectral glide may be accomplished through a wah-wah, mute, or pedal, or through the modification of one's vocal tract while speaking, singing, or playing an instrument such as the didgeridoo.
Lip-vibrated 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.
Karlheinz Stockhausen specifies the use of a trumpet wa-wa mute in his Punkte (1952/1962/64/66/93) 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.