A spectral glide is a "modification of the vowel quality of a tone" (Erickson 1975, p. 72). Since the vowel quality of a tone

Pitch (music)

Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...

 is determined by the overtone

Overtone

An overtone is any frequency higher than the fundamental frequency of a sound. The fundamental and the overtones together are called partials. Harmonics are partials whose frequencies are whole number multiples of the fundamental These overlapping terms are variously used when discussing the...

s, spectrum

Spectrum

A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

, or timbre

Timbre

In music, timbre is the quality of a musical note or sound or tone that distinguishes different types of sound production, such as voices and musical instruments, such as string instruments, wind instruments, and percussion instruments. The physical characteristics of sound that determine the...

 (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 modification of one's vocal tract

Vocal tract

The vocal tract is the cavity in human beings and in animals where sound that is produced at the sound source is filtered....

 while speaking, singing, or playing an instrument such as the didgeridoo

Didgeridoo

The didgeridoo is a wind instrument developed by Indigenous Australians of northern Australia around 1,500 years ago and still in widespread usage today both in Australia and around the world. It is sometimes described as a natural wooden trumpet or "drone pipe"...

. 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

Karlheinz Stockhausen

Karlheinz Stockhausen was a German composer, widely acknowledged by critics as one of the most important but also controversial composers of the 20th and early 21st centuries. Another critic calls him "one of the great visionaries of 20th-century music"...

 specifies the use of a trumpet mute in his Punkte

Punkte

Punkte is an orchestral composition by Karlheinz Stockhausen, given the work number ½ in his catalogue of works.-History:Punkte originated as a punctual orchestral work which was begun in September in Hamburg and had reached a first-draft stage by 30 September...

(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. Loren Rush

Loren Rush

Loren Rush is a U.S. composer. His works include the drone piece Hard Music for three amplified pianos. The piece features no melodic figuration but rather clouds created by only one note, the low D above cello C, repeated quickly enough by each player to be heard as nearly continuous...

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)

Source

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