A spectral glide is a music-composition concept, consisting of a "modification of the vowel quality of a tone". Since the vowel quality of a tone is determined by the overtones, spectrum, or timbre of that tone, a spectral glide is a move from a spectrum characteristic of one vowel to a spectrum characteristic of another vowel. 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. Strings have the smallest range.