Vowels are speech sounds (1) during whose production “the tongue is held at such a distance from the roof of the mouth that there is no perceptible frictional noise” and “a resonance chamber is formed which modifies the quality of tone” (Jones, Pronunciation 12). Gimson defines vowels (2) as a “category of sounds … normally made with a voiced egressive air-stream, without any closure or narrowing such as would result in the noise component characteristic of many consonantal sounds” (Introduction 35).
Fant gave a list of several correlates in speech sound classification (Speech pp. 153-155). What follows is a compiled overview of properties a sound should have, according to Fant, to be classified as a vowel. The first condition is that a vowel must have sound energy visible in sound spectrum, and that the source of the acoustic energy originates from the vocal folds vibration. A vowel should also have “vowelike correlate” in speech production, which means an unobstructed pass of airstream. Waveform analysis of a “vowelike sound” implies that “at last F1 and F2 [are] detectable”, while F3 should be visible if F1/F2 are not at their lower ends (156). To classify a vowel as a diphthong, the speech sound must satisfy the “glide” correlate, which in the production context means “moderate speed within a segment”, seen as a “relatively slow [spectrum change] rate but faster than for mere combination of two vowels” (156). The picture below shows a spectrogram of a diphthong, satisfying Fant’s requirements for the classification.
We will give one more description of vowels (3), as described by Laver, who says that two of the distinctions for classifying speech sounds are place of articulation and degree of stricture, both related to the medial phase of a segment. Place of articulation refers to “the location of the articulatory zone in which the active articulator is closest to the passive articulator during the medial phase of a segment” (166). Degree of stricture identifies the degree of closure between the two articulators in the medial phase. Thus, he defines vowels as a group of sounds articulated in places of neutral articulation (167), when “the potential active articulators … lie in their neutral anatomical position” (166) opposite their passive articulators. In discussion about degree of stricture Laver says that in resonants “the stricture is one of open approximation” (168), allowing unrestrained pass of energy from the vocal folds.
1 They are also discussed in terms of being “purely linguistic units, counters which do a certain job, irrespective of how they sound” (O’Connor, Phonetics, 199) but that is a more phonological approach.
2 Gimson refers to vowels in the introductory chapters as “the vowel type” of sounds, “described in mainly auditory terms” (Introduction, 35). When discussing the vowel versus the consonant distinction he notes: “It will be found that the phonemes of a language usually fall into two classes, those which a typically central (or nuclear) in the syllable and those which are non-central (or marginal). The term ‘vowel’ can then be applied to those phonemes having the former function and ‘consonant’ to those having the latter.” (53).
3 Laver (pp. 167-172) gives a detailed description of several articulation aspects.
This post is based on a draft for one of the introductory chapters in my paper. For cited works please visit the page Books & References.