Question

47.Apply the principles of plosive production to clinical disorders. 48. Outline the articulatory and aerodynamic and acoustic events for typical fricative production. 49.Describe the different sound sources associated with fricative production.

Answer 1

47. Stops are sounds in which the flow of air which is active in creating the sound is completely blocked for a short interval of time. In the most common type of stop sound, known as a plosive, air in the lungs is briefly blocked from flowing out through the mouth and nose, and pressure builds up behind the blockage. The sounds that are generally associated with the letters p, t, k, b, d, g in English words such as pat, kid, bag are examples of plosives. At least some sounds of this type are found in every known language spoken by humans. In phonetic terminology the sounds associated with p, b are called bilabial plosives because their pronunciation requires bringing the two lips together; the sounds associated with t, d are called dental or alveolar plosives because the tongue closes against the upper teeth or the skin covering the roots (alveoli) of the teeth; the sounds associated with k, g are called velar plosives because the tongue closes against the soft part of the palate, technically known as the velum. Bilabial, dental, alveolar and velar are known as places of articulation.

The letters p, t, k represent voiceless plosives, that is, those in which the vocal folds in the larynx are apart and the air is able to flow quite freely from the lungs into the mouth cavity. The letters b, d, g represent voiced plosives, that is, those in which the vocal folds are in a position close together so that, provided the air pressure in the lungs is sufficient, the vocal folds will be caused to vibrate against one another (as they do, for example during humming).

48. Acoustic events begin by first identifying events or landmarks in the speech signal, such as at consonant-vowel (CV) and vowel-consonant (VC) boundaries. Once landmarks are located, signal processing and analysis can be focused in their vicinity, where change is occurring and information is concentrated. The strategy is to extract appropriate acoustic information in the vicinity of these landmarks, and also in the intervals when the vocal tract is most and least constricted.

Fricative consonants are distinguished from other speech sounds by their manner of production. Fricatives are produced by forming a narrow constriction in some region along the length of the vocal tract. Airblown through this constriction becomes turbulent inflow, typically near an obstacle in the airstream or at the walls of the vocal tract. The acoustic result of this turbulence is the generation of noise. This noise is then filtered by the vocal tract, with the acoustic cavity in front of the constriction contributing the greatest influence on the filtering. The eight fricatives in English, are distinguished by the location of the consonantal constriction: labiodentals /f, v/, dentals /0, d/, alveolars /s, z/, and palatals /, The first of each pair is voiceless and the second is the voiced cognate, in which the voicing source is superimposed on the noise. One approach for characterizing these sounds is to describe them by a set of distinctive features. These features specify the articulators, such as the tongue, lips, and larynx, that are used in producing the sounds, and describe how these articulators are adjusted in forming constrictions in the vocal tract (Jakobson, Fant, and Halle, 1965; Chomsky and Halle, 1968; Halle and Stevens, 1991; Stevens and Keyser, 1992).

During spoken communication, the vocal tract alternately opens and closes. The major features that subdivide speech sounds into classes focus on the articulatory and acoustic events that occur at different phases of this opening and closing. Vowels are produced when the vocal tract is least constricted and the vocal folds are positioned so that spontaneous voicing occurs.

The following acoustic events are associated with fricatives produced in an intervocalic context:

* An interval of frication noise with a spectrum that is shaped by the location of the constriction.

* Formant transitions into adjacent vowels that provide an additional place of articulation information.

* Detailed events, which occur during the transition from noise production to voicing onset, that signal the distinction between voiced and voiceless fricatives and contribute to naturalness.

49. Fricatives are the kinds of sounds usually associated with letters such as f, s; v, z, in which the air passes through a narrow constriction that causes the air to flow turbulently and thus create a noisy sound. The other classes of consonants which are found in the majority of languages (nasals, "liquids" and vowel-like approximants) are voiced in the overwhelming majority of cases.