Vocal Tract And Vowels

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Vowels are generally those that are produced with an open vocal tract and consonants are those that are produced with a constriction anywhere in the vocal tract. Vowels are the most sonorant, or intense, and the most audible of sounds in speech. Vocal fold vibration is the sound source for vowels. The vocal tract above the glottis acts as an acoustic resonator affecting the sound made by the vocal folds. The shape of this resonator determines the quality of the vowel: [i] versus [u] versus [a], for example. There are four main ways in which speakers can change the shape of the vocal tract and thus change vowel quality:
• raising or lowering the body of the tongue
• advancing or retracting the body of the tongue
• rounding or not rounding the
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(Vowel quadrilateral figure)

Acoustic waveforms
All audible sound is the result of variations in air pressure that produce vibration. In vibration, the pressure in a particular place becomes alternately higher and lower. This is usually described in terms of wave motion. If the vibration happens rapidly, it is said to have a high frequency. If it happens less rapidly it is said to have a lower frequency. If the vibration is regular, repeating its pattern over and over it is called periodic, while a pattern of vibration which does not have such a pattern of regular vibration is called aperiodic. If the amount of energy is large it is said to have high amplitude.
In acoustic analysis, the process of breaking down a complex waveform into simple waveforms is called spectral analysis. The resultant image of the waveform after its spectral analysis is called a spectrogram.

Spectrogram figure
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Hence, estimations of speakers’ vowel space area (VSA) provide a promising avenue for assessing speech motor control. However, VSA measurements have traditionally demonstrated limited success in distinguishing healthy and disordered speech. One factor that plays a role is the formant measurement point. Research in speech motor control almost universally takes measurements of formant values from vowels’ temporal midpoint. It provides measurement consistency, and the influence of vowels’ phonetic context rarely results in symmetrical formant trajectories, and thus midpoint values exhibit considerable variation across word tokens. There is also increasing evidence that a vowel’s temporal midpoint reflects a different stage of articulation in faster and slower speech. For people with motor speech disorders, significant reductions in speech rate are common. It has been hypothesized that the extraction of formant values from a static time point could be responsible, in part, for the failure of studies to consistently reveal reductions in the VSA of this population – despite evidence of reduced lip, tongue and jaw movements. The acoustic consequences of such vowel production deficits includes centralization of formant frequencies, reduction of vowel space area (i.e., mean working vowel space), and abnormal formant frequencies for

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