The Acoustics of Normal and Nasal Vowel Production

Abstract

The number of investigators in the area of speech pathology has increased considerably in the last two decades. At the same time, the variety of instruments capable of performing sophisticated acoustic analyses has also increased. The result has been a rapid expansion of the literature dealing with acoustic analyses of defective speech. One of the areas which has received considerable research interest is that of nasality. Acoustic studies have been made of functional (8), simu— lated (5), organic (7), and synthesized (6) nasal quality, and the results of theSe investigations, while varied, have indicated certain basic simi— larities. The purpose of this article is to discuss these similarities and to consider some of their implications. The initial focus will be upon a consideration of the acoustics of the normal vowel (nasality is essentially a vowel—based phenomenon). With this information providing points of reference, four kinds of spectrum distortion which result from acoustic coupling of the nasal cavity will be considered. Comparative normal-nasal spectrum curves for several vowels will be derived and explanations to account for each of the prominent within—vowel differences will be offered. From a consideration of these differences, the paper will conclude with a discussion of possible approaches to the development of an objective measure of nasality. The Acoustics of the Normal, Non-Nasal Vowel A normal vowel is the product of three acoustic influences: a) the sounds produced by the larynx; b) the effect on these sounds of passage through the oral-pharyngeal resonances chambers; and c) a radiation effect present at the lips as the sounds leave the head. Each of these factors influences the shape of the overall vowel spectrum in a characteristic manner; each, therefore, will be considered separately. THE LARYNGEAL SOUND SOURCE. It is well known that the " pitch " of the speaking voice is determined by fundamental frequency. The average frequency of the voice of an adult American male is 132 Hz, of an adult female, 224 Hz, and of a child, 265 Hz (10).