In normal speech, nasality is referred to as nasalization and is a linguistic category that can apply to vowels or consonants in a specific language. The primary underlying physical variable determining the degree of nasality in normal speech is the opening and closing of a velopharyngeal passageway between the oral vocal tract and the nasal vocal tract. In the normal vocal tract anatomy, this opening is controlled by lowering and raising the velum or soft palate, to open or close, respectively, the velopharyngeal passageway.
In some types of abnormal speech, the term nasality, or more formally hypernasality, refers to an auditory impression about the speech, and is not a precise physical variable. Excess nasality is often observed in the speech of hearing impaired persons, who cannot adequately monitor this subtle characteristic of speech (examples). It is also common with persons having a cleft palate, for whom another passage or passages may exist between the oral and nasal vocal tract, or distortion of the palate prevents or hinders an adequate velopharyngeal closure. Since nasality is a subtle characteristic of speech, it is sometimes not readily controlled in learning a foreign language or in child language learning.
Hypernasality is generally segmented into so-called 'resonance' effects in vowels and some voiced or sonorant consonants and the effects of excess nasal airflow during those consonants requiring a buildup of oral air pressure, such as stop consonants (as /p/) or sibilants (as /s/). The latter nasal airflow problem is termed 'nasal emission', and acts to prevent the buildup of air pressure and thus prevent the normal production of the consonant. In testing for resonance effects without the aid of technology, speech pathologists are asked to rate the speech by listening to a recorded sentence or paragraph, though much variability in such subjective ratings, for at least two reasons. First, the acoustic effect of a given velopharyngeal opening varies greatly depending on the degree of occlusion of the nasal passageways. (This is the reason why a stuffy nose from an allergy or cold will sound more nasal than when the nose is clear.) Secondly, for many persons with hypernasal speech, especially hearing impaired, there are also mispronunciations of the articulation of the vowels. It is extremely difficult to separate the acoustic effects of hypernasality from the acoustic effects of mispronounced vowels (examples). Of course, in speech training of the hearing impaired, there is little possibility of making nasality judgments aurally, and holding a finger to the side of the nose, to feel voice frequency vibration, is sometimes recommended.
Without the use of a technological aid, nasal emission is sometimes judged by listening for any turbulence that may be produced by the nasal airflow, as when there is a small velopharyngeal opening and there is some degree of mucous in the opening. More directly, methods recommended include looking for the fogging of a mirror held near the nares or listening through a tube, the other end of which is held in or near a nares opening.
There have been many attempts to use technological augmentation more than a mirror or tube to aid the speech pathologist or provide meaningful feedback to the person attempting to correct their hypernasality. Among the more successful of these attempts, the incompleteness of velopharyngeal closure during vowels and sonorants that causes nasal resonance can be estimated and displayed for evaluation or biofeedback in speech training through the nasalance of the voice, with nasalance defined as a ratio of acoustic energy at the nostrils to that at the mouth, with some form of acoustic separation present between the mouth and nose. In the nasalance measurement system sold by Kay-Pentax, the acoustic separation is provided by a solid flat partition held against the upper lip, while in the system sold by Glottal Enterprises the acoustic separation can be by either a solid flat partition or a two-chamber mask.
However, devices for measuring nasalance do not measure nasal emission during pressure consonants. Because of this, a means for measuring the degree of velopharyngeal closure in consonants is also needed. A commercially available device for making such measurements is the Perci-Sar system from Microtronics. The Nasality Visualization System from Glottal Enterprises allows both the measurement of Nasal Emission and Nasalance. In the presence of a cleft palate, either of these systems can be helpful in evaluating the need for an appliance or surgical intervention to close the cleft or the success of an appliance or a surgical attempt to close the cleft.
- ^ a b R.J. Baken, Robert F. Orlikoff. Clinical Measurement of Speech and Voice San Diego: Singular, 2000
- ^ a b Kummer, A. W. Resonance disorders and nasal emission: Evaluation and treatment using "low tech" and "no tech" procedures. The ASHA Leader (2006 Feb 7) 11(2), pp. 4, 26.
- ^ Watterson T, Lewis K, Brancamp T. "Comparison of Nasalance scores obtained with the Nasometer 6200 and the Nasometer II 6400." Cleft Palate Craniofac J. 2005 Sep;42(5):574-9 PMID 16149843
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