Effects of Nasal Port Area on Perception of Nasality and Measures of Nasalance Based on Computational Modeling

Abstract

Objective

The purpose of this study was to extend previously published modeling work examining the relation between nasal port opening, measures of nasalance, and perceptual ratings of nasality by experienced listeners for three simulated English corner vowels, /i/, /u/, and /ɑ/.

Design

Samples were generated using a computational model that allowed for exact control of nasal port size and direct measures of nasalance. Perceptual ratings were obtained using a paired stimulus presentation.

Participants

Four experienced listeners.

Main Outcome Measures

Nasalance and perceptual ratings of nasality.

Results

Findings indicate that perceptual ratings of nasality and nasalance increased for samples generated with nasal port areas up to and including 0.16 cm² but plateaued in samples generated with larger nasal port areas. No vowel differences were noted for perceptual ratings.

Conclusions

This work extends previously published work by including nasal port areas representative of those reported in the literature for clinical populations, however, continued work using samples with varied phonetic context, and varying suprasegmental and temporal characteristics are needed.

Introduction

Perceptual rating is considered the gold standard for assessment of oral-nasal resonance. Numerous studies have attempted to show the relation between measures of velopharyngeal orifice size and perceived nasality (e.g., Carney and Sherman, 1971; Carney and Morris, 1971; Dalston, Warren, and Dalston, 1991; Kummer, Briggs, and Lee, 2003, Brancamp, Lewis, and Watterson, 2010). These studies have failed to show a clear, linear relation between the measures, but indicate that continued research is needed to better understand the relationship and ultimately, to guide clinical management. Bunton and Story (2011) reported on a set of data generated using a computational model and found a high correlation between ratings of nasality by experienced clinicians and direct measures of nasalance for nasal port areas ranging from 0–0.05cm². While this study demonstrated the usefulness of modeling to understand the perceptual phenomena of nasality, the range of nasal port sizes was not representative of velopharyngeal opening sizes reported in the literature for patient populations (e.g., Laine, Warren, Dalston, and Morr, 1988). The goal of the present study is to extend this work to include larger nasal port area sizes (0.04–0.5 cm²) and further understanding of the relation between perceptual ratings of nasality, measures of nasalance and nasal port area.

Method

Simulation of Audio Samples

Vowel samples with varying degrees of nasal port coupling were generated with a computational model. Details of the modeling can be found in Bunton and Story (2011). In brief, three vowels /i/, /u/, and /ɑ/ were simulated with 24 equally incremented values of the nasal port area that ranged from 0.04cm² to 0.5 cm². Waveforms were generated for each vowel condition and used as stimuli for the listening experiment. Three audio files containing all stimuli for the three different vowels (Vow[i]series, Vow[u]series, Vow[ɑ]series) are available as supplemental online content.

Measures of Nasalance

Nasalance was calculated from these signals according to the equation,

$$\text{Nasalance}=100\times P_n/(P_o+P_n)$$

where P_n and P_o are the RMS pressures at the nares and lips, respectively. For each vowel, two nasalance values were calculated. First with the raw signals, which are referred to as “full bandwidth,” and then with the pressure signals at the nares and lips bandpass filtered (4^th order Butterworth) with cutoff frequencies of 350 Hz and 650 Hz prior to determining the RMS values of Pn and Po. The second condition emulated the filtering typically performed by the Nasometer system (Fletcher et al., 1989).

Auditory Perceptual Scaling of Nasality

The listening panel consisted of 4 speech-language pathologists with greater than 6 years clinical experience. All listeners were female, native English speakers, and passed a hearing screening. Procedures were are identical to those reported by Bunton and Story (2011) where each speaker heard a pair of vowels and was asked to determine which sample they perceived as having greater nasality. Responses were recorded using a horizontally oriented “slider scale,” with the slider button positioned at the scale’s midpoint (Hillenbrand & Gayert, 1994). The vowel pair included one sample where the nasal port area was set to zero (i.e., not coupled to the vocal tract) and one where the nasal tract was coupled to the vocal tract with an area between 0.04–0.5 cm². An overall rating for each stimulus was then derived on the basis of the mean rating across the listeners. Interrater reliability was assessed using the intraclass correlation coefficient (Shrout & Fleiss, 1979) and was 0.95.

Results

Nasalance

Measures of nasalance are presented in Figure 1. In panel (a) nasalance calculated across the entire bandwidth is plotted against nasal port area whereas in panel (b) the nasalance was calculated based on the Nasometer bandwidth (Fletcher et al., 1989). Within each panel a bolded reference line at 32 percent nasalance is shown (Dalston, Warren, and Dalston, 1991).

Figure 1.

Direct measures of nasalance plotted against nasal port area (cm²) for the full bandwidth (a) and the Nasometer bandwidth (b)

For the full-bandwidth the nasalance values rose steadily as nasal port areas increased from 0.04cm² to 0.2cm². For larger nasal port areas, nasalance values plateaued. The nasalance values based on the Nasometer bandwidth increased until the nasal port area reached 0.18 cm² and then plateaued. This general pattern was consistent across vowels, with nasalance values highest for the vowel /i/, followed by /u/ and /ɑ/.

Auditory Perceptual Rating of Nasality

Mean nasality ratings across listeners are plotted in Figure 2 as a function of nasal port area. Samples with larger numbers were judged by the listeners to be more nasal than samples with smaller numbers. Ratings of nasality increased for all three vowels until the nasal port area reached 0.16 cm². Nasality ratings for samples generated with larger nasal port areas had a shallow slope or plateaued. It appears that samples generated with nasal port areas greater than 0.16 cm² did not represent an increase in nasality for listeners. This finding is interesting given that listeners did not use the full rating scale to rate nasality. A range of 0 to 500 was allowed in the user interface; however, mean ratings were between 0 and 350.

Figure 2.

Mean nasality ratings across listeners plotted against nasal port area (cm²). Individual vowels are represented by shape and line type within the figure.

Individual listener data, shown in Figure 3, shows variability in ratings across listeners. Listener 1 had the greatest variability in nasality ratings across the range of nasal port areas. This listener also consistently rated nasality lower for /i/ compared to the /ɑ/ and /u/. Listeners 2 and 3, those with the most experience (>10 years), showed similar overlap in ratings and vowels. Listener 4 rated the vowel /ɑ/ with consistently lower values than the other two vowels. Interestingly, this listener had the least clinical experience (6 years) and specialized in voice rather than resonance disorders.

Figure 3.

Individual listener nasality ratings plotted against nasal port area (cm²). Individual vowels are represented by shape and line type within the figure.

Nasalance and Nasality

To examine the relation between measures of nasalance and mean perceptual nasality ratings, correlation coefficients were calculated. Separate coefficients were calculated for the full bandwidth nasalance scores and the limited bandwidth scores. Correlation coefficients were greater than 0.94 for all vowels and bandwidths.

Discussion

The aim of the current study was to examine the relation of perceptual ratings of nasality, measures of nasalance, and nasal port area using a computational model to precisely control area. The current study extends previously published modeling work by including nasal port areas representative of those reported in the literature for clinical populations (i.e., 0.04–0.5cm²).

Vowel differences

Nasalance measures were similar for the full bandwidth versus the Nasometer bandwidth. Nasalance was highest for the high front vowel /i/ followed by the high back vowel /u/ and finally the low back vowel /ɑ/. This is consistent with data published by Bunton and Story (2011) for small nasal port area.

Perceptual ratings of nasality showed no vowel differences based on the group data. A difference in vowels was seen for individual listeners; however, it was variable. Two listeners (2 & 3) rated nasality for the three vowels similarly. Listener 4, on the other hand, rated /i/ and /u/ similarly and /ɑ/ as less nasal. Listener 1 rated /u/ and /ɑ/ similarly and /i/ less nasal across nasal port area size. Bunton and Story (2011) reported consistently higher ratings for the high vowels /i/ and /u/ compared to the low vowel/ɑ/.

Inconsistency in vowel ratings is noteworthy given previously published data reporting low vowels are perceived as more nasal than high vowels in normal speakers (Lintz and Sherman, 1971). The vowels simulated in the present study were based on area functions generated from a normal male speaker (Story, 2008) but with precisely controlled oral-nasal coupling. One explanation for the different findings could be that vocal tract shape did not vary as nasal port area increased. A normal speaker would have the flexibility to move the articulators to change the acoustic impedance of the oral cavity relative to the acoustic impedance of the nasal cavity, thus adjusting the degree of nasality in a given production (Fant, 1960).

Nasality ratings and Nasalance

High correlations were found between auditory-perceptual ratings of nasality and measures of nasalance for data reported in the present study and Bunton and Story (2011). These values are higher than the modest correlations (r=.31; Watterson et al., 1993) or the moderate correlations (r=.63; Brancamp, Lewis, and Watterson, 2010) previously reported. Differences in study design may help explain the findings. One assumption made when correlating perceptual ratings and instrumental measures is that they rely on the same factors. Listener ratings of nasality, however, may include variables such as suprasegmental features such as pitch, rate, and context. Nasalance measures are strictly calculated based on the radiated acoustic energy. In the simulated samples, suprasegmental variables were held constant across the vowel (except f0 which varied 15 Hz). A higher correlation between the two measures might therefore be predicted. For a real speaker producing a connected speech sample these variables change over time, therefore, correlation between measures may not be as high.

Nasal Port Area

If we collectively examine the dataset published by Bunton and Story (2011) and the present data set, it appears that listeners were able to detect nasality in sustained vowel samples generated with a nasal port area greater that 0.01 cm² and the nasality ratings increased until that area reached 0.16 cm². Nasality ratings were relatively flat for samples generated with nasal port areas greater than 0.16cm². These findings are consistent with reports on the clinical category of gross inadequacy and perceived hypernasality defined as a velopharyngeal area greater than 0.2cm². Speakers with velopharyngeal areas ranging from 0.1 to 0.19 cm² are clinically classified as borderline/inadequate and it has been reported that speech is not perceived as hypernasal and normal aerodynamic patterns are seen for speakers in this category (Warren, 1979). Data from the present study suggests that listener’s are able to perceive nasality in vowel samples generated with nasal port openings comparable to this clinical category. It is also interesting that ratings plateaued for samples with nasal port areas greater than 0.16cm², this could represent a ‘saturation’ point for vowels. Ratings of alternative speech samples, such as connected speech, could explain the difference in findings.

Conclusions and Future Directions

Results of the present study in conjunction with previously published data (Bunton and Story, 2011) document nasal port areas that correlate with distinct regions of nasality ratings. Nasal port area needs to exceed 0.1 cm² for listeners to detect nasality in a sustained vowel sample. Samples generated with nasal port areas greater than 0.16cm² do not appear to result in increased nasality ratings. Continued work in this area, including samples produced with varied articulatory configurations, respiratory effort, suprasegmental characteristics, and changes in oral and nasal impedance will allow for a more complete explanation of the relation between auditory-perceptual ratings of nasality, nasal port area and the acoustic characteristics of nasalized vowels.