JSLHR
Research Note
Effects of Computer System and Vowel Loading on Measures of Nasalance Shaheen N. Awan,a Kristin Omlor,b and Christopher R. Wattsc
Purpose: The purpose of this study was to determine similarities and differences in nasalance scores observed with different computerized nasalance systems in the context of vowel-loaded sentences. Methodology: Subjects were 46 Caucasian adults with no perceived hyper- or hyponasality. Nasalance scores were obtained using the Nasometer 6200 (Kay Elemetrics Corp.), the Nasometer II 6400 (Kay Elemetrics Corp.), and the NasalView (Tiger DRS, Inc.) for sentences loaded with mixed, high front, high back, low front, or low back vowels. Results: Measures of nasalance obtained with the NasalView were significantly higher than those obtained with the Nasometer 6200, and the measures of nasalance obtained with the Nasometer 6200 were significantly higher than those obtained with the Nasometer II 6400. However, similar effects of vowel loading
R
obust vowel effects have been reported to influence objective measures of nasalance (the ratio of nasal to oral sound-pressure level [SPL]). Kummer (2005) used the Nasometer 6200 (Kay Elemetrics Corp., 1989a, 1989b) to study the effect that vowel content had on nasalance scores. Results indicated that nasalance scores from the high front vowel /i / were significantly higher than the scores from the low back vowel /A /. The vowel /i/ was again reported to have substantially higher nasalance than the vowel /A / by Kummer (2005) using the Nasometer II 6400 (Kay Elemetrics Corp., 2003a, 2003b). Also using the Nasometer 6200, Lewis, Watterson, and Quint (2000) collected speech samples
a
Bloomsburg University of Pennsylvania, Bloomsburg, PA Northern York County School District, Dillsburg, PA c Texas Christian University, Fort Worth, TX Correspondence to Shaheen N. Awan:
[email protected] Editor: Anne Smith Associate Editor: David Zajac Received July 22, 2010 Revision received October 24, 2010 Accepted January 21, 2011 DOI: 10.1044/1092-4388(2011/10-0201) b
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on measures of nasalance were observed, regardless of system. For all systems, the high front vowel sentence tended to result in higher measures of nasalance than did the high back, low front, and low back vowel sentences—the mixed vowel sentence tended to have a higher degree of nasalance than did any of the other sentences. Conclusions: Although nasalance data computed using different systems are not readily comparable, all three systems that were evaluated produced similar effects of vowel loading on nasalance. Increased nasalance for high front versus low back vowels may be due to factors such as increased oral impedance, reduced radiated oral sound pressure, possible increases in airflow via the nasal cavity, and increased transpalatal nasalance. Key Words: nasalance, Nasometer, NasalView, vowels
from 19 children with velopharyngeal dysfunction (VPD) and 19 children without history of a communication disorder. The authors compared nine different stimulus items that were controlled for vowel content (five sentences that contained high front, high back, low front, low back, and a mixture of vowel types; sustained vowels /i /, /u /, /æ /, and /A / ). Results from sentence productions indicated that high front /i /–loaded sentences and high back /u /–loaded sentences produced significantly greater nasalance than did those sentences loaded with low front and low back vowels in VPD subjects. In the non-VPD group, high front–loaded sentences were again observed to have the highest nasalance, and high back–loaded sentences were observed to be significantly higher than low front–loaded sentences but not significantly different from low back–loaded sentences. For sustained vowels from both VPD and non-VPD groups, Lewis et al. (2000) also reported significantly higher nasalance in the vowel /i / than in all other vowels, and the high back vowel /u/ was reported to have significantly Disclosure Statement Shaheen N. Awan is the developer of the NasalView system used in this study. All rights and algorithms for the NasalView were acquired by Tiger DRS, Inc., in September 2011.
Vol. 54 • 1284–1294 • October 2011 • D American Speech-Language-Hearing Association
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higher mean nasalance than the low front or low back vowel productions. These authors explained the effect of vowel production on nasalance by stating that an increase in nasalance could result from a decrease in oral intensity and/or an increase in nasal intensity. In a study investigating whether nasalance scores would be altered by nares occlusion, GildersleeveNeumann and Dalston (2001) elicited speech samples (the Zoo Passage and sustained vowels /i, A, u / using the Nasometer 6200) obtained with and without nares occlusion from a group of 60 healthy adult speakers of varied gender, dialect, and racial background. A significant decrease in nasalance was observed for all elicited samples when the nares were occluded. In the unoccluded condition, a significant difference was observed between productions of /i / and /A / (M = 31% vs. 16%), whereas the two vowels were highly comparable with nares occluded (M = 6% vs. 4%). The results of this study suggested that the majority of acoustic energy that was detected via the nasal microphone during the production of nonnasal utterances by nonnasal speakers was the result of sound transmission emanating from the nose as a result of energy transfer across palatal structures. Gildersleeve-Neumann and Dalston (2001) speculated that the vowel /i/ may have increased nasalance because of (a) high resistance to airflow and sound transfer via the mouth and (b) increased palatal surface area exposed to acoustic vibration and, therefore, increased efficiency of sound transfer via the nasal cavity (Zajac, 2000, as cited in GildersleeveNeumann & Dalston, 2001). The substantial reduction in nasalance for vowel /i / with and without nares occlusions suggested that /i / may have the greatest degree of transpalatal nasalance. These authors also reported lower nasalance values for sustained high back /u / versus high front /i /, as well as for low back /A /, and speculated that the high back tongue placement for /u / in the vicinity of the velum may dampen velar oscillations and reduce transpalatal acoustic transfer. The views of Gildersleeve-Neumann and Dalston (2001) were supported by Bundy and Zajac (2006), who examined the degree of nasalance attributable to transpalatal transfer of acoustic energy during the production of voiced-stop consonant productions. These authors also speculated that oral cavity impedance may affect transpalatal nasalance and indicated that increased oral opening (as occurs in the low back vowel /A / ) may be beneficial in reducing this effect. A study by Kendrick (2004) reported on nasalance scores obtained using the Nasometer 6200-2 (Kay Elemetrics Corp.) for 40 healthy adult subjects producing vowels from all locations on the vowel quadrilateral. Kendrick’s data indicated that front vowels produced higher nasalance values than did back vowels, with /i / resulting in the highest nasalance and /u/ the lowest. A
study by Fowler (2004) showed results similar to those found in the Kendrick study. Nasalance samples were collected from healthy women who were trained singers and from healthy women who were nonsingers. The data were collected via the Oronasal Nasality System (Glottal Enterprises, Inc.), with each subject being asked to sing the vowels /i /, /æ /, /u /, and /A /. The results showed that both groups had significantly higher nasalance scores for the front vowels (/i / and / æ / ) versus the back vowels (/A / and /u / ), with /i / having the highest nasalance and a tendency for /u/ to have the lowest nasalance. Fowler (2004) reported that the significant effect of vowel was due to factors such as the degree of tongue advancement and differences in oral impedance for different vowels (e.g., increased oral impedance for the vowel /i/). A study by Johnson Jennings and Kuehn (2008) investigated the effects of frequency range, vowel, dynamic loudness level, and gender on nasalance in normal amateur and classically trained adult singers. Using data obtained with the Nasometer II 6400, these authors also reported that the vowel /i/ produced significantly higher nasalance than did other vowel productions, and there was a strong tendency for the lowest nasalance to occur on the high back vowel /u/. As reported in the previous literature review, much of the nasalance data that have been used to investigate the effects of vowel on nasalance have been recorded and analyzed using the Nasometer 6200 system. However, since the mid-1990s, other systems have become commercially available for the measurement of nasalance— for example, the NasalView (Tiger DRS, Inc.), the OroNasal System and the Nasalance Visualization System (both by Glottal Enterprises, Inc.), and the updated Nasometer II 6400. Although some of these systems have been used in previous work to examine the effect of vowel production on nasalance (Fowler, 2004; Kummer, 2005; Johnson Jennings & Kuehn, 2008), there has been some indication that other nasalance analysis systems may provide differing results regarding the possible effect of vowel type on nasalance. Using the same five sentences that were used for stimuli by Lewis et al. (2000), Lewis and Watterson (2003) attempted to determine whether the pattern of changes for these sentences was the same for the Nasometer 6200 and the NasalView. Results obtained from 50 children with typically developing speech (14 males and 36 females, approximate ages 5–12 years) indicated significantly different scores for four of the five sentences, with differing scores not always in the same direction. In particular, the authors observed that the high front–loaded sentence had the highest nasalance using the Nasometer 6200, whereas the high back–loaded sentence had the highest nasalance with the NasalView. Lewis and Watterson suggested that the differences between scores from the Nasometer and the NasalView were due to differences
Awan et al.: Effects of Computer System and Vowels on Nasalance
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in the filtering system (the Nasometer uses a 300-Hzwide bandpass filter with a center frequency of 500 Hz, whereas the NasalView does not apply a filter to the input signal). If the various acoustic, aerodynamic, and physiological hypotheses regarding nasalance expectations for various vowels are sound, they should be reflected in the nasalance measurements obtained via various systems. Although different nasalance systems have reportedly resulted in different and nondirectly comparable absolute measurements (Awan, 1998; Bressman, Klaiman, & Fischbach, 2006), it is important that these different systems reflect the expected relative variations in measurements, as may be expected in examples of healthy and disordered speech. Therefore, the purpose of this study was to determine similarities and differences in nasalance scores observed using three different nasalance measurement systems (the Nasometer 6200, the Nasometer II 6400, and the NasalView) in the context of vowel-loaded sentences.
Method Subjects There were 46 subjects in this study: 22 males (M = 21.18 years, SD = 2.51) and 24 females (M = 23.95 years, SD = 2.21). All subjects were Caucasian adults, were native American English speakers, and were native to the northeastern Pennsylvania region. The subjects had no perceived hyper- or hyponasality (as judged by a trained speech-language pathologist), had no history of cleft lip and/or palate, and passed a hearing screening at 25 dB HL at 0.5, 1.0, 2.0, and 4.0 kHz.
Instrumentation The three nasalance systems used were the Nasometer 6200, Nasometer II 6400, and NasalView. All three systems produce measurements of nasalance (%; Fletcher, 1970), and all of these systems obtain measurements by having the subject wear headgear, which incorporates a separator plate and nasal and oral microphones. Nasalance is calculated via the following formula: n 100; nþo
ð1Þ
in which n refers to the nasal sound pressure and o refers to the oral sound pressure. The Nasometer 6200-1 was an original model run on an Apple IIe computer (Apple Corp.). In the Nasometer 6200-1, the nasal and oral microphone signals are separately preamplified and then fed to bandpass filters
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(center frequency = 500 Hz; –3-dB bandwidth of 300 Hz) to capture the lower frequency region of the speech spectrum. The data acquisition routines in the Nasometer 6200-1 software sample the root-mean-square (RMS) level of the nasal and oral microphone signals at a rate of 120 Hz at 8 bits of resolution (Fletcher, Adams, & McCutcheon, 1989). The Nasometer II 6400 (Version 2.70) incorporated several changes to the original Nasometer. Although the headgear (separator plate and microphones) and the bandpass filtering procedure were maintained, the oral and nasal microphone signals were sampled at 11025 Hz per channel at 16 bits of resolution. Nasalance is then calculated from the digitized data using an 8-ms averaging frame to approximate the procedure used in the Nasometer 6200 (personal communication, S. Crump of KayPentax). In addition, changes in the microphone calibration procedures and the capability for signal playback were incorporated into the Nasometer II 6400. The Nasometer II 6400 Version 2.7.0 was run under Windows XP on a Gateway 2000 EV 500. The NasalView Version 1.30 software was introduced by Tiger DRS, Inc., in 1996 and also uses a separator plate and two microphones to record one oral and one nasal signal. At the time of its release, the NasalView was intended as a lower cost, portable alternative to the Nasometer 6200 that would use a computer’s internal sound card for the recording and playback of signals. In addition, the NasalView is capable of using the higher sampling rates available in current analog-todigital converters (up to 44100 Hz per channel) and provides a visual display of both nasal and oral sound waves in addition to the computation of nasalance (%). Hogen Esch and Dejonckere (2004) state that the NasalView’s edit/zoom capabilities for analysis of the recorded speech samples and accompanying nasalance contour may allow for detailed information on the characteristics of velopharyngeal function/dysfunction. The default sampling rate for the NasalView is 22050 Hz at 16 bits of resolution. An important difference between the NasalView and the Nasometer systems is that the incoming signals are not bandpass filtered, potentially allowing for the full spectrum up to approximately half the sampling rate to be captured and analyzed (Awan, 2001). In addition, the NasalView software incorporates a silence criteria setting that allows the user to remove lowamplitude background noise during signal analysis. Other differences between the NasalView and Nasometer systems include calibration procedures, the design of the separator plate, and the characteristics of the microphones. The NasalView was run under Windows XP on a Dell Optiplex Gs. A prototype version of the NasalView was validated in studies by Bressmann and colleagues (1998, 1999, 2000). In addition, the NasalView has been used to assess nasality of speech in healthy
Journal of Speech, Language, and Hearing Research • Vol. 54 • 1284–1294 • October 2011
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subjects and in subjects with speech disorders in studies by Awan (2001); Islami, Sokoli, Bicaj, and Staka (2002); Hogen Esch and Dejonckere (2004); and Widdershoven, Stubenitsky, Breugem, and MinkvanderMolen (2008).
Speech Stimuli The stimuli consisted of five sentences that were loaded with mixed, high front, high back, low front, or low back vowels (Lewis et al., 2000; Lewis & Watterson, 2003). The sentences were as follows: He had two rock lizards. (mixed vowel content) Bill sees the sleepy kid. (high front emphasis) Sue took the old blue shoes. (high back emphasis) Bess has Ella’s red cat. (low front emphasis) Father got all four cards. (low back emphasis)
Procedure The three systems were calibrated according to the manufacturers’ calibration procedures before data collection. Batteries were checked and replaced when necessary for the NasalView amplifier unit. Each subject was seated in a sound-treated room in front of the respective computer screens for the Nasometer 6200, Nasometer II 6400, or the NasalView. The respective headgear were sterilized using Cavicide (Metrex Research Corp.) prior to fitting. Headgear apparatus was placed on each subject according to the respective system’s instructions. On all three systems, subjects were asked to read the five stimuli sentences three times each at a comfortable pitch and loudness. The order of the systems and sentences was randomized for each subject. Following data collection, we isolated the middle trial of the sentence to be analyzed by using either cursors (Nasometer systems) or the selection box (NasalView) to reduce the influence of any extraneous signals. We analyzed recordings in terms of mean nasalance using the software provided with each of the aforementioned systems. All systems were set to their default settings. The silence criteria setting on the NasalView was set to 9.
Results This study was a completely within-subjects, repeatedmeasures design with three levels of system (Nasometer 6200, Nasometer II 6400, and the NasalView) and five levels of sentence type (mixed, high front, high back, low front, and low back). To test for a possible gender effect, an initial independent-samples t-test was applied to the data for males (M = 15.3%, SE = 1.1) and females (M = 16.9%, SE = 1.0), pooling across all levels of system and sentence type. Results indicated no statistical difference in nasalance values between male and female participants, t(44) = 1.08, p = 0.29. Because a gender effect was not found for the sample data used, the remaining analyses were computed with data pooled across participants of both genders. We used parametric statistical analyses to investigate the presence or absence of significant main effects and an interaction effect. We applied an initial two-way (System × Sentence Type) repeated-measures analysis of variance (ANOVA) to the data. Table 1 illustrates the results of this analysis. The assumption of sphericity was not met for the sentence type variable, resulting in a Greenhouse–Geisser correction being used for interpretation of the probability value for this factor. Results indicated significant main effects for system, F(2, 90) = 145.33 ( p < .001, h2 = .76) and sentence type, F(3.18, 143.08) = 41.65 ( p < .001, h2 = .48) and a significant interaction effect, F(8, 360) = 2.58 ( p = .009, h2 = .05). Effect sizes were very large for the system and sentence type factors, suggesting that a large amount of the variability within the data was explained by those two variables (76% and 48% of the variability, respectively). The amount of variability explained by the interaction effect (5%) was far less in comparison. The discrepancy in the effect sizes between the two main effects and the interaction calls into question the practical significance of the latter. Because of this large disparity in effect sizes, the interaction was not investigated further. Post hoc analyses using paired-samples t-tests were applied separately to the data for system and sentence type. The total number of post hoc comparisons was 13 (three for the system factor, 10 for the sentence type
Table 1. Analysis of variance table demonstrating significant main effects for system and sentence type, as well as a significant interaction effect, with very large effect sizes for system and sentence type. Source
SS
df
MS
F
p
Partial h2
System Sentence System × Sentence
10,461.47 1,611.01 90.69
2, 90 3.18, 143.08 8, 360
5,230.73 506.66 11.34
145.33 41.65 2.58
< .001 < .001 .009
.76 .48 .05
Note. SS = sum of squares; MS = mean squares.
Awan et al.: Effects of Computer System and Vowels on Nasalance
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factor). To control for Type I error while maintaining a respectable level of power, the level of significance was lowered to .01 for each of these comparisons (i.e., a modified Bonferroni correction was applied). Table 2 illustrates averaged raw nasalance data for each level of both factors (system and sentence type). When pooled across all sentence types, nasalance measures were highest for the NasalView system (M = 20.4%, SE = 0.66) and lowest for the Nasometer II 6400 (M = 11.2%, SE = 0.89). As can be seen, SE was low for all measurements, and there was no overlap in confidence intervals for any two systems. These results suggested the possibility of an effect of system on nasalance measures. Figure 1 provides box-and-whisker plots for the three levels of system. Statistical results indicated a significant difference ( p < .01) between all combinations of this factor. This indicated that nasalance measures were significantly higher with the NasalView system compared with the two Nasometer systems and were significantly lower with the Nasometer II 6400 compared with the other two systems. Table 2 also provides the averaged raw nasalance scores pooled across analysis system for each level of the second independent variable, sentence type. Post hoc statistical results indicated significant differences ( p < .01) between various combinations of the sentence type factor, except when comparing the low back sentences with the high back sentences (t = –2.32, p = .02) and low front sentences (t = 0.364, p = .72) sentences. These results indicated a significant effect on nasalance scores from the type of vowels embedded within sentences (except for low back sentences vs. high back and low front sentences) when pooled across system type. Nasalance measures were highest for the mixed sentences (M = 18.4%, SE = 0.74). In terms of more specific vowel loading, nasalance measures were highest for the high front sentences (M = 17.2%, SE = 0.89) and were lowest for the high back sentences (M = 14.3%, SE = 0.75), with some overlap in the confidence intervals for most levels of this factor. As with the system data, SE was low for each level, and the data clearly suggest an
Figure 1. Box-and-whisker plots showing the median (central horizontal line), 75th and 25th percentiles (box), and 90th and 10th percentiles (whiskers) for the three levels of the system factor. All comparisons were significantly different.
effect of sentence type on nasalance measures. Figure 2 displays the mean nasalance values plotted as a function of system and sentence. This graph demonstrates the appearance of a large effect for both system and sentence, with no overlap of any condition given the scaling factor of the y-axis.
Discussion The results of this study indicate that measures of nasalance that were computed using different systems are not readily comparable. Measures of nasalance obtained via the NasalView system tended to be higher than those computed using either of the Nasometer systems, and the Nasometer 6200 system produced significantly higher nasalance values than did the Nasometer II 6400 system. However, regardless of the nasalance system used, similar differential effects of vowel loading on
Table 2. Individual vowel-loaded sentence means (Ms; %) and standard errors (SEs) for each nasalance system. System type M (SE ) Sentence type Mixed High front High back Low front Low back Pooled Ms
NasalView
Nasometer 6200
23.04 (0.81) 21.29 (0.72) 18.62 (0.55) 19.56 (0.62) 19.63 (0.61) 20.43 (0.66)
19.15 (1.11) 18.20 (1.07) 15.19 (0.89) 15.07 (1.00) 15.10 (0.98) 16.54 (1.01)
Nasometer II 6400 13.16 (0.93) 12.18 (0.89) 9.20 (0.81) 10.99 (0.94) 10.62 (.89) 11.23 (0.89)
Note. Pooled means across system and sentence type are also provided (see bold).
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Journal of Speech, Language, and Hearing Research • Vol. 54 • 1284–1294 • October 2011
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Pooled M (SE ) 18.45 (0.74) 17.22 (0.89) 14.34 (0.75) 15.21 (0.85) 15.12 (0.83)
Figure 2. Mean nasalance (%) plotted as a function of system (NasalView, Nasometer 6200, and Nasometer II 6400) and sentence (mixed, high front, high back, low front, and low back).
measures of nasalance were observed. For all systems, the high front vowel sentence tended to result in higher measures of nasalance than did the high back, low front, and low back sentences. In addition, an interesting result was that the mixed vowel sentence tended to have a higher degree of nasalance than did any of the other sentences.
Computer System Effects Consistent with normative data published in the Nasometer II 6400 manual (Kay Elemetrics Corp., 2003b) as well as in publications by Kummer (2005) and Awan and Virani (2010), the results of this study show that measures of nasalance obtained with the Nasometer II 6400 are significantly lower than those obtained with the NasalView and, perhaps more important, the Nasometer 6200 (Kay Elemetrics Corp., 1989a) system. These results are in stark contrast to those of Watterson, Lewis, and Brancamp (2005), who compared nasalance scores from the Nasometer 6200-3 and the Nasometer II 6400 obtained from 60 healthy adult speakers and reported that nasalance measures obtained with the Nasometer II 6400 were significantly higher than those obtained with the Nasometer 6200 for the Turtle Passage (comparable to the nonnasal Zoo Passage) and Mouse Passage (comparable to the phonetically balanced Rainbow Passage). Although Watterson et al. concluded that different systems/machines introduce a source of variation in nasalance measurement and that eight nasalance points should be considered
“normal variation” when comparing these two systems, these authors also stated that “it would appear that the old Nasometer and the Nasometer II compare quite well” (2005, p. 579). Even though they reported nasalance values for the Nasometer II 6400 to be consistently lower than those for the Nasometer 6200, Kummer (2005) also stated that any differences between systems would not be considered clinically significant. However, the results of this study indicate that there is actually a clear and substantial difference in mean nasalance across the various sentence stimuli between the Nasometer 6200 and Nasometer II 6400 (pooled mean difference = 5.31%; see Table 2). Considering that system difference (in this case, the Nasometer 6200 vs. the Nasometer II 6400) is a known and consistent source of variation when interpreting nasalance values, our view is that normative data examples obtained with the Nasometer 6200 may not be appropriate for use with the Nasometer II 6400, or vice versa. We recognize that this study has focused on specific nonnasal vowelloaded sentences that differ somewhat from commonly used nonnasal passages (e.g., Zoo Passage) used in the clinical assessment of hypernasal speech; however, differences in nasalance values between the Nasometer 6200 and Nasometer II 6400 systems, as reported here, may be of particular importance when consulting normal versus hypernasal cutoff scores as determined via sensitivity and specificity analyses. For example, Dalston, Neiman, and Gonzalez-Landa (1993) reported that a threshold nasalance score of 28% on the Nasometer 6200 was optimal for the identification of patients with and without clinically significant hypernasality. However, this cutoff may be inappropriate for use with the lower nasalance values provided by the Nasometer II 6400, resulting in poor sensitivity to the presence of milder levels of hypernasality. Therefore, it is only prudent to consult norms that have been developed for the specific system that is being used (Nasometer 6200, Nasometer II 6400, or NasalView) and/or to develop preliminary normative data for clinical and research use when appropriate norms are not available. We hope that future studies will provide a greater database of normative and disordered nasalance examples for the Nasometer II 6400 for consultation by clinicians and researchers. It may be expected that the various hardware and software differences among the three systems would affect characteristics of the recorded signals and, therefore, the results of nasalance calculations. Bressman (2005) and Bressman et al. (2006) reported that the nasalance scores from different systems are not interchangeable and cannot be directly compared, even though the alternative systems such as those used in this study may still provide acceptable nasalance measures for diagnostic use. The cumulative effects of even small hardware and/or software differences among systems can result
Awan et al.: Effects of Computer System and Vowels on Nasalance
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in significant differences in similar measurements. Even though all three systems use a similar computational formula for the measurement of nasalance, there are substantial differences across the systems in terms of hardware components and software. As mentioned earlier in the Method section, the score differences may be attributed to a combination of factors such as differences in calibration procedures, separator plate design, sampling rates, bits of resolution (i.e., detail of the amplitude scale during digitization), overall hardware design (e.g., amplifier and A-to-D converter, filtering vs. no filtering), and differences in computational algorithms. In fact, there are so many differences among these three systems (including the differences between the Nasometer 6200 and 6400 systems) that it would be extraordinary if these systems did, in fact, produce the same values. It has been well established in computer analysis of voice signals that it may not be appropriate to compare acoustic measures such as fundamental frequency variability, jitter, and shimmer obtained from different systems (Awan & Scarpino, 2004; Karnell, Scherer, & Fischer, 1991; Naufel de Fellipe, Grillo, & Grechi, 2006); this also appears to be the case when dealing with alternative systems for the measurement of nasalance.
Vowel Effects The results of this study indicate that high front vowel loading tends to produce higher mean nasalance. When examining the results of all three nasalance systems combined (Nasometer Models 6200 and 6400 and the NasalView), the high front vowel loading produced significantly higher nasalance than did the high back vowel–, low front vowel–, or low back vowel–loaded sentences. Although the actual mean nasalance differences between various vowel-loaded sentences are relatively small (greatest mean difference = 2.88% between high front vowel– and high back vowel–loaded sentences), these results do provide us with important information regarding the possible individual and interactive effects of articulatory physiology, aerodynamics, and acoustics on nasalance. The magnitude of mean nasalance values observed in this study between different vowelloaded sentences is similar to the nasalance differences between same sentences, as reported in Lewis et al. (2000). It is interesting to note that the differential effects of vowel loading on nasalance are much smaller when elicited via sentence production than when examined via isolated sustained vowel production or CV syllables. Kummer (2005) reported a difference between CV syllables containing /i / versus /A / of approximately 10%, and a difference of 13% between prolonged /i / versus /A /. Lewis et al. (2000) also reported a difference of 10% nasalance between high front /i/ and the vowels /u, æ, A /. Smaller magnitudes of nasalance differences 1290
in vowel-loaded sentences versus high front versus low back vowels in isolation or in simple CV syllables may be because in healthy speakers, nasalance values are mitigated by the presence of surrounding oral consonant productions, and the relative duration of vowel segments in continuous speech samples is much shorter (and, therefore, has less of an effect on the overall nasalance calculation) than what is observed in sustained vowels or in CV syllable productions. The vowel effects reported in this study are consistent with a number of previous studies (using sentences and sustained vowels) that have also observed the tendency of high front vowels to elicit the highest nasalance scores (Fowler, 2004; Gildersleeve-Neumann & Dalston, 2001; Kendrick, 2004; Kummer, 2005; Lewis et al., 2000; Lewis & Watterson, 2003). There are several reasons high vowels may be expected to produce higher nasalance. Vowel differences in oral impedance. Oral impedance to airflow may be higher for high front vowels than for back vowels (particularly low back vowels) due to increased oral constriction (Gildersleeve-Neumann & Dalston, 2001; Raphael, Borden, & Harris, 2007). Increased oral impedance will result in weaker radiated sound pressure from the oral cavity as compared with a low vowel such as /A/ (Denes & Pinson, 1993), and Lewis et al. (2000) concluded that the “natural differences in oral and nasal sound intensity would sum in the direction of increased nasalance on high vowels” (p. 588). Oral versus nasal airflow. Hajek (1997) has discussed the possibility that increased oral impedance during the production of a high vowel may result in increased airflow into the nasal cavity. This possibility has been at least partially supported by Young, Zajac, Mayo, and Hooper (2001), who reported increased nasal airflow and increased nasal to oral-plus-nasal airflow ratios during /i / as compared with /A / in women. Increased nasal airflow and resulting acoustic disturbance may be detected via the nasal microphone and result in increased nasalance. Transpalatal nasalance. Gildersleeve-Neumann and Dalston (2001) and Bundy and Zajac (2006) have speculated that oral cavity impedance may affect transpalatal nasalance. Anterior oral cavity impedance may result in increased posterior oral cavity sound transfer to the nasal cavity via the increased exposed palatal surface area reported for the vowel /i /. As previously mentioned, the substantial reduction in nasalance reported by Gildersleeve-Neumann and Dalston (2001) for the vowel /i / with and without nares occlusion suggests that /i / may have the greatest degree of transpalatal nasalance. Bundy and Zajac (2006) have suggested that increased oral opening (as observed in low back vowel production) may reduce the effect of transpalatal nasalance.
Journal of Speech, Language, and Hearing Research • Vol. 54 • 1284–1294 • October 2011
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Acoustic interaction between oral and nasal cavity resonances. The first formant frequency for high front vowels such as /i / or /I / may be close to the nasal cavity resonance (the “nasal murmur”) or the resonant frequency of the soft palate (Bundy & Zajac, 2006) and, therefore, result in a greater degree of excitation within the neighboring nasal cavity for this type of vowel. It may be that a combination of these aforementioned effects results in a higher degree of measured nasalance for sentences loaded with high front vowels. In addition, these aforementioned effects also provide possible explanations for the seemingly contradictory observation that high vowels have increased velar height and increased velopharyngeal closure (Bzoch, 1968; Moll, 1962) but also have a tendency for increased nasalance. Across systems, reduced levels of nasalance were observed for the high back, low back, and low front vowels (no significant difference was observed across systems for these three conditions). Low vowels (such as /A /) are produced with increased oral opening, reduced oral impedance, and greater radiated acoustic power and vocal intensity than are high vowels (Black, 1949; Denes & Pinson, 1993; Fletcher, 1953). Because of the attenuation effect of the separator plate between the nasal and oral microphones, increased oral intensity may result in relatively increased SPL measured via the oral versus the nasal microphone, resulting in reduced nasalance. In addition, because low back vowels such as /A/ tend to have high first formant and low second formant frequencies, the concentration of spectral energy for these vowels may be outside of the resonant frequency range of the soft palate (approximately 350–750 Hz as reported by Bundy & Zajac, 2006), resulting in reduced transpalatal nasalance. Therefore, the inherent spectral characteristics of low back vowels such as /A / may dictate lower nasalance for these production types. The results of this study also showed a consistent trend across systems for high back–loaded sentences to produce the lowest measured nasalance. This result is consistent with the findings of Gildersleeve-Neumann and Dalston (2001), Kendrick (2004), Fowler (2004), and Johnson Jennings and Kuehn (2008). GildersleeveNeumann and Dalston (2001) speculated that the high back tongue placement for vowels such as /u/ may dampen velar oscillations, reduce transpalatal acoustic transfer, and thereby produce a lower nasalance than even low back vowels such as /A/. Kendrick (2004) noted that, for the vowel /u /, the tongue is in an elevated and retracted position that may allow for greater velar elevation and lower nasalance scores. In addition, a relatively stronger oral acoustic signal may be produced for the vowel /u / by an articulatory position in which the lip protrusion and raising of the tongue dorsum has a tendency to resonate the fundamental frequency and lower
harmonics (Raphael et al., 2007). The observation of reduced nasalance in high back vowel–loaded sentences is in contrast to findings reported by Lewis et al. (2000) and Lewis and Watterson (2003). Lewis et al. (2000) observed that high tongue positions (regardless of front or back) tended to produce higher nasalance values than did low tongue positions. A similar result was observed by Lewis and Watterson (2003), who reported a tendency for higher mean nasalance in high back versus either low front–loaded or low back–loaded sentences using the Nasometer 6200. It may be speculated that the differences in findings between the current study and the previous studies by Lewis and colleagues may be due to factors such as the age of the subjects (adults in the current study vs. children in the Lewis et al. studies), possible effects of differing microphone sensitivity both between and even within systems (Zajac et al., 1996), and dialectical variations in the subjects tested. In this study, we evaluated individuals from northeastern Pennsylvania, whereas Lewis et al. (2000) and Lewis and Watterson (2003) studied individuals from the western United States region. In previous studies, researchers reported that dialectical variations may affect nasalance scores (Leeper, Rochet, & MacKay, 1992; Rochet, Rochet, Sovis, & Mielke, 1998; Seaver, Dalston, Leeper, & Adams, 1991). Further studies are needed to document possible dialectical effects on nasalance values from the varied geographical and cultural regions of the United States. It is of interest that, in this study, the mixed vowel sentence tended to have an even higher degree of nasalance than the high front vowel sentence across all three systems evaluated. It may be speculated that the high degree of measured nasalance in the mixed vowel sentence productions may be due to intonation patterns in which relatively more stress (increased loudness and duration) may have been produced on syllables containing high front vowels. In addition, because the mixed sentence contains an alternating sequence of high and low vowels, it is possible that either (a) tongue or velar height or (b) formant frequencies and relationships were affected differently for this sentence versus for the high front sentence, in which all stressed vowels are in the same relative articulatory position. Future studies that further investigate the possible effects of intonation patterns and coarticulation on nasalance will be valuable in addressing these issues. The effects of vowel loading as observed for the three different nasalance systems used in this study are remarkably similar (see Figure 2). This finding indicates consistency in the effects of vowel loading regardless of the hardware/software differences among systems. This finding is in contrast to that of Lewis and Watterson (2003), who reported differential effects of vowel loading, depending on the system used (NasalView
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vs. Nasometer 6200; see Figures 3A and 3B). Figure 3A shows mean nasalance data per sentence obtained using the NasalView in the current study as compared with the data reported by Lewis and Watterson (2003). The data presented in Figure 3A show opposing results for the degree of nasalance per sentence, as observed in this study. One may speculate that methodological differences (e.g., use of the selection window to isolate the utterance being measured, use of different silence criteria, consistent battery check for the NasalView amplifier, possible differences in calibration and equipment setup, differences in microphone sensitivity) as well as the aforementioned dialectical variations may be responsible for the differences in NasalView data. Figure 3B shows similar trends for the effects of vowel loading on nasalance scores obtained via the Nasometer 6200 in the current study and the Lewis and Watterson (2003) study,
Figure 3. Comparison of mean nasalance scores from the current study with those of Lewis and Watterson (2003) for the NasalView (Panel A) and Nasometer 6200 (Panel B) systems.
although it is of interest that the mean nasalance values obtained in the current study for both the Nasometer 6200 and the NasalView are substantially higher than those reported in the Lewis and Watterson (2003) study. Higher nasalance values in the current study may be due to the use of adult subjects in the current study versus children in the Lewis and Watterson (2003) study. Several studies have reported that nasalance tends to increase with age (Awan, 2001; Brunnegård & van Doorn, 2009; Kendrick, 2004; Rochet et al., 1998). Although the nasalance values in the current study are higher than those reported in the Lewis and Watterson (2003) study, the mean nasalance values obtained for the nonnasal productions in this study are consistent with normative means for adult speakers reported for the Nasometer 6200 (Bressman, 2005; Bressman et al., 2006; Dalston, Neiman, & Gonzalez-Landa, 1993; Gildersleeve-Neumann & Dalston, 2001; Seaver et al., 1991) and for the NasalView (Awan, 1996, 2001; Bressman, 2005; Bressman et al., 2006).
Conclusion Even though all three systems examined in this study (Nasometer 6200, Nasometer II 6400, and NasalView) use a similar computational formula for the measurement of nasalance, the results of this study indicate that the various hardware and software differences among the systems can produce significant differences in measures of nasalance. The result is that nasalance data computed via the use of different systems are not readily comparable. Therefore, it is essential that any normative nasalance expectations and/or normal versus disordered nasalance cutoff scores be considered with full knowledge of the specific system that was used to acquire the nasalance data. In addition, intrasubject changes in nasalance can be validly assessed only when test-versus-retest measurements have been acquired through use of the same nasalance system. Although there are substantial differences in mean computed nasalance per system, results indicate that all three systems show a great deal of similarity in nasalance trends for the effect of vowel loading. It is important that clinicians consider the possible effects of vowel loading on nasalance scores when interpreting measures of nasalance from different speech samples, particularly when nasalance is assessed in shorter stimuli such as words or short phrases (Lewis et al., 2000). The vowel context in which nasalance measures are obtained presents an important extraneous variable that must be considered and controlled when evaluating possible nasalance differences between groups or within subjects in conditions such as pre- versus posttreatment comparison.
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Journal of Speech, Language, and Hearing Research • Vol. 54 • 1284–1294 • October 2011
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Acknowledgments We would like to thank Stephen Crump, of KayPentax (formerly Kay Elemetrics), for his help in describing the Nasometer II 6400 system.
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