# Influence of the Nasometric Instrument Structure on Nasalance Score
## 3 Nasalance Characteristics according to the Structure of the Nasometric Instrument
### Experimental Results  
The frequency characteristics of measured voice signals from the separate plate- and mask-structure nasometric instruments were analyzed using the MATLAB program. The analysis results are shown in Figures 8 and 9. Figures 8 and 9 are the frequency characteristics for the emitted /a/,/mama/, and /mimi/ sounds of males and females, respectively. The measured voice signal (Figures 8d-f and 9d-f) through the mask-type nasometric instrument was suppressed in the range above 500 Hz relative to the measured voice signal (Figures 8a-c and 9a-c) through the separation-type nasometric instrument. This result reflects the frequency filter characteristics of the mask-type structure in the nasometric instrument and is similar to the simulated result based on the electrical-mechanical-equivalent circuit. It can be observed that it is less sensitive to the influence of acoustic feedback than the separation plate type. In addition, the correlation coefficient between the collected speech signals through the separation plate-type and mask-type nasometric instruments is expressed by Equation (9). In general, the correlation coefficient can be calculated using the sample mean \(\overline{\mathrm{x}}\) and \(\overline{\mathrm{y}}\), and standard deviation of the two signals \(x\) and \(y\)[4, 12], where \(x\) and \(y\) are the voice signals measured through a microphone in the nasal and oral directions, respectively. Correlation coefficients were analyzed using the two measured vocal signals through the oral and nasal microphones when speaking the same word. Thus, these values are used to show the similarity of voice data in the nasal and oral directions measured by the microphone in the separation and mask structure. \[\textit{Correlation coefficient}=\frac{\sum_{i=1}^{n}(x_{i}-\overline{x})(y_{i}- \overline{y})}{\sqrt{\sum_{i=1}^{n}(x_{i}-\overline{x})^{2}\sum_{i=1}^{n}(y_ {i}-\overline{y})^{2}}} \tag{9}\]  
Figure 7: (**a**) Experimental schematic and (**b**) environment picture using the nasometric instruments of the separation-type structure and mask-type structure. The nasalance of the healthy control group using the two nasometric instrument structures is expressed as Equation (10), in which t\({}_{1}\) and t\({}_{2}\) are the lengths of the measured vocal signals through the oral and nasal microphones, respectively. \[\textit{Nasalance}\left[\%\right]=\frac{\sum_{n=1}^{t1}\textit{Nasal}}{\sum_{n=1 }^{t1}\textit{Nasal}+\sum_{n=1}^{t2}\textit{Oral}}\times 100 \tag{10}\]  
The calculated correlation coefficients and nasalance results by Equations (9) and (10) are shown in Table 3.  
In general, the vowel extension/a/ was less than 20% in nasalance, and the nasal words /mama/, /mimi/ were about 40% and 60% to 80%, respectively. In the case of /a/, there was no consonant component affecting the nasalance, and /mama/ and /mimi/ were somewhat higher as a standard nasal term. The reason why the nasalance of /mama/ and /mimi/ is different is that the /a/ component of the /mama/ word is a posterior low vowel characteristic and the tip of tongue is located at the lower side. Furthermore, the /i/ component of the /mimi/ word has anterior high vowel characteristics, which is expected to be the result of a mechanism that enhances the separation of the nasal and oral cavity [21]. The differences in the calculated index through the separation plate-type and mask-type nasometric instruments are shown in Figure 10.  
Figure 8: Measured spectrum of vocal signal of males using the separation-type structure for (**a**) /a/, (**b**) /mama/, and (**c**) /mimi/, and measured spectrum of vocal signal of males using the mask-type structure for (**d**) /a/, (**e**)/mama/, and (**f**)/mimi/ (the red line is the microphone output in the oral direction and the blue line is the microphone output in the nasal direction). In the case of the male mean nasalance for /a/, /mama/, and /mimi/, it was measured about 5% to 10% lower than that of the mask-type nasometric instrument.