## 6 Evaluation Experiments
### Experimental Results and Discussions  
Figures 13-16 show spectrograms of the signals observed using the conventional and proposed methods for air turbines with large rotational speed fluctuations, air turbines with smaller rotational speed fluctuations, scalers, and contra-angles, respectively. Figure 3 shows the original sound sources of the dental treatment noise for four types of dental instruments. As shown in Figures 13-16, the harmonics of the proposed method were smaller than those of the conventional method. This is because the proposed method uses triangular waves as the masker source. In higher-frequency bands, strong harmonics contribute to the reduction in discomfort caused by auditory masking, but overly strong harmonics degrade comfort. On the other hand, low harmonics in the proposed method produce a comfortable sound, although the performance in discomfort reduction is expected to degrade. As shown in Figure 15, the proposed method was able to follow the peak frequencies, and the melodies were continuous, with the chord progressions and the melodies in harmony. \begin{table}
\begin{tabular}{l l} \hline \hline \multirow{4}{*}{Score for comfort} & 5: Very comfortable sound \\  & 4: Quite comfortable sound \\  & 3: Neutral \\  & 2: Slightly comfortable sound \\  & 1: Not so comfortable sound \\ \hline \multirow{4}{*}{Score for discomfort reduction} & 5: Greatly reduces discomfort \\  & 4: Slightly reduces discomfort \\  & 3: Neutral \\ \cline{1-1}  & 2: Does not reduce discomfort \\ \cline{1-1}  & 1: Is not able to reduce discomfort at all \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Evaluation indices for comfort and discomfort reduction. Figure 17 shows the match rate of the peak frequency and nearest neighbor harmonics. The vertical axis represents the peak frequency match rate, and the horizontal axis shows the methods. As shown in the peak frequency match rates in Figure 17, the proposed method was superior to the conventional method for the air turbine with large rotational speed fluctuations, the air turbine with small rotational speed fluctuations, and the scaler and performed similarly to the conventional method for the contra-angle. This may be attributed to the fact that the two peak frequencies were closely located, as shown in Figure 3d. Figure 16: Spectrograms of signals observed from a contra-angle with the conventional and proposed methods. Figure 14: Spectrograms of signals observed from an air turbine (rotational speed fluctuation: small) with the conventional and proposed methods. Figure 13: Spectrograms of signals observed from an air turbine (rotational speed fluctuation: large) with the conventional and proposed methods. Figure 15: Spectrograms of signals observed from a scaler with the conventional and proposed methods. Figure 18 shows the scores for comfort and Figure 19 shows the scores for discomfort reduction. These figures represent box-and-whisker diagrams. The vertical axis represents the mean opinion score (the average score), and the horizontal axis shows the methods. The values indicate the mean opinion score. As shown in the comfort scores in Figure 18, the proposed method outperformed the conventional method for an air turbine with large rotational speed fluctuations and an air turbine with small rotational speed fluctuations and performed similarly to the conventional method for a scaler. On the other hand, the proposed method was inferior to the conventional method for a contra-angle. This may be attributed to the fact that the peak frequency fluctuations were minimal, a condition that would not have caused performance degradation even with the conventional method. Figure 17: Match rates of peak frequency and nearest neighbor harmonics. Figure 18: Scores of comfort. The proposed method outperformed the method without the melody for an air turbine with small rotational speed fluctuations, a scaler, and a contra-angle and performed similarly to the method without the melody for a air turbine with large rotational speed fluctuations.