# Application of Soundproofing Materials for Noise Reduction in Dental CAD/CAM Milling Machines
## 5 Application and Results of the Noise Reduction Method  
A previous study reported that the sound absorption rate of soundproofing materials differs according to the state of attachment [19; 20; 21]. It also depends on the conditions of the backside of the material (the presence/absence of a back wall of the material, the characteristics of the wall, the presence of an air layer between the material and the wall, the thickness of the air layer, etc.) Although the soundproofing material is thin, sound absorption is most effective when it is installed with some intervening space with respect to the wall. In order to soundproof low-pitched sounds, the soundproofing materials are installed at some distance from the wall, which usually reduces room space. However, as there is no free space in existing milling machines, the sound-absorbing material and -insulating material should be attached to the path through which noise is transmitted, with the minimum thickness that can be applied to milling machines. The combination of Thinsulator and a triple-soundproofing mat (butyl + aluminum + noise insulating material) was selected as most appropriate, as described in Section 4.2. The combination of Thinsulator and the triple-soundproofing mat (butyl + aluminum + noise insulating material) was attached to the milling machine housing for final evaluation, as shown in Figure 12. In the final evaluation, the temperature, the material thickness, and the attachable method were considered and applied in the test.  
In the case of Thinsulator, the volume decreased by up to about 3 mm when it was compressed to the maximum. After applying 10 mm of the triple-soundproofing mat, the final thickness increased by approximately 1.3-1.5 cm (see Figure 10A,B). Figure 2 implies that it is impossible to apply this to all sides of the milling machine housing. For this reason, the areas for which attachment to the housing was not possible because of, e.g., a monitor, were excluded, as indicated in Figure 12C. Moreover, wide use of soundproofing materials may cause problems of heat generation due to the characteristics of milling machines. Therefore, it is necessary to consider the attachment space along with the heat and the final weight. After applying this, the weight increased by approximately 4 kg, and when it was measured under the same conditions as Figure 10, the noise decreased by 7.9 dB, from 78.5 to 70.6 dB. Regarding the heat problem, the temperature was independently measured and was observed to be maintained without any significant change. The final results after application are shown in Table 3.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Without Soundproofing** & **Triple-Soundproofing Mat** & \begin{tabular}{c} **Thinsulator +** \\ **Triple-Soundproofing Mat** \\ \end{tabular} \\ \hline Average noise level (dB) & 78.7 & 73.8 & 70.4 \\ \
\begin{tabular}{c} Milling machine noise \\ level (SPL) \\ (dB, 10 s on average) \\ \end{tabular} & – & 5 & 8.3 \\
\begin{tabular}{c} Target frequency noise \\ level at 560 Hz (SPL), \\ (dB, 10 s on average) \\ \end{tabular} & – & 8 & 9 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summarized results of the noise reduction level.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Without Soundproofing Material** & **After Applying Soundproofing Material** \\ \hline Average noise level (dB) & 78.5 & 70.6 \\ Temperature (\({}^{\circ}\)C) & \(45\pm 2\) & \(45\pm 2\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Final selection and application of sound-absorbing and -insulating material.  
Figure 12: Optimal application of soundproofing materials to a milling machine. (**A**) the combination of Thinsulator and the triple-soundproofing; (**B**) thickness of each material; (**C**) the scheme of application.