# Application of Soundproofing Materials for Noise Reduction in Dental CAD/CAM Milling Machines
## 3 Target Noise Source Setting and Prevention Technology Selection
### Setting the Target Noise Source and Determining the Noise Reduction Method  
The selected target noise spot is where the strongest level of noise occurred, and the peak frequency range of generated noise refers to the target frequency range. The target noise was set based on the range of frequencies corresponding to the strongest level of noise in Figures 5 and 6. The method for controlling noise may vary depending on a variety of conditions of the noise, such as the frequency characteristics of the noise sources, regular or irregular noise sources, and repetition [17]. Thus, it is critical to establish what the most effective target noise source is, and apply the most appropriate method that is fitting for the noise source. In this study, the target noise source was established in consideration of the noise control effect depending on the noise controllability and human hearing characteristics. Human ears are most sensitive to frequencies between about 500 Hz and 6 kHz, and are less sensitive to frequencies above and below these (see Figure 7B). [PERSON] et al. [18] reported that an active noise control method can be effectively applied to reducing noise below the 500 Hz frequency band (Figure 7A). Thus, we concluded that the reduction of the target noise source at 560 Hz is more effective than the other target noise source of 60 Hz. Moreover, passive noise control using the difference in soundproofing performance is considered effective for the milling machine with the 560 Hz band as the peak frequency. Following the analysis result, the selected method of noise reduction was attachment of the soundproofing material to the inside of the housing. The most appropriate soundproofing material was selected based on the test results for noise-absorbing performance centering on the target frequency band of 560 Hz for each soundproofing material (Table 1).  
* B. Results of Spindle Motors and pumps measurement to identify and analyze the main peak frequency sources
* Point1: Part of the motor system (spindle device), front  
Figure 6: Spindle motor and pump measurements to identify and analyze peak frequency. **(A)** measurement method; **(B)** results of the spindle motor and pump measurement to identify and analyze the main peak frequency.  
_Appl. Sci._**2020**, _10_, 2768  
\begin{table}
\begin{tabular}{c c c c c} \hline  & & **1. Target Frequency** & **2. Recorded Milling** & **Thickness** \\  & **Type** & **Noise Level** & **Machine Noise Level** & **(mm)** \\  & & **(SPL (dB) at 560 Hz)** & **(SPL (dB), 10 s on Average)** & **(mm)** \\ \hline A & Artuboard & 64.1 & 75.4 & 9 \\ B & Rubber noise insulation & 57.0 & 76.4 & 2 \\ C & Sponge & 76.6 & 78.9 & 5 \\ D & Aluminum & 78.9 & 82.5 & 2 \\ E & Sponge & 72.8 & 79.0 & 5 \\ F & Thinsulator & 68.2 & 76.8 & 10 \\  & Triple soundproofing mat & & & \\ G & (Butyl + Aluminum + & 58.2 & 72.1 & 10 \\  & Sound-insulating material) & & & \\ \hline \end{tabular}
\end{table}
Table 1: Performance evaluation results of various materials.  
Figure 7: Target noise source setting and prevention technology selection. (**A**) operational range of active and passive noise reduction (active NR and passive NR); (**B**) frequency weighting curves. A-weighting is commonly used for a rough estimate of the frequency sensitivity of human hearing.