# Application of Soundproofing Materials for Noise Reduction in Dental CAD/CAM Milling Machines
## 4 Determining the Best Soundproofing Materials  
### Performance Evaluation of Soundproofing Materials  
A soundproofing material performance test was conducted to select the most suitable soundproofing material according to the frequency characteristics of milling machine noise. Thicker material and higher frequency sound increase sound absorption efficiency. Moreover, inserting an air layer increases sound absorption efficiency [19; 20; 21]. For evaluating the performance of soundproofing materials, the experiment was focused on three conditions: the scope of soundproof frequency, the thickness, and the applicability of each soundproofing material.  
Since soundproofing materials do not absorb sound at all audible frequencies, pure tones with a target frequency of 560 Hz were generated, and the recorded noise of the milling machine was then played to measure the noise levels and the noise reduction effects of the individual soundproofing materials (see Figure 8). For the purpose of securing the basic data to be used as the selection criteria for the sound-absorbing/-insulating materials, the noise reduction performance was evaluated for seven types of various sound-absorbing/-insulating products (an artboard, rubber sound insulation, a sound-absorbing sponge, aluminum, a sponge, Thinsulator, and a triple soundproofing mat (butyl + aluminum + sound-insulating material)) (see Figure 9). Firstly, the sound-absorbing/-insulating materials were selected according to the characteristics of the products, and the sound-absorbing/-insulating characteristics of various materials were analyzed against the noise from the milling machine. The measurement method is described in Figure 8A. As shown in Figure 8B, a sample (2.5 m \(\times\) 4 m) case with a size of 1400 mm \(\times\) 700 mm \(\times\) 700 mm was manufactured. Sound sources were installed in one corner of the case, and these were measured using a microphone (Dayton Audio EMM-6 electret measurement microphone) and a noise level meter (Lutron Sl-4013) in the opposite spot to record the changes of the soundproofing materials. The change in noise was then recorded for each material. All experiments were independently repeated three times. It is normal to calculate a sound absorption coefficient for each frequency (1/3 octave) to evaluate the sound absorption efficiency and sound absorption performance of soundproofing materials inside a reverberation chamber. This study, however, aimed to observe changes in noise released outside the milling machine, and not to compare the soundproof characteristics of soundproofing materials. Therefore, we monitored the actual changes in the noise from the milling machine in general environments such as a hospital, and not in a reverberation chamber.  
In order to obtain basic data, a performance evaluation of varying materials was performed for their noise-absorbing effect. Seven types of soundproofing materials--an artboard, rubber sound insulation, a sound-absorbing sponge, aluminum, a sponge, a Thinsulator, and a triple soundproofing mat (butyl + aluminum + sound-insulating material)--were employed, as shown in Figure 5. The result of the noise reduction level for each of the seven types of soundproofing material is shown in Table 1. The soundproofing material with the lowest overall noise level was found to be the triple soundproofing mat (butyl + aluminum + sound-insulating material), followed by artboard, rubber sound insulation, and Thinsulator, in that order. However, among them, it was found that the artboard was difficult to attach to the curved inner side of the machine due to its hardness which also represents a difficulty in terms of commercial or sale purposes. Thus, it was excluded from further consideration.