# Noise Reduction Using Active Vibration Control Methods in CAD/CAM Dental Milling Machines
## 5 Discussion  
For these experiments, noises produced from the milling machine varied according to the conditions of use: (1) types of materials used in processing, and (2) operation conditions (wet or dry). In Table 3, the use of a dust collector for CAD/CAM milling produced the highest levels of noise--about 60 Hz. Therefore, in this paper, a focus was placed on the noise analysis and reduction of the dust collectors attached to milling machines. The experiment indicated the results of the noise coming from the milling machine were being influenced by the vibrations of the dust collector. As mentioned earlier, it seems that the noise is generated as vibration from the dust collector, which is then transmitted to the external case by the compressor inside the dust collector. It is anticipated that this can be applied not only to CAD/CAM milling machines for dental clinics but also to any machine using a dust collector and a compressor. The key point in designing a successful controller is to first find the appropriate place for an actuator to be set. Second, the type of available and accessible sensors and actuators should be specified. In this paper, we have chosen an electro-mechanic transducer called an exciter (also called a tactile transducer) as an actuator. The exciter is normally a moving coil device, which is carefully positioned and designed to optimally excite the natural resonant modal structure of the panel [16]. Figure 11: Result of noise reduction (frequency analysis and noise sound spectrum (linear scale)). \begin{table}
\begin{tabular}{c c c} \hline \hline Sound pressure level (dB(A)) & \
\begin{tabular}{c} Before \\ 82.5 \\ \end{tabular} &
\begin{tabular}{c} After \\ 71.4 \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Result values before and after. Tactile transducers create and transmit low frequency vibrations to create tactile sound. Tactile sound is present in the vibratory signature of musical instruments, automobiles, aircrafts, and refrigerator compressors [17; 18]. In addition, when a sound is generated by a tactile transducer, it performs a linear motion and can create strong vibrations (see Figure 12). As an actuator of a linear motor type speaker, tactile transducers are able to control vibrations by sound with ease and accuracy. In addition, linear actuators, in contrast to electric motors, are able to deliver energy not only in the form of vibrations, but also in the form of actual motion. The use of the tactile transducer (vibration speaker or exciter) makes it possible to generate vibration without designing a complex structure for vibration reduction. Besides, this can be applied to various devices and is very efficient to control and in reducing the cost. Noise in the dental practice setting causes an unpleasant experience [19]. As shown in the study by [PERSON] et al. [20], the highest dental practice noise level at the distance of 100 cm is 77 dB, and 82.5 dB for milling machines. This suggests that the emergence of milling machines may cause a louder noise, which calls for attention. It was revealed that in this study, the noise reduction plan using the vibration reduction method can be effectively applied to the noise characteristics of the milling machine. However, the internal and external noises, including events while systems are on or off, were not considered in the present experiment. It is, therefore, thought that additional measures for reducing the noise caused by unpredictable and uncertain events would be needed. In addition, the quality and characteristics of the remaining sound after control were not evaluated. Therefore, the proposed method in this paper does not fully specify a solution to this issue. In many practical active noise control applications, the undesired primary noise consists of multiple harmonic-related tones, which can be effectively reduced by internal model control feedback ANC systems [21].