# Application of Soundproofing Materials for Noise Reduction in Dental CAD/CAM Milling Machines
## 1 Introduction  
The noise from machines such as handpieces in dental environments has become a cause of hearing loss in dentists and has evoked fear in patients [1, 2]. Many different methods have been tried and tested to improve the satisfaction of patients and staff and generate a more pleasant environment at the hospital or clinic, and the reduction of unnecessary noise, among others, has been recognized and studied as an essential element [3, 4, 5]. Recently, various advanced medical machines have been introduced and concurrently recognized as new noise sources in dental environments. In particular, the emergence of three-dimensional computer-aided design/computer-aided manufacturing (3D CAD/CAM) has enabled the conversion of an existing manual process to a new digital process with the advantages of reduced cost and saved time, as a dental prosthesis can now be designed and manufactured in a continuous workflow [6]. However, despite having an effective manufacturing method, the milling machine is difficult to place within the dental environment, and noise control in the work environmentmust still be implemented to suppress the excessive and unpleasant noise due to cutting that is still experienced even if it is placed in an alternative location, such as a separate room [7]. Thus, our study aims to analyze the noise characteristics of the 3D CAD/CAM milling machine, which causes a new type of noise in the dental environment, and to propose an effective noise reduction plan. Noise reduction methods include both active and passive noise control. Active noise control is a technique that actively reduces noise by analyzing the wave form of the sound in real time and generating a control sound. Though it is effective for low frequencies, it has a high cost and is difficult to use in three-dimensional spaces [8]. On the other hand, passive noise control is a technique of passively reducing noise through a direct application to the noise source. It has been widely used due to its cost-effectiveness [9]. Therefore, in this paper, we present an effective noise control method considering the accurate identification of the characteristics of the noise source as well as the environment requiring noise control. The fundamental solution to addressing the noise of machines in a medical environment is to remove the cause of noise during the manufacturing process while considering the usage environment. However, in the case of already commercialized products, precise analysis of the characteristics of the noise generated from a machine is needed. Consequently, it is critical to identify the noise transmission paths and the causes of such noise in a milling machine [10]. Our previous study suggested an active noise control method by identifying the noise characteristics of the milling machine but also reported the disadvantage of having to generate the control sound depending on where the machine is installed [11]. For the application of the passive noise reduction method, the basic noise reduction effects of the soundproofing and sound-insulating material that are applicable to the machine were analyzed. Due to the characteristics of the milling machine noise, in various work environments, the passive method of reducing noise by attaching noise-absorbing or sound-insulating materials to the pathways delivering noise from the noise source of the machine to the motor, housing, or bracket may be more effective than the active noise control method. However, it is critical to keep the use of soundproofing materials to a minimum by only using them in the optimal places, otherwise problems such as overheating may result. There have been an increasing number of studies on the noise-reducing effect of soundproofing and sound-insulating materials on medical machines and in the installation environment for the improvement of medical services and medical environments. However, there have been hardly any studies on dental environments [12; 13; 14; 15].