Noise Reduction Using Active Vibration Control Methods in CAD/CAM Dental Milling Machines

applied sciences

Article Noise Reduction Using Active Vibration Control Methods in CAD/CAM Dental Milling Machines

Eun-Sung Song 1 , Young-Jun Lim 2,* , Bongju Kim 1 and Jeﬀery Sungjae Mun 3

1 Clinical Translational Research Center for Dental Science, Seoul National University Dental Hospital, Seoul 03080, Korea; [email protected] (E.-S.S.); [email protected] (B.K.) 2 Department of Prosthodontics and Dental Research Institute, School of Dentistry, Seoul National University, Seoul 03080, Korea 3 Department of Biology, Swarthmore College, Swarthmore, PA 19081, USA; [email protected] * Correspondence: [email protected]; Tel.: +82-02-2072-2040

Received: 28 February 2019; Accepted: 8 April 2019; Published: 12 April 2019

Abstract: Used in close proximity to dental practitioners, dental tools and devices, such as hand pieces, have been a possible risk factor to hearing loss due to the noises they produce. Recently, additional technologies such as CAD/CAM (Computer Aided Design/Computer Aided Manufacturing) milling machines have been used in the dental environment and have emerged as a new contributing noise source. This has created an issue in fostering a pleasant hospital environment. Currently, because of issues with installing and manufacturing noise-reducing products, the technology is impractical and insuﬃcient relative to its costly nature. In this experiment, in order to create a safe working environment, we hoped to analyze the noise produced and determine a practical method to attenuate the noises coming from CAD/CAM dental milling machines. In this research, the cause for a noise and the noise characteristics were analyzed by observing and measuring the sound from a milling machine and the possibility of reducing noise in an experimental setting was examined using a noise recorded from a real milling machine. Since a milling machine generates noise mainly due to vibration of the dust collector, the possibility of reducing noise was examined by controlling vibration. This study was conducted to understand the cause for noise from the milling machine and verify the possibility of improving noise by a tactile transducer.

Keywords: noise reduction; active vibration control; tactile transducer; anti-vibration; dental milling machine

1. Introduction Workplace noise exposure directly inﬂuences the health of workers. Dental professionals are exposed to high levels of noise while working in dental clinics or laboratories. Both the intensity and long duration of noise from dental work environments are known to contribute to hearing loss [1]. Recent studies have assessed noise exposure and hearing problems in dental practices on students from a dental college located in the United Arab Emirates (UAE); about 80% of students experienced some noise, 54% reported that they have hearing problems, and 10% reported hearing loss. The study recommended the use of sound absorbing materials in the construction of dental oﬃces and laboratories in order to reduce the noise levels [2]. Dental professionals who regularly used high-speed hand pieces had worse hearing than those who did not use these hand pieces [3]. In addition to the existing noises, studies by Fabry, Gijbels et al. and Messano and Petti indicate that the introduction of new technologies such as three-dimensional (3D) CAD/CAM milling machines have contributed to the existing noise source of dental clinics and laboratories, causing greater than expected hearing loss in dental professionals than before [4,5].

Appl. Sci. 2019, 9, 1516; doi:10.3390/app9081516 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 1516 2 of 12

In general, there are two ways to reduce environmental noise: passive and active. The passive method uses techniques such as enclosures, barriers, and silencers [6]. While these passive noise control techniques can help facilitate noise control of mid and high frequencies, they are inefficient and costly at lower frequencies. The active method uses an active noise control (ANC) system to cancel the unwanted noise based on the principle of superposition [7]. Specifically, identical amplitude and antiphase anti-noise is created and actively integrated with the first noise, thereby triggering the elimination of both noises [8]. Several studies were published on the application of ANC methods (such as active noise canceling for headphones using a hybrid structure, active noise control in headsets by using a low-cost microcontroller, a low-power broad-bandwidth noise cancellation for in-ear headphones, adaptive feedback active noise control headset and sound quality of the machine) [9–15]. Although various problems are raised and relevant studies are constantly increasing to address noise problems, studies on noise in the dental treatment room including the medical industry are inadequate. In addition, efficient production methods considering costs and methods are needed to apply this noise elimination method into machines aiming for commercialization [12,14]. We recently investigated a more effective approach to noise reduction in the CAD/CAM milling machine using tactile transducers to produce tactile sound. This study was designed to determine the main noise source generated from CAD/CAM milling machines installed in dental clinics or the laboratory, and to create a pleasant clinic environment by applying a noise reduction method.

2. Methodology The ﬁrst step to reducing the noises produced by CAD/CAM milling machines was through an investigation of the causes and sources of noise level through experimentation in a closed environment. After determining the cause and location of the noise source, we identiﬁed the Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 13 main target area for noise reduction. Using the information on the targeted area, a noise control method was proposed. A block diagram description of the method is shown in Figure1.

Figure 1. Noise improvement process.

Dental CAD/CAM milling machines can be categorized as drying or wetting type depending on the use of water during the milling process. A more suitable method is determined by considering the type of material and milling process used. The wet type does not require a dust collector because it uses water. On the other hand, the drying type requires a dust collector because it produces dust, but the dust collector causes noise. Therefore, the process conditions were classified as follows and the investigation methods of milling machine noise were selected accordingly (Table 1).

Table 1. Investigation of milling machine noise.

Process Conditions Use Conditions Investigation Methods Dry milling process Use of dust collector Vibration and noise level Wet milling process No use of dust collector Noise level

Dental CAD/CAM milling machines can be categorized as drying or wetting type depending on the use of water during the milling process. A more suitable method is determined by considering the type of material and milling process used. The wet type does not require a dust collector because it uses water. On the other hand, the drying type requires a dust collector because it produces dust, but the dust collector causes noise. Therefore, the process conditions were classiﬁed as follows and the investigation methods of milling machine noise were selected accordingly (Table1).

Table 1. Investigation of milling machine noise.

Process Conditions Use Conditions Investigation Methods Dry milling process Use of dust collector Vibration and noise level Wet milling process No use of dust collector Noise level

2.1. Equipment The CAMeleon CS®CAD/CAM milling machine (NeoBiotech, Seoul, Korea) was employed to obtain all test results presented in this study (Figure2). The noise level was measured while the machine was running with a diamond bur cutting a zirconia block. The noise was analyzed by sound pressure level, frequency spectrum and vibration. The sound level intensity was measured with a Digital Sound Level Meter (Lutron Sl-4013®, Lutron, Taipei, Taiwan) by using the dB scale. The frequency spectrum of noise was investigated by a half-inch microphone (ECM8000®, Behringer, Willich, Germany). The vibration frequency of noise was analyzed by a Vibration Tester (AV-160B ®, Vetus, Guangdong, China). The equipment used for noise level Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 13 evaluation are presented in Table2. Table 2. Equipment list used for noise level evaluation. Table 2. Equipment list used for noise level evaluation. Equipment Microphone Sound level Meter Vibration Tester EquipmentBehringer Microphone SoundLutron Level Meter VibrationVetus Tester Company/Model ECM8000Behringer (USA) Sl-4013(Taiwan)Lutron A-160B(China)Vetus Company/Model FrequencyECM8000 (USA) Sl-4013 (Taiwan) A-160B (China) Measurement range: 30–130 Frequency range: 10 Hz–1 Specifications Frequencyrange: range: Measurement range: Speciﬁcations dB Frequency range:kHz. 10 Hz–1 kHz. 15–20,00015–20,000 Hz Hz 30–130 dB

Figure 2. Equipment used to measure and record; noise dust collector and milling machines Figure 2. Equipment used to measure and record; noise dust collector and milling machines (CAMeleon CS® ). (CAMeleon CS®). 2.2. Noise Survey Method To determine the cause and location of the noise, the sound level intensity was measured with a Digital Sound Level Meter. Noise investigation can be conducted in various aspects for hearing loss, talking disorders, noisiness, and improving noise. This study was carried out for investigating noise to identify the noise source in order to improve noise. Measurement location and measuring methods were therefore selected as comparatively easier ways to compare test results, not to measure the absolute sound level serving as the criterion of hearing loss or noisiness, but to identify locations of noise source and causes. Three measuring points (front, left, and right) were selected and measured in an appropriate measuring perspective to identify the sound causes and locations of milling machines. The noise meter repeatedly measured three times per 10 seconds from 1.2 m above the ground. The place of highest noise level was chosen as a target noise source. The measurement locations were held constant during the experiment as shown in Figure 3. Appl. Sci. 2019, 9, 1516 4 of 12

2.2. Noise Survey Method To determine the cause and location of the noise, the sound level intensity was measured with a Digital Sound Level Meter. Noise investigation can be conducted in various aspects for hearing loss, talking disorders, noisiness, and improving noise. This study was carried out for investigating noise to identify the noise source in order to improve noise. Measurement location and measuring methods were therefore selected as comparatively easier ways to compare test results, not to measure the absolute sound level serving as the criterion of hearing loss or noisiness, but to identify locations of noise source and causes. Three measuring points (front, left, and right) were selected and measured in an appropriate measuring perspective to identify the sound causes and locations of milling machines. The noise meter repeatedly measured three times per 10 seconds from 1.2 m above the ground. The place of highest noise level was chosen as a target noise source. The measurement locations were held constant during the experiment as shown in Figure3. Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 13

FigureFigure 3. 3.ExperimentalExperimental arrangement arrangement for for measurement measurement location. location.

TheThe determination determination of of the the mechanical mechanical vibration vibration and and acoustic acoustic noise noise produced produced by by the the dust dust collector collector systemsystem of of the the milling milling machine machine consists consists of of two two steps. steps. Initially, Initially, tw twoo measurements measurements were were performed performed for for fivefive minutes minutes while while the the dust dust collector collector was turned on:on: (1)(1) measurementmeasurement of of the the acoustic acoustic noise noise level level in in the thedust dust collector; collector; (2) (2) measurement measurement of of the the vibration vibration around around the the center center of theof the dust dust collector. collector. Then,Then, in in order order to to discern discern thethe characteristicscharacteristics of of the the noise noise and and vibration vibration of theof the dust dust collector, collector, the noisethe noisesignals signals were were recorded recorded using using both a both microphone a microphone and vibration and vibration sensor simultaneously sensor simultaneously during the during period theof period time at of which time theat which dust collectorthe dust wascollector running. was running. In other words,In other recorded words, recorded frequency freq anduency vibration and vibrationfrequency frequency were compared were compared by measuring by measuring vibrations andvibrations recording and noise recording as a way noise to seeas a whether way to majorsee whethernoise sources major arenoise created sources by vibration.are created The by recording vibration. microphone The recording and microphone sound level meterand sound were locatedlevel meterat a distance were located of 30 cm at horizontally,a distance of in 30 the cm positions horizontally, of A (Figure in the4 positions). Noise was of A analyzed (Figure by4). sampling Noise was the analyzedsignal of by the sampling microphones the signal with a of frequency the microphones up to 44.1 with kHz. a A frequency spectral analysis up to 44.1 was kHz. performed A spectral using analysisthe Fast was Fourier performed Transform using (FFT) the algorithm Fast Fourier to identify Transform dominant (FFT) frequencies. algorithm to The identify vibration, dominant as well frequencies.as the noise The measurement, vibration, as was well realized as the noise by real-time measurement, recording was of realized the signal by real using-time a vibration recording tester of thefor signal five minutes using a vibration during operation tester for offive the minutes dust collector. during operation The location of the of dust the vibrationcollector. measurementThe location ofpositions the vibration was determinedmeasurement in thepositions same mannerwas determined as those ofin thethe noisesame measurementmanner as those points. of the Vibration noise measurementsensors were points. attached Vibration to the flat sensors surface were on theattached steel cover to the near flat thesurface precipitator on the steel hoses cover at point near A the (see precipitatorFigure4) where hoses the at strongestpoint A (see vibration Figure is4) triggered.where the strongest vibration is triggered. Appl. Sci. 2019, 9, 1516 5 of 12 Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 13

Figure 4. Dust collector noise level recording points. Figure 4. Dust collector noise level recording points. 3. Experimental Result 3. Experimental Result 3.1. Analysis of Noise Characteristics of Milling Machines 3.1. AnalysisNoise from of Noise milling Characteristics machines of wasMilling recorded Machines as 80.2 dB(A) on the front, 82.4 dB(A) on the left, Noiseand 78.8 from dB(A) millingon machines the right was when recorded precipitators as 80.2 dB(A) are used on the in front, the case 82.4 ofdB(A) normal on the operation. left, and When78.8 dB(A) precipitators on the are right not when used, precipitators sound pressure are on used the frontin the is case 71.2 dB(A), of normal 72.4 operation.dB(A) on the When left, precipitatorsand 67.8 dB(A) are on not the used, right. sound Table 3pressure shows sound on the pressures front is 71.2 of milling dB(A), machines72.4 dB(A) and on Figure the left,4 presents and 67.8 dB(A)results on as the causes right. of generatedTable 3 shows noise sound and locations pressures of of major milling noise machines sources and after figure investigating 4 presents noise. results as causes of generated noise and locations of major noise sources after investigating noise. Table 3. Noise level of the milling machine using and not using a dust collector. SPL = sound Tablepressure 3. Noise level. level of the milling machine using and not using a dust collector. SPL = sound pressure level. Noise SPL (dB(A)) Front Left Right NoiseNot using SPL dust(dB(A)) Front Left Right 71.2 72.4 67.8 Not usingcollector dust collector 71.2 72.4 67.8 UsingUsing dust dust collector collector 80.280.2 82.582.5 78.878.8

As shown in Table 3, the milling machine generates a louder noise when using a dust collector As shown in Table3, the milling machine generates a louder noise when using a dust collector than than when it was not used. Therefore, the use of a dust collector indicated the target noise of when it was not used. Therefore, the use of a dust collector indicated the target noise of CAD/CAM CAD/CAM machines. In this study, a focus was placed on the noise reduction of the dust collectors machines. In this study, a focus was placed on the noise reduction of the dust collectors attached to attached to milling machines. milling machines.

3.2. Analysis Analysis of of Noise Noise Characteristics of Dust Collectors Table4 4 indicates indicates thatthat thethe vibrationvibration frequencyfrequency atat pointpoint AA waswas 59.459.4 HzHz whenwhen thethe noisenoise fromfrom thethe dust collector was recorded and measured. It It approached approached 60 60 Hz Hz around around the the hose hose of of the the dust dust collector, collector, which is where the center and the most powerful energy sound sound pressure pressure level level (dB) (dB) was was observed. observed. Seeing that that 60 60 Hz Hz and and the thenoise noise of harmonics of harmonics accord accord with the with number the of number revolutions of revolutions of the compressor of the compressor inside the dust collector in the bottom of the milling machine, it is supposed that the Appl. Sci. 2019, 9, 1516 6 of 12

Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 13 inside the dust collector in the bottom of the milling machine, it is supposed that the noise was noise was generated because the vibration of the compressor was transmitted to the dust collector generated because the vibration of the compressor was transmitted to the dust collector case. case. Table 4. The sound pressure level of the dust collector at the recording point A. Table 4. The sound pressure level of the dust collector at the recording point A. Dust Collector A Dust Collector A Sound pressure level (dB(A)) 82.5 Sound pressure level (dB(A)) 82.5 Vibration frequency (Hz) 59.4 Vibration frequency (Hz) 59.4

Figure5 5 shows shows the the power power spectrum spectrum analyzed analyzed using using data data recorded recorded around around point point A, where A, where the noise the noiselevel oflevel the of dust the collector dust collector is the highest. is the highest. In this spectrum,In this spectrum, frequency frequency changes changes according according to the length to the of lengththe noise of period, the noise and period, it can beand seen it can that be the seen noise that is generatedthe noise is at generated a constant at level a constant and concentrated level and concentratedin the frequency in the band frequency of 60 Hz. band As aof discrete 60 Hz. As tone, a discrete the noise tone, of the the dust noise collector of the dust has acollector fundamental has a fundamentalfrequency of nearlyfrequency 60 Hz of andnearly a regular60 Hz and sound a regular pressure sound variation pressure to bring variation about to the bring sense about of pitch the senseconsisting of pitch of multiple consisting harmonics. of multiple When harmonics. it comes to When frequency, it comes this noiseto frequency, has a line this spectrum noise has (Figure a line5). spectrumIt was (Figure found 5 that). the prominent band of the frequency spectrum obtained from the microphone and theIt was main found peak that of the the vibration prominent frequency band of obtained the frequency from the spectrum vibration obtained test coincided from the at microphone about 60 Hz. andBased the on main these peak results, of the the vibration vibration frequency generated fromobtained the dust from collector the vibration was confirmed test coincided as the at main about noise 60 Hz.source. Based Vibration on these and results, noise the generated vibration from generated precipitators from the are dust considered collector as was noise confirmed caused by as vibrationthe main noisetransferred source. from Vibration compressors. and noise Generated generated causes, from locations, precipitators and resultsare considered can be analyzed as noise as caused shown by in vibrationFigure6 depending transferred on from test results.compressors. Generated causes, locations, and results can be analyzed as shownAs in a consequenceFigure 6 depending of analysis on test of noise results. characteristics, the noise from the dust collector is caused by vibrationAs a transmittedconsequence from of analysis around of the noise hose characteristics, located on the side.the noise The structurefrom the ofdust the collector dust collector is caused (see byFigure vibration7) makes transmitted it di fficult from to add aroun a noised the reductionhose located device on the (such side. as The rubber structure/damper) of the inside. dust collector (see FigureTherefore, 7) makes we propose it difficult a noise to add control a noise method reduction using anti-vibrationdevice (such as as rubber/damper) the most efficient inside. method to reduceTherefore, the undesired we propose noise. a noise control method using anti-vibration as the most efficient method to reduce the undesired noise.

Figure 5. FrequencyFrequency analysis and noise sound spectrum (linear scale). Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 13

Appl. Sci. 2019, 9, 1516 7 of 12 Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 13

Figure 6. Identifying the main noise source of milling machines. Figure 6. Identifying the main noise source of milling machines. Figure 6. Identifying the main noise source of milling machines.

Figure 7. Internal structure of the dust collector. Figure 7. Internal structure of the dust collector. Figure 7. Internal structure of the dust collector. 4. Noise Reduction Solution 4. Noise Reduction Solution 4. Noise Reduction Solution Experimental Verification by Vibration Reduction Method Experimental Verification by Vibration Reduction Method ExperimentalA final set ofVerification experiments by Vibration was conducted Reduction to Method measure the sound cancellation achieved through A final set of experiments was conducted to measure the sound cancellation achieved through experimentationexperimentationA final set in ofa inexperiments laboratory a laboratory setting. wassetting. conducted In In order order to to to verify measure verify the thevalidity the validity sound of noise cancellation of noisereduction reduction throughachieved through the through theexperimentation vibrationvibration control control in method, method,a laboratory a recording setting. andIn and order noise noise to measurement measurementverify the validity environment environment of noise were reduction were constructed constructed through the throughvibrationthrough a computer a control computer program. method, program. The a The recording program program and was was created noisecreated measurement so so that that the the noise noise environment reductio reductionn results results were could constructed could be be analyzedthroughanalyzed after a computer after creating creating anprogram. an experimental experimental The program environment environment was created for for vibration vibration so that control controlthe noise using using reductio recorded recordedn noise results noise files. could files. be SinceanalyzedSince the strongest the after strongest creating vibration vibration an occurredexperimental occurred around around environment the the hosehose area for of ofvibration the the dust dust collector, control collector, theusing the tactile tactilerecorded transducer transducer noise files. was attached to the area in which the strongest vibration (A) was generated to record the greatest wasSince attached the strongest to the area vibration in which occurred the strongest around the vibration hose area (A) of was the generateddust collector, to record the tactile the greatesttransducer sound pressure. A tactile transducer that was used in manufacturing the equipment and can generate soundwas pressure.attached Ato tactilethe area transducer in which that the was strongest used in vibration manufacturing (A) was the generated equipment to andrecord can the generate greatest vibrationvibration is also is called also called a ‘vibration a ‘vibration speaker’. speaker’. Based Based on the on principle the principle that in that the in domain the doma of lowin of frequency, low soundfrequency, pressure. it is Apossible tactile to transducer feel with one that’s wasbody, used not within manufacturing one’s ears, this the speaker equipment can transmit and can low generate- it is possible to feel with one’s body, not with one’s ears, this speaker can transmit low-frequency vibrationfrequency is vibration also called to the a surface ‘vibration it is attached speaker’. to, Basedsince this on has the no principle diaphragm. that in the domain of low vibrationfrequency, toIn theaddition it surfaceis possible to the it is tactile to attached feel transducer, with to, one since ’as sensor thisbody, has wasnot no placedwith diaphragm. one’s in the ears, area thisin order speaker to determine can transmit the low- frequencyInvibrational addition vibration frequency. to the to tactile theThe surface transducer,speaker it reproduced is attached a sensor a to, wasrecorded since placed thissound inhas thesource no area diaphragm. to insimulate order tothe determine vibration the vibrationalandIn noise addition frequency. generated to Thethe by tactile speakerthe dust transducer, collector. reproduced The a sensoratactile recorded transducers was soundplaced reproduced source in the to area simulate anti in-vibration order the to vibration with determine the and the noisevibrational generated frequency. by the dust The collector. speaker The reproduced tactile transducers a recorded reproduced sound source anti-vibration to simulate with the the vibration sound of theand same noise frequency generated and by the size dust emitted collector. by the The dust tactile collector transducers to control reproduced the vibration anti on-vibration the other with side. the The experimental set up is shown in Figure8 and the noise control method and diagram are shown in Figures9 and 10. Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 13

sound of the same frequency and size emitted by the dust collector to control the vibration on the Appl.other Sci. side. 2019 , The9, x FOR experimental PEER REVIEW set up is shown in Figure 8 and the noise control method and9 ofdiagram 13 are shown in Figures 9 and 10. sound of the same frequency and size emitted by the dust collector to control the vibration on the Appl. Sci. 2019, 9, 1516 8 of 12 other side. The experimental set up is shown in Figure 8 and the noise control method and diagram are shown in Figures 9 and 10.

Figure 8. Experimental configuration.

Figure 8. Experimental configuration. Figure 8. Experimental conﬁguration.

Appl. Sci. 2019, 9, x FOR PEERFigure REVIEW 9. The diagram for the vibration reduction method. 10 of 13

FigureFigure 9. 9.The The diagram diagram for forthe the vibration vibration reduction reduction method. method.

FigureFigure 10. 10.Vibration Vibration speakerspeaker installed at at the the dust dust collector collector..

The result of the test demonstrated that it was possible to reduce the level of noise by only reducing vibration, because when the dust collector of the milling machine was used, vibration was transmitted to the surface of the dust collector by the compressor and the noise generated this time was dominant. Table 5 and Figure 11 indicate the result.

Table 5. Result values before and after.

Before After Sound pressure level (dB(A)) 82.5 71.4

Figure 11. Result of noise reduction (frequency analysis and noise sound spectrum (linear scale)). Appl. Sci. 2019, 9, x FOR PEER REVIEW 10 of 13

Appl. Sci. 2019, 9, 1516 9 of 12

Figure 10. Vibration speaker installed at the dust collector. The result of the test demonstrated that it was possible to reduce the level of noise by only reducing vibration,The because result when of the the test dust demonstrated collector of the that milling it was machine possible was to used, reduce vibration the level was of transmitted noise by only to thereducing surface vibration, of the dust because collector when by the the compressor dust collector and of the the noise milling generated machine this was time used, was dominant.vibration was Tabletransmitted5 and Figure to 11the indicate surface the of the result. dust collector by the compressor and the noise generated this time was dominant. Table 5 and Figure 11 indicate the result. Table 5. Result values before and after. Table 5. Result values before and after. Before After Sound pressure level (dB(A)) Before After Sound pressure level (dB(A)) 82.5 71.4 82.5 71.4

FigureFigure 11. Result 11. Result of noise of noise reduction reduction (frequency (frequency analysis analysis and noiseand noise sound sound spectrum spectrum (linear (linear scale)). scale)).

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 Appl. Sci. 2019, 9, 1516 10 of 12 positioned and designed to optimally excite the natural resonant modal structure of the panel [16]. Tactile transducers create and transmit low frequency vibrations to create tactile sound. Tactile sound isTactile present transducers in the vibratory create andsignature transmit of lowmusical frequency instruments, vibrations auto tomobiles, create tactile aircrafts, sound. and Tactile refrigerator sound compressorsis present in [ the17,18] vibratory. In addition, signature when of musicala sound instruments, is generated automobiles, by a tactile transducer, aircrafts, and it performs refrigerator a linearcompressors motion[ 17and,18 can]. In create addition, strong when vibrations a sound (see is generated Figure 12). by aAs tactile an actuator transducer, of a itlinear performs motor a lineartype speaker,motion and tactile can tcreateransducers strong are vibrations able to (see control Figure vibrations 12). As an by actuator sound of with a linear ease motor and accuracy. type speaker, In addition,tactile transducers linear actuators, are able in tocontrast control to vibrations electric motors, by sound are withable to ease deliver and accuracy.energy not In only addition, in the linearform ofactuators, vibrations, in but contrast also in to the electric form motors,of actual are motion. able to The deliver use of energy the tactile not transducer only in the (vibration form of vibrations, speaker orbut exciter) also in makes the form it possible of actual to generate motion. Thevibration use of without the tactile designing transducer a complex (vibration structure speaker for orvibration exciter) reduction.makes it possible Besides, to this generate can be vibration applied without to various designing devices a complex and is very structure efficient for vibration to control reduction. and in reducingBesides, thisthe cos cant. be applied to various devices and is very eﬃcient to control and in reducing the cost.

Figure 12. Voice coil type tactile transducer and basic principles of exciter technology. Figure 12. Voice coil type tactile transducer and basic principles of exciter technology. Noise in the dental practice setting causes an unpleasant experience [19]. As shown in the study by LeeNoise et al. in[ the20], dental the highest practice dental setting practice causes noise an unpleasant level at the distanceexperience of 100[19]. cm As is shown 77 dB, inand the 82.5 study dB byfor Lee milling et al. [20], machines. the highest This dental suggests practice that thenoise emergence level at the of distance milling of machines 100 cm is may 77 dB, cause and a82.5 louder dB fornoise, milling which machines. calls for This attention. suggests It wasthat revealedthe emergence that in of this milling study, machines the noise may reduction cause a plan louder using noise, the whichvibration calls reduction for attention method. It can was be erevealedffectively that applied in this to the study, noise the characteristics noise reduction of the milling plan using machine. the vibrationHowever, red theuction internal method and external can be noises,effectively including applied events to the while noise systems characteristics are on or of o theff, were milling not machine.considered However, in the present the internal experiment. and external It is, therefore, noises, including thought that events additional while systems measures are for on reducing or off, werethe noise not considered caused by in unpredictable the present experiment. and uncertain It is, events therefore, would thought be needed. that additional In addition, measures the quality for reducingand characteristics the noise caused of the remainingby unpredictable soundafter and controluncertain were events not evaluated.would be needed. Therefore, In addition, the proposed the 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]. In this study, we also created a simulated environment using tactile transducers and sensors to confirm our prediction of the effectiveness of anti-vibrations in decreasing sound pressure levels. However, our experiment did not completely cover significant aspects, which must have been taken into account if experimentation had been done with real-time milling machine vibrations. In order to resolve real-time noise levels, a feedback system should be established, which would be able to consider factors such as the material, type of milling, and model type. In addition, it is necessary to use additional sensors and identify the optimal algorithm to reduce noise errors and in response to unexpected noise. Further research could focus on an in depth analysis of the quality and characteristics needed in future studies after the removal of sound. Furthermore, additional studies that could be done could focus on the improvement of clinical noise levels in terms of sound quality and control. Our experiment indicated a decrease in noise levels. However, determination of sound frequency disruptiveness varies from person to person, causing subjectivity. To alleviate this issue, surveys and further research must be done to identify the types of noises at which dental clinicians and patients find the most discomfort. Appl. Sci. 2019, 9, 1516 11 of 12

6. Conclusions The noise generated by CAD/CAM milling machine operation was reduced by 9 dB using the proposed vibration control method. The noise intensities and frequencies of the milling machines were mainly characterized by low frequency range vibrations (about 60 Hz) coming from the dust collector. Our method signiﬁcantly decreased noise levels in order to create a better clinical environment for both dental professionals and patients. Further research could be done on not only identifying and decreasing the intensity of noise, but also evaluating the quality and characteristics of the noise on the qualitative level and developing a feedback system.

Author Contributions: Conceptualization, E.-S.S.; methodology, Y.-J.L.; software, E.-S.S.; validation, B.K.; investigation, B.K.; data curation, E.-S.S. and B.K.; writing—original draft preparation, E.-S.S.; writing—review and editing, Y.-J.L. and J.S.M.; visualization, E.-S.S.; supervision, Y.-J.L. Funding: This work was supported by grant no. 04-2017-0092 from the Seoul National University Dental Hospital Research Fund, Seoul National University Dental Hospital. The APC was funded by “Dental Research Institute, School of Dentistry, Seoul National University, Seoul, Republic of Korea”. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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