PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Applications of psychoacoustics in dental drill noise evaluation
Tomomi YAMADA1; Sonoko KUWANO2; Mikako HAYASHI3
1, 2, 3

Osaka University, Japan

ABSTRACTThe sound produced by dental drills such as a dental air turbine handpiece can markedly influence the soundenvironment in a dental clinic. Indeed, many patients report that the sound of a dental drill elicits unpleasantfeeling. Comfortable sound environment for patients is pursued in a field of medical care as well as in daily
life. The sounds emitted by dental drills at dental clinics have high frequency components up to 20,000 Hz
with high sound pressure level. In this study, the effect of sound level or frequency components inhigh-frequency region on the subjective impression of dental noise was systematically investigated in order
to find clues to create comfortable environment in dental clinics. In addition, we also performed thepsychological evaluations for the effect of masker on the sounds emitted by dental drills and the results
suggested that mixing frequency components may improve the subjective impression of the dental drill noise.
Keywords: Dental drill, Sound quality, Psychoacoustics

1. INTRODUCTION
The sound emitted by a dental drill has a powerful influence on the sound environment in a dental
clinic. According to a questionnaire survey regarding comfortable dental environment for patients,
approximately half of the respondents reported having experience of unpleasant feelings on hearing
the sound of a dental drill (1). Moreover, the fear induced by the sound of a dental drill had a strong
influence on dental anxiety level (1). A comfortable sound environment at dental clinics is likely to
encourage patients to visit for dental evaluations and treatment, and may facilitate the maintenance ofdental health, and accordingly, quality of life.Several kinds of dental apparatuses are used for cutting
teeth and removing dental materials in a dental clinic, including dental high-speed air turbines anddental electrical motors (2). The sound emitted by a dental high-speed air turbine is the representativesound of dental treatment and would have a powerful influence in comparison to other devices on thesound environment in a dental clinic. Therefore, in this paper, we considered the sound emitted bydental high-speed air turbines as dental drill noise.
Various psychological measurements have been used to examine the relationship between physicalvalues and subjective judgment in noise research and sound quality engineering for improving thesound environment. In our previous study (3), we assessed the physical values associated with thesubjective impressions of 24 original sounds emitted by 12 kinds of dental drills under idling anddrilling conditions. We found that the impressions of the sounds of dental drills changed substantiallybetween the idling and active drilling conditions, and that the impressions of the sounds of dental drills
consisted of two factors classified as “metallic and unpleasant” and “powerful.” Among the examined
physical metrics, LAeq values were positively correlated with the “powerful impression” factor; thecalculated sharpness was related to the adjective scale values for the “metallic impression” factor; andboth LAeq and sharpness predicted the “unpleasant impression” of the sound of dental drills. We
performed multiple regression analyses of the scale of “unpleasant adjectives” and, LAeq and sharpness,and compared the results of the multiple regression analyses with that of Comfort Index (CI), which iscomposed of LAeq and sharpness (4). In that comparison, the correlations of CI with “unpleasantadjectives” were almost same as that of the multiple regression analyses (3).
12yamada@dent.osaka-u.ac.jp
kuwano@see.eng.osaka-u.ac.jp7737

In order to identify cues to improve the sound quality of dental drills for patients, it is necessary toexamine the effect of the physical properties of the sounds of dental drills on their impressions.
Generally, the main target for improving noise from various sources, particularly machines, is thereduction in sound pressure levels related to loudness (5). In this study, we prepared 13 sound stimuli
under active drilling condition processed to have different sound levels or calculated sharpness valueof the sounds emitted by a dental drill using sound software and conducted psychoacoustic experiment.

2. MATERIALS AND METHOD
2.1 Sound Stimuli for Psychological Experiment
Thirteen sound stimuli for psychological experiment were prepared as shown in Table 1. The soundof a dental air turbine handpieces was recorded during drilling of an artificial tooth at Osaka
University Dental Hospital in a quiet environment. The sounds emitted by the dental drills wererecorded using a sound level meter (LA-5560; Ono sokki) and a Digital recorder (DR7100; Ono sokki).
We edited the duration of the dental drill noise recording to approximately 5 s using sound software(Auditon 2.0; Adobe) and named it Stimulus No.1. The average spectrum of Sound Stimulus No.1 isshown in Figure 1. Stimuli No.2 and No.3 were made from No.1 attenuated by 5 dB and 10 dB.
Stimulus No.4 was made by attenuation of the frequency components over 9 kHz of No.1and Stimuli
No. 5 and No. 6 were attenuated by 5 dB and 10 dB. Stimuli No.7, 10 and13 were made by mix-pasting
noise A, B and pink noise, and Stimulus No.1in the original sound level , and adjusted the sound level
using the sound software. Noise A was a dental vacuum noise and it had broad band frequency
components up to 20 kHz, especially high level from 500 Hz to 10 kHz as shown in Figure 1. Noise B
had only frequency components below 3.5 kHz of Noise A. The analyses, processing and calculating
sharpness were conducted using sound quality software (7698, 5265; Brüel and Kjær).
Table 1 – Sound stimuli used in this experiment
Stimuli
No.

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LAeq (dB)

Sharpness(acum)

Fluctuation
Strength (vacil)

1

Original70

4.7

2.7

2

Attenuated of No.1 by 5 dB65

4.72.63Attenuated of No.1 by 10 dB60

4.72.24Attenuated of No.1 over 9 kHz54

3.02.35Attenuated of No. 4 by 5 dB50

2.92.46Attenuated of No. 4 by 10 dB45

2.8

1.97Mix No.1 and Noise A63

4.1

1.98Attenuated of No. 7 by 5 dB58

4.3

1.79Attenuated of No.7 by 10 dB54

4.01.610Mix No.1 and Noise B

65

3.9

2.6

11Attenuated of No.10 by 5 dB60

4.02.412Attenuated of No. 10 by 10 dB553.72.1

13Mix No.1 and Pink noise

65

4.2

2.6

Figure 1 – The average spectra of Stimulus No.1 and the Noise used in the experiment.
2.2 Procedure of Psychological Experiment
We conducted psychological measurements using a semantic differential scale with 15 adjectivepairs (see Figure 2) selected on the basis of the pre-experimental results (3). Eleven female and fourmale participants aged between 24 and 29 years (average, 26.3 years) participated. All of theparticipants were Japanese and had experienced dental treatment. Participants were seated alone in aroom and instructed to judge the impression of the sound stimuli delivered through headphones(SRM-313; STAX). Before exposure to the test stimuli, participants were informed that the test stimuli
consisted of the recordings of dental drill sounds. Participants were then asked to judge theirimpressions of the sound stimuli twice in two separate sessions.
The experiment was approved by the Ethics Committee of the Osaka University Graduate
School of Dentistry. All participants provided written informed consent before taking part in theexperiments.
2.3 Statistical Analysis
All statistical analyses were performed using SPSS statistical software (SPSS, Inc.), and p values of<0.05 were considered statistically.

3. RESULT and DISCUSSION
3.1 Subjective Impressions of the original sound stimuli
We found a significant correlation between the results from the two psychological evaluation
sessions in all participants (Spearman’s rank correlation, correlation coefficient r = 0.73). This result
indicates that the judgments of all participants were reliable. Therefore, we combined the results of thetwo psychological evaluation trials in the subsequent analyses.
Figure 2 shows the results of subjective impressions to Stimuli No.1-3. As shown in Table 1, thecalculated values of these stimuli were the same as 4.7 acum and the LAeq values were 70, 65 and 60 dB,respectively. There are statistically differences among the scale values of “loud” or “clamorous“ forStimuli No. 1-3. However the “unpleasant- pleasant” impressions hardly improved. Figure 3 shows therelation between the adjective scaled of “soft- loud” and LAeq of all stimuli. Overall, attenuation of thesound level seemed to improve the impression of “loud” in this experiment. The correlation
coefficients between the “soft-loud” adjective scales and LAeq were 0.94.
Next, we compared the stimuli to examine the effect of the maskers. Figure 4 shows the spectra
and the average of the FFT analysis of Stimuli No.2, 7 and 11. We can observe characteristic soundwaveforms changing with time around 6 kHz clearly in Stimuli No.2 and 11, however, it is difficult
to see the fluctuating waveform in Stimuli No.7. Figure 5 shows the results of the subjective
impressions of Stimuli No. 2, 7 and 11. The score of the adjective “loud” of Stimuli No.2 and 7(65dB and 64 dB, respectively) were evaluated nearly close, however they got different subjective
evaluations on the other several adjectives. Stimulus No.11 (60 dB) was evaluated almost the sameof the judgement of Stimulus No. 2.

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Figure 2 – Profiles of subjective impressions of the sound stimuli No.1-3.

Figure 3– Relation between loudness judgement and LAeq values

Figure 4 – Spectra and the average of FFT analysis Stimuli No, 2, 7 and 117740Figure 5 – Profiles of subjective impressions of the sound stimuli of Stimuli No.2(Red),No.7(Yellow) and No. 11(Green)

Figure 6 – Relation between the scales of “sharp- dull” and sharpness or Fluctuation strength.
Figure 6 (left) shows the relationships between the scales of “sharp-dull” and sharpness values ofall stimuli. Stimuli No. 7, 8 and 9 were evaluated better on the several adjectives such as“distinct-vague”, “harsh-smooth”, “painful-not painful” than the other stimuli. The values ofadjectives sharp” of No. 7, 8 and 9 were evaluated duller than Stimuli No.10-13 as the close sharpness
values. And they seemed to deviate from the approximate line of the other values. The correlations
between calculated sharpness and the adjective evaluations “sharp-dull” among all the stimuli (r=0.31).
Figure 6 (right) shows the relationships between the scales of “sharp-dull” and Fluctuation strength of
all stimuli. You can find the good correlation (r=0.98). It suggests that masking the characteristicfrequency components changed fluctuation strength and affected the impression of the adjectives“distinct-vague”, “harsh-smooth”, “painful-not painful”, “pleasant-unpleasant” and so on.
Although we recognize that technical concerns often limit the capacity to reduce sound pressurelevels and frequency components in reality, we conducted the experiment under the conditions of thestimuli prepared. We will examine the effect to sound quality in more details to realize theimprovement of dental drills into consideration.

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4. FINAL REMARKSIn this paper, we examined sounds emitted by dental high-speed air turbines as dental drill noise.
We conducted psychological experiment using semantic differential to facilitate potential
modifications of the sounds of dental drills.
The results showed that overall attenuation of the sound level and attenuation of the frequency
components over 9 kHz of the original test stimuli statistically improved the scale of “soft-loud” incomparison with the original sound stimuli. Attenuation of the frequency components over 9 kHz andmixture of the frequency components were effective of the scale of “pleasing- unpleasing”.
The present findings suggest that reducing the sound pressure level and refining the frequency
characteristics of sounds emitted by dental drill considering acoustical characteristics is important for
creating a comfortable sound environment in dental clinics.

ACKNOWLEDGEMENTS
This work was supported by a grant-in-Aid for Scientific Research (No. T15K111110 and19K10148) from the Japan Society for the Promotion of Science.

REFERENCES
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2. International Organization for Standardization. ISO 14457: Dentistry—Handpieces and motors.
International Standard. Geneva, Switzerland, 2017.
3. Yamada T, Kuwano S, Ebisu S, Hayashi M. Statistical analysis for subjective and objective evaluationsof dental drill sounds. PLoS One 2016;11:e0159926.
4. Kuwano S, Namba S, Takehira O, Fastl H. Subjective impression of copy machine noises: An
examination of physical metrics for the evaluation of sound quality. Proc. InterNoise.09 23-26 August;Ottawa, Canada 2009.
5. Fastl H. Section 3: Psychoacoustics and product sound quality. In: Kuwano S, editor. Recent topics inenvironmental psychoacoustics, Osaka: Osaka University Press; 2008, p. 63–87.

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