PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

The effect of 40 kHz ultrasonic noise exposure on human hearing
Andrew DI BATTISTA11 Ultrahaptics Ltd, UKABSTRACT
Purpose: High-Intensity ultrasound is being used for various applications in the latest consumer technology,
including mid-air haptic feedback. International guidelines concerning maximum permissible levels (MPLs)
for 40 kHz noise are contentious. This pilot audiometry study aimed to determine the immediate effects noise
exposure at 40 kHz has on human hearing and to help inform concerns about noise exposure levels in future
investigations. This study was conducted with cooperation from the NHS Occupational Health Service (UK)and was ethically approved by the Health Research Authority (HRA). Methods: 16 people (10 experiment, 6control) (ages 24-67 years) were exposed to high-intensity ultrasound emitted from a haptic device (40 kHz,SPL values ranging from 100-120 dB SPL) for 5 minutes. Pure Tone Audiometry (PTA) was conducted prior
to and immediately after exposure. Results: PTA test indicate no significant change in hearing sensitivity at
any of the tested frequencies (statistical power >80% in all cases, with 5 dB threshold shift set as a clinicallysignificant risk factor). Conclusion: Further studies are required to consider different age groups and to
consolidate appropriate sensitivity levels to high frequency sound. Initial results suggest levels as high as 120
dB SPL could be used in future investigations.
Keywords: Ultrasound, Audiometry, Hearing

1. INTRODUCTION
Exposure to loud audible noise can cause temporary threshold shifts (TTS), a recoverable loss inhearing sensitivity. Repeated occurrences of TTS can ultimately lead to permanent hearing loss (1).
For ultrasonic noise, these audiological effects are normally only attributed to SPLs in excess of 145
dB (2). To date, there has been no demonstration that 40 kHz ultrasound below 120 dB SPL has anyeffects on hearing (3). Nonetheless, ultrasound SPL in excess of 110 dB has been known to cause
subjective effects in humans, and it is for this reason that many international guidelines have adopted
this as their maximum permissible level (MPL) for ultrasound exposure. Currently, there is debate overthe standing of these guidelines, having been based on limited and outmoded data (4). Therefore, there
is a strong scientific motivation for updating current knowledge about the effects of ultrasonic noise.
Ultrahaptics is a technology that uses high-intensity 40 kHz ultrasound to produce mid-air haptic
feedback (Figure 1). In order to achieve this, an array of transducers is focused at a point to produce anSPL in excess of 145 dB; a requirement for perceptible tactile sensation on the skin (5). This SPL drops
significantly when away from the focus.

Figure 1 – Visualization of mid-air haptics using an array of ultrasonic transducers
What is of particular interest in terms of safety, is whether the incidental ambient noise (away fromthe focal point and reaching the ear) produced from the array is of a level that might produce TTS. With
regards to quantifying and defining hearing loss, a standard threshold shift (STS) has been defined by1andrew.di.battista@ultrahaptics.com

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OSHA as a 10 dB loss in hearing sensitivity (6,7). In contrast, threshold shifts greater than 5 dB can beconsidered of clinical relevance (8). Thus, this paper will need to demonstrate that the experiment had
sufficient statistical power to detect a change as small as 5 dB.
Collecting accurate and meaningful SPL measurements of ultrasonic noise is challenging;ultrasonic acoustic fields are highly variable in terms of spatial and temporal distribution. Moreover,complex interference patterns can be generated by multiple coherent sources (e.g. an Ultrahaptics
array). In terms of occupational health and safety for workplace monitoring, there currently exists nointernational standard for performing ultrasonic noise measurements (9). Since a single stationary
measurement is not appropriate, it has been recommended to use a statistical approach, reporting also
on the measurement uncertainty. An AU frequency weighting has also been adopted (10) but only for
audible noise measurements in the presence of ultrasound.
This paper first describes measurements that were carried out to estimate the ambient SPL levels of
mid-air haptics technology. It then describes a clinical trial that was conducted to determine if thesenoise levels would produce significant TTS within a general user-case scenario.

2. METHODS
2.1 SPL Estimates
In order to estimate ultrasound exposure levels, measurements were made using a B&K head andtorso simulator (HATS) equipped with a single TYPE 4191 microphone (in the left ear). A dataacquisition system comprising of a Picoscope (DrDAQ, Pico Technology) was used to collectreal-time SPL measurements with digital implementations of a microphone equalization filter and a1/3-octave band-pass filter centered at 40 kHz. SPL values were exponentially time weighted using a1s time constant; equivalent to the ‘SLOW’ setting on a standard sound level meter.
As array output, a focal point was generated from the array at a height of 20 cm, centered over thearray and tracing a 3 cm radius circle rotating at 100 rps. This is a typical output to create a circle
haptic sensation. The HATS was kept approximately within ‘arms-length’ (~60cm) and waspseudo-randomly translated and rotated manually in the transverse plane. Results shown in Figure 2are illustrative of a typical SPL exposure (Leq = 118 dB SPL). Also shown are statistical measures
L10,50 and 90 that describe levels as percentile thresholds.

Figure 2 – (left) HATS setup with permissible motion paths. (right) SPL measurements (1/3-octave band ,fc=40kHz) over time. Leq =118 dB, Lmax =125 dB, L10 = 122 dB, L50 = 116 dB, L90 = 111 dB.
2.2 Audiometry
Pure Tone Audiometry (PTA) was carried out at an NHS clinic (Avon Partnership Occupational
Health Service). Prior to this experiment, ethical approval was obtained through the UK Health4784Research Authority (HRA) by the Southwest Research Ethics Committee. The experimental design
consisted of a simple before-and-after noise exposure comparison of PTA results. A total of 16
participants (1 female, 15 males), ages [24-67], recruited from Ultrahaptics employees, were randomlysplit into experimental (n e =10) and control (nc =6) groups. All recruits were self-reported to be free of
any underlying otological health issues and underwent a standard otoscopy examination to check forany blockages.
For each participant, a baseline audiogram was produced from a PTA test (BILSOM CA850 Series
3 Automatic Audiometer) which covered a standard test frequency range (500, 1k,2k,3k,4k,6k,8k) Hzin both left and right ears, separately.
Baseline tests were followed immediately by 5 minutes exposure to an actively emitting
Ultrahaptics array. For the control group, nothing was emitted but a LED indicator light continued toflash on the array, as in normal operation.
Participants were instructed to sit at a table, arm’s length from the array, and to avoid leaning
over/placing hands over the array (Figure 3). The setup aimed to mimic that of the HATS experiments
for SPL estimates. All participants were supervised during this phase of the experiment to ensure
conformity with safety aspects of the protocol.

Figure 3 – (Left) anechoic chamber for PTA. (right) participant setup.
After the noise exposure phase, an immediate second PTA was conducted. Additionally,participants were also given a private consultation from the audiologist about their general
audiological health. All research data was assigned to an anonymous participant study ID number for
subsequent analysis.

2.3 Exclusions
Data from one individual complaining of moderate tinnitus after the baseline test was excluded
from this study (ne=9). In some cases, participants did not produce consistent responses to the PTA test
at particular frequencies, despite a repeated attempt. This occurred in both baseline and secondaudiograms, across both experimental and control groups. Due to testing constraints, these data points
were dropped; in total 6 data points were omitted from the analysis. These were accounted for insubsequent statistical calculations with the appropriate value for sample number.

3. RESULTS
The box-plot in Figure 4 amalgamates all baseline audiograms of both experiment and control
group, in both left and right ears. Figure 5 and 6 depict the threshold shift (pre exposure audiogram
subtracted from the post exposure audiogram) for control and experimental group respectively. Notethat hearing loss is associated with positive values of threshold shift in these figures.

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Figure 4- Box-plot of all baseline audiograms (experimental and control groups, left and right ears)

Figure 5 – Shift in hearing threshold (second audiogram – baseline) for the control group

Figure 6 – Shift in hearing threshold (post exposure – pre exposure) for the experimental group4786

3.1 Statistical Analysis
A one-sided paired t-Test (α=0.05) was used to test the significance of the threshold shifts observed
within the experimental group. The main hypothesis underlying this experiment can be stated asfollows:
Ho : Ultrasound Exposure has no effect on PTA results (𝜇o = 0)
Ha : Ultrasound Exposure causes a threshold shift (𝜇a ≥5 dB)
Each PTA frequency was analyzed independently. Moreover, each ear (right and left) wasaccounted for individually, resulting in a total of 30 ears (18 experimental, 12 control). The t-statistic
was calculated at each frequency using the pooled variance from both groups. In all cases, the P-value
far exceeded 0.05, indicating that Ho could not be rejected (Table 1). However, this result alone does
not rule out the possibility of some undetectable TTS having occurred due to the limitations of theexperiment’s sensitivity. It is important to demonstrate that a significant TTS could be detected with areliably high probability (i.e. statistical power) in the event it did occur. Conventionally, the minimum
requirement for statistical power is 80%. Subsequent retrospective analysis indicated the ability of theexperiment to detect a shift as low as 4 dB at all frequencies (for an STS of 10 dB, statistical powerincreased to 99.9%).
Table 1 – P-value and mean threshold shift (experimental group) at each PTA frequency
f (kHz)

0.5

12

3

4

6833333
𝑇𝑇𝑆-1.5

0.24

0.22

-0.56

-0.33

0.78

-1.56

P

0.99

0.29

0.350.8

0.72

0.23

0.88

4. DISCUSSION
4.1 SPL Estimates
It has been identified that the pinna/head related transfer function played an important role inultrasound noise exposure as SPL levels varied greatly with head orientation (Figure 2). The HATS
used in this experiment possessed only one variant of a realistic human pinna model. Moreover, thesimulated workstation experiment was restricted to a single plane of motion. It was also not practicalto conduct exhaustive testing on all possible haptic focal point outputs from the array.
It is important to note that in a real mid-air haptics user-case scenario, the ultrasound phased array
output would be intermittent, switching on only to the presence of a hand. The hand itself would scatterthe acoustic energy back towards the array. This would likely greatly reduce the overall Leq that would
reach the ear. However, removing hand gesture interference from this scenario provided moreconsistency throughout the experiment. It is not yet clear if other statistical measures (Lmax , L90 , L10 ,etc.) should also play a role in assessing risk factors to ultrasonic noise exposure.
4.2 Audiometry
Initial results suggest that high-intensity 40 kHz ultrasound of approximately 120 dB SPL does notcontribute to TTS in standard PTA testing. However, there are several limitations to what can beconcluded from this pilot study:
It was not practicable to refine recruitment for different age groups and/or demographics. Only
one woman participated in the study, and no one under the age of twenty years old was tested; thus,
potential vulnerable groups to ultrasonic noise were not adequately represented.
Intergroup comparison was not readily applicable due to the size and uneven distribution of age
across the experimental and control groups. Moreover, the large age range is evident in the rapid
increase in hearing thresholds above 4 kHz (Figure 4). Extended high-frequency PTA was not available
at the audiometry clinic; however, it is clear that several participants would not have been able tocomplete a PTA test for frequencies above ~10 kHz, in any event.

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PTA is heavily dependent on the test subject’s ability to maintain focus throughout testing. For thisreason, repeated data acquisition attempts on the same person would likely reduce reaction time andtest-retest reliability, rather than actually improve the statistical certainty of results. It was for thisreason that missing data points were not exhaustively chased-up.
Nevertheless, this study demonstrated the sensitivity to detect shifts as low as 4 dB using a
relatively small sample size. An STS of 10 dB would have been detected with near statistical certainty.
These results will help inform designs of future studies; it is likely that 5 dB TTS will be used as thebenchmark for indications of hearing loss.

5. FINAL REMARKSThis experiment was conducted to ascertain what if any immediate effects high-intensity 40 kHz
ultrasound has on hearing sensitivity. Results suggest that SPL levels of approximately 120 dB SPL
does not contribute to TTS. It is hoped that these results can serve as a useful baseline in conducting
future work that will include more controlled participation, with emphasis on potential vulnerable
groups and testing using extended high-frequency PTA.

ACKNOWLEDGEMENTS
The author would like to acknowledge the help and support from the audiologist and staff of theAvon Partnership NHS Occupational Health Service for providing their premises and expertise. Theauthor would also like to acknowledge the contributions of the Frenchay REC in reviewing andimproving the protocols used in this experiment.

REFERENCES
1. Ryan A et al. Temporary and Permanent Noise-Induced Threshold Shifts: A Review of Basic ClinicalObservations. Otol Neurotol. 2016 September; 37(8): e271-e275.
2. OSHA Technical Manual, Section III: Chapter 5 – Noise Measurement. U.S. Department of Labor,
Occupational Safety and Health Administration. URL http://www.osha.gov/dts/osta/otm/new_noise.
3. Health Canada. Guidelines for the Safe Use of Ultrasound: Part II – Industrial and CommercialApplications. Safety Code 24; 1991 ISBN 0-660-13741-0. p. 14.
4. Lawton B. Damage to Human Hearing by Airbourne Sound of very high frequency or ultrasonicfrequency. HSE; UK 2001.
5. Carter T et al. Ultrahaptics: multi-point mid-air haptic feedback for touch surfaces. Proc 26th Sym
ACM-UIST; 2013. p.505-514
6. Margolis R.W. Distribution Characteristics of Normal Pure-Tone Threshold. Int J Audiol. 2016.
7. Ryan A.K. Temporary and Permanent Noise-Induced Threshold Shifts: A Review of Basic and ClinicalObservations. Otol Nerotol. 2016.
8. De Mello L.D. Test-Retest variability in the pure tone audiometry: comparison between two
transducers. Audiol Cmmun Res; 2015. p. 239-245.
9. Radosz J, Pleban D. Ultrasonic noise measurements in the work environment. JASA; 144, 2018.
p.2532-2538.
10. Wolff A. Airbourne ultrasound at german workplaces. Inter-Noise; 2016. p.2863-2867

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