1

1

2

Assessment of short-term exposure to an ultrasonic rodent repellent device 3

4

Astrid van Wieringen1 and Christ Glorieux2 5

KU Leuven – University of Leuven, 1Department of Neurosciences, Experimental ORL & 6

2Laboratory of Acoustics, Division of Soft Matter and Biophysics, Department of Physics and 7

Astronomy, KU Leuven, Celestijnenlaan 200D , B-3001 Heverlee, Belgium 8

9

10

11

Conflict of interest: this study was carried out for the Belgian Federal Public Service, Health 12

Food chain safety and Environment (no DG5/PB_PP/IVC/13026) 13

14

15

Keywords: ultrasound exposure; auditory sensitivity; spectral properties; adverse factors 16

2

ABSTRACT 17

The objectives of the present study were to investigate the acoustical properties of the very 18

high frequencies and/or ultrasound signals produced by a repellent device and to investigate 19

potential adverse factors as a result of short-term exposure to these signals. Potential adverse 20

effects were evaluated perceptually with 25 young and 25 middle-aged persons, all with normal 21

hearing thresholds, in a quiet room using different outcome measures, including a 15-item 22

survey presented before and immediately after each condition. Spectral analyses showed that, 23

besides emitting frequency modulated sounds in the expected frequency ranges, a faint but 24

audible sound in the 4-5 kHz range was present. On average, relatively short exposure to the 25

sound produced by a repellent device did not lead to significant adverse effects. Yet, when the 26

signal was perceived, as it was frequently for the younger population at the two lower 27

frequencies settings (12-14 kHz, 25-25 kHz) and with 2 sources emitting, it was considered to 28

be disturbing by several participants. Given the increasing usage of ultrasonic devices as well 29

as the much longer exposure of high frequency and ultrasound in domestic usage, careful 30

consideration and better guidelines are required, especially for those who are most sensitive to 31

sound. 32

33

34

35

36

3

I. INTRODUCTION 37

Pest repellent devices are often purchased for deterring birds, rodents and insects by emitting 38

high-frequency sounds (Aflitto and DeGomez, 2014). Dependent on the setting of the device, 39

the frequency of the sound ranges from 8 kHz to 40 kHz, with the frequency range up to 17.8 40

kHz termed ‘very high frequency sound’ (VHFS) and above 17.8 kHz onwards termed 41

‘ultrasound sound’ (US) (Leighton, 2007, 2016, 2017). Exposure to VHFS/US emitted by 42

public address systems, repellent devices, automatic doors, etc., is common, but it has not been 43

accompanied by research on potential harm or disturbance for adults and children. Undesirable 44

effects are mainly reported for high exposure levels, i.e. higher than 100 dB SPL for industrial 45

ultrasonic sources (e.g. Acton and Carson, 1967; Ahmadi et al., 2012; Smagowska and 46

Pawlaczyk-Łuszczyńska, 2013). Ueda et al. (2014a, 2014b) have been among the few to 47

measure the ultrasonic field produced by a single rodent repellent device in a public space. The 48

sound pressure of the 19-20 kHz sound was around 130 dB re 20 µPa right under the sound 49

source and still 90 dB re 20 µPa or more at 14 m distance, thereby dissipating a high sound 50

pressure level over a wide area. In parallel with these measurements, a survey was filled out by 51

all participants. The survey showed that a 100% of respondents (aged 20-50 yrs) could hear the 52

sound and that 89% of those indicated they could hear it clearly. Several persons experienced 53

‘pain in the ear’, ‘restlesness’, and few experienced nausea and dizziness. 54

The human auditory system is very sensitive to sound: a healthy young person can hear 55

sound frequencies between approximately 20 Hz to 20 kHz (Durrant and Lovrinic, 1984). The 56

upper frequency limit of hearing decreases with age, but can still be relatively high, as was 57

recently reported by Jilek et al. (2014) and by Rodriguez et al. (2014). Young persons (19-58

29 yrs) can perceive a 20 kHz tone if it is presented at 100 dB SPL, while middle-aged persons 59

(50-60 yrs) would only be able to detect a 16 or 18 kHz tone at the same sound pressure level. 60

For more common sound pressure levels, i.e. between 60-70 dB SPL, healthy middle-aged 61

4

persons perceive up to about 14 kHz. It is well known that auditory sensitivity can vary 62

considerably in normal hearing persons and that some adults and children have significantly 63

better hearing thresholds than expected by their age range. However, very little is known about 64

exposure to ultrasound on hearing sensitivity. A recent study reported that exposure to 65

ultrasound (between 20-40 kHz) damages hearing in the frequency range between 9-18 kHz. 66

Damage may already appear after less than five years of exposure, and may be accompanied 67

by other problems such as tinnitus (Maccà et al., 2015). Several studies focus on exposure in 68

industrial settings (e.g. Grzesik and Pluta, 1986). It is not clear, however, whether public 69

address systems and/or repellent devices are less harmful, just because they emit sound within 70

permitted sound level ranges. Following several complaints by the public and an objective 71

evaluation (AIB-Vinçotte, 2011), the Federal Public service in Belgium for Health, Food chain 72

safety and Environment commissioned a two-part study in order to inform advice regarding 73

the potential nuisance caused by a repellent device. The main aims of the study were to 74

characterize the acoustical properties produced by a KEMO ultrasonic repellent device (type 75

M175-02-18) and to investigate possible undesirable effects caused by short-term exposure to 76

it. Analyses were carried out for levels of exposure lower than 100 dB SPL, following 77

prescriptions of the specifications sheet. The device has 10 different settings with center 78

frequencies between 8 kHz and 43 kHz. According to the producer, the most effective 79

frequency range lies between 10-12 kHz for birds, between 20-30 kHz for rodents, and between 80

30-40 kHz for insects. Three of the frequency settings were evaluated, with nominal frequency 81

ranges 12-14 kHz, 25-28 kHz and 35-41 kHz, corresponding to settings 5, 8, 9 of the device, 82

respectively. The device came with two loudspeaker settings to boost the sound pressure level 83

over an extended area, 1) with an internal loudspeaker (source 1), and 2) with an external 84

loudspeaker (source 1+2). It was expected that potential undesirable effects would appear more 85

prominently for persons with normal hearing (as described in ISO-389-8 (2004) and ISO 389-86

5

5 (2006)) than for those with (high frequency) hearing impairment. Moreover, due to declining 87

hearing sensitivity with increasing age, potential adverse effects were expected to be more 88

pronounced for young than for middle-aged persons. Measurements were carried out in a very 89

quiet environment, which is expected to provoke more undesirable effects than a more 90

realistic/nosier outdoor scenario. A more realistic environment could potentially mask the 91

target signal. 92

First, the sound pressure levels of the Kemo-device, type M175-02-18, without and with the 93

external L020 loudspeaker, were determined for frequencies ranging between 20 Hz and 46 94

kHz (Experiment 1). Moreover, we investigated the influence of the positioning of the device 95

on sound propagation, its directivity, the shadowing effect of the human head, and the 96

contribution of reflections by concrete or soil covered by grass to the total sound pressure level. 97

Second, a perceptual evaluation of the different sound conditions was carried out in young and 98

middle-aged persons, all with normal hearing thresholds between 125 Hz and 8 kHz 99

(Experiment 2). The study was approved by the Medical Ethical Committee of the University 100

Hospital Leuven (B322201318943). 101

102

II. EXPERIMENT 1: Characterization of very high frequency sound (VHFS) and 103 ultrasound (US) 104

A. Settings 105

The frequency range of the Kemo device could be set by a dial connected to a potentiometer. 106

An indicator on the dial pointed to a scale from 0 to 10. A rough numerical indication of the 107

corresponding VHFS/US frequency range was provided by the specification sheet. Since the 108

rotation of the dial was continuous, its position is subject to frequency variations, typically in 109

the order of 1 kHz compared to the values listed in the specifications sheet. 110

6

In order to ensure reproducibility of the frequency range during all conditions, and to allow 111

switching the device settings by the operator inaudibly and invisibly for the test person, the 112

original potentiometer was replaced by a circuit containing 3 analogous potentiometers These 113

could be selected remotely by means of an electronically controlled switch with 3 positions. 114

The potentiometers were set to reproduce dial positions 5 (nominal frequency range 12–115

14 kHz), 8 (frequency range 25–28 kHz) and 9 (frequency range 35–41 kHz). The sound 116

pressure levels of the device were determined for these 3 positions, without and with activation 117

of the additional L020 loudspeaker, resulting in 6 scenarios. 118

119

B. Semi-anechoic rooms and equipment 120

The characterization of the sound produced by the Kemo device was performed in a semi-121

anechoic measurement room, mimicking free-field conditions. The background noise level in 122

this semi-anechoic room was less than 0 dB SPL. Exposure tests were performed in a larger 123

semi-anechoic room, which was better adapted to host people comfortably. In this room the 124

average background sound pressure level was 21.5 dBA. In both rooms the total sound pressure 125

level in the frequency range covering the VHFS/US emission band was dominated by the sound 126

field of the device, except when the device was switched off. 127

For the objective characterization of exposure conditions evaluated perceptually in Experiment 128

2, the Kemo-device and the L020 loudspeaker were placed at 6.5 m from the measurement-129

device and about 3.8 m apart from each other. 130

In order to determine absolute sound pressure levels and signal spectra (linear and in 1/3 131

octave-bands between 20 Hz and 46000 Hz), the following equipment was used: 132

7

A calibrated condenser 1/8” microphone B&K type 4138 (flatness of 1 dB between 20 Hz and 133

50000 Hz). It was factory calibrated by the supplier, Brüel and Kjaer, and the calibration was 134

taken from the B&K calibration chart. 135

A B&K measurement amplifier type 2606 (bandwidth 200 kHz) 136

A Roland Studio Capture audio interface (sampling frequency 96 kHz, 24 bits, -90 dBu noise 137

level) 138

B&K dual channel real-time frequency analyzer type 2144 for calibration and noise-level 139

performance of the measurement chain up to 20 kHz) 140

DANAK Calibrated B&K pistophone-calibrator as level-reference at 250 Hz. 141

B&K 4231 1 kHz, 94 dB SPL calibrator for reference calibration. 142

C. Spectral content of the very high frequencies and ultrasounds 143

In addition to the absolute sound pressure levels, we also investigated the spectral content of 144

the sound. Inspection of the spectra and spectrograms (Figure 1) revealed a strong tonal 145

character with peaks in the ranges 12-14 kHz (setting 5), 25-28 kHz (setting 8) and 35-41 kHz 146

(setting 9), and with smaller peaks around the harmonics (harmonic multiples of the 147

fundamental frequency). These ranges are somewhat different from the ones in the producer’s 148

specification sheet, which mentions ca. 26-30 kHz and ca. 38-43 kHz for settings 8 and 9, 149

respectively. For settings 5, 8, 9 the frequency of the main peak was modulated between 12-150

14 kHz, 25-28 kHz and 35-41 kHz, respectively, at a modulation rate of about 2 Hz. Given the 151

slow modulation, the character of the sounds was tonal on a short time scale, with a well-152

defined spectral peak. The long-term averaged spectra contained bands within the ranges 153

mentioned before. 154

8

155

Fig. 1. Fourier spectra (left) and spectrograms (right) of Kemo-generated 5.28 sec (ultra)sound, 156

in nominal settings 12-14 kHz, 25-28 kHz and 35-41 kHz, sampled at 96 kHz. The frequency 157

modulation period is 0.44 sec. The arrows in the spectrograms point to a faint peak in the 158

audible frequency range between 4.2 kHz and 5.8 kHz. In order to make the spectra levels 159

intuitive to interpret, the spectra were normalized so that the average peak levels corresponds 160

with the total sound pressure level of the energy in the emission band 161

162

These properties were audible when listening to recordings played at a lower sampling 163

frequency. Interestingly, in addition to the high frequency content, spectral analysis shows the 164

presence of a faint but distinct spectral content at frequencies well within the normal range of 165

human hearing i.e. between 4 kHz and 5 kHz, in the audible part of the spectrum. It was verified 166

9

by the experimenters that this 4-5 kHz sound component, whose existence is not mentioned in 167

the device specifications and is probably unintentionally produced by device, was always 168

present, albeit fluctuating in frequency and magnitude. 169

170

D. Sound pressure levels 171 172

For each scenario of interest, a recording was made and analysed spectrally. In order to scale 173

the amplitude of the signals and spectra, a reference wave file was recorded while placing the 174

microphone in a 1 kHz, 94 dB SPL calibrator, using the same settings for the hardware and the 175

data processing. For some of the measurements, the energy of the broadband electronic 176

background noise, cumulated over the whole frequency range, was not negligible compared to 177

the energy in the (ultra)sound emission band. Therefore, the level of the VHFS/US was 178

determined by only integrating the spectral energy within the frequency window at which the 179

device was emitting. In this way, possible contributions of the background noise to the 180

determined sound pressure level were discarded. Two approaches were taken to determine the 181

sound pressure level experienced by the test persons during exposure sessions. In the first 182

approach, the measurement microphone was positioned at 6.5 m from the loudspeaker(s), i.e. 183

at 0.5 m within the distance used during the perceptual tests. In the second approach, which 184

was done to verify the values of the first one, the sound pressure level was determined at 1 185

meter from the source to obtain a better signal to noise ratio (the noise being mainly of 186

electronic nature). Assuming far-field conditions in a point source approximation (which is 187

valid for distances much larger than the loudspeaker diameter D=6 cm and the wavelength 188

(between 1 cm and 3 cm), Kino, 2000, ch. 3, p 165), the sound pressure level at 6.5 m along 189

the axis perpendicular to the loudspeaker membrane was calculated from the one at 1 m by 190

applying a geometrical reduction factor 191

10

10

6.5R 20log 16.3 dB

1

, and a frequency dependent atmospheric absorption factor, 192

which was taken to be 193

2

fA 0.002 dB/m x 6.5 m-1 m

1000 Hz

, with f the frequency. 194

Since, in reality, ultrasound attenuation is strongly temperature and humidity dependent (ISO 195

9613-1, Harris, 1966) the latter equation only yields a rough approximation of the real value. 196

Table 1 lists the sound pressure values for the different conditions. Overall, the values of the 197

approaches were consistent within the experimental uncertainty, provided one takes into 198

account inaccuracies of the geometrical attenuation model (ideal point source), of the 199

atmospheric absorption model (simplified frequency dependence, and neglecting humidity and 200

temperature effects), and of spurious contributions of electronic noise. The difference between 201

the values obtained by the two approaches was highest for setting 5. This could be a 202

consequence of the contribution of reflections by the concrete floor of the laboratory, whose 203

impact of coherence is expected to be higher at lower frequencies. Such a situation with fairly 204

coherent sources, can lead to interference between the sound waves impinging on a person's 205

body and the respective reflected waves, with an increase of up to 6 dB, or a (partial) 206

cancellation. Therefore, we made sure that the subject’s head orientations were the same for 207

the different conditions during the perceptual experiment (Experiment 2). 208

209

Table 1. Average sound pressure levels and standard deviations (SPL x) of the Kemo-device 210

type M175-02-18 in different scenarios. The table lists the FM emission range, number of 211

sources (nr srces), the SPL at 1 meter, calibrated with a 1k Hz tone at 94 dB, the air attenuation 212

11

correction (AAC), the extrapolated values at 6.5 m, including the geometrical and attenuation 213

correction, and the values determined at 6.5 m from the source(s). 214

FM emission range

(device setting)

Nr

srces

dB SPL at

1 meter

AAC Extrapolated

values

6.5 m from

the srces

12-14 kHz (setting 5) 1 802 -1.7 62 3 67 2

1+2 802 -1.7 62 3 70 2

25 -28 kHz (setting 8) 1 762 -6.9 53 2 57 2

1+2 812 -6.9 58 2 62 2

35- 41 kHz (setting 9) 1 752 -13.5 46 2 45 1

1+2 782 -13.5 49 2 49 1

215

The sound pressure levels for 2 sources listed in Table 1 were, on average, 0 dB to 5 dB higher 216

than for 1 source. For non-coherent sounds, we expect, for 2 equal sources at the same distance 217

to the receiver, an increase of 3 dB. In the Kemo device, the sounds had a narrowband character 218

with a relatively slow frequency modulation cycle duration of 0.44 sec. This condition, with 219

fairly coherent sources, can lead to interference between the sound wave impinging on a 220

person's body and the respective reflected wave, with, as in the case of floor reflections, in an 221

increase up to 6 dB, or a (partial) cancellation. 222

E. Parameters influencing the sound pressure levels at the listening position 223

In addition to the scenarios described above, we also determined sound levels emitted from the 224

repellent device for intermediate distances between source(s) and the measurement/listening-225

12

position. As mentioned above, due to the increasing geometrical expansion of the wave front 226

with increasing distance from the source, the sound pressure level decreased with increasing 227

distance. In the approximation of a point source, the wave fronts were spherical and the sound 228

pressure amplitude decreased inversely proportional with distance, resulting in a decrease of 229

6 dB per doubling of the distance or 20 dB per tenfold distance. The additional attenuation of 230

sound waves by air-dissipation resulted in an additional decline of 0.3 dB/m, 1.3 dB/m and 231

2.5 dB/m, respectively for 12-14 kHz, 25-28 kHz and 35-41 kHz, respectively. For a source-232

distance of 6.5 m, this amounted to 2.0 dB, 8.5 dB and 16.3 dB for the same frequencies, 233

respectively. The increasing attenuation with increasing frequency explains the sound level 234

values of Table 1 at 6.5 m distance for the higher frequencies. 235

236

Since the dimensions of the Kemo device were larger than the wavelengths of the 3 frequency 237

settings for the frequencies under test, their radiation pattern was not isotropic. The sound was 238

emitted most efficiently (up to 25 dB stronger) in the direction along the speaker membrane 239

axis. The directivity of the source for the setting 5, with emission in the 12-14 kHz range, is 240

shown in Figure 2. Most of the sound energy was radiated within a solid angle of 60°-80°. 241

During the exposure tests, the device membrane pointed towards the test person (00 in Figure 242

2). 243

244

13

245

Fig. 2. Dependence of the relative sound pressure level in dB (measured at 5m from the Kemo 246

device), with respect to the level in the direction along the connecting line between the source 247

and the measurement location (00: front of the source pointed to the measurement location) for 248

setting 5: 12-14 kHz. 249

250

Sound pressure levels were also measured with a microphone of a Head Measurement System 251

(HMS, Head Acoustics) dummy head at about 1 cm of the opening of the ear canal, in order 252

to assess the acoustical head shadow effect. The (symmetrical) pinna and orientation of the 253

head with respect to the connecting line between the head and the source influence the 254

effective sound level. The artificial head was rotated in the horizontal equator plane of the 255

source. A distinct increase of the level was determined when the microphone at the ear canal 256

was positioned in the direction of the source, compared to the level for the diametrical position, 257

on the other side of the artificial head. As expected, diffraction around the head declined, i.e. 258

14

acoustical shadowing increased, with increasing frequency: the front-back ratio was about 259

10 dB at setting 5 and 20 dB at setting 8. In experiment 2 the forehead of the test persons was 260

pointed towards the source, with the left- and right ear at 90° and 270°, respectively. 261

262

We also noticed that, with the microphone being positioned at 1-5 cm from the artificial head, 263

interference effects occurred due to the coherent, tonal character of the sound. Subtle details in 264

head orientation and distance between the microphone and the head determine whether the 265

combination of incoming waves and sound waves reflected off the head-surface, resulted in 266

full or partially destructive or constructive interference. Together with interference, due to 267

reflections from the floor and (in some cases) the presence of two sources, this induced 268

variations amounted up to 10 dB for positional changes of the order of magnitude of the 269

acoustic wavelength (±1 cm). 270

271

F. Reflections in real-life conditions 272

Besides reaching a person via the shortest possible route (direct sound wave component), the 273

sound of a source also reaches the person through reflections by objects in the surroundings. 274

In a reverberant space, this can lead to substantial enhancement of the sound pressure level(s). 275

If the Kemo device is placed outdoors, the reflection of the ground may might change the sound 276

pressure level. In case of a concrete floor, the reflection coefficient approaches 100%. In order 277

to assess the effect of reflections in a garden setting, we also determined the high frequency 278

sound reflection coefficient of a soil covered by grass. A setup was built in which broadband 279

sound (5-50 kHz) was generated by a homemade spark source, reaching the microphone 280

directly and via reflection off a slab of soil covered by grass. The spark source and microphone 281

were placed at about 13 cm above the soil covered by grass and about 30 cm from each other. 282

By analyzing the spectrum of the reflected and direct wave packet and by taking into account 283

15

the travelled distance, the reflection coefficient of the soil covered by grass was about 25% 284

across the whole frequency range of interest (12 kHz-41 kHz). The potential increase in sound 285

pressure level due to reflection off a grass surface was therefore limited to a maximum 286

amplification of 20log10(1+0.25) = 2 dB (potential increase in case of constructive 287

interference, and neglecting the path length difference between the direct wave and the 288

reflected wave). In the case of a hard concrete soil, the ultimate increase would be between 289

20log10 (1+1) = 6 dB (in the case of coherent sound), and 10log10 (1+1) = 3 dB (in the case of 290

incoherent sound). It is also possible that a (partial) cancellation occurs. 291

292

III. EXPERIMENT 2: Perception of VHFS/US 293

Potentially adverse factors due to short-term exposure to the VHFS/US produced by the 294

ultrasonic repellent from KEMO, type M175-02-18 were investigated perceptually for the two 295

different loudspeaker settings (source 1, and with the external loudspeaker, source 1+2). The 296

addition of the external loudspeaker resulted in an increase of 3 dB (Table 1). This corresponds 297

to the change of level when spatially averaging the acoustic energy over sites of constructive 298

(+6 dB) and destructive (cancellation) interference (separated ½ wavelength apart in the order 299

of a couple of mm). The two sources were mounted at a height of 2.40 m on two pillars in the 300

corners of a semi-anechoic room, with their front surface pointing towards the listener. The 301

distance between the two pillars was 4.6 m. Both sources were positioned at 6.5 m from the 302

subject, in accordance with the specification sheet. The sound pressure levels were measured 303

and cross-validated with the other measurements (Experiment 1) and found to be consistent 304

within ± 2 dB. The participants were seated behind a table, while the test leader controlled the 305

conditions outside of the room. Three frequencies * 2 sources + 2 dummy conditions were 306

presented in random order. 307

16

308

A.High frequency audiometry 309

Hearing thresholds were determined separately of the left and right ear for frequencies between 310

1 kHz and 16 kHz to ensure normal hearing. This was done using a portable high frequency 311

audiometer (Orbiter 922 version 2, Madsen Electronics) and a Sennheiser HDA200 312

circumaural headphone. The equipment was calibrated according to the manufacturer’s 313

recommendations and ISO 389-5 (2006). 314

315

B. Custom-made Survey 316

A 15-item survey was designed on the basis of the most likely complaints reported by persons 317

and/or by the literature (Acton and Carson, 1967; Lawton, 2001; Maccà et al., 2014). 318

Participants responded to yes/no or open questions and they rated their level of agreement or 319

disagreement on a symmetric 5-point agree (=1) to disagree (=5) scale for a series of 320

statements. This Likert-type rating method is one of the most widely used approaches to scale 321

responses (Burn and Burns, 2008). Prior to testing the participants replied to a few general 322

questions concerning gender, date of birth, and medical history with regard to hearing and 323

vestibular function. Moreover, they had to indicate whether they had experience with ultrasonic 324

repellents (yes/no). Subsequently, they responded to items 1 and 2 of the survey concerning 325

sensitivity to loud and high sounds (see appendix). 326

After exposure to each of the 8 conditions, the participants were requested to respond to items 327

3-15 of the survey. These items referred to audibility of the exposure (yes/no), experience of 328

the sound (nice, neutral, unpleasant, very unpleasant), acclimatization to the sound? (yes - I 329

don’t know- no), and description of the sound. Subsequently, the subject responded to items 330

4-15 on a Likert scale. 331

17

332

C. Participants 333

Participants included young adults, between 18 and 25 yrs old (12 male, 13 female), and 334

middle-aged persons (between 46 and 58 yrs, 11 male, 14 female). They were recruited through 335

personal contacts, flyers, the Federal Public service Health intranet and Facebook page. In 336

order to qualify for participation their hearing thresholds had to be < 25 dB HL on octave 337

frequencies between 125 Hz and 8000 Hz. Approximately two-thirds of the initially screened 338

middle-aged persons did not qualify for further testing. We expected persons with excellent 339

hearing thresholds to be most sensitive to the VHF/US sound emitted by a repellent device. In 340

general, young persons have better hearing than older ones (ISO 7029, 2000). However, to 341

avoid hearing loss as a confounding factor, we intentionally recruited persons with good 342

hearing acuity in both age groups. 343

Participants signed an informed consent. They were paid 30€ for approximately 4 hours in 344

total. Data were analyzed anonymously. 345

346

D. Protocol 347

The protocol consisted of 3 parts: pre-exposure, exposure and post exposure. 348

Pre-exposure: Thresholds for high frequency sinusoids were obtained before commencing the 349

perceptual experiment, and participants responded to items 4-14 of the survey in order to have 350

a baseline. 351

Exposure: the subjects were exposed to 8 experimental conditions in random order: 3 352

frequencies x 2 loudspeaker settings + 2 dummy conditions (= no signal). Each condition lasted 353

20 minutes. During exposure the subject was allowed to read or use a tablet. 354

18

Post exposure: Immediately after each 20-minute exposure, each participant was required to 355

respond to items 3-15-of the survey. Pure tone thresholds were only determined before 356

exposure, mainly because the step size/error of the audiometer is in the order of 5 dB. 357

Moreover, a pilot test with 3 participants showed no significant pre-post difference in hearing 358

sensitivity. 359

360

E. Audibility of high frequency sounds 361

Figure 3a displays the average threshold values in the normal frequency range for young and 362

middle-aged participants, separately. Although the threshold values were significantly lower 363

for the middle-age participants than for the younger ones (t-test, p < 0.001), they were within 364

the ISO norm of normal hearing (between 0 and 20 dB HL). 365

366

Fig. 3. Left: Average hearing thresholds of the young and middle-age participants in the 367

normal/speech frequency range between 1 and 8 kHz. Right: Average hearing thresholds of the 368

younger and middle-aged participants for 10 kHz, 12.5 kHz, and 16 kHz. Data are given in 369

terms of dB SPL. 370

371

19

372

Average threshold values in the high frequency region (> 10 kHz) are plotted in Figure 3b in 373

terms of dB SPL (not dB HL) in order to compare with the sound pressure levels listed in Table 374

1 and with the literature. In accordance with data of Rodriguez et al. (2014) the younger 375

participants could perceive the pure tone clearly at 16 kHz (60 dB SPL). However, the middle-376

aged participants required, on average, at least 100 dB SPL to ‘just’ perceive this frequency. 377

Moreover, ten participants could not perceive this frequency at all. 378

379

380

F. Discrepancy between threshold values and self-report from the survey 381 382

Unexpectedly, an audible sound component was perceived by all the young persons and by 383

63% of the middle-aged participants (item 3 of survey, reported in Table 2). This was the case 384

even when the (in principle) inaudible ultrasound in setting 8 (25-28 kHz frequency range) was 385

emitted through two sources (approximately 65 dB SPL, Table 1). With only 1 source active, 386

approximately 30% of the young and middle-aged participants reported that they heard a signal. 387

At the highest frequency, nearly 50% of both age groups reported that they perceived a signal 388

when two sound sources were on. Most persons, including those experimenters who perceived 389

it, described this signal as a high frequency varying chirp (‘cricket’), more faint with 1 source 390

than with 2 sources, but still ‘distressing’ or ‘very distressing’. In the middle-frequency range 391

the sound was described as a faint intermittent chirp sound, more neutral and only disturbing 392

for four young participants and five middle-aged participants with two sources. With 1 source 393

on, the sound was described as neutral. In the high frequency region those who reported hearing 394

the signal (= mostly with 2 sources) described the sound as a neutral soft buzzing sound, only 395

20

disturbing to 1 person in the middle-aged group. Given that 65 dB SPL at about 25-28 kHz is 396

below hearing threshold, we suspect that the test persons did not hear the 25-28 kHz sound 397

component, but the faint signal component in the 4 kHz-5 kHz range that was discussed before. 398

The level of this faint sound is within the audible range of hearing for most young and several 399

middle-aged persons. 400

401

Table 2. Percentage of participants who reported that they had heard a signal (item 3 of survey). 402

Nominal frequency range

/setting

18-25 yrs

Source 1

18-25 yrs

Source 1 + 2

46-58 yrs

Source 1

46-58 yrs

Source 1 + 2

12-14 kHz /setting 5 100 100 100 92

25 -28 kHz /setting 8 24 100 36 63

35- 41 kHz /setting 9 8 48 12 48

No signal/dummy 4 4 0 20

403

G. Potentially undesirable effects 404

The main aim of this study was to determine possible undesirable effects after exposure to high 405

frequency (ultra) sound produced by the product under investigation. Tables 3 and 4 present 406

the average Likert scores (and standard deviations) for the 12 items (4-15) for 25 young and 407

middle-aged participants, respectively. Values are listed separately for the pre-exposure 408

condition, the 3 frequencies x 1 or 2 sources and 2 dummy conditions. Since most values were 409

below 2, it was concluded that, on average, very little to no undesirable effects were 410

experienced in the different conditions. Paired t-tests were performed between the pre- and post 411

scores for items 4-15, for each of the 8 testing conditions separately, and for the young and 412

middle-aged persons separately. After correcting for multiple comparisons (using the false 413

21

discovery rate correction) only 2 items appeared significant (p<0.01) at the lowest frequency 414

setting (12-14 kHz) with 2 sources, and only with the young adult participants. These two are 415

marked by an asterisk in Table 3 and refer to item 5 (‘I have a headache’), and item 7 (‘I have 416

pain in my ears’). The younger, not the older, participants also reported that they felt that they 417

could not concentrate well during exposure of the lowest frequency, with 1 and 2 sources (item 418

15 of survey). 419

Three middle-aged participants specifically volunteered to participate because of undesirable 420

effects during exposure of an ultrasonic repellent in their home environment (neighboring 421

property). Their responses did not differ from the average data. 422

423

Table 3. Average Likert score of 25 young participants, per item, with standard deviation 424

between brackets. This is for the pre exposure condition (PRE), 3 exposure frequencies, with 425

either 1 or 2 sources (src(s)), and the two dummy (DUM) conditions. The Likert score varies 426

between 1 (= absolutely not) and 5 (very severe). Items significant from pre-exposure condition 427

are marked in bold. 428

PRE 12-14

kHz

1 src

25-28

kHz

1 src

35-41

kHz

1 src

12-14

kHz

2 srcs

25-28

kHz

2 srcs

35-41

kHz

2 srcs

DUM DUM

I feel nauseous 1.0(0.2) 1.2(0.5) 1.1(0.3) 1.0(0) 1.1(0.3) 1.0(0.2) 1.0(0.2) 1.1(0.3) 1.1(0.3)

I have a headache 1.2(0.5) 1.7 (1.0) 1.3(0.7) 1.3(0.7) 1.5(0.8) 1.3(0.7) 1.2(0.5) 1.3(0.7) 1.4(0.8)

I am dizzy 1.0(0) 1.1 (0.3) 1.0(0.2) 1.0(0.2) 1.0(0) 1.0 (0) 1.0 (0) 1.0 (0) 1.0 (0)

Pressing feeling ears 1.2(0.4) 1.4(0.6) 1.2(0.4) 1.1(0.3) 1.4(0.8 1.2(0.4) 1.2(0.5) 1.1(0.3) 1.2(0.6)

Pain in my ears 1.0(0.2) 1.4 (1.0) 1.2(0.5) 1.1(0.4) 1.5(0.8) 1.1(0.6) 1.1(0.4) 1.2(0.5) 1.2(0.6)

I have tinnitus 1.0(0) 1.5 (0.8) 1.0(0.2) 1.1(0.3) 1.4(0.8) 1.0(0.2) 1.0(0.2) 1.0(0.2) 1.1(0.3)

22

I feel tense 1.4(0.6) 1.1(0.3) 1.0(0) 1.0(0) 1.3(0.7) 1.0(0.2) 1.0(0) 1.0(0) 1.1(0.3)

I feel tired 1.8(0.8) 1.8 (0.7) 1.8(0.8) 1.7(0.7) 1.7(0.8) 1.7(0.7) 1.9(0.7) 1.8(0.6) 1.9(0.7)

I feel warm. 1.6(0.8) 1.4(0.8) 1.4(0.6) 1.3(0.6) 1.4(0.8) 1.4(0.7) 1.5(0.8) 1.4(0.7) 1.4(0.7)

I feel uneasy 1.1(0.2) 1.3 (0.8) 1.0 (0) 1.0(0) 1.0(0.2) 1.0(0) 1.0(0) 1.0(0) 1.0(0.2)

I feel frightened 1.0(0.2) 1.0(0) 1.0(0.0) 1.0(0) 1.0(0.2) 1.0(0) 1.0(0) 1.0(0) 1.0 (0)

Could not concentrate 2.3(1.1 1.2(0.4) 1.1(0.3 2.5(1.3) 1.4(0.8 1.4(0.7) 1.250.5) 1.4 (0.8)

429

430

Table 4. Average Likert score of 25 middle-age participants, per item, with standard deviation 431

between brackets. This is for the pre exposure condition (PRE), 3 exposure frequencies, with 432

either 1 or 2 sources (src(s)), and the two dummy (DUM) conditions. The Likert score varies 433

between 1 (= absolutely not) and 5 (very severe). 434

PRE 12-14

kHz

1 src

25-28

kHz

1 src

35-41

kHz

1 src

12-14

kHz

2 srcs

25-28

kHz

2 srcs

35-41

kHz

2 srcs

DUM DUM

I feel nauseous 1.0(0.2) 1.2(0.4) 1.0(0.2) 1.1(0.4) 1.1(0.4) 1.0(0) 1.0(0) 1.0(0) 1.0(0.2)

I have a headache 1.2(0.5) 1.3(0.6) 1.1(0.5) 1.1(0.4) 1.3(0.7) 1.1(0.3) 1.1(0.3) 1.2(0.5) 1.0(0.2)

I am dizzy 1.0(0.2) 1.0(0) 1.0(0.2) 1.0(0) 1.1(0.3) 1.0(0) 1.0(0) 1.0(0) 1.0(0)

Pressing feeling ears 1.2(0.5) 1.3(0.6) 1.1(0.3) 1.2(0.6) 1.4(0.6) 1.3(0.6 1.2(0.5) 1.2(0.4) 1.0(0)

Pain in my ears 1.0(0.2) 1.2(0.5) 1.0(0.2) 1.0(0.2) 1.0(0) 1.0(0.2) 1.0(0.2) 1.0(0.2) 1.0(0)

I have tinnitus 1.2(0.4) 1.5(0.8) 1.2(0.4) 1.2(0.5) 1.4(0.6) 1.2(0.5) 1.2(0.4) 1.1(0.3) 1.1(0.3)

I feel tense 1.2(0.5) 1.2(0.5) 1.0(0.2) 1.1(0.4) 1.2(0.4) 1.1(0.4) 1.1(0.3) 1.0(0.2) 1.0(0)

I feel tired 1.3(0.7)

1.2(0.5) 1.2(0.4) 1.3(0.5) 1.4(0.6) 1.4(0.7) 1.2(0.4) 1.3(0.4) 1.3(0.5)

23

I feel warm. 1.1(0.3) 1.0(0) 1.0(0) 1.1(0.3) 1.0(0) 1.0(0.2) 1.0(0) 1.0(0) 1.1(0.3)

I feel uneasy 1.1(0.4) 1.4(0.6) 1.0(0) 1.2(0.6) 1.4(0.6) 1.1(0.4) 1.0(0) 1.0(0) 1.0(0)

I feel frightened 1.0(0.2) 1.0(0.2) 1.0(0) 1.0(0) 1.0(0.2) 1.0(0) 1.0(0) 1.0(0) 1.0(0)

Could not concentrate 1.5(0.9) 1.1(0.3) 1.2(0.5) 1.6(1.0) 1.2(0.5) 1.2(0.6) 1.0(0.2) 1.1(0.4)

435

436

IV. DISCUSSION 437

The main component of the sound emitted by the Kemo rodent repellent device is a frequency 438

modulated VHFS/US. However, different from our initial expectations, spectral analyses 439

revealed a faint spectral content at frequencies around 4-5 kHz, in the audible part of the 440

spectrum. This sound was audible to several listeners, especially the younger ones. The sound 441

levels at 6.5 meters from the device ranged from 45 to 67 dB SPL for the 3 investigated device 442

settings (5, 8, 9). Depending on the spatial geometry, the frequency settings, and effects of 443

reflections by the head, the addition of a second loudspeaker led to an increase that ranged 444

between 0 dB and 5 dB. Level differences between different device settings were due to the 445

increasing attenuation at higher frequencies, which is substantial at 6.5 meters. In addition to 446

the distance between the source and the person, sound levels depend on the orientation of the 447

device and of the test person’s head, and on the contribution of the reflection by concrete 448

surface or soil covered by grass to the total sound pressure level. 449

Perceptual evaluation (Experiment 2) showed that 20-minute exposure to the investigated 450

device settings did not lead to hearing damage nor to significant undesirable effects such as 451

headache, tinnitus, nausea in the two different age groups. With settings 8 and 9, however, the 452

presence of the 4-5 kHz signal component that accompanied the ultrasonic 25-28 kHz signal, 453

24

was found disturbing by several persons. These findings suggest that persons should be advised 454

about exposure and that use of these devices must be considered carefully. 455

A. Mounting and sound levels 456

In the current study only three settings of the specific ultrasonic repellent were evaluated, 457

namely setting 5 (12-14 kHz), setting 8 (25-28 kHz) and setting 9 (35-41 kHz). Exposure levels 458

were relatively low, i.e. between 45 and 70 dB SPL. Settings lower than ‘5’ were not included, 459

as their respective frequencies were clearly in the audible range, and expected to be perceived 460

as even more disturbing than the tested ones, especially with two sources on (or with one source 461

at a shorter distance from the subject). While we evaluated short-term exposure and sound 462

levels according to the specification sheet, it is very likely that the lower frequencies and/or 463

higher sound pressure levels are used in domestic devices to repel animals. Some participants 464

claimed that the sound they perceived at the neighbor’s home was much clearer and louder than 465

the sound they perceived during the laboratory tests. Differences between devices and 466

differences in mounting may account for undesirable effects, as was shown in the studies by 467

Ueda and colleagues (2014a, 2014b). Continuous exposure to the ultrasonic sound may affect 468

health on the long-term (Andringa and Lanser, 2013), as hearing is not only damaged by the 469

level, but also by the duration of exposure. Recently, Chopra et al. (2016) reported significant 470

reductions in overall hearing sensitivity in dentists who were exposed to ultrasonic scalers. 471

Exposure levels of the 25-28 kHz sound produced by the scalers ranged between 84 dB and 91 472

dB for a single device and exceeded 130 dB SPL for multiple devices. In addition to an 473

immediate temporary shift in hearing thresholds, some dentists reported mild ear pain or 474

tinnitus. It is possible that rodent repellent devices also pose a potential hazard to the hearing 475

of citizens if they continuously emit sound exceeding 80 dB SPL and/or if more than one is 476

used simultaneously. Moreover, they may present a hazard for children who's parents are 477

25

unaware of the exposure, as is also pointed out in Leighton (2016) and discussed in more detail 478

in the following section. 479

B. Sensitivity to sound: specific demographics within the population 480

In the current study potential adverse effects were investigated for normal hearing young and 481

middle-age adults. One should be cautious with generalizing the above reported findings 482

concerning absence of undesirable effects, as those persons who dislike high-pitched sounds 483

may not have volunteered to participate in Experiment 2 (with the exception of the 3 persons 484

mentioned before). Hearing sensitivity varies from person to person and undesirable effects are 485

not predictable by hearing thresholds alone. For instance, we are aware of clinical significant 486

hyperacusis occurring in approximately 1.75% of the general population (Jastreboff and 487

Jastreboff, 2015). Hyperacusis, an increased sensitivity to (environmental) sound that would 488

not trouble a normal individual, can be devastating for a person (Katzenell & Segal, 2001; 489

Baguley, 2003; Khalfa et al., 2004; Tyler et al., 2014; Paulin et al., 2016). Persons with 490

hyperacusis experience an enhanced awareness and discomfort to sound with strong emotional 491

responses such as fear and pain. Basner et al. (2014) presented a review of observational and 492

experimental studies showing that noise exposure leads to annoyance. Increased environmental 493

noises, including those produced by repellent devices, can severely affect the health (e.g. sleep, 494

concentration, emotion) of many persons in daily life (Stansfeld and Matheson, 2003; Baliatsas 495

et al., 2016). Individuals with high noise sensitivity are more likely to experience physical 496

and/or mental diseases. As these issues are not negligible, it is important to gain an in-depth 497

understanding of how noise, including VHFs and US, affects public health (Park et al., 2017). 498

Children form another important demographic subset within the population. Young children 499

are very sensitive to very high frequency sounds (Lee et al., 2012; Beahan et al., 2012), and are 500

expected to experience unpleasant effects at relatively modest sensation levels (Lawton, 2013). 501

26

Hearing sensitivity declines after the age of 20 (Groh et al., 2006), which is the youngest age 502

included in the current study. With increasing air-borne ultrasound devices in public space 503

(Leighton, 2016), including rodent repelling devices, we need to be aware that children can 504

hear more sources than adults. 505

C. Future directions 506

The current study does not allow solid conclusions to be drawn regarding the safety of exposure 507

of very high frequencies and ultrasound produced by repellent devices. However, for the 508

investigated device, it did show that the emitted signal contains certain components that are 509

audible to young adults and middle-aged persons, and suggests that audible emissions are too 510

loud and annoying for prolonged continuous exposure in children and/or persons who are 511

susceptible to noise. The review by Leighton (2016) argues that the current state of knowledge 512

is inadequate to make assessments concerning the safety of such exposures, and/or with regard 513

to the source of the complaints put forward by citizens. Research should carefully consider the 514

acoustical characteristics of the different devices producing very high frequencies and 515

ultrasound, the exposure levels and duration of exposure in daily life. Evidence of non-auditory 516

effects on environmental noise exposure on public health is growing, but the current 517

understanding is not substantial enough to draw conclusions regarding the psychological and 518

physiological influences of the VHFS/US across the life span. As Leighton (2016, 2017) 519

indicates, this research is needed to update and support existing guidelines (see also Howard et 520

al. (2005) and Leighton et al. (2007). Current guidelines, often based on older studies, are not 521

sufficient to cope with the current mass exposure. Moreover, Leighton (2016) pointed out that 522

existing guidelines are based on one-third octave frequency bands, and commented that this 523

approach may not be adequate for the narrow band sounds produced by many of the ultrasound 524

devices. Leighton (2016) listed 14 requirements that need to be addressed in order to be able 525

to formulate safety guidelines, not only of rodent repellent devices, but also for the increasing 526

27

number of application using airborne ultrasound (Dahl et al., 2014). These include appropriate 527

guidelines for both public and occupational exposure, surveys of the occurrence and properties 528

of devices that expose the public and workers, standardization of procedures for reporting 529

levels very specifically, and measurements following international protocols that take account 530

of the particular features of using ultrasonic equipment (directionality, scattering etc). 531

532

V. CONCLUSIONS 533

This study showed that, unlike the manufacturer’s device specifications, the device generates 534

sound components during some settings that are audible to some people. When the signal was 535

perceived – and this occurred more frequently with the two lower frequency settings and with 536

2 sources – it was considered to be disturbing by several participants. Acoustical measurements 537

suggest that for high frequency settings of the investigated design, sound was audible due to 538

the presence of a faint but audible 4-5 kHz component in addition to the high frequency 539

components. Given the wide usage of rodent repellents, positioned in (domestic) gardens 540

frequented by children and pets, we cannot conclude that exposure to ultrasound is less 541

hazardous than audible sound until more research has been done. 542

543

ACKNOWLEDGEMENTS 544

We thank Marina Lukovnikova and Isabel Van Coppenolle of the FOD (Belgium) for their 545

comments and suggestions throughout the research. The authors are also grateful to Geert 546

Dierckx, for carrying out the microphone measurements, and to Sandra Delbeek, Rosanne De 547

Jongh, Christopher Häggblom, and Anne-Sophie Ooghe en for carrying out the listening tests. 548

549

28

REFERENCES 550

Acton, W.I., and Carson, M.B. (1967). “Auditory and subjective effects of airborne noise from 551

industrial ultrasonic sources,” Br J Ind Med. 24:297–304. 552

Ahmadi, F., McLoughlin, I.V., Chauhan, S., and ter Haar, G. (2012). “Bio-effects and safety 553

of low-intensity, low frequency ultrasonic exposure”, Prog Biophys Mol Biol. 108, vol 554

3, 119-138. 555

AIB-Vinçotte (2011). “Karakterisatie van (ultra) geluidsemissies van ultrasone 556

dierenverdrijvers,” Meetverslag, in opdracht van FOD volksgezondheid, voedselketen 557

en leefmilieu, DG5/RB/ML/11031. Dutch only. 558

Aflitto, N., and DeGomez T. (2014). “Sonic pest repellents,” The university of Arizona, 559

College of Agriculture and Life Sciences,” https://extension.arizona.edu/pubs/sonic-560

pest-repellents. 561

Andringa, T.C., and Lanser, J.J. (2013). “How pleasant sounds promote and annoying sounds 562

impede health: a cognitive approach,” Int J. Environ. Res. Public Health 10, 1439-1461. 563

Baguley D.M. (2003). “Hyperacusis,” J R Soc Med. 96(12):582-5. 564

Baliatsas C., van Kamp I., Swart W., Hooiveld M., and Yzermans J. (2016). “Noise sensitivity: 565

Symptoms, health status, illness behavior and co-occurring environmental 566

sensitivities,” Environ Res. 150:8-13. 567

Basner M., Babisch W., Davis A., Brink M., Clark C., Janssen S., and Stansfeld S. (2014). 568

“Auditory and non-auditory effects of noise on health,” Lancet. 383(9925):1325-32. 569

Burns, A., Burns, R. (2008). Basic Marketing Research (Second ed.). New Jersey: Pearson 570

Education, p. 45. 571

29

Chopra A., Thomas B.S., Mohan K., and Sivaraman K. (2016). “Auditory and Non auditory 572

Effects of Ultrasonic Scaler Use and Its Role in the Development of Permanent Hearing 573

Loss,” Oral Health Prev Dent. 14(6): 493-500. 574

Dahl T., Ealo J.L., Bang H.J., Holm S., and Khuri-Yakub P. (2014). “Applications of airborne 575

ultrasound in human-computer interaction,” Ultrasonics. 4(7):1912-21. 576

Durrant, J.D., Lovrinic, J.H. (1984). Bases of Hearing Science, 2nd edition, Williams and 577

Wilkins, 276p. 578

Grzesik, J, Pluta, E. (1986). “Dynamics of high-frequency hearing loss of operators of 579

industrial ultrasonic devices,” Int Arch Occup Environ Health 57: 137-142. 580

Harris, C.M. (1966). “Absorption of sound in air versus humidity and temperature,” J Acoust 581

Soc Am. 40(1), 148-159. 582

Howard, C.Q., Hansen, C.H., and Zander. A.C. (2005). “Review of current recommendations 583

for airborne ultrasound exposure limits,” Review of Acoustics, Australian Acoustical 584

Society, 341-343. 585

ISO 7029 (2000). Acoustics - Statistical distribution of hearing thresholds as a function of age. 586

(International Organization for Standardization, Geneva). 587

ISO 389-8 (2004). Acoustics—Reference Zero for the Calibration of Audiometric Equipment—588

Part 8: Reference Equivalent Threshold Sound Pressure Levels for Pure Tones and 589

Circumaural Earphones (International Organization for Standardization, Geneva). 590

ISO 389-5 (2006). Acoustics—Reference Zero for the Calibration of Audiometric Equipment. 591

Part 5—Reference Equivalent Threshold Sound Pressure Levels for Pure Tones in the 592

Frequency Range 8 kHz to 16 kHz. (International Organization for Standardization, 593

Geneva). 594

30

Jastreboff P.J., and Jastreboff M.M. (2015). “Decreased sound tolerance: hyperacusis, 595

misophonia, diplacousis, and polyacousis,” Handb Clin Neurol. 129:375-87. 596

Jilek M., Šuta D., and Syka J. (2014). “Reference hearing thresholds in an extended frequency 597

range as a function of age,” J Acoust Soc Am. 136(4):1821-30. 598

Katzenell U., and Segal S. (2001). “Hyperacusis: review and clinical guidelines,” Otol 599

Neurotol. 22(3):321-7 600

Khalfa S., Dubal S., Veuillet E., Perez-Diaz F., Jouvent R., and Collet L. (2002). “Psychometric 601

normalization of a hyperacusis questionnaire,” ORL J Otorhinolaryngol Relat Spec. 602

64(6):436-42. 603

Kino, G.S. (2000). Acoustic Waves: Devices, Imaging, and Analog Signal Processing, 604

Prentice-Hall Signal Processing Series, 688 pages. 605

Lawton B.W. (2013). “Exposure limits for airborne sound of very high frequency and 606

ultrasonic frequency,” ISVR Technical Report No. 334, Univ of Southampton, 607

Southampton, UK, 16 p. 608

Lee J., Dhar S., Abel R., Banakis R., Grolley E., Lee J., Zecker S., and Siegel J. (2012). 609

“Behavioral hearing thresholds between 0.125 and 20 kHz using depth-compensated 610

ear simulator calibration,” Ear Hear. 33(3): 315-29. 611

Leighton, T.G. (2007). “What is ultrasound?,” Prog Biophys Mol Biol., 93, 3–83. 612

Leighton T.G. (2016). “Are some people suffering as a result of increasing mass exposure of 613

the public to ultrasound in air? ,” Proc.R.Soc.A 472:20150624. 614

Leighton, T.G. (2017). “Comment on ‘Are some people suffering as a result of increasing mass 615

exposure of the public to ultrasound in air?,” Proc R. Soc. A 473:20160828. 616

31

Maccà I., Scapellato M.L., Carrieri M., Maso S., Trevisan A., and Bartolucci G.B. (2015). 617

“High-frequency hearing thresholds: effects of age, occupational ultrasound and noise 618

exposure,” Int Arch Occup Environ Health. 88(2):197-211. 619

Park J., Chung S., Lee J., Sung J.H., Cho S.W., and Sim C.S. (2017). “Noise sensitivity, rather 620

than noise level, predicts the non-auditory effects of noise in community samples: a 621

population-based survey,” BMC Public Health. 17(1):315. 622

Paulin J., Andersson L., and Nordin S. (2016). “Characteristics of hyperacusis in the general 623

population,” Noise Health. 18(83):178-84. 624

Rodriguez, Valiente, A., Trinidad, A. Garcia Berrocal, J.R., Gorriz,C, and Ramirez Camacho, 625

R. (2014). “Extended high-frequency (9-20 kHz) audiometry reference thresholds in 626

654 healthy subjects,” Int J Audiol. 53(8), 1-15. 627

Smagowska, B., and Pawlaczyk-Łuszczyńska, M. (2013). “Effects of Ultrasonic Noise on the 628

Human Body—A Bibliographic Review ,” Int J Occup Saf Ergon 19, No. 2, 195–202 629

Stansfeld S.A., and Matheson M.P. (2003). “Noise pollution: non-auditory effects on health,” 630

Br Med Bull. 2003;68:243-57 631

Tyler R.S., Pienkowski M., Roncancio E.R., Jun H.J., Brozoski T., Dauman N., Dauman N., 632

Andersson G., Keiner A.J., Cacace A.T., Martin N., and Moore B.C. (2014). “A review 633

of hyperacusis and future directions: part I. Definitions and manifestations,” Am J 634

Audiol. 23(4):402-19. Review. 635

Ueda, M., Ota, A., and Takahashi, H. (2014a). “Investigation on high-frequency noise in public 636

space,” In INTERNOISE 2014 - 43rd International Congress on Noise Control 637

Engineering: Improving the World Through Noise Control, Australian Acoustical 638

Society. 639

32

Ueda, M., Ota, A., and Takahashi, H. (2014b). “Investigation on high-frequency noise in public 640

space. We tried noise abatement measures for displeasure people,” In Proceedings of 641

Forum Acusticum. (Vol. 2014-January). European Acoustics Association, EAA. 642

643

644

33

645

Appendix 646

15 items of the survey (Experiment 2). Items 1 and 2 were scored as ‘yes/no’ and only 647

administered once, prior to commencing exposure. Items 4-14 were presented prior to testing 648

to have a baseline scores. Items 3-15 were administered after each condition. They were scored 649

on a Likert scale between 1 (absolutely not) to 5 (very severe). 650

Yes No

1 I am sensitive to loud sounds

2a I am sensitive to high sounds

2b I am sensitive to low sounds

3a I could hear a sound

3b If yes, did it sound nice, neutral, unpleasant, very unpleasant?

3c If yes, please describe

3d If yes, did you get used to it?

1 2 3 4 5

4 I feel nauseous

5 I have a headache

6 I am dizzy

7 I have a pressing feeling in my ears

8 I have pain in my ears

9 I have tinnitus

34

10 I feel tense

11 I feel tired

12 I feel warm

13 I feel uneasy

14 I feel frightened

15 During the test I could not concentrate on what I was doing

651

652

35

653

654

655

656

657

658