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

Exposure levels for parametric arrays in light of guidelineambiguities
Craig N. DOLDER 1; Mark D. FLETCHER1; Sian Lloyd JONES2; Benjamin LINETON1; Timothy G
LEIGHTON1
12

Institute of Sound and Vibration Research, Southampton

Department of Audiology and Hearing Therapy, Royal South Hants Hospital

ABSTRACT
Recent revelations about ambiguities in guidelines for very-high frequency sound and ultrasound require
many everyday devices to be revisited in terms of guideline compliance. One device that has a highultrasound component but is designed for audio broadcast is a parametric array. Acoustic radiation forcommercial parametric arrays was measured at a distance of 3.5 m in an anechoic chamber. The output of thedevice is compared against three international regulations and guidelines for ultrasound exposure. Theauthors found that the device was not compliant under the conditions tested in the laboratory.
Keywords: Ultrasound, Noise Exposure, Guidelines

1. INTRODUCTION
1.1 The rise of ultrasoundRecently, the number of devices that exploit ultrasound in our daily lives have increased (1). The
availability of a wide variety of ultrasonic devices has increased as the cost for such devices have
dropped. Ultrasonic parking sensors have revolutionized the automotive industry and we might soonsee virtual keyboards with haptic feedback provided by ultrasonics (2).
The possible adverse effects that are claimed to be due to ultrasound exposure include annoyance,dizziness, nausea, anxiety, and hearing loss (3-5). The thresholds for onset of these symptoms and thefrequencies to which individuals are sensitive vary widely (6, 7), making the determination ofexposure guidelines difficult. Many of the guidelines that are in existence are based on limited data
and lack a broad participant pool (1).
Despite the large number of devices used in public, there are few reported source levels forultrasonic sources. Recent studies have been seeking to fill that gap (8-11). These studies provide half
the key to safeguarding wellbeing, the other half being studies that determine the threshold of adverse
effects (3, 12, 13). This paper expands on a previous study of ultrasonic deterrents (8) to include
parametric arrays.
1.2 Parametric arrays
A parametric array is a sound source that uses high amplitude non-linear effects to create an
acoustic signal with a very tight beamwidth. The first parametric arrays were underwater sources andhave been heavily studied since the theory was introduced in the 1960s (14). The first known study ofparametric arrays in air (15) was published in 1975, however they are now widely available
commercially. Parametric speakers generally have a very narrow beamwidth, which makes them ideal
for public spaces such as museums or positioned next to advertisements where the owner wants thesignal to be audible to only a subset of an audience in a room.
1.3 Guidelines and ambiguities
Several reviews exist concerning regulations and guidelines intended for ultrasound and audible
1C.N.Dolder@soton.ac.uk

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signals (1, 8, 16). Here guidelines concerning ultrasound in air above the 20 kHz third-octave-band(TOB) will be reviewed briefly. The expected highest levels produced by parametric arrays (all dBwith a reference pressure of 20 μP) are going to be above the 20 kHz TOB. It is important to note,however, that most audible guidelines also apply to ultrasonic signals, since ultrasound extends down
to 17.8 kHz and most guidelines and regulations concerning audible sound extend up to 22.4 kHz. The
Occupational Safety and Health Administration (OSHA) in the USA publishes a set ofrecommendations (17) for ultrasound exposure in the workplace. The exposure level for each thirdoctave band is assessed against a ceiling value (CV) which is defined as the “exposure limit thatshould not be exceeded even instantaneously” (18). This level is not the instantaneous or peak soundpressure level (SPL), but the TOB SPL from a sound level meter set to the slow detection setting,which indicates an averaging over a one second period. These CVs are shown in Table 1. One
ambiguity in these guidelines is that they report they are based upon implies these levels can beincreased by 30 dB if the subject’s head is not in direct contact with the source (for example via water).
Increasing the levels by that amount has been highly criticized (1, 8).
Table 1 – Ceiling values for OSHA and ICNIRP guidelines for occupational (Occ.) and public (Pub.)
exposure.
OSHAICNIRPICNIRP

TOB [kHz]

Occ. [dB]Occ. [dB]

Pub. [dB]20105757025110110100

31.511511010040115

110100

Other guidelines(19) are published by the International Non-Ionizing Radiation Committee of theInternational Radiation Protection Association (ICNIRP). Unlike the OSHA guidelines, these containrecommended CV levels for both occupational and public exposure as summarized in Table 1.

2. METHODOLOGY
The measurements presented here were recorded at the same time as measurements previously
published (8) elsewhere. Recordings were made in the anechoic chamber at the Institute of Sound andVibration Research at the University of Southampton, which was commissioned to have free fieldradiation up to 20 kHz, where it is certified to a 1 dB tolerance. The source was mounted on a pedestal
which was covered with absorptive material to minimize reflections. A microphone was positioned on
axis from the source 3.5 meters away. This is a practical distance at which such a speaker could beused, but a practical application would be in an environment with significant reflections. An anechoicchamber was used to minimize the effect of interference from reflections on the measurements.
A calibrated B&K type 4191 microphone was used in conjunction with a type 2669 preamplifierand a type 2690 Nexus signal conditioner. The Nexus has a built-in 100 kHz low-pass filter. The
analog signal was digitized on a National Instruments USB-6212 16-bit recording system with asampling rate of 250 kHz. The microphone was calibrated between 20 kHz and 50 kHz by the National
Physical Laboratory (NPL). Calibration tones were recorded before and after the measurement session
with an agreement within 0.02 dB.
The parametric array was a generic commercial unit that is expected to be representative of many
commercial devices available. Measurements were made with a broadband input noise on both highand low gain settings.

3. RESULTS
A summary table of the sound pressure levels (SPLs) measured are presented in Table 2. There areinherent ambiguities in the application of an A-weighting and Z-weighting. Here both are applied witha rectangular filter with a high-frequency cut-off at the top of the 20 kHz TOB. The difference in theA-weighted output from the device between the low and high gain settings is 10 dB. Despite that,
there is only 1 dB difference in the pump signal, which is at 40 kHz. A spectrum showing the6360

broadband noise, the modulated signal, and the pump frequency are shown in Fig. 1.
Table 2 – SPL in dB (re: 20 μPa) of the A and Z weighted signal between the 20 Hz and 20 kHz TOBS andthe SPL of four TOBs.
Source Condition

A-SPL

Z-SPL

20 kHz

25 kHz31.5 kHz

40 kHzParam. - H gain

6466

5356

69118

Param. - L gain5459535663117

Figure 1 – Spectral density of microphone recording 3.5 m away from parametric array outputting
broadband noise.

4. CONCLUSIONSThe pump signal recorded exceeds all guidelines discussed for the 40 kHz TOB. The only
possibility for passing the OSHA guideline is if one interprets the guideline such that 30 dB can beadded. The device exceeds the public exposure guidelines by at least 17 dB. The author is unaware of
any legal requirements to prevent this exposure in public spaces and standard SPL meters are notrequired to be sensitive above 20 kHz. If someone suffered an adverse reaction for exposures to theselevels it is unlikely that they would be able to identify the speaker as the source of the problem. Arecent review of ultrasound exposure (1) suggested manufacturers should be held responsible forwhether devices meet international guidelines and should be required to test and publish the results.
More studies need to be conducted to determine the appropriate safe exposure levels.

ACKNOWLEDGEMENTS
The time of T.G.L., P.R.W., B.L., and M.D.F. spent on background to this paper was funded by theColt Foundation, and time on reviewing results was funded by EU funded project EMPIR 15HLT03“Ears II: Metrology for modern hearing assessment and protecting public health from emerging noisesources.”

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