Page 18 - Fall2020

EXPOSURE TO ULTRASOUND IN AIR
bands (TOBs), set the same maximum permissible levels (MPLs) for 17.8-22.4 kHz (the TOB centered on 20 kHz; Leighton, 2017, 2018). This is a difficult band in which to set MPLs because of the huge spread in hearing thresh- old level (HTL; the lowest detectable sound pressure level [SPL] of a pure tone) across listeners. That spread is 85 dB between the 5th and 95th percentile HTLs at 20 kHz for 20-29 year olds (Rodríguez Valiente et al., 2014). Indeed, Ashihara et al. (2006) recorded pure-tone HTLs at 24 kHz as low as 88 dB SPL (all SPLs in this article are Z-weighted and expressed in dB re 20 μPa). Here, we define three frequency regions for humans delimited by TOB boundaries, specifically, the lower audio frequency range below 11.2 kHz, the very high frequency sonic (VHFS) range from 11.2 to 17.8 kHz, and the ultrasonic range above 17.8 kHz (Leighton, 2017).
After receiving Figure 1 from Leighton, Mrs. Zawatski told him:
“Two school district maintenance men came to help. They had heard of another case of this at a different school in the district — the sound was driving the (younger) teacher crazy but they were unable to hear it. I downloaded the app on their phones, adjusted the factory settings, and they went to work. Within short order, they had climbed onto a ladder and discovered the source of the sound. It was a defective motion sensor for the classroom lights that was supposed to be functioning at 40 kHz. They removed the motion sensor and problem solved. The students could immediately tell the difference (of course) and they were so happy and relieved.”
The Context of This Case Study
Because the SPLs displayed by smartphone apps are not reli- able, it was impossible for Leighton to compare the recorded level in the WAV file used for Figure 1 with the recom- mended MPLs for this public exposure published by the International(includingtheUnitedStates)Commissionon Non-Ionizing Radiation Protection (ICNIRP; INIRC-IRPA, 1984). For tonal outputs in the range from 17.8 to 22.4 kHz (20-kHz TOB), the MPL is 70 dB SPL. Public exposures should never be authorized by comparison with MPLs for occupational exposures where knowledge of the age, preex- isting conditions, hearing protection and health degradation of the subject, and duration and location of the exposures, allow supervision. The ICNIRP’s public exposure MPLs allow no amplitude/duration trade-off.
It would have been illuminating to discover whether the SPL of the sound recorded in Figure 1 exceeded the 70 dB MPL. If the SPL of this motion sensor exceeded 70 dB, it would indicate a need for greater vigilance in the signal levels that commercial equipment can generate in classrooms. However, if the SPL was less than 70 dB, it would indicate that these interim MPLs are inadequate to protect young people. This may be the case, given that the ICNIRP’s public MPLs were based on subtracting 30 dB from occupational MPLs that, in turn, were based on tests of a small number of adults. If 70 dB had indeed been causing an adverse reaction in Mrs. Zawalski’s class- room, it may appear surprising that such low levels might generate headaches. However, if one can hear ultrasonic frequencies, the usable dynamic range (between what is perceptible and what causes adverse effects) is likely to be unexpectedly small in many individuals (Leighton, 2017).
What Is Known About Ultrasonic Effects on Humans?
Past questions of adverse effects from airborne ultrasound had focused on measurable changes in HTLs as a result of occupational exposures, either as temporary or permanent hearing threshold shifts (TTSs and PTSs, respectively). However, this article is concerned with public exposures where, to date, the SPLs have tended to be low enough that TTS and PTS are unlikely. Consequently, our focus is on adverse effects provoked at these lower SPLs, including (but not limited to) discomfort, failure to concentrate or perform tasks, tinnitus, nausea, dizziness, and a feeling of pressure in the ear. Unlike PTS, these symptoms do not necessarily indicate hearing damage but may arise from brain responses to an audible stimulus.
However, persistent exposure could conceivably cause similar health effects (e.g., sleep deprivation and cardiovascular disorders) to those associated with envi- ronmental noise exposure at lower audio frequencies (Murphy, 2017). Most claims of adverse effects of air- borne VHFS and ultrasound on humans are anecdotal. This is because controlled experiments and epidemio- logical studies of chronic exposure are rarely funded, and when they are, they are difficult to conduct. For decades, ultrasonic pest deterrents and cleaning baths exposed humans to ultrasound in the air, and a plethora of national and international guidelines for MPLs for occupational exposure was produced (Leighton, 2016a). These tended to recommend SPLs of 110 dB for the TOBs
18 Acoustics Today • Fall 2020