Page 22 - Fall2020

EXPOSURE TO ULTRASOUND IN AIR
of HTLs (in dB) at 18 kHz is around 3 times that at 1 kHz (Rodriguez Valiente et al., 2014). This variability appears to arise partly from the cochlea and middle ear but also from the sound field in the ear canal that becomes increas- ingly complex at high frequencies and depends critically on the dimensions and shape of the pinna, ear canal, and tympanic membrane (Leighton, 2016a).
Human Adverse Effects
The authors have their own anecdotes of people adversely affected by audible ultrasound (e.g., discomfort, headaches, failure to concentrate, and falling off chairs at the sudden onset of a tone). We also have reports from people who are convinced that they have been affected but are unlikely to have been (e.g., they report that the sound follows them from country to country). Laboratory studies of the symp- toms listed above are further complicated by a reliance on subjective ratings which audio frequency studies indicate are affected by nonacoustic factors such as the context in which sounds are presented and the subject’s mood and attitudes (Miedema and Vos, 1999). Despite these com- plications, laboratory tests are able to reveal differences between responses to ultrasound and responses to lower audio frequency exposure, thereby controlling for non- auditory effects. Such tests have consistently found that audible stimuli become more aversive as their frequency increases into the VHFS and ultrasonic ranges when con- trolling the stimulus level either for ratings of subjective loudness or for sensation level (where sensation level is defined as the level of the stimulus above the individual's HTL). Subjects rated VHFS emissions and ultrasound to be worse than lower audio frequency sounds on scales of unpleasantness, discomfort, annoyance, and distract- edness (Fastl and Zwicker, 2007; Kurakata et al., 2013; Fletcher et al., 2018a). This is consistent with the case study described in The Context of This Case Study.
Moreover, the usable dynamic range (i.e., the difference in SPL between extreme unpleasantness and inaudibil- ity) reduces as frequency increases, so subjects can find
VHFS stimuli extremely unpleasant and annoying even when only at a sensation level of 10 or 20 dB above HTL (Kurakata et al., 2013; Leighton, 2017; Fletcher et al., 2018a; EARS II, 2019). There also appears to be a sub- population of unknown size who find VHFS exposure particularly annoying despite not suffering from hyper- acusis (a low tolerance of everyday loud sounds; Fletcher et al., 2018a; EARS II, 2019).
Why Are Higher Frequency Sounds Aversive?
Why higher frequency sounds are more aversive is unknown. Suggested reasons include their rarity, associations with human or animal vocalizations, or differences in physiology between higher and lower frequency receptive areas in the cochlea. Many commercial sources of airborne ultrasound are tonal. Applying either amplitude or frequency modu- lation to a pure tone at lower audio frequencies is found to increase perceived unpleasantness (Fastl and Zwicker, 2007; Kumar et al., 2008). However, in the VHFS and ultra- sonic ranges, modulation is often perceived differently due to the limited dynamic range available (the difference between audible and tolerable SPLs) and the steep gradient of the HTL-frequency curve, which strongly couples any frequency modulation to modulations of audibility or sub- jective loudness. Hence, in our studies of unpleasantness and discomfort, we used unmodulated pure tones (with durations of at least 0.5 s; Fletcher et al., 2018a).
Symptoms Associated with
Airborne Ultrasound
Other symptoms anecdotally attributed to audible airborne
ultrasonic exposure are even more difficult to study because they appear only after some time following the stimulus onset (Leighton, 2016a). These include nausea, headache, dizziness, ear pain, fatigue, and anxiety. So far, controlled studies (at the limited exposures allowed) have not found that audible ultrasound can reliably provoke these specific symptoms. Those self-diagnosing as symptomatic from previous public exposures had difficulty concentrating and greater annoyance and, like the group who did not self-diag- nose in this way, showed greater discomfort (Fletcher et al., 2018a). For our laboratory testing, we worked within an eth- ical framework that limited human exposures to an 8-hour equivalent continuous SPL of 76 dB per day. This equates to 85 dB per hour. Because this is far lower than some SPLs to which the public can be exposed day-to-day, our labora- tory data of adverse human responses cannot explore the exposures to which some subjects attribute adverse effects.
The symptoms attributed to airborne ultrasound may arise from processes similar to those arising at lower audio frequencies. Epidemiological studies show that chronic environmental noise exposure at lower audio frequencies is a likely cause of several adverse effects in humans, including severe annoyance, sleep disturbance, cardiovascular disease, and impaired cognitive devel- opment in children (WHO, 2011; Murphy, 2017). One
22 Acoustics Today • Fall 2020