Page 21 - Fall2020

Figure 3. The spatial distribution of LZeq,40kHz (the unweighted equivalent continuous sound pressure level in the TOB centered on 40 kHz) plotted in a vertical (a) and a horizontal (b) plane through the focus where the human is meant to interact with an ultrasonic field to produce haptic feedback (such that b is measured at z = 200 mm) at maximum setting. Figure was taken at the Physikalisch-Technische Bundesanstalt (PTB) using a 1⁄8-inch microphone (from Liebler et al., 2019). Reproduced from Leighton et al., 2019, with permission.
increasing more sharply with a frequency above 10 kHz (Ashihara et al., 2006). Testing 18-33 year olds, Ashihara et al. (2006) found some individuals having HTLs around 90 dB SPL at 24 kHz, although this upper frequency was determined partly by limits on the SPL imposed by the equipment or safety regulations. Good hearing at ultra- sonic frequencies can persist into older adulthood. Of those tested, Rodriguez Valiente et al. (2014) found that 5% of 40-49 year olds had better HTLs at 20 kHz than the median 20-29 year old. The increase in HTL with frequency in the VHFS range is due to (1) increasing middle ear attenuation and (2) the fact that the stimulus
frequency begins to exceed the highest natural frequency of the basilar membrane (which is found at its base), leading to a reduction in the size of the receptive region of the cochlea (Yasin and Plack, 2005). Measured VHFS HTLs reflect the sensitivity to sound of the ear rather than of the mechanoreceptors in the skin, which are far less sensitive, particularly above about 1 kHz (Boothroyd and Cawkwell, 1970).
Subharmonics generated by source or propagation are discussed in Table 1. Ultrasonic stimuli at sufficiently high SPLs may also generate subharmonics in the middle ear vibration, leading to perception arising from other regions of the basilar membrane. However, stud- ies employing masking stimuli suggest that at the SPLs considered in this article up to 24 kHz, it is the funda- mental frequency component of ultrasound rather than the subharmonics that is perceived (Ashihara et al., 2006). Consequently, at least up to 24 kHz, excitation of this basal cochlear region is thought to be the most likely origin of the auditory sensations discussed here.
Aging, ototoxic drug exposure, and (possibly) noise expo- sure all appear to cause damage initially at the cochlear base, showing up as increases in HTLs first at very high frequen- cies, then later at lower audio frequency HTLs. HTLs in the ultrasonic range are generally lower in children than in adults (Rodriguez Valiente et al., 2014), and, consequently, adults in decision-making positions may be unaware of problems that only affect children and teenagers.
In addition, the variability in HTLs between individuals increases greatly with frequency. Thus, in the population of healthy young adults, the intersubject standard deviation
Table 2. Maximum permissible levels according to different guidelines for TOBs between 20 and 40 kHz
TOB
OSHA Occupational Level
ICNIRP Occupational Level
ICNIRP
Public Exposure Level
20
105
75
70
25
110
110
100
31.5
115
110
100
40
115
110
100
ICNIRP, International Commission on Non-Ionizing Radiation Protection occupational levels can be increased for shorter exposures, whereas public exposures levels cannot. TOB is in kHz; levels are in dB re 20 μPa. Reproduced from Dolder et al. (2019).
Fall 2020 • Acoustics Today 21