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as being so highly annoying, may not be easily explained by measurements with an A-weighted sound level meter. Rather, the low-frequency and infrasound levels need to be considered as contributing to the perceived phenomenon. Subjectively, the perceived fluctuation from an amplitude modulated sound and from a low-frequency biased sound are identical even though their mechanisms of generation are completely different. For the subject, the summed effects of both types of amplitude modulation will contribute to their perception of modulation. Acousticians therefore need to be aware that the degree of modulation perceived by humans and animals living near wind turbines may exceed that detected by a sound level meter.
2. Endolymphatic Hydrops Induced by Low Frequency Tones
As mentioned above, endolymphatic hydrops is a swelling
of the innermost, membrane bound fluid compartment of the inner ear. Low-frequency tones presented at moderate to moderately-intense levels for just 1.5 to 3 minutes can induce hydrops (Figure 3), tinnitus (ringing in the ears) and changes in auditory potentials and acoustic emissions that are physi- ological hallmarks of endolymphatic hydrops (Salt, 2004, Drexl et al. 2013).
Unlike the hearing loss caused by loud sounds, the symptoms resulting from endolymphatic hydrops are not permanent and can disappear, or at least fluctuate, as the degree of hydrops changes. Return to quiet (as in Figure 3) or relocation away from the low-frequency noise environment allow the hydrops, and the symptoms of hydrops, to resolve. This which would be consistent with the woman’s description of her symptoms given earlier. As hydrops is a mechanical swelling of the membrane-bound endolymphatic space, it affects the most distensible regions first – known to be the cochlear apex and vestibular sacculus. Patients with saccular disturbances typi- cally experience a sensation of subjective vertigo, which would be accompanied by unsteadiness and nausea. As we mentioned above, an ear that has developed endolymphatic
hydrops becomes >20 dB more sensitive to infrasound be- cause the helicotrema becomes partially obstructed (Salt et al. 2009). The possibility of a positive feedback – low-frequency induced hydrops that causes the ear to be more sensitive to
Figure 3 : Brief exposures to low-frequency tones cause endolym- phatic hydrops in animals (Salt, 2004) and tinnitus and acoustic emission changes consistent with endolymphatic hydrops in humans (Drexel et al, 2013). The anatomic pictures at the right show the difference between the normal (upper) and hydropic (lower) cochleae The endolymphatic space (shown blue) is enlarged in the hydropic cochlea, generated surgically in this case.
low frequencies – has to be considered. To date, all studies
of low-frequency tone-induced hydrops have used very short duration (1-2 min) exposures. In humans, this is partly due to ethical concerns about the potential long-term consequences of more prolonged exposures (Drexel et al., 2013). Endolym- phatic hydrops induced by prolonged exposures to moderate levels of low-frequency sound therefore remains a real pos- sibility.
3. Excitation of Outer Hair Cell Afferent Nerve Pathways
Approximately 5-10% of the afferent nerve fibers (which
send signals from the cochlea to the brain - the type II fibers mentioned above) synapse on OHCs. These fibers do not respond well to sounds in the normal acoustic range and they are not considered to be associated with conscious hearing. Excitation of the fibers may generate other percepts, such as feelings of aural fullness or tinnitus. Moreover, it appears that infrasound is the ideal stimulus to excite OHC afferent fibers given what has been learned about these neurons from in vitro recordings (Weisz et al, 2012; Lichtenhan and Salt, 2013). In vivo excitation of OHC afferents has yet to be attempted with infrasound, but comparable fibers in birds have been shown to be highly sensitive to infrasound (Schermuly and Klinke, 1990). OHC afferents innervate cells of the cochlear nucleus that have a role in selective attention and alerting, which
may explain the sleep disturbances that some people living
24 | Acoustics Today | Winter 2014