Page 9 - Fall2013

force generated by the
inertia of a mass. The
answer is that both
otoliths measure force
generated by an inertial
mass (Grant and Best,
1987), and that the utric-
ular otolith which meas-
ures primarily horizontal
acceleration responds to
the force generated by an
inertial mass virtually
identically to the way it
would respond when
immersed in acoustic
pressure. As recent
research has shown
(Uzun-Coruhlu et al. 2007), this result occurs because, the utricular otolith is only “hinged” to the temporal bone, whereas the saccular otolith and the classical model for both otoliths has their whole base firmly attached to the temporal bone (Grant and Best). In the classical model, the base con- tains the hair cells which extend into a gel region that sits above the base. Above the gel region is a region filled with calcium carbonate crystals called otoconia, such that this layer has about double the density of the gel layer and the flu- ids that fill the inner ear. So acceleration of this mass creates shear forces in the gel layer and these forces cause the hair cells to generate signals. But in the utricular otolith variations in the outer surface properties will result in localized stretch- ing either as a result of the inertial mass or its being “immersed” in a pressure field.
One can show that the force generated on the otolith is roughly identical between an acoustic pressure of 70 dB at 0.6 Hz and an acceleration of 2 m/s2 at 0.6 Hz. So we have the fol- lowing facts: (1) The otoliths sense acceleration in the region of about 10 Hz and below, (2) Accelerations of about 2 m/s2 generate about the same force on the otoliths as do acoustic pressures of about 70 dB, (3) The utricular otolith senses force in about the same way for both acceleration and pressure and (4) There is evidence of a tuned response to accelerations that relate to seasickness. Therefore we come to two questions. The first question is, if the otoliths sense force generated by accel- eration of a mass virtually identically to how they sense the force generated by acoustic pressures, and the force generated is essentially the same between moderate wind turbine pres- sures and moderate accelerations, how does the ear sense these accelerations if it cannot sense acoustic pressures that create the same force; both in the presence of the same atmospheric noise? The second question is to what extent does signal pro- cessing in the ear and/or in the brain play in the detection of these signals? From the ability of the utricular otolith to sense the force resulting from acceleration in the presence of random acoustic noise, one would expect the same signal processing to detect signals generated by the same otolith in the same way by an acoustic tone in the presence of the same random acoustic noise. Moreover, as Figure 1 shows, current wind turbine fre- quencies are well into the frequency range of what can be
thought of as a tuned filter that affects the prevalence of seasickness.
In terms of the ear/brain signal process- ing capability, the entire inner ear occupies 1 to 2 cubic centimeters and essentially performs a Fourier transform of audio signals, senses linear acceleration in three directions in addition to the acceleration due to gravity, senses angular acceleration on three orthogonal axes, and for
noise immunity the sensing of angular acceleration and hori- zontal acceleration operate push-pull between the two ears. So the assertion that the ear detects signals in the presence of noise with no tuned circuit or “smart” signal processing is a questionable assertion.
The second assertion is that internal body-generated infrasound is in the same frequency range as is the wind tur- bine infrasound, and is higher in level by 20 dB or more. As noted, these signals enter the inner ear right next to the round window and couple only a fraction of their available energies to the full space of the inner ear. And again, the assertion is that it is a “dumb ear” and a “dumb brain”, both using just the levels of the signal (wind-turbine-generated acoustic pressures) and the level of the noise, both without any signal processing. But in this case, unlike the random atmospheric noise case, the body generates the heartbeat and regulates respiration; the brain knows when these signals are occurring. Professor Leventhall's assertion, with absolute certainty, is that the brain doesn't use this information.
Professor Leventhall goes on to assert with absolute cer- tainty that no problems are generated by low frequency sound whatsoever, and rather that all of the public outcries result from people being told that they have a problem. He decries people who have spoken out that there is a problem, but says nothing of the wind developers that have told the public that the sound is “less than a quiet refrigerator.” The point is that for every extreme individual opposed to wind farms there are equal actions by proponents of wind farms. The proponents use what can only be termed “scientific spin” with such slogans as “If you can't hear it, it can't hurt you.” As if, if you can't hear ultrasonic noises, they can't hurt you, or if you can't see infrared or X-rays, they can't hurt you. The sit- uation is rapidly becoming airport-esque. Soon the siting of every new wind farm will be a battle like the siting of a new airport or even just a new runway. The answer is there is truth on all sides. Certainly some people are influenced by what they read and hear and their response is partially or even fully based on non-acoustic factors. But the assertion that every single person world-wide is being affected only by non-acoustic factors in totally unfounded.
The solution can only come from a broad research pro-
Figure 1. The Navy's nauseogenic region (after Kennedy et al., 1987)
8 Acoustics Today, October 2013