Page 37 - Volume 9, Issue 3

Fig. 5. Low frequency hearing thresholds.
Fig. 6. Action of the ear. Adapted from (Maroonroge, Emanuel et al. 2009)
sions and the cochlea is continuously exposed to infrasound from heartbeat and breathing at similar, and lower, frequen- cies to wind turbine rotational infrasound.
Traboulsi and Avon detected pressure peaks in the ear at 0.2Hz from breathing and at 1Hz from heartbeat.(Traboulsi and Avon 2007). Recent work has measured the transfer function between pulsations occurring in the carotid artery in the neck and the consequent pressure detected in the ear canal, when it was occluded by a microphone (Furihata and Yamashiti 2013). A 600 second analysis of the pressure detected in the ear canal is shown in Fig. 7, where the heart rate is close to 60bpm (1Hz) and two harmonics are shown . Below 1Hz there is infrasound from breathing and other body processes. The pressure in the small volume of the occluded ear canal was 95 -100dB, corresponding to an aver- age ear drum displacement of nearly 0.1μm.
The pressures produced in the inner ear fluid by exter- nal infrasound and internally generated infrasound, can be compared by ear drum displacements, allowing for forward gain and reverse losses. The reverse losses in the ear are greater than the forward gain, possibly 20-40dB greater (Hudde and Engel 1998, Puria 2003, Cheng , Harrington et al. 2011). By considering these, it was concluded that the levels of infrasound in the inner ear from internal body sources are greater than those from external infrasound from wind turbines (Leventhall 2013). The body, and vestibular systems, have developed to avoid disturbance from the high levels of infrasound which are produced internally from the heartbeat and other processes. In fact, the hearing mechanisms and the balance mechanisms, although in close proximity, have evolved to minimise interaction. (Carey and Amin 2006).
Assessment
Internally generated infrasound from heartbeat and breathing, which enters the inner ear via the cochlear aque- duct, is greater than that received externally from wind tur- bines at similar frequencies, perhaps by 20dB or more. Levels of infrasound received from wind turbines at typical residen- tial distances are well below hearing threshold and also main- ly below the outer hair cell threshold, proposed by Salt and Hullar as a possible onset level of adverse effects. There is no evidence that this wind turbine infrasound is harmful, whilst there is evidence from atmospheric infrasound that it is not. For example, microbaroms at around 0.2Hz may be of high- er level than wind turbine infrasound at that frequency. Microbaroms have been measured at a power spectral densi- ty of 120dB at 0.2Hz (Shams, Zuckerwar et al. 2013).
Certainty is never 100%, especially when biological dif- ferences are involved, but all indications are that the Wind Turbine Syndrome is based on fallacious reasoning and that inaudible infrasound from wind turbines is not a problem for residents.
However, some people are convinced that they are harmed by infrasound from wind turbines, but this appears to be because they have been told, repeatedly, in publicity opposing wind turbines, that harm will occur. Frequent rep- etition of an incorrect fact does not make it correct although,
Fig. 7. Spectrum of infrasonic pressure in the occluded ear canal.
the cochlea to the cerebrospinal fluid, permitting bidirec- tional flow of fluid and allowing pressure equalisation of the cochlea, Fig. 6. The cochlear aqueduct offers a high resistance to high frequencies, but passes the low frequency pressure pulses from the cerebrospinal fluid into the fluid of the inner ear (Traboulsi and Avon 2007). This effect is strong enough to drive the ear in reverse so that infrasound pulses generat- ed by heartbeat and breathing, which enter the cerebrospinal fluid and transmit to the inner ear via the cochlear aqueduct, may be detected with a microphone in the ear canal. The detection has similarities to detection of otoacoustic emis-
36 Acoustics Today, July 2013