Page 36 - Volume 9, Issue 3

Fig. 4. Narrow band analysis (0.05Hz) of wind turbine noise.
somatosensory phenomenon.
In our study, cortical activation patterns
appeared to be similar for all frequencies applied, suggesting that LFT are processed in a similar way as frequencies of our main hearing range (200 to 5000Hz).
We presented the 12Hz stimuli at three different levels. Tone bursts of 120 and 110 dB resulted in cortical activation. The 90dB stimulus did not induce a significant response of the auditory cortex in group analysis which, in agreement with the findings of Møller and Pedersen (2004), indicates that this SPL is below the estimated perception threshold for 12 Hz. (Møller and Pedersen 2004)
This shows that low frequency tones and infrasound are per- ceived through the normal auditory pathways, the same pathways as for higher frequencies.
Furthermore, sounds, including infrasound, which are below the hearing threshold, do not produce a response in the auditory cortex, as is also the case for sub-threshold high- er frequencies. Whilst the lowest frequency used was 12Hz, the regular slope of the hearing threshold indicates that sim- ilar processes are likely to apply at lower frequencies. For example, Hensel et al showed that a biasing tone at 6Hz, 130dB was detected by the cochlea and that there was no abrupt change in response in the transition from infrasound to low frequency sound (Hensel, Scholz et al. 2007).
The ear is a bi-directional device
The ear operates in both forward and reverse directions. In normal, forward operation, sound waves excite the ear drum, which drives the ossicles to impart vibrations to the cochlear fluid (perilymph) via the oval window. (Fig. 6) These vibrations propagate up and down the cochlea to the pressure release of the round window, causing waves along the basilar membrane and exciting the inner hair cells, which send signals via the auditory nerve to the auditory cortex, where they are interpreted as sound The system is mechani- cal up to the oval window and largely hydrodynamic within the cochlea.
Reverse action of the ear was demonstrated through otoacoustic emissions (OAE) in which “ringing” of the cochlear amplifier, which is based in the outer hair cells, sends vibrations back through the oval window and ossicles to excite the ear drum. Vibrations of the ear drum can then be detected by a microphone in the ear canal (Kemp 2002).
The cochlear aqueduct and internally generated infrasound
The brain produces a fluid (cerebrospinal fluid) which bathes the brain and the spinal cord, providing protection, lubrication and an egress for metabolic wastes. The cere- brospinal fluid, which can be sampled by lumbar punctures, carries infrasonic pressure pulses resulting from heartbeat and breathing. A small duct, the cochlear aqueduct, connects
Concerns About Infrasound from Wind Turbines 35