The Ear Detects Infrasound at Levels that are not Heard.

In humans an infrasonic tone at 5 Hz must be presented at approximately 109 dB SPL in order to be heard. In guinea pigs, measured hearing sensitivity to low frequency tones indicates their ears are approximately 10 dB LESS sensitive than the human, requiring 10 dB higher sound levels. It can therefore be inferred that for a guinea pig, a 5 Hz tone would need to be presented at approximately 119 dB SPL in order to be heard.

The graph below shows the amplitude of cochlear responses to 5 Hz tones in the guinea pig. These responses were recorded from an electrode inserted into scala media of the third cochlear turn. Responses were band-pass filtered around 5 Hz and in each case the responses to 20 stimulus presentations were averaged to reduce background noise. Using this methodology, it was found that consistent 5 Hz cochlear microphonic responses were recorded with sound levels down to 55 dB SPL, which is OVER 60 dB below the level that is heard by the guinea pig. This demonstrates that the mammalian inner ear responds well to infrasounds, even at levels well below those that are heard.

The explanation for this difference is provided by the fact that the cochlear microphonic responses are generated by the outer hair cells (OHC) of the cochlea while hearing is mediated by the inner hair cells (IHC) of the cochlea. The sensory hairs of the OHC are embedded in the gelatinous tectorial membrane so they respond to basilar membrane displacements. In contrast, the sensory hairs of the IHC do not contact the tectorial membrane but remain in the fluid filled subtectorial space. As a result, the two types of cell respond differently to low frequency stimuli

While the OHC respond to displacement, the IHC respond to the velocity of the stimulus. This is illustrated below for 3 stimuli of different frequency at the same presentation level. The OHC would be stimulated equally by the 3 stimuli of equal displacement, while the IHC would be stimulated less by the the lower frequency stimulus, because the maximum velocity of the the stimulus is reduced as frequency decreases. For this reason, the IHC (and hearing) becomes less sensitive at lower frequencies, while OHC responses such as the cochlear microphonic are better maintained.

The figure below shows low frequency sound levels generated by wind turbines (blue, cyan) compared with human hearing sensitivity (purple) and also the sound level which elicits cochlear responses in guinea pigs. Human cochlear responses would be expected to occur with sounds at similar levels or lower than those in the guinea pig.

It can be concluded that the ear responds to infrasonic sound stimuli at levels well below those that are heard. The infrasonic sounds generated by wind turbines are sufficient to stimulate the ear and elicit physiologic responses.