Alec Salt, a Washington University scientist who studies the inner ear, has discovered that outer ear cells may respond to very low frequency infrasound, well below the frequencies that are audible or otherwise consciously perceptible.  Salt suggests that his discovery may help explain why some individuals seem to be more dramatically affected by low frequency wind turbine noise than would be expected.

The study, funded by the National Institutes of Health’s National Institute on Deafness and other Communicative Disorders, is a literature survey that looks especially at the physiological responses of guinea pigs exposed to infrasound down to 5Hz.  Humans can generally hear sounds as low as 20Hz; sounds below this frequency are called infrasound.  Guinea pigs are often used in lab studies, since their hearing mechanisms are similar to those in humans; in fact, human ears are more sensitive to low frequencies than are guinea pigs.  The crux of his findings center on the ways that hair-like cells in our ears, the Outer Hair Cells (OHCs) and Inner Hair Cells (IHCs), work together to translate sound pressure at various frequencies into the perception of sound in our brain.  For audible frequencies, the OHCs amplify the vibrations they receive from sound waves, triggering hair-like structures on the IECs to ripple and bend; it is this movement of IHCs that create the electrical (neural) impulses that our brain perceives as sound.

The surprise in Salt’s study was that OHCs also react to infrasound.  Rather than, as might be expected, simply not being affected by infrasound, OHCs are “highly sensitive” to it, and respond to very low frequencies by actively working to prevent IHC movements. In fact, the paper finds that “the OHCs could be stimulated at levels up to 40dB below those that stimulate the IHCs.” (i.e. at 40dB below the threshold for hearing; this is crucial, as very low frequencies and infrasound around wind turbines are often 20-40dB below hearing thresholds.) As described in an NIH press release:

“So, while the brain may not hear the sound, the OHC responses to it could influence function of the inner ear and cause unfamiliar sensations in some people. Salt and his colleagues still aren’t sure why some people are sensitive to infrasound and others aren’t. It could be the result of anatomical differences among individual ears, or it could be the result of underlying medical conditions in the ear that cause the OHCs to be ultrasensitive to infrasound.”

While the author suggests that these factors could be in play near wind farms, it’s important to stress that this paper should be considered part of science’s long-term process of gradually learning more about physiological responses to low frequency sounds.  While it does not prove that these effects are occurring in humans near wind farms, it does provide a viable avenue for more study and likewise affirms that we can’t say with assurance that infrasound effects are non-existent. Some community groups are citing this paper as if it’s more concrete than it really is; more to the point would be its role in pointing out that we still have much to learn about physiological responses to sound, especially low-frequency and infrasound that is a bit below the thresholds of conscious perception.

For more, read the press release or the paper itself.