Cochlear Fluids Research Laboratory, Washington University in St. Louis
Alec Salt Ph.D., Revised August 30th, 2010
Our recently-published paper reviews well-established publications about low frequency hearing by leading scientists in the field of auditory physiology.
It concludes that low frequency sounds that you cannot hear DO affect the inner ear. The commonly held belief that “if you can't hear it, it can't affect you” is incorrect.
The paper shows how the outer hair cells of the cochlea are stimulated by very low frequency sounds at up to 40 dB below the level that is heard.
It shows that there are many possible ways that low frequency sounds may influence the ear at levels that are totally unrelated to hearing sensitivity.
As some structures of the ear respond to low frequency sound at levels below those that are heard, the practice of A-weighting sound measurements grossly underestimates the possible influence of these sounds on the ear. Studies that focus on measurements in the “audio frequency range” (i.e. excluding infrasound) will not provide a valid representation of how wind turbine noise affects the ear.
The high infrasound component of wind turbine noise may account for high annoyance ratings, sleep disturbance and reduced quality of life for those living near wind turbines.
Wind turbines are becoming increasingly important to our society, providing a “green” form of energy generation. As a result, the size and the numbers of wind turbines being built are rapidly increasing.
The noise generated by wind turbines has been reported to be substantially more annoying than most forms of transportation noise (airplanes, railways, roads, etc) (Pederson and Persson Wayne, 2004; Pederson and Persson Wayne, 2007; Pedersen et al, 2009). It has also been reported that some people with wind turbines located in the vicinity of their homes are upset by the noise and some have reported a variety of symptoms that only occur within the vicinity of wind turbines ( Pierpont 2009; Nissenbaum, 2010)
The noise generated by wind turbines is rather unusual, containing high levels (over 90 dB SPL) of very low frequency sound (infrasound) as shown in the Figure (Van den Berg 2006; Jung and Cheung 2008).
There has been a widely held view that the infrasound at the levels produced by wind turbines cannot influence the ear because they are below the threshold for human hearing. Our study shows this view is incorrect.
But as a result, most measurements of wind turbine noise are A-weighted (i.e. adjusted according to the sensitivity of human hearing).
According to the British Wind Energy Association, the A-weighted sound level (in which the high infrasound component has been taken out) generated by wind turbines is 35-45 dB SPL. They state that “Outside the nearest houses, which are at least 300 metres away, and more often further, the sound of a wind turbine generating electricity is likely to be about the same level as noise from a flowing stream about 50-100 metres away or the noise of leaves rustling in a gentle breeze. This is similar to the sound level inside a typical living room with a gas fire switched on, or the reading room of a library or in an unoccupied, quiet, air-conditioned office.”
From this description, wind turbines would appear to be incredibly quiet.
So no one would expect emitted sound at this level to be a problem.
This characterization of wind turbine noise totally ignores the high infrasound component of the noise. A-weighting or G-weighting sound measurements are perfectly valid if hearing the sound is the important factor. But, as sensory cells in the ear are stimulated at levels substantially below those that are heard, A-weighted measurements do not adequately reflect the true effect of the sound on the ear.
The research performed in our laboratory covers a number of areas related to inner ear function and the physiology of the cochlear fluids (apparent from the rest of the Cochlear Fluids website). Our group has for years been using infrasonic tones to study how the ear works. These are often described as “biasing tones”, because they allow the structures of the ear to be displaced slowly while measurements are made. For almost 10 years we have been using infrasonic 5 Hz bias tones at levels as low as 85 dB SPL (shown as the green diamond in the graph at the right) to manipulate cochlear responses in guinea pigs. The guinea pig is LESS sensitive to low frequencies than the human, so this makes you realize that low frequency infrasonic sounds ARE AFFECTING THE FUNCTION OF THE EAR at levels well below those that are heard by humans. (shown as blue symbols in the graph). Also shown for comparison (red line) is the calculated sensitivity of the inner hair cells (IHC) of the cochlea – the cells that you hear with.
So, the question remains, how can infrasonic bias tones affect cochlear responses at levels well below those that should be heard by the guinea pig.
The answer is complex and requires an understanding of the physiology of the ear and how it responds to low frequency stimuli. It is the subject of our paper titled:
Responses of the Ear to Low Frequency Sounds, Infrasound and Wind Turbines
Alec N. Salt and Timothy E. Hullar
Some of the points made by our paper include:
The Outer Hair Cells Respond to Infrasound
The estimated outer hair cell sensitivity curve for humans is shown as the brown line in the figure at the left, and compared to the spectrum of wind turbine noise (shown as the blue line and the red line). The outer hair cells are far more sensitive to infrasound than previously appreciated. In addition, the outer hair cells are known to be mechanically motile (these cells contract when you stimulate them). They are the mechanical “amplifiers” of the inner ear and contribute to making your hearing as sensitive as it is. They can be thought of as miniature “muscles” that amplify vibrations for the higher frequencies that you hear. However, another function of these cells may be to mechanically counteract very low frequency, infrasonic vibrations - to help make sure you don't hear them. This would represent a biological form of active noise cancellation. So, these cells are not insensitive to infrasound. Instead, they transduce the signal and then actively cancel it out at the inner hair cell so you don't hear it. So a high infrasonic component in a noise would at best be expected to give the outer hair cells “a darned good workout”. And you wouldn't necessarily be aware of what they were doing, because their role may be to cancel out the sound so you don't hear it. This raises the POSSIBILTY that the dislike / disturbance of individuals by wind turbine noise may be related to the long-term stimulation of the outer hair cells with infrasound.
It cannot yet be concluded that this type of stimulation causes specific symptoms in people. More research needs to be performed in this area. It does, however, suggest that the infrasound component of wind turbine noise should be studied further as a possible cause of people's symptoms, rather than being dismissed as an impossible cause. There is a need to collect more direct evidence from humans. For example, it is possible to reduce the infrasound sensitivity of the ear in humans by placing a tympanostomy tube in the eardrum. The tympanostomy tube provides a tiny perforation so that sound pressure is shunted across the eardum. Because infrasound changes pressure rather slowly it gets equilibrated across the eardrum more easily than high frequency sound, so the low frequencies will no longer stimulate the ear as much (Voss et al, 2001). If the symptoms of patients who were sensitive to wind turbine noise were alleviated by placement of tympanostomy tubes, then this would support the case that the infrasound component of the noise was the source of the problem.
Jung SS, Cheung W. Experimental identification of acoustic emission characteristics of large wind turbines with emphasis on infrasound and low-frequency noise. J Korean Physic Soc 2008; 53:1897-1905.
Nissenbaum 2010 The Society for Wind Vigilance
Pedersen E, van den Berg F, Bakker R, Bouma J. Response to noise from modern wind farms in The Netherlands. J Acoust Soc Am. 2009;126:634-643.
Pedersen E, Waye KP. Perception and annoyance due to wind turbine noise--a dose-response relationship. J Acoust Soc Am. 2004;116:3460-3470.
Pedersen A, Persson Waye K. Wind turbine noise, annoyance and self-reported health and well-being in different living environments. Occup Environ Med 2007;64:480-486.
Voss SE, Rosowski JJ, Merchant SN, Peake WT. Middle-ear function with tympanic-membrane perforations. I. Measurements and mechanisms. J Acoust Soc Am. 2001 ;110:1432-44.