The Bottom Line
The commonly held belief that low frequency sounds you cannot hear cannot affect the inner ear is incorrect. Our paper reviews well-established publications by the leading scientists in the field of auditory physiology and concludes that 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.
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.
But often such advances do not come without a cost.
The noise generated by wind turbines has been reported to be substantially more annoying than most forms of transportation noise (airplanes, railways, roads, etc) (Pedersen and Persson Waye, 2004; Pedersen and Persson Waye, 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)
Wind Turbine Noise
The noise generated by wind turbines is rather unusual, containing high levels (over 90 dB SPL) of very low frequency sound (infrasound) as shown for two studies in the Figure (Van den Berg 2006; Jung and Cheung 2008).
It is 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.
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 how could emitted sound at this level possibly be a problem?
It is our view that the wind industry has unfortunately stumbled into an area of inner ear physiology that no-one could have foreseen. Although the ear acts like a microphone, converting sound waves into electrical signals that are sent to the brain, it performs this task in a very complex manner, the significance of which has not been fully appreciated until now.
Research By Our Group
The research performed in our laboratory covers a number of areas related to inner ear function and the physiology of the cochlear fluids (as is apparent from the rest of this website). Our group has for years been using infrasound as a tool to study how the ear works. These are often described as “low frequency biasing tones”, because the low frequency bias tone is used to displace the structures of the ear slowly enough to allow induced changes in the cochlear properties to be observed. For almost 10 years we have been using infrasonic 5 Hz bias tones to manipulate cochlear responses in guinea pigs. The guinea pig has a shorter cochlear than the human and hearing sensitivity measurements show that it is approximately 17 dB LESS sensitive to low frequencies than the human. In our studies, we found that 5 Hz bias tones effectively altered cochlear responses at sound levels as low as 85 dB SPL (The green diamond shown on the graph at the right). In humans the threshold of sensitivity for 5 Hz is 109 dB SPL (Møller and Pedersen, 2004), as also shown on the graph at the right as the blue symbols representing hearing sensitivity. Also shown by the red line is the calculated sensitivity of the inner hair cells (IHC) of the cochlea. In brief, you “hear” with your inner hair cells. BUT, if the guinea pig is less sensitive than the human, how can such a low level 5 Hz tone, which the guinea pig would not be expected to hear, influence function of the ear?
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
Hearing Research. In press.
Some of the points made by our paper include:
- The OHC (i.e. the outer hair cells of the cochlea, which are the sensory cells which act as the active “amplifier” of the ear) are stimulated with low frequency sounds at levels much LOWER than the IHC. It is likely that the OHC are stimulated in some people by infrasounds at the levels generated by wind turbines. Thus the infrasound component of wind turbine noise may be the cause of the increased annoyance of some individuals to wind turbine noise. It also has to be considered that if there are health effects in some individuals, then the infrasound component of wind turbine noise could be involved.
- Stimulation of the OHC occurs at infrasound levels substantially below the levels that are heard. We calculate that stimulation of the OHC occurs at approximately 30-40 dB below sensation level depending on frequency. The concept that sounds that you cannot hear can have no influence on the inner ear is incorrect. Infrasounds that cannot be heard DO influence inner ear function.
- The practice of A-weighting measurements of wind turbine noise underestimates the influence of this noise on the inner ear.
- Some clinical conditions (endolymphatic hydrops and “third window” pathologies, such as superior canal dehiscence) make the ear hypersensitive to infrasound stimulation. In both hydrops and SCC dehiscence it is possible to have the condition and be asymptomatic. This leads to the possibility that some “apparently normal” (asymptomatic) individuals may be hypersensitive to infrasound.
- In order to more fully understand why infrasound affects the ear at levels that are not heard, you will have to read the paper!
The estimated OHC (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. Our paper shows that the inner ear is far more sensitive to infrasound than previously thought. This raises the POSSIBILTY that the dislike / disturbance of individuals by wind turbine noise may be related to the stimulation of the ear with infrasound at levels below those that are heard. It CANNOT be concluded from our study that infrasound is the cause of people’s symptoms. More work needs to be done in this area. It does, however, suggest that the infrasound component should be studied further as a possible cause, rather than being excluded 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. For those that think placement of tympanostomy tubes is an “extreme” solution, it should be pointed out that thousands of these tubes are placed in children’s ears every day, to relieve middle ear infections and the risk to the ear is low.
At present there seems to be considerable distrust between the public, hearing of problems in some communities from wind turbine noise, and the industry, claiming that wind turbine noise is completely unobtrusive.
If the infrasound component is (from further study) determined to contribute to the problem, it could lead to the following actions:
- Installation of systems in the home capable of monitoring infrasound and audible sounds over time,so that the scope of the problem can be quantified.
- Modification or redesign of wind turbines for use in the urban environment to alleviate infrasound problems.
If, by these actions, the undesirable attributes of wind turbines are reduced or eliminated, this would restore the trust between communities and the industry that would INCREASE the acceptance of these devices in the community.
Our goal is that by understanding the true nature of the problem related to annoyance and potential health effects of wind turbine noise, it is then possible to solve it.
Harry A. Wind turbines, noise and health. 2007.
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. The Society for Wind Vigilance. 2010.
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.
Pierpont N. Wind turbine syndrome. 2009.
Van den Berg GP. The sound of high winds: the effect of atmospheric stability on wind turbine sound and microphone noise. PhD Dissertation, University of Groningen, Netherlands.
Voss SE, Rosowski JJ, Merchant SN, Peake WT. Middle-ear function with tympanic-membrane perforations. I. Measurements and mechanisms. J Acoust Soc Am 2001;110:1432-1444.
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