Definitions of infrasound and low-frequency noise are discussed and the fuzzy boundary between them described. Infrasound, in its popular definition as sound below a frequency of 20 Hz, is clearly audible, the hearing threshold having been measured down to 1.5 Hz. The popular concept that sound below 20 Hz is inaudible is not correct. Sources of infrasound are in the range from very low-frequency atmospheric fluctuations up into the lower audio frequencies. These sources include natural occurrences, industrial installations, low-speed machinery, etc. Investigations of complaints of low-frequency noise often fail to measure any significant noise. This has led some complainants to conjecture that their perception arises from non-acoustic sources, such as electromagnetic radiation. Over the past 40 years, infrasound and low-frequency noise have attracted a great deal of adverse publicity on their effects on health, based mainly on media exaggerations and misunderstandings. A result of this has been that the public takes a one-dimensional view of infrasound, concerned only by its presence, whilst ignoring its low levels.
The paper reviews perception of low frequency noise (LFN) and the responses of people to LFN. Sometimes, when there are complaints of LFN and its effects, a specific noise cannot be measured. The possibilities are considered for development of enhanced sensitivity, for alternative (non-aural) receptors at very low frequencies and for false perceptions. The way in which we measure LFN may contribute to the problems. INTRODUCTIONThere is considerable misunderstanding about low frequency noise and infrasound Some of this is due to misleading presentations by scientists 30 or 40 years ago (Gavreau, 1968;Gavreau et al., 1966), which were interpreted selectively by the media in order to give eye-catching news stories. A succession of "reinterpretations" over a period of five or six years led infrasound and low frequency noise to be described in popular texts as a cause of death, whilst also possessing the ability to knock down buildings. (Watson, 1974). This aura of mystery and danger still persists today, deep in the minds of many people, where it waits for a trigger to bring it to the surface. The most recent trigger has been wind turbines.Although we know a great deal about low frequency noise, there are aspects which we cannot yet explain. We know about how people hear low frequency noise and that some have a low tolerance to it. We believe that low frequency noise may, in general, be more annoying than higher frequency noise, but do not know why this is so. We do not know why some people complain of a low frequency noise which cannot be measured separately from the background noise.It is also possible that there are subtle effects of low frequency noise on the body, which we do not yet understand.
A small group (n=9), whose complaints of low frequency noise had not been resolved by Environmental Health Officers and related care professionals, were invited to attend a series of intervention sessions led by a psychotherapist. The aims of the sessions were to improve the participants' coping strategies and their quality of life, in order to relieve them from some of the distress caused by their belief that they were exposed to low frequency noise. Prior to the psychotherapy sessions the group was evaluated on a number of self report questionnaires, which measured individual responses for reaction to low frequency noise, quality of life, quality of coping and a personality questionnaire (Insights tm Discovery Preference Evaluator). A before and after, within group analysis of responses was based upon repeat measures of the three behavioural response questionnaires for noise reactivity, quality of life and coping. A general reduction in the subjects' stress levels was shown, suggesting positive effects of psychotherapy upon symptoms that had, in this group's case, proved resistant to improvement by conventional local authority and specialist interventions. This 'therapeutic' approach to LFN interventions could lead to improved health and effectiveness and fewer demands on local services. Although the techniques of tinnitus management were informative, analogy between the problems of low frequency noise sufferers and those of tinnitus sufferers fails at the point where low frequency noise sufferers believe that an external agency is the cause of their problems.
Setbacks for wind turbines have been established in many jurisdictions to address potential health concerns associated with audible noise. However, in recent years, it has been suggested that infrasound (IS) and low-frequency noise (LFN) could be responsible for the onset of adverse health effects self-reported by some individuals living in proximity to wind turbines, even when audible noise limits are met. The purpose of this paper was to investigate whether current audible noise-based guidelines for wind turbines account for the protection of human health, given the levels of IS and LFN typically produced by wind turbines. New field measurements of indoor IS and outdoor LFN at locations between 400 and 900 m from the nearest turbine, which were previously underrepresented in the scientific literature, are reported and put into context with existing published works. Our analysis showed that indoor IS levels were below auditory threshold levels while LFN levels at distances >500 m were similar to background LFN levels. A clear contribution to LFN due to wind turbine operation (i.e., measured with turbines on in comparison to with turbines off) was noted at a distance of 480 m. However, this corresponded to an increase in overall audible sound measures as reported in dB(A), supporting the hypothesis that controlling audible sound produced by normally operating wind turbines will also control for LFN. Overall, the available data from this and other studies suggest that health-based audible noise wind turbine siting guidelines provide an effective means to evaluate, monitor, and protect potential receptors from audible noise as well as IS and LFN.
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