The methodology employed in the Cape Bridgewater study started from resident’s diaries of disturbances whilst noise monitoring was occurring. The procedure then took weather data, wind farm operating data and noise data as post-processed data to compare with the diaries to find trends where specific wind farm operations corresponded to the report disturbances. A similar procedure occurred for investigating “noise” complaints from residents concerning a coal-fired power station and a large ventilation fan for an underground coal mine. Limitations in obtaining high-quality full-spectrum wave files were encountered. Typical Class 1 sound level meters have storage limitations of 1 – 2½ days for such high-quality samples. Using multichannel systems such as a Bruel & Kjaer Pulse are expensive and lead to very large storage requirements. Utilising the study procedure resulted in the development of a relatively low cost, two-channel, full-spectrum data recorder for field use, coupled with simultaneous biometric monitoring. The methodology has been successfully employed/developed. The opportunities now available for more detailed processing of this data, together with linking the disturbances to the startle reflex is discussed.
Typically, compliance monitoring in relation to industrial noise sources is assessed in terms of permit conditions that generally are expressed in the A-weighted Leq value. Depending upon the noise source of concern the permit conditions may specify that such noise is to be broadband and free tonal or intermittent components, whilst in relation to wind farms there is no such specification. Some regulations or standards for wind farms may require adjustments due to special audible characteristics.
When dealing with complaints associated with such noise sources, Environmental Authorities or the operator of the noise source in question only look to the criteria in the permit conditions – generally in dB(A). However, investigations in response to complaints require a different methodology if one is serious about identifying, resolving and preventing those problems.
When the noise source is low-frequency music from nightclubs or similar, one can undertake on-off tests for identifying the occurrence and source of the noise in the complainant's premises. Whether the noise is audible or inaudible to the persons conducting measurements is not relevant, because in some situations the complainant may have developed a heightened sensitivity to the low-frequency music that is audible to them, but can be inaudible to testing authorities or the consultant employed. The complaint can be investigated by simple on off testing of the music, without the complainant being aware of what is occurring.
In relation to large industrial complexes that may have thousands of noise sources, a more detailed approach is required in relation to investigating the complaint. It is very important to take account of the operating scenarios at which the time the disturbances are alleged to have occurred. In such situations, the benefit of a diary format from residents that can be compared with the operation of the plant is essential.
Going back 30 years ago one relied upon audibly tracking down low-frequency noise and/or the use of graphic level recorders or similar, to monitor the linear output of the sound level meter and/or vibration as part of the investigation, supplemented (if possible) by narrow band analysis.
Successfully identifying noise sources of infrasound or low-frequency noise (“ILFN”) complaints in industrial premises was primarily trial and error, and being aware of the various relatively large noise sources on the site that operate at low operating speeds or involve pressure fluctuations that could be the source of the complaint.
It has been observed that in many cases of ILFN noise complaints the matter of perceived vibration and noise in the infrasound and low-frequency regions are interchangeable. Often there is an incorrect identification by the complainant's (and in some cases by consultants or regulatory authorities) of noise versus vibration.
Such a situation has occurred with the development of wind farms in relatively close proximity to residential premises, and similarly with large power stations or underground mines that have induction fans or ventilation fans respectively, running at low speed and transferring large volumes of air.
Our work to date in using multi-channel extended monitoring (to obtain simultaneous inside/outside measurements) under different operating scenarios of ILFN sources has found that modulation of low-frequency noise at an infrasound rate that occurs at or near the threshold of hearing is able to trigger a response in individuals.
Narrowband analysis has identified the presence of discrete infrasound “components” for repetitive pulsations (“dynamically pulse amplitude modulation”) that may simply be the result of the analysis, not necessarily the presence of such a signal.
The presence of amplitude modulation in the low frequency region, that modulates at an infrasound rate, at or near the threshold of hearing has been identified and may support the following proposal:
“Wind Turbine Syndrome, I propose, is mediated by the vestibular system—by disturbed sensory input to eyes, inner ears, and stretch and pressure receptors in a variety of body locations. These feed back neurologically onto a person's sense of position and motion in space, which is in turn connected in multiple ways to brain functions as disparate as spatial memory and anxiety. Several lines of evidence suggest that the amplitude (power or intensity) of low frequency noise and vibration needed to create these effects may be even lower than the auditory threshold at the same low frequencies. Re-stating this, it appears that even low frequency noise or vibration too weak to hear can still stimulate the human vestibular system, opening the door for the symptoms I call Wind Turbine Syndrome.” Pierpont 2009
With the latest development of technology and analysis that is available the provision of multichannel data recorder for use in the field provides the acoustic starting point (when coupled with the appropriate analysis), that permits the medical investigations necessary for ascertaining the impacts of infrasound and low-frequency noise.