L Huson & Associates Pty has been commissioned to review the recommended noise conditions outlined in an Approval dated 25 November 2013 by the NSW Department of Planning & Infrastructure for the Flyers Creek Wind Farm.
This report presents infrasound measurements and results from attenuation testing of rural properties in Australia and New Zealand that indicate that the Director General’s Environmental Assessment report dated 25 November 2013 may have been ill informed. Suggestions have been made to redress the perceived problems with modifications to the proposed noise conditions.
Issues such as amplitude modulation and infrasound should be included within the noise conditions and the noise targets to protect the health of residents should be clearly identified. The Proponent has not addressed all of the issues raised that warranted further information by SKM. Accordingly, the Proponent has not satisfied its obligations in relation to the Director-General’s requirements and South Australian EPA – Wind Farms – Environmental Noise Guidelines, 2003.
APPROPRIATE TARGET NOISE LIMITS
The first misunderstanding in the DGEAR (page 21) is the statement that an indoor sound level of 30 dBA will be achieved for an outdoor sound level of 45 dBA with windows open. This headline statement is commonly used to support wind farm developments and the Director-General’s department has been misled in this regard. The following extract is from the WHO (Berglund & Lindvall) Community Noise Guidelines pertaining to sleep disturbance, consideration of vulnerable groups and the derivation of outside noise levels that correctly reflect the indoor sound level target. A fuller reference was included in our review (Huson review) of the Vipac Environmental Assessment.
…..Inside bedrooms the sound pressure level should not exceed 30 dB LAeq for steady-state continuous noise, and for a noise event not exceed 45 dB LAmax, preferably even lower (maybe 40 dB LAmax). Still lower levels may be annoying depending on the nature of the noise source. At nighttime, sound pressure levels outdoors should not exceed 45 dB LAeq, so that people may sleep with bedroom windows open. Even lower levels may be required pending the design of the window opening, maybe 35-37 dB LAeq outdoors.
The target ‘steady state’ noise parameter indoors (for sounds not containing high low frequency content for non-vulnerable people) of 30 dB(A) is in terms of the Leq. The sound power calculations and measurements from wind turbines are also in terms of the Leq parameter. The generally accepted conversion from Leq to L90 is 2 dB. For the sake of simplicity, we can assume that the correction from Leq to L90 is 2dB (Leq = L90 + 2 dB). This means that the steady state target internal noise limit of 30 dB(A), Leq becomes an internal target noise limit of 28 dB(A), L90.
It should be recognised that the difference between Leq and L90 becomes greater if there is amplitude modulation and that the L90 statistic greatly underestimates the impact of modulated sound. For this reason, we would not recommend the use of LA90 as either an outside or inside noise criteria. Thankfully, the recommended conditions in G7 use LAeq.
We have measured actual attenuation from dwellings with windows open in Queensland and Victoria and have found that outside to inside attenuation results vary depending upon the frequency of the external sound. The overall A-weighted attenuation from outside to inside a sample of four different bedrooms surveyed in Queensland was between 4.1 dB(A) and -1.2 dB(A).
Our recent research on amplitude modulation will be presented at a conference on amplitude modulation hosted by the Institute of Acoustics in the UK in March 2014. We have identified that for a typical dwelling in Victoria the attenuation of wind turbine sound from outside to inside a room is governed by the acoustic modes of the room. At the room mode frequencies around 50 Hz and between 120Hz and 500Hz there can be no attenuation at all. Furthermore, the broadband swish noise outside a dwelling is modified to have a particular frequency characteristic that can be classed as tonal indoors and is dependent upon the receiving room dimensions. Amplitude modulation levels can also be enhanced indoors.
In summary, attenuation of wind turbine sounds from outdoors to indoors is certainly much less than 15 dB for an open window. However, if outdoor sound levels are to be chosen for permit conditions then it is a simple task to assess the real attenuation of potentially affected dwellings from measurement and correct accordingly.