Author: | Australia
Proceedings of Acoustics 2013, 17-20 November 2013, Victor Harbor, Australia
Development of a technique to minimise the wind-induced noise in shielded microphones
S.V. Alamshah, A.C. Zander
School of Mechanical Engineering, University of Adelaide, Adelaide, South Australia, Australia
Science & Assessment Division, SA Environment Protection Authority, Adelaide, South Australia, Australia
Environmental noise measurements are usually performed in atmospheric conditions where wind and thus turbulent flow over the microphone is present. In such conditions the measured noise is highly affected by the wind-induced noise generated by turbulence structures present in the flow and microphone generated wakes. A novel approach has been employed to distinguish the contribution of wind-induced noise from the acoustic signal using Incoherent Output Power analysis between two microphone signals. Various experimental arrangements were investigated to examine the influence of the experimental parameters on the results obtained. The technique was successfully tested and validated in a series of indoor experiments in a small anechoic wind tunnel. Finally, a new approach for minimising wind-induced noise using the Coherent Output Power between two shielded microphones is proposed and tested.
Meteorological stability impacts on wind turbine noise assessments
Bill Dawson and Neil Mackenzie
Building Sciences, Aurecon, Adelaide, Australia
Current wind farm noise regulations stipulate wind speed dependent criteria (referenced to wind speed at the hub height of the turbines), under the assumption that during high-wind speed conditions (when wind turbines generate higher noise levels), there will be a corresponding high wind speed and masking noise level at nearby receivers. However, under very stable conditions, high wind speeds at the turbine hub height will create significant noise, while low wind speeds at the receiver will not be sufficient to provide a masking effect. This has been considered in assessment guidelines by filtering day/night background data, but this approach ignores the impact of changes in the level and spectral content of turbine noise due to high shear velocities across turbine blades. This paper examines meteorological data in the vicinity of an undisclosed future wind farm site in South Australia, which was used to filter noise and wind speed data based on stability criterion, and discusses the potential impact on the noise criteria used for wind farm developments.
Forecasting low frequency noise from wind farms
Rhys Brown, Andrew Mitchell, and Luis Najera Medina
AECOM, Brisbane and Melbourne, Australia
Wind farm noise is commonly forecast as an overall A-weighted noise level for comparison against legislative requirements. In Australia various regulatory bodies have begun considering low frequency noise criteria which can be applied to industrial facilities, including wind farms. One such example is the NSW Draft Wind Farm Noise Guidelines which presents criteria of 60 dB(C) for the night-time and 65 dB(C) for the daytime. Whilst the forecasting of overall A-weighted noise levels from wind farms is well documented and validated against measured wind farm noise levels, the forecasting of low frequency noise (noise above 20Hz but below 200Hz) has not been widely validated. This paper analyses wind farm compliance noise monitoring adjacent wind farms in Australia and compares the measured low frequency noise levels against forecast low frequency noise levels. The influence of various factors of the monitoring and modelling chain is discussed, including the effect of wind noise on the measurement microphone. It was found that modelling of C-weighted noise levels can be performed using the same model as used for forecasting A-weighted noise levels and the results obtained would likely be a conservative estimate of C-weighted wind farm noise levels in most cases.
Effects of different meteorological conditions on wind turbine noise
Jonathan Cooper and Tom Evans
Resonate Acoustics, Adelaide, Australia
The accuracy of wind turbine noise predictions is sometimes the subject of debate during assessments of proposed wind farms. Theoretical questions are raised about the potential effects of different meteorological conditions on noise emission and propagation. In particular, periods of higher wind shear, temperature inversions and inflow turbulence have been raised as concerns. This paper presents noise measurement data gathered at operational wind farm sites where the meteorological wind shear, temperature gradient, turbulence and inflow angle variables are monitored. The effect of these factors on both noise emission and noise propagation from modern wind turbines are investigated and it is found that there is only a small influence on noise emission and negligible influence on noise propagation for the range of operating conditions of typical wind farms. An increase in noise emission was identified at lower frequencies when a turbine was operating under inflow turbulence but this only occurred at low wind speeds with no difference observed when the wind speed increased. Noise propagation from wind turbines was not found to increase with either wind shear or temperature gradient. It is theorised that this may be due to the height of the noise source as well as the fact that the operation of turbines would disrupt stable conditions immediately downwind of the blades.
Automated detection and analysis of amplitude modulation at a residence and wind turbine
Jonathan Cooper and Tom Evans
Resonate Acoustics, Adelaide, Australia
A small degree of amplitude modulation is a normal feature of wind turbine noise but most assessment guidelines for wind farm noise state that, where excessive amplitude modulation occurs, an additional penalty should be applied to the measured noise. Excessive amplitude modulation is typically defined as a situation where the peak to trough levels (either overall or in particular frequency bands) exceed a nominated level. The assessment of amplitude modulation outdoors at receptor locations near wind farms over a wide range of wind conditions can be difficult due to the need to undertake unattended measurements in an environment where background noise regularly interferes with the measurements. This paper describes a methodology for the assessment of amplitude modulation over an extended period at a residence, and the specific techniques used to identify amplitude modulation resulting from the wind farm. The methodology has been employed at an operational wind farm and the results at both a residence and wind turbine assessed to identify conditions which contribute to modulation judged to be ‘excessive’ using the modulation test provided in New Zealand Standard 6808:2010.
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