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Www.hoarelea.com Wind Farm Noise Impact Assessment NOISE PREDICTIONS – SOURCE DATA AND PROPAGATION.

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Presentation on theme: "Www.hoarelea.com Wind Farm Noise Impact Assessment NOISE PREDICTIONS – SOURCE DATA AND PROPAGATION."— Presentation transcript:

1 Wind Farm Noise Impact Assessment NOISE PREDICTIONS – SOURCE DATA AND PROPAGATION

2 Source Propagation Receiver The situation to be assessed ….

3 VHVH V 10 SWL v = SPL v + 10 x log ( 4 x p x r 2 ) – 6dB Source Sound Power (IEC ) r H

4 ISO : Frequency based data Third octave SWL Narrow band tonal assessment

5 Sound power values? Tested sound power (IEC ) with reported test uncertainty σ Warranted values: includes “safety margin” of typically up to 2dB(A) …… but sometimes not (commercial considerations) Turbine specification: additional uncertainty in warranty? Declared values: procedure of IEC for adding uncertainties (1.654 σ) N.B: test results are L Aeq data, so L A90 prediction is obtained by subtraction of 2dB

6 Source Propagation Receiver The situation to be assessed ….

7 Propagation models ‘Exact’ numerical methods (e.g. parabolic equation, fast field program) Approximate semi-analytical methods (e.g. ray tracing) Empirical ‘engineering’ methods (e.g. ISO , Concawe) Input data Calculation implementation Output results

8 Factors affecting sound propagation

9 Factors affecting sound propagation - 1 Spherical spreading of noise Atmospheric attenuation

10 Factors affecting sound propagation - 1

11 Factors affecting sound propagation - 1

12 Factors affecting sound propagation - 2

13 Factors affecting sound propagation - 2 In ISO : modelled by ground factor coefficient G (0 to 1)

14 Factors affecting sound propagation - 2 G = 1 : soft, porous ground ground covered by grass or trees cultivated farming land  Not used as it will often under-predict G= 0: hard ground paving, water ice, concrete, etc. Industrial ground  Robust but can over-predict, particularly with warranted source data G= 0.5: mixed ground  Used with warranted source data or tested data + uncertainty

15 Factors affecting sound propagation - 3 Refraction effects…

16 Factors affecting sound propagation - 3 Hot Cold Temperature lapse Upwind

17 Factors affecting sound propagation - 3 Hot Cold Temperature inversion Downwind

18 Factors affecting sound propagation - 4

19 Factors affecting sound propagation - 4 Sound energy enters ‘shadow region’ via turbulent scattering

20 Factors affecting sound propagation – 3 + 4

21 Comparison of PE and ISO 9613 results 20m 200m 1000m Harmonoise PE model (based on annual measured meteorological conditions [Salomons]) ISO 9613 (based on typical downwind propagation conditions) Percentage of time below stated noise level, % Calculated noise level, dB

22 Factors affecting sound propagation - 6

23 Factors affecting sound propagation - 6 In downwind conditions: limited to no more than 2 to 3 dB Terrain = noise barrier?

24 Factors affecting sound propagation - 6 “Valley effect”: +3 dB h m ≥1.5 abs(h s -h r )/2

25 Measurement studies 3 Separate wind farm sites, both located in rural environments and each comprising more than 20 turbines Two-speed turbines rated at over 2MW peak generating capacity, 60 to 70m hub height Type 1 sound meters continuously sampling statistical and equivalent data at varying distances, with double wind-shield arrangements Published

26 Measurement locations: from approximately 100m to 750 m from nearest turbine Very flat terrain Minimal vegetation Propagation path covered by peat bog Ground prone to flooding – saturated for duration of survey Wind Farm Envelope N Site B

27 Site B Single site wind speed (G = 0 / 754m)

28 Turbine specific wind speed (G = 0 / 754m) Site B

29 Turbine specific wind speed (G = 0 / 754m) Site B

30 Turbine specific wind speed (G = 0 / 754m) Site B

31 Turbine specific wind speed (G = 0 / 754m) +2dB(A) “warranty” margin added ~5dB Site B

32 Main measurement locations: from approximately 100m to 820 m from nearest turbine Lightly undulating but acoustically flat terrain Minimal vegetation surrounding turbines, forestry close to locations Ground cover of mixed soft ground and flooded areas N Wind Farm Envelope P1 P2 P3 P4 P5 Predictions are made for: high speed mode only G=0.5 tested sound power data + 1dB (Stated test uncertainty) Site C

33 Location P4: 700 m distance Turbine only (predicted) Turbine + background Site C

34 Additional research Evans and Cooper Comparison of predicted and measured wind farm noise levels and implications for assessments of new wind farms Paper Number 30, Proceedings of ACOUSTICS November 2011, Gold Coast, Australia “Steady slope”“Concave”

35 Recommendations For use and application of ISO to WTN L A90 = L Aeq – 2 dB Use spectrum data if available 4m receiver height 10 degrees / 70% humidity Do not use G=1 G=0.5 recommended, with source levels which incorporate test uncertainties (as in most warranties or “declared“ values as per IEC ) For propagation across a valley add +3dB Terrain screening: no more than -2dB State all assumption and input data

36 Thank you


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