Wind Power in the UK (PDF). - Sustainable Development Commission

sd.commission.org.uk

Wind Power in the UK (PDF). - Sustainable Development Commission

Annex DNoiseHow noise is measuredSound is always associated with small scalechange in pressure, which produces sensations(i.e. is ‘heard’) at the human ear. Because of thewide range of sound pressures to which the earresponds, sound pressure is an inconvenientquantity to use in graphs and tables and sonoise is measured on a logarithmic scale indecibels (dB). The decibel is a measure of thesound pressure level, i.e. the magnitude of thepressure variations in the air.A change in sound level of 1 dB cannot beperceived, except under laboratory conditions.Doubling the actual energy of a sound source ordoubling the number of identical sound sourcescorresponds to a 3 dB increase. A 3 dB changein sound level is considered a barely discernibledifference, outside the laboratory.The noise that a machine such as a wind turbinecreates is normally expressed in terms of itssound power level. Although this is described indB(A), it is not a measurement of the noiselevel but of the power emitted by the machine,which then creates the sound pressure levelwhich can be heard and measured using asound meter.Sources of wind turbine noiseStanding next to a wind turbine, it is usuallypossible to hear a noise often described as awhoosh or a swish as the blades rotate. Thewhirr of the gearbox and generator may also beaudible, depending on the type of turbine.There are plenty of detailed reviews of thesources and noise generation processes of windfarm noise 1 , but in general, the sources of noiseemitted from operating wind turbines can bedivided into two categories, mechanical andaerodynamic.Mechanical noiseMechanical noise is transmitted along thestructure of the turbine and is radiated from itssurface. The hub, rotor, and tower can all act asloudspeakers, transmitting the mechanical noiseand radiating it. Because it is associated withturning machinery, this noise can be heard at adistinct constant frequency, described as ‘tonal’.Aerodynamic noiseThe biggest contributor to the total sound powerfrom a turbine is the aerodynamic noise whichis produced by the flow of air over the blades.The proportion of noise from each source istypical of modern wind turbines. A large numberof complex flow phenomena occur which cangenerate aerodynamic noise. There is muchongoing research into these phenomena.Broadband noise is often caused by theinteraction of wind turbine blades withatmospheric turbulence, and is also described asthe characteristic ‘swishing’ or ‘whooshing’sound of wind turbines. Airfoil noise alsoincludes the noise generated by the air flowright along the surface of the airfoil. This type ofnoise is typically of a broadband nature, buttonal components may occur due to blunttrailing edges, or flow over slits and holes.Low frequency noiseLow frequency noise, with frequencies in therange of 20-100 Hz, is mostly associated with‘downwind turbines’, with the rotor on thedownwind side of the tower. It is caused whenthe turbine blade encounters localised flowdeficiencies due to the flow around a tower.When a rotating blade encounters this, pulses oflow frequency noise are generated. Turbinesthat have their rotors upstream of the tower,except in very rare circumstances, do notgenerate such pulses since there is nothingblocking the flow upwind of the rotor. When itdoes occur, because of the low rotational rates148 Wind Power in the UK sustainable development commission

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