Infrasound Emission from Wind Turbines

A critical survey of all known published measurement results of infrasound from wind turbines has been made. The survey indicates that wind turbines of contemporary design with an upwind rotor generate very faint infrasound with a level far below the threshold of perception even at a rather short distance. From considerations on propagation and transmission of infrasound it is concluded that infrasound from such upwind turbines can be neglected when evaluating the environment effects of wind turbines. Turbines with downwind rotors produce 10 30 dB higher infrasound levels, and these may exceed relevant assessment criteria for dwellings in the immediate neighbourhood. When longer distances are considered, neither downwind nor upwind turbines are capable of violating assessment criteria for infrasound. This paper considers whether other aspects of the noise than the infrasound can explain the indicated adverse public reactions to large downwind turbines. 1.INTRODUCTION 1.1 Noise from wind turbines Wind turbines constitute a very distinctive group of noise-producing devices. During the last 20 years or so a considerable insight has been gained into the noise mechanics of wind turbines, mainly for the purpose of making quieter turbines to enable a better exploitation of wind energy. An excellent overview of the noise sources of wind turbines can be found in [1] . They may be split into two groups, machinery noise and aerodynamic rotor noise. 1.2 Machinery noise The machinery that transforms the rotation of the rotor blades into electricity chiefly generates the machinery noise: the gear box and the generator. The noise from these components frequently contains more or less prominent tones, whose amplitude and sometimes also frequency fluctuates slightly in rhythm with the blade passing frequency of the rotor. Additional sources of machinery noise are ventilation equipment for the machinery compartment, hydraulic pumps, and yawing machinery. It is common to all these sources that the mechanisms responsible both for the noise generation and the radiation are known from other fields of application, and they are well described. Also the means of controlling machinery noise have been known for some time, [2], though it is not to say that it can be easily obtained. 1.3 Aerodynamic rotor noise The aerodynamic noise from the rotor is less well-known, though a better understanding has been gained both through development of theories for the aerodynamic noise sources of a wind turbine [3, 4] and through experiments [5, 6]. Design rules for quiet rotor blades exist in most manufacturing companies, and it is rare nowadays to listen to the bleating, squeaking, whining, or whistling sounds that were characteristic for the earlier types of wind turbines. The rotor noise from a well designed wind turbine would have a broadband character and a characteristic ampliVol. 24 No. 3 2005 JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL Pages 145 – 155 145 LF 24(3)-Jako 10/1/06 10:15 am Page 145