1	Diagnosing Noise Radiation Mechanisms from Wind Turbines M F Harper, M Thompson Mecon Limited, Cambridge, UK
2	First: Wind Turbines aren’t Noisy! A typical half-megawatt machine radiates only 5 milliwatts of noise to far field A great deal of design goes into minimising noise radiation Outstanding problems are difficult to cure
3	Noise Radiation Mechanisms Aerodynamic noise from airflow over blades: mostly near tips. Short range. Gearbox noise radiated from all parts of structure. Noise is tonal, at gear tooth meshing frequencies. Low frequencies carry long distances
4	How does the gearbox noise escape? Most nacelles have ventilator openings Nacelle skin is forced by internal noise Blades have large surface areas Tower has even larger area
5	We need to know how the noise is escaping in order to reduce it! Possible methods: Inference from level measurements Excite individual radiating surfaces If noise is broadband and distributed and structural modal densities are high, consider Statistical Energy Analysis If noise is tonal, use Transmission Path Audit If system can be dismantled, use transfer function measurements
6	Non-Invasive Analysis: Transmission Path Audit
7	Applying Transmission Path Audit Apply acoustic and mechanical forcing to noise source at many points sequentially Record path excitations and farfield noise each time Use simultaneous equations to estimate path strengths One frequency at a time Predict far field noise from results and compare with observation as a check
8	Case Study ½ Megawatt machines in a rural European environment Background noise levels extremely low Aerodynamic noise not a problem Gearbox tones audible at dwellings at night at least 500m away Operator required by local authorities to reduce levels or cease operation
9	Case Study: Nacelle Instrument fit
10	Case Study: Parameters 17 Excitations at each of three frequencies in range 100 to 400Hz 12 Mechanical excitations at 170N RMS 5 acoustic excitations at 90dB Excitations last 1 minute to provide high data quality Results continuously monitored: excitations stop when results stabilise
11	Case study: Path Groupings Turbine Blades Sensor channels 1-3 Tower Sensor channels 4-6 Nacelle Walls Sensor channels 7-9 Air in Nacelle Sensor channels 10-13
12	Case study: Results
13	Case Study: Results of subsequent noise reduction work on blades
14	Conclusions Transmission Path Audit has been shown to work very well in a difficult application Tonal noise transmission can be analysed non-invasively Noise Control Engineer can predict results of path treatments in advance Noise reduction measures can be tailored to achieve noise reduction targets cost-effectively