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Third International Meeting on Wind Turbine Noise Aalborg Denmark 17 19 June 2009Long distance sound propagation over a sea surfaceB.L. Andersson1, K. Bolin2, A. Cederholm1 & I. Karasalo1,2 Addresses: 1) FOI - Swedish Defence Research Agency, Stockholm, Sweden 2) KTH - Royal Institute of Technology, Stockholm, Sweden e-mails: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
AbstractResults from measurements of sound propagation over a sea surface to a 10 km distant receiver are compared to modelling with the Greens Function Parabolic Equation (GFPE) method by Gilbert and Di. The purpose is to assess the accuracy of prediction of atmospheric sound propagation by methods that use detailed knowledge of the local geographical and meteorological conditions. Experimental data were collected during a one-week period in June 2005, and consist of data on the transmission loss (TL) of narrow band signals with frequencies 80, 200 and 400 Hz. Meteorological data were provided from radio sounding and balloon tracking up to 2-4 km in height at the receiver location and from meteorological sensors mounted on a 90 m high mast at the emission point. An atmospheric model including a laminar and a superimposed turbulent wind field was fitted to the meteorological data. Comparisons between the experimentally observed TL and predictions by the GFPEmodel are presented. A satisfactory agreement is observed of the model-predicted transmission loss as a function of time to the experimental data.
IntroductionIn the light of global warming the transition to renewable power sources is a crucial challenge to today's society. A power source that will probably play a major role in the future is wind turbine power. Until now most of the wind turbines are land based, however large off-shore farms are under construction or planned all over the world. These will exploit the vast wind resources available in at sea and by 2020 50 GW of installed capacity is planned in Europe . Off-shore wind turbines are often located near a coast due to cost increases with increasing water depth. Such installations have given rise to concerns for noise pollution on shore, often in recreational regions unaffected by community noise. Since atmospheric sound propagation is highly Long distance sound propagation over a sea surface Page 1 of 9
dependent upon the changing meteorological conditions, the level of such noise varies strongly with time. Measurements of long distance sound propagation over water surfaces concurrently with meteorological observations have been performed by Konishi and Tanioku [3,4]. However, the meteorological data were registered up to a few hundred meters height only, while knowledge of the meteorological conditions (wind velocity, humidity and temperature) further up in the atmosphere is in general needed for accurate predictions of the soundfield. The aim of this paper is to present measurements of sound propagation at 10 km distance  and evaluate the reliability of a sound propagation model with detailed knowledge of the meteorological and geographical conditions at the measurement times. This has been conducted by comparing transmission loss (TL) of the predictions to the measurements.
MeasurementsThe measurements were performed between the 15th and the 21st of June 2005 in the Kalmar strait and the island land in the Baltic Sea. A motivation for this choice of experimental period was that most annoyance from wind turbine noise could be expected in the summer due to increased recreational outdoor activity.
FIG. 1: Measurement setup. Acoustical measurements
Source site Two sound sources were placed on Utgrunden lighthouse located 9 km from shore. The sources were mounted at height 30 m on the lighthouse roof, with reference microphones placed 1 m front of respective source for recording the emitted signals.
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The first source was a compressed-air-driven siren (Kockum Sonics Supertyfon AT150/200 with Valve Unit TV 784). It produced a 10-second signal with an average level of 130 dB and average frequency 200 Hz. Both this fundamental frequency and the first harmonic at 400 Hz were used in the analysis. The signal presented variations of the order of 1% in frequency and about 20 dB in sound level within each sound pulse, caused by the decreasing pressure of the compressed air driving the siren. The second source consisted of a sound generator coupled to a loudspeaker and a 1.2 m-long resonator tube. It produced a 1 minute long 80 Hz tone with constant sound pressure level of 113 dB at 1 m distance. Both sound sources were employed simultaneously.
Receiver siteThe receiver site was on the island land 750 m from shore with ground height 7 m above sea level (see Fig. 1 for the experimental setup). The site was adjacent to the houses closest to the shoreline, in a quiet residential area. The receiver was a horizontal linear array of eight -inch microphones oriented parallel to the direction to the source. The array was placed at 1.7 m height accordingly to ISO 1996. The distance between the microphones was set to 40 cm, equal to half the wavelength at 400 Hz, to ensure a beam pattern free from grating lobes at all three frequencies. The signals were transmitted through a preamplifier to an UA100 analyzer and then processed in Matlab as explained in Ref. . Meteorological measurements
Source siteThe wind speed was measured at 38, 50, 65, 80 and 90 m above sea level on a meteorological mast at the emission point. The wind direction was determined with wind vanes at 38 m and 80 m heights. The temperatures were measured at five heights: 6, 38, 50, 65 and 80 m. The relative humidity was measured at 38 m height. Data from these sensors were registered at 10 minutes intervals, and the average and standard deviations were recorded.
Receiver siteDuring the measurements performed in June 2005, wind profiles at the receiver site were measured during the day using radio probes and theodolite tracking of free flying balloons . These measurements were performed by staff from the Department of Earth Sciences, Uppsala University. Wind velocity (horizontal components), humidity and temperature were measured up to 3500 m height.
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GFPE Model The GFPE method was developed by Gilbert an Di [7,8] and later slightly improved by Salomons . The method is particularly designed for atmospheric sound propagation and can use considerably longer range-steps than conventional PE methods. Because of its computational efficiency, the GFPE model was used in this study. Model description The method computes a 2D field in the rz-plane where r is the radial distance from the source and z is the vertical axis. From the 3D Helmholtz equation for the sound pressure, p, in cylindrical coordinates combined with a variable substitution =exp(-i k0 r) pr0.5 two expressions (1) and (2) can be derived [7,9]
(r + r , z ) = exp(i
rk 2 ( z ) ) 2k r(1)
1 ik ' z 2 2 ((r , k ' ) + R(k ' )(r ,k ' )) exp(ir ( k r k ' k r ))e dk ' + 2 i z 2 2 + 2i (r , ) exp(ir ( k r k r ))e
where r is the horizontal step size, k(z)=/c(z) is the wave number, kr is a reference wave number (kr=k0=k(0) in this paper ), R(k)=(k' Zg-kr)/(k' Zg+kr)) is the planewave reflection coefficient, Zg is the normalized ground impedance, =kr/Zg is the surface-wave pole in the reflection coefficient and is given in Eq.(2)
(r , k ) = exp(ikz ' ) (r , z ' )dz '0
which combined constitute the fundamental step in the GFPE-algorithm. Parameter selection The parameters were selected by suggestions from Ref. [7,9,10]. The horizontal step was set to 10 and the vertical step size was 0.1 in accordance to recommendations in Ref. . An absorption layer that suppress spurious reflections from the upper limit of the numerical integration has an absorption parameter, A, calculated according to Ref.  with a depth of 75. The attenuation coefficients were calculated according to ISO/DIS 9613-1 . To calculate the surface impedance the model by Embleton et. al.  was used. The ground impedance is determined by the sound frequency and a flow resistivity parameter, which was selected to a value representative of grass in a rough pasture . The water surface was treated as perfectly reflecting. Turbulence Effects of wind and temperature fields were included in the GFPE model following the approach in Appendix I and J of Ref. . Thus, the turbulent components of Long distance sound propagation over a sea surface Page 4 of 9
these fields are modelled as homogeneous random fields with von Karman type horizontal wavenumber spectra. The effect of such turbulence on the GFPE solution is represented by including a random z-dependent phase factor in the GFPE propagator, without requiring explicit computation of realizations of the fields. According to this turbulence model the transmission loss to the receiver is a stochastic variable, and statistics of the transmission loss were determined by carrying out 50 Monte Carlo runs for each frequency at every hour of the experimental week. Meteorological assumptions Meteorological inputs to the GFPE model were balloon measurements (horizontal wind velocity), radio balloon (relative humidity and temperature) and the anemometers on the mast (standard deviation of wind speed and temperature). The wind and radio balloon data were used as meteorological parameters (U(z), rh, T) of the laminar atmosphere. Linear interpolation was used between measurement points in the vertical direction as well as in time. The mast data (horizontal wind speed and the variances of wind velocity and temperature) were used to estim