* 

NASA Contractor Report 177956 

PREDICTION OF FAR FIELD NOISE 

FROM WIND ENERGY FARMS 

{NASA-CR-177956) PREDICTICN CP THE PAR N86-252 15 

EfELD ROISE FACE EINC ENEKGY EAiiHS 

IBionetics COIF.) 34 HC A03/MF A01 

CSCL 20A Unclas 

G3/71 42918 

Kevin P. Shepherd and Harvey H. Hubbard 

THE BIONETICS CORPORATION 

Hampton, Virginia 

Contract NAS 1- 16978 

April 1986 

National Aeronautics and 

Space Administration 

Langley Research Center 

Hampton, Virginia 23665 
TABLE OF CONTENTS 

TITLE 

INTRODUCTION ........................ 1 

CALCULATION METHODOLOGY AND ASSUMPTIONS ........... 1 

Single Machine Reference Spectrum ......... 1 

Directivity of the Source ............. 2 

Spherical Spreading and Atmospheric Absorption . . 2 

Refraction .................... 3 

Freque ncy We igh t i ng Cons idera t ions 3 

Wind Energy Farm Configuration .......... 3 ........ 

SOUND PRESSURE LEVEL CALCULATIONS .............. 4 

Effects of Distance ................ 4 

Effects of Additional Rows of Machines ...... 5 

Effects of Row Length and Number of Machines ... 5 

Effects of Row Spacing .............. 5 

Effects of Machine Power Rating .......... 5 

Farm Directivity Considerations .......... 6 

Sound Pressure Level Contours ........... 6 

"A"-Weighted Composite Spectra .......... 7 

CONCLUDING REMARKS ..................... 7 

REFERENCES ......................... 8 

APPENDIX .......................... 9 

FIGURES ........................... 12 

i 
INTRODUCTION 

Wind turbine generators are being grouped in large numbers to 

establish wind energy farms and there is concern for possible 

adverse environmental impact of the radiated noise on nearby residents. Thus there is a need for generally accepted methods for 

predicting the noise from such wind energy farms for a variety of 

configurations and operating conditions. 

The present paper includes a review of the basic physical 

factors involved in making predictions; and outlines an approach 

which allows for differences in the wind turbine generators, configurations of the wind energy farms and propagation conditions. 

Example calculations are presented to illustrate the sensitivity 

of the radiated noise to such variables as machine size, spacing 

and numbers; and such atmospheric variables as relative humidity, 

temperature, wind velocity and wind direction. 

This effort is part of the Department of Energy program 

managed by the Solar Energy Research Institute. 

CALCULATION METHODOLOGY AND ASSUMPTIONS 

Relevant characteristics of the noise sources and factors 

which influence sound propagation are reviewed and evaluated. 

Included are such items as the reference noise spectrum for the 

particular machine used, the directivity of the noise source, 

a tmo s p he r i c at te nu at ion , f reque ncy we ig h t i ng cons ide ra t ions a nd 

the geometric arrangement of the wind energy farm. The methodology used to predict the noise from a large number of sources 

such as a wind energy farm is described along with the simplifying assumptions which are valid and pertinent. 

Single Machine Reference Spectrum 

The most basic information required for the prediction of 

noise from a wind energy farm is the noise output of a single 

machine. Its noise spectrum may be predicted based on knowledge 

of the geometry and operating conditions of the machine (refs. 1 

and 2) or it may be measured at a reference distance. An example 

of spectral data for a particular size range of machines is given 

in figure 1. The measured spectrum is taken from ref. 3 and represents a 50 kw downwind machine having a three blade rotor with 

a diameter of 56 ft. The hatched area encompasses the range of 

available unpublished data for several machines rated at 50 to 

100 kw for both downwind and upwind configurations, having 2 and 

3 bladed rotors with rotor diameters ranging from 48-61 ft. Also 

shown in figure 1 is the spectrum which is used subsequently in 

this paper in example calculations to represent a machine having 

a rated power of approximately 100 kw. It is the straight (solid) 

line having a decrease in sound pressure level with increasing 

frequency of 1 dB per one third octave band or 10 dB per decade. 

-1- 
This shape is considered to be generally representative of the 

aerodynamic noise of wind turbine generators, however predictions 

for a specific wind energy farm should be based if possible, on 

data for the particular types of machines of which it is comprised. 

Directivity of the Source 

Measurements of aerodynamic noise for a number of large horizontal axis wind turbines (refs.4-8) indicate that the source 

directivity depends on the significant noise generating mechanisms. 

For broad band noise sources such as those due to inflow turbulence 

and blade boundary layer/blade trailing edge interactions, the 

sound pressure level contours are approximately circular at distances close to the machine. The above higher frequency sources 

are common to all types of machines. Lower frequency impulsive 

noise which is due to the interactions of the blades with the 

tower wake, radiates most strongly in the upwind and downwind 

directions. This latter source is characteristic of machines for 

which the blades are located downwind of the supporting tower. 

Even though most wind turbine sites have a prevailing wind 

direction, it is not uncommon for the wind vector to vary over a 

range of 90" in azimuth angle during normal operations. Thus one 

of the simplifying assumptions made in the calculations to follow 

is that each individual machine behaves like an omnidirectional 

acoustic point source. 

Spherical Spreading and Atmospheric Absorption 

For the case of non-directional single sources and closely 

grouped multiple sources, spherical spreading may be assumed in 

the far radiation field. Circular wave fronts propagate in all 

directions from the source and the sound pressure levels decay at 

the rate of 6 dB per doubling of distance. This latter decay rate 

is illustrated by the straight line of figure 2, and is valid for 

situations where no other attenuation mechanisms are operative. 

Absorption of sound by the atmosphere is usually substantial 

for wind turbine noise and must be considered. The resulting 

effects are illustrated in figure 2 which shows the decay of sound 

pressure level with distance for various noise frequencies. The 

top line, as noted above, represents zero atmospheric absorption, 

a condition that would apply only for low frequency components. 

At higher frequencies, as indicated by the dashed lines, the decay 

in sound pressure level with distance becomes larger. 

The curve of figure 3 was plotted from the tabulated values 

of ref. 9 and details the changes in atmospheric absorption as a 

function of frequency. For the examples shown later in this paper, 

the ambient temperature was assumed to be 20°C and the relative 

humidity was assumed to be 70% (see figure 3). Atmospheric 

absorption values for other conditions of ambient temperature and 

relative humidity can be obtained from the extensive tables of 

ref. 9. 

-2- 
Refract ion 

Refraction can cause nonuniform propagatiori as a function 

of azimuth angle around a source. An example of the effects of 

refraction due to a mean wind gradient is given in figure 4 for 

an elevated point source. Note that in the downwind direction the 

rays tend to bend toward the ground due to the wind gradient whereas in the upwind direction the rays curve upward away from the 

ground. This results in the formation of a shadow zone upwind of 

the source within which the noise is greatly attenuated as a function of distance. In a situation where there is a prevailing wind, 

refraction effects may thus be used to advantage in the siting of 

wind energy farms. If however the direction of the wind is variable, this advantage is lost. Note that temperature gradients may 

also be present and may add to or subtract from the effects due 

to wind illustrated in figure 4. It is believed that in general 

wind effects will dominate the temperature effects in noise propagation from wind energy farms. 

Frequency We ight i ng Considerat ions 

For the evaluation of the direct effects of noise on communities the "A"-weighted metric expressed in dB(A) is in widespread 

use. The nature of this metric is shown in figure 5. The assumed 

single machine reference spectrum for a distance of 30 m is reproduced from figure 1 as the solid line. The equivalent *A"-weighted 

spectrum at the same distance is shown as the topmost dashed curve. 

It can be seen that this particular weighting emphasizes the higher 

frequencies and de-emphasizes the lower frequencies. At increased 

distances, as illustrated by the bottom two solid curves, the 

levels of the higher frequency components decay at a faster rate 

than those of the lower frequencies, due to atmospheric absorption. 

The result is that the mid-range of frequencies (100-1000Hz) tend 

to dominate the "A"-weighted spectrum at large distances. Frequencies higher than lOOOHz will generally not be important at large 

distances due to the effects of atmospheric absorption. Those 

frequency components below lOOHz may not be significant for ''A"- 

weighting considerations but they can be significant for indirect 

effects such as noise induced building vibrations, which occur 

generally at structural resonances below 100 Hz. 

Wind Energy Farm Configurations 

A number of different geometric arrangements of multiple noise 

sources has been considered for the purpose of representing example 

wind energy farms. These different arrangements are shown schematically in figure 6. Configuration A is the baseline with which the 

others are compared. It consists of 31 machines per row, each 

machine having a power of approximately 100 kw, and a rotor diameter 

of 15 m. The spacing between machines is 30 meters, the total row 

length is 900 m and the spacing between rows is 200 m. The number 

of rows was varied from one to eight. Configuration B is comparable 

to configuration A except that the row spacing is reduced to 100 m. 

-3- 
Configuration C involves machines having 4 times the rated power 

and the machine separation and row s e p a r a t i o n d i s t a n c e s are twice 

those of the baseline (config. A). 

Configurations D and E both have row lengths that are twice 

that of the baseline. Because of differences in machine spacing, 

configuration E has the same total number of machines as the 

baseline whereas configuration D has twice the total number of 

machines. 

SOUND PRESSURE LEVEL CALCULATIONS 

A series of parametric sound pressure level calculations has 

been performed based on the arrays illustrated in figure 6. For 

the data of figures 7 through 11 the observer is located on the 

line of symmetry perpendicular to the rows and the observer distance is measured from the nearest row of machines. In figure 11 

the observer is located on the line of symmetry parallel to the 

rows. 

For figures 7 through 12 the sound pressure level values 

were calculated by summing, on an energy basis, the contributions 

of each individual machine (see appendix). It was assumed that 

each machine radiated noise equally in all directions and was 

represented by the reference spectrum shown in figures 1 and 5. 

The contribution of each machine was based on its distance to the 

observer position. The calculation of the total sound pressure 

level assumed that the sources were -incoherent (i.e., random phase). 

Four values of atmospheric absorption,a, were used in the calculations: 0, 0.10, 0.27, 0.54 dB/100 m. These may be considered 

to represent one-third octave band center frequencies of 50, 250, 

500, and 1000 Hz respectively provided that the temperature is 

2OoC and the relative humidity is 70% (see figure 3). These frequency values were chosen because they encompass the range believed 

to be important for evaluating the environmental impact of wind 

turbine noise in adjacent communities. 

Effects of Distance 

Figure 7 shows calculated sound pressure level values for configuration A as a function of distance downwind for several different values of atmospheric absorption. Figure 7(a) shows the characteristic spreading from an extended source for zero atmospheric 

absorption. The decay rate as a function of distance is less than 

for the single source of figure 2. At intermediate distances the 

array acts like a line source for which the theoretical decay rate 

is 3 dB per doubling of distance or 10 dB per decade. Only at the 

decay rate approach the single source value of 6 dR per doubling 

of distance or 20 dB per decade. For figures 7 ( b ) , 7 ( c ) , and 7(d) 

for which the atmospheric absorption is greater than zero the decay 

rates are obviously greater than for the zero absorption case and 

increase as the absorption coefficient increases. 

extreme distances, greater than one row length or 900 m, does the 6 

-4- 
Effects of Additional Rows of Machines 

The calculations of figure 7 were made for one, two, four 

and eight rows of machines, thus illustrating the effect of progressively doubling the number of machines. For the case of zero 

atmospheric absorption and at receiver distances which are large 

compared to the array dimensions, a doubling of the number of rows 

results in an increase in sound pressure level of 3 dB. This 

simply reflects a doubling of acoustic power. At shorter distances 

the closest machines dominate, and the additional rows result in 

only a small increment in the sound pressure level. 

In the case of non-zero atmsopheric absorption, the row spacing is significant even at large receiver distances. Doubling the 

number of rows results in less than a 3 dB increase in sound pressure level. This is particularly apparent at the highest frequencies and for the larger number of rows (figure 6(d)). 

Effects of Row Length and Numbers of Machines 

The effects on the sound pressure levels of doubling the lengths 

of the rows is illustrated in figures 8 and 9. In figure 8 comparisons are given between configurations A and E. In configuration E 

the same numbers of machines are involved but the spacing between 

machines is increased. Comparative data for a single row and for an 

array of eight rows are shown for absorption coefficients of 0 and 

0.54 dB per 100 m. At extreme distances the same levels are achieved 

for configurations A and E because there is the same number of machines 

in each row. At the shorter distances the levels are higher for configuration A because of the closer spacing of the machines. 

Figure 9 shows similar data for configurations A and D. For 

these comparisons the machine spacing is constant and the row 

lengths are doubled by doubling the numbers of machines per row. 

For these comparisons the sound pressure levels at the shorter distances are equal because of the equal machine spacing. At extreme 

distances the levels for configuration D are higher by 3 dB because 

the numbers of machines per row are doubled. 

Effects of Row Spacing 

The effects on sound pressure levels of a change in the row 

spacing is seen in figure 10. Comparisons are shown for configurations A and B for atmospheric absorption values of 0 and 0.54 dB 

per 100 m, and for arrays of 2 and 8 rows. Configuration B has the 

same numbers of machines per row but the spacing between rows is 

half the distance of that for configuration A. At all distances 

the sound pressure levels are higher for the more compact array. 

These observed differences are however small at extreme distances. 

Effects of Machine Power Rating 

Configurations A and C are compared in figure 11 to illustrate 

the effects of machine power rating. For these comparisons the 

-5- 
machine spacing and row spacing for the larger machines are double 

those for configuration A. Data are given for atmsopheric absorption coefficients of 0 and 0.54 dB per 100 m and for single rows 

and arrays of 8 rows. The assumptions are made that the reference 

spectrum shape for the larger machine is the same as for the smaller 

machine but that the levels are 6 dB higher (implying four times 

the acoustic power). For the single row of 16 large machines, the 

sound pressure levels are about 3 dB higher than for a single row 

of 31 small machines, due to the fact that the total acoustic power 

is essentially doubled. For the larger arrays the higher sound 

pressure levels are also associated with the larger machines, due 

to the greater acoustic power per row of machines. Different 

results would be obtained if the larger machines had reference 

spectra which differ in shape from that of figure 1. 

Farm Directivity Considerat ions 

Even though the individual machines have been treated as if 

they radiate sound equally in all directions, an array of such 

sources may not have uniform directivity characteristics. Figure 

12 shows a comparison of the sound pressure levels for configuration A as viewed from two different directions. Data are presented 

for an observer located on the line of symmetry perpendicular to 

the rows, and also for an observer located on the line of symmetry 

parallel to the rows. These two conditions are referred to as downwind and crosswind, respectively in figure 12. At extreme distances 

the differences in sound pressure level are small. However at the 

shorter distances the downwind levels are higher because of the 

closer spacing of the machines in the row. 

Sound Pressure Level Contours 

Sound pressure level contours for a wind energy farm have been 

estimated, and are presented in figure 13. The farm geometry was 

chosen to be configuration A with five rows of 31 machines each, 

thus yielding an approximately square array. This results in 

essentially circular sound pressure level contours. 

Figure 13 gives sound pressure level contours of 40, 50 and 

60 dB for four values of atmospheric absorption. The contours are 

circular if there are assumed to be no propagation effects due to 

the wind gradient. These contours thus represent a worst case 

scenario. It is readily apparent that the low frequency contours 

extend to much greater distances than do the high frequency contours. 

Under the assumption of a fixed wind direction, the distance 

to contours in the upwind direction is greatly reduced, as indicated by the dashed curves in figure 13. These upwind contours 

are derived from computed distances to the acoustic shadow zone 

and excess attenuation occuring within it (see ref. 8). The 

formation of an acoustic shadow zone results in greatly reduced 

distances to particular noise level contours for all frequencies 

above about 60 Hz. 

-6- 
A more dramatic method of illustrating the differences of 

figure 13 is to compare the ground areas that are exposed to particular sound pressure levels. Figure 14 presents such data, for 

the two downwind quadrants only of figure 13, for a range of 

atmospheric absorption values from 0 to 0.54 dB per 100 m. The 

most obvious result is that the areas exposed to the lower levels 

of noise radiation are much larger than those exposed to the higher 

levels. It can also be seen that the exposed areas decrease rapidly as atmospheric absorption increases. This effect is most 

noticeable for the lower level contour areas for which large distances are involved. 

"A"-We ighted Compos ite Spectra 

The data of figures 7 through 14 are derived for a few 

selected values of atmospheric absorption and illustrate how the 

noises from individual machines sum together for various arrays 

of machines. Figure 15 illustrates the effects of "A"-weighting 

the composite sound spectrum from wind energy farm configuration 

A. It can be seen that as for the single machine example of figure 

4, at large distances the mid range of frequencies dominate the 

"A"-we ighted spectrum. 

CONCLUDING REMARKS 

A proposed model to predict noise from wind energy farms is 

presented. Various assumptions are made concerning characteristics 

of the noise sources and propagation phenomena. The wind turbines 

are assumed to be omnidirectional noise sources which add together 

in random phase. Propagation is assumed to be controlled by 

spherical spreading and atmospheric absorption, with refraction 

effects included in the upwing direction. 

Very few data are available with which to compare the results 

of this analytical study. As other data become available it may 

be shown that other factors such as ground absorption, refraction 

in the downwind direction and noise source directivity may need 

to be incorporated as refinements to this model. 

-7- 
REFERENCES 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

Grosveld, F.W. : Prediction of Broadband Noi.se from Horizontal 

Axis Wind Turbines. AIAA Journal of Propulsion and 

Power. Vol. 1, No. 4, July-August 1985. 

Viterna, L.A.: The NASA-LeRC Wind Turbine Sound Prediction 

Code. Presented at the Department of Energy Wind Turbin 

Dynamics Conference 24-27 February 1981, NASA CP 2185, 

1981. 

Hubbard, Harvey H., and Shepherd, Kevin P. : Noise Measurements 

for Single and Multiple Operation of 50 kw Wind Turbine 

Generators. NASA CR 166052, December 1982. 

Shepherd, Kevin P. and Hubbard, Harvey H. : Measurements and 

Observations of Noise from a 4.2 Megawatt (WTS-4) Wind 

Turbine Generator. NASA CR 166124, May 1983. 

Grosveld, F.W., Shepherd, K.P. and Hubbard, H.H. : Measurement 

and Prediction of Broadband Noise from Large Horizontal 

Axis Wind Turbine Generators. Presented at NASA/DOE 

Workshop on Horizontal Axis Wind Turbine Technology, 

Cleveland, Ohio, May 8-10, 1984. 

Shepherd, Kevin P. and Hubbard, Harvey H.: Sound Measurements 

and Observations of the MOD-OA Wind Turbine Generator. 

NASA CR 165856, February 1982. 

Hubbard, Harvey H., Shepherd, Kevin P. and Grosveld, F.W.: 

Sound Measurements of the MOD-2 Wind Turbine Generator. 

NASA CR 165752, July 1981. 

Shepherd, Kevin P., and Hubbard, Harvey H.: Sound Propagation 

Studies for a Large Horizontal Axis Wind Turbine. NASA 

CR 172564, March 1985. 

Anonymous: Method for the Calculation of the Absorption of 

Sound in the Atmosphere. ANSI S1.26, 1978. 

Rathe, E.J.: Note on Two Common Problems of Sound Propagation. 

Journal of Sound & Vib., vol. 10, no. 3, 1969. 

. 

-8- 
APPENDIX 

CALCULATION OF SOUND PRESSURE LEVEL DUE TO MULTIPLE SOURCES 

Two methods are presented for calculating the sound pressure 

level due to multiple noise sources at any arbitrary receiver distance. Both methods assume that each source radiates equally in 

all directions and include attenuation due to atmospheric absorption. The first method has no limitations on either the number 

of machines or their geometric arrangements. The second method 

(derived from ref. 10) which is computationally more efficient, 

requires that the sources be approximately equally spaced within 

rows and that the number of sources per row be greater than three. 

Method A. Summation of Contributions from Each Source 

The required input is a sound pressure level spectrum, either 

narrow band or one-third octave band, for a single machine. This 

spectrum should be measured or predicted for a distance M, meters 

from the machine, where M is approximately equal to the tower 

height plus the rotor radius. 

The following procedure should be performed for each frequency 

band : 

1. The contribution from a single machine at a distance, R, 

from a receiver is given by: 

Sound Pressure Level Si = L - 20 log(~./M) -UR 

where L is sound pressure level at distance M, and 01 is atmosperic 

absorption in dB/m (ref. 9). 

Note that 0 varies as a function of frequency, temperature and 

re 1 at ive humid i ty . 

2. Calculate contributions from all other sources as in step 1. 

The total sound pressure level at a particular receiver location is given by: 

n 

Total Sound Pressure Level = 10 loglo i=l c ,,Si/lO 

where n is the number of sources. 

This procedure should be repeated for all frequency bands, 

thus yielding a sound pressure level spectrum at the receiver 

location. Noise measures such as A-weighted sound pressure Level 

may subsequently be calculated for each receiver location. 

-9- 
Method B. Summation of Contributions from Each Row of Machines 

If the sources are arranged in rows, the following procedure 

reduces the computations required to predict the sound pressure 

level at a receiver location. 

As for Method A, the required input is a sound pressure 

level spectrum for a single source for a distance M. A finite 

row of sources, spaced "b" meters apart, results in a sound 

pressure level at a distance R given by: 

Receiver 

where et is atmospheric absorption in dB/m and R1 is distance from 

receiver to closest source. 

Note that en, 61 are in radians and may take positive or negative 

values, but en must be greater than 81. For the case where the 

receiver is on the line of symmetry perpendicular to the row, 81 = 

-en and R1 = R and the equation reduces to: 

Contributions from other rows should be calculated similarly and 

summed as in step 2 of Method A (n will be the number of rows). 

This procedure will not work if R=O; however a small number, 

approaching zero, will overcome this problem. This procedure 

gives extremely accurate results (errors less than 0.2dR) for 

atmospheric absorption equal to zero. For non zero absorption 

values, accuracy is excellent in the downwind direction (perpend- 

-10- 
icular to rows) but may lead to errors of 1-2dB in the crosswind 

direction at high frequencies. 

-11- 
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( c) configuration C 

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Figure 6. - Schematic diagrams of the geometric layouts of wind 

energy farms for which noise calculations are 

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I I I 11111 I I I IIIII 

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Distance, m 

Figure 7. - sound pressure levels for various assumed absoprtion 

coefficients as a function of distance downwind from 

the example wind energy farm of configuration A. 
No. 

of rows 

, 

80 

70 

60 

50 

40 

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Distance, m 

Figure 7. - Concluded. 
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Figure 8. - Effects of machine spacing and row length on the 

calculated sound pressure levels downwind of wind 

energy farms corresponding to configurations A and E. 
80 

.. 70 

60 

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v, 40 

30 

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Figure 9. - Effects Of row length on the ca 

levels downwind of wind energy 

configurations of A and D. 

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Figure 10. - Effects of row spacing on the calculated sound pressure 

levels downwind of a wind enerqy farm corresponding 

to configurations A and B. 
I I I I I111 I I I IIIII 

100 1000 10 000 

Distance, m *Or 

m I 

I I I 1 I Ill1 I \I I IIIII 

100 1000 10 000 

Distance, m 

Figure 11. - Comparisons of sound pressure level as a function Of 

distance in the downwind direction for wind energy 

farms incorporating two different sizes of machines. 
Crosswind 

I I I I I I I11 I I 1 1 I 1111 

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Figure 12. - Comparisons of sound pressure levels as a function 

of distance in the downwind and cross wind directions 

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, 
. 1. Report No. 2. Govrnmant AcreKw, ' No. 3. Rcipiont'r etab NO. 

4. Title and Subtitle 

NASA CR-177956 

5. R~port Date 

Prediction of Far Field Noise from W€nd Energy Farms 

7. Author(sl 8. hrforming Organization Report No. 

Kevin P. Shepherd 

Harvey H. Hubbard 

The Bionetics Corporation 

10. Work Unit No. 

9. Performing Organization Name and Address 

18 Research Drive 11. Contract or Grant No. 

Hampton, VA 23666 NAS1-16978 

, 13. Type of Report and Period Covad 

12. Sponsoring Agency Name and Address NASA Contractor Report 

15. Supplementary Notes 

This report was prepared jointly by Kevin P. Shepherd and Harvey H. Hubbard of 

the Bionetics Corp. under contract NAS1-16978. Langley Technical Monitor: 

Clemans A. Powell 

Included in this paper are a review of the basic physical factors involved in 

making predictions of wind turbine noise; and an approach which allows for 

differences in the machi'nes, the wind energy farm configurations and propagation 

conditions. Example calculations are presented to illustrate the sensitivity 

of the radiated noise to such variaGles as machine size, spacing and numbers; 

and such atmosphere variables as ab'sorption (relative humidity and temperature) 

and wind direction. 

16. Abstract 

Calculated far field distances to particular sound level contours are greater 

for lower values of atmospherlc absorption, for a larger total number of 

machines, for additional rows of machines and for more powerful machines. At 

short and'intermediate distances, lifgher sound pressure levels are calculated 

for closer machine spacings, for more powerful machines, for longer row lengths 

and for closer row spacings. 

b 

17. Key Words (Suggclted by Author(sl1 18. Distribution Statement 

Wind Turbine 

Noise Prediction Unclassified - Unlimited 

Parametric Study 

Subject Category - 71 

19 Securiry Classif. (of this report) 20. Security Clrwf. (of this prey) 21. No. of Pages 22. Rice 

Unclassified Unclassified 33 A0 3 i 

11-305 Fw sale by the National Technical Information Service, Springfield. Vlrglnia 22161 
I 

I 

4. Title and Subtitle 

Prediction of Far Field Noise fromwind Energy Farms 

1. Report No. 2. Gownnunt Accawm . No. 3. Raipiont's Catah NO. 

NASA CR-177956 

5. Report 0110 

April 1986 

6. Morming Orpnizatim 

7. Author(s) 

Kevin P. Shepherd 

Harvey H. Hubbard 

The Bionetics Corporation 

- 

9. Perforrnlng Organization Name and Address 

18 Research Drive 

Hampton, VA 23666 

12 Sponsoring Agency Name nd Address 

National Aeronautics and Space Administration 

Washington, DC 20546 

8. kformiq Or-niration R- NO. 

10. Wak Unit No. 

11. Contract or Grant No. 

NAS1-16978 

13. Tvp of Rlpoct and Period Covcrd 

NASA Contractor Report 

Ycod. '' ?%6y-'i;f 0 2 

776-31-41-51 

Oistribution Statcnwnt 

5 I Unclassified - Unlimited 

SuGject Category - 71 

I Unclassified Uncla s s if ied 

11-305 For sale by the National Technical information Service, Springfield, Virginia 22161