Acoustic Tests of Small Wind 

Turbines

Preprint 

October 2003 • NREL/CP-500-34662 

P. Migliore, J. van Dam, and A. Huskey 

To be presented at the 2004 Wind Energy Symposium 

Reno, Nevada 

January 5–8, 2004 

National Renewable Energy Laboratory 

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1

ACOUSTIC TESTS OF SMALL WIND TURBINES*§

P. Migliore, J. van Dam and A. Huskey 

National Renewable Energy Laboratory, National Wind Technology Center 

1617 Cole Boulevard, Golden, Colorado 80401, USA 

paul_migliore@nrel.gov; jeroen_van_dam@nrel.gov; arlinda_huskey@nrel.gov

 ABSTRACT*§

Eight small wind turbines ranging from 400 watts to 

100 kW in rated power were tested for acoustic emissions at the U.S. Department of Energy’s National Renewable Energy Laboratory. Rigorous test procedures 

based on international standards were followed for 

measurements and data analyses. Results are presented 

in the form of sound pressure level versus wind speed, 

where the sound was recorded downwind of the turbine 

at a distance equal to the hub height plus half the rotor 

diameter. When there was sufficient separation between wind turbine noise and background noise, the 

apparent sound power level was calculated. In several 

cases, this was not possible. The implications of this 

problem are discussed briefly. Some of the configurations tested were specifically developed to reduce the 

noise level of their predecessors. Test data for these 

machines demonstrate marked progress toward quieter 

turbines. 

INTRODUCTION

Until recently, wind turbine manufacturers and operators were challenged by the tasks of keeping machines 

operating reliably and improving energy capture. Although dramatic improvements have been made in both 

areas, there have been occasions when acoustic emissions proved so vexing they overshadowed performance 

and reliability issues. For example, some wind turbines 

suffer an unfavorable reputation for noise problems 

associated with high tip speeds, furling, or blade flutter. 

The U.S. Department of Energy (DOE) and its National 

Renewable Energy Laboratory (NREL) are engaged in 

several turbine research and demonstration projects 

focused on reducing the cost of energy at low wind 

speed sites. Recent analyses have shown that this effort, if successful, will lead to the installation of wind 

turbines in large numbers. In this circumstance, it is 

essential that the turbines available for deployment are 

quiet. This suggests there should be an effort by NREL 

 

*

 This work was performed at the National Renewable Energy 

Laboratory in support of the U.S. Department of Energy under Contract No. DE-AC36-99GO10337. 

§

 This material is declared a work of the U.S. Government and 

is not subject to copyright protection in the United States. 

to measure the acoustic signature of existing turbines 

and work diligently to reduce (below the state of the 

art) the signatures of new turbines being developed. 

Coincidentally, with recent energy shortages and the 

ensuing statewide deployment incentives, there is resurgent interest in small wind turbines for distributed 

generation. Because of the potential for installation 

near residences, noise may be even more important for 

small turbines than for large turbines installed in wind 

power plants. 

Because small wind turbines are sold in large numbers 

and located close to people, there is a need for reliable 

noise data. If it was available, homeowners and local 

authorities could use the information to develop expectations regarding noise production before the turbines 

are actually installed. Furthermore, based on field test 

observations and the influence of the parameters investigated, improvements to the turbines might be made 

with relative ease and low cost. 

As part of its aeroacoustic research program, NREL 

performed acoustic tests [1] on eight small wind turbines with power ratings from 400 W to 100 kW. The 

goals of these tests were to develop a database of acoustic signatures to compare new and existing turbines and 

to establish targets for low-noise rotors. Test results 

will be documented and disseminated in the form of 

NREL reports, technical papers, seminars, and colloquia. This is part of broader effort to support the U.S. 

wind industry in applying rational acoustic-design principles to the development and deployment of advanced 

wind turbines. 

 

Tests were conducted on two Bergey Excel and one 

XL.1 turbines, one Southwest Windpower Whisper 

H40 and two AIR turbines, an Atlantic Orient Corporation AOC 15/50, and a Northern Power Systems North 

Wind 100. In some cases, more than one configuration 

was tested to demonstrate noise reduction techniques. 

Measurements were made according to procedures described in the International Electrotechnical Commission (IEC) standard for acoustic noise measurement 

techniques [2] with minor modifications that were necessary for small turbines. In addition to the acoustic 

signals, wind speed and direction, turbine power and 

rotor speed were measured. In this paper, results are 
 

2

presented as sound pressure level and apparent sound 

power level for several wind speeds of interest. In the 

NREL report [1], noise spectra of sound pressure level 

versus imission1

 frequency are also provided. 

MEASUREMENTS AND DATA ANALYSES

Acoustic tests were conducted at the National Wind 

Technology Center (NWTC) near Boulder, Colorado. 

The site is located in somewhat complex terrain at an 

approximate elevation of 1850 m above sea level. The 

soil is covered with grassy vegetation and measurements indicate that the roughness length is approximately 0.05 m. A gravel mine and concrete plant to the 

west are the main sources of background noise, although passing automobiles and airplanes also contribute. The prevailing wind direction is 292o

 relative to 

true north. 

Data were collected and analyzed according to the IEC 

standard [2] and NREL’s quality assurance system [3] 

where possible. A reference microphone was located 

downwind of the turbine at a distance equal to the hub 

height plus half the rotor diameter. The microphone 

was placed on a circular plywood ground board that is 

one meter in diameter and 13 mm thick. The ground 

board was placed on a flat surface with no cavities beneath and the edges of the board were covered with dirt. 

Three additional microphones and ground boards were 

placed around some turbines for special tests. For this 

study, only data from the reference microphone were 

considered. 

Wind speeds of 6–10 m/s were measured, although 

measurements were taken outside this range for some 

turbines. Data were obtained for both the operating and 

parked conditions to allow correction for background 

noise. In circumstances of intrusive background noise, 

such as airplanes, automobiles or animals, the test data 

were discarded. 

In addition to the acoustic pressures, wind speed and 

direction were measured. Both were essential to the 

subsequent analysis, and particular importance is assigned to having the reference microphone downwind 

of the turbine. For some tests, rotor speed and power 

were also measured with the expectation that these data 

might provide insight regarding noise-generating 

mechanisms. 

 

1

 In the study of acoustics, the term “imission” refers to the 

noise level perceived by an observer at a receptor location. 

This is in contradistinction to the term “emission” which 

means ‘something sent forth by emitting’ and refers to the 

strength of the acoustic source. 

Acoustic data were recorded on an 8-channel digital 

audiotape (DAT). All other data were recorded on a 

digital data logger. The analog microphone signals were 

recorded (digitally) on the DAT and then played back 

as analog inputs to a signal analyzer. Depending on the 

desired averaging period, either 1-minute or 10-second 

average sound pressure2

 levels were calculated. Although the IEC standard prescribes 1-minute averages, 

10-second averages seem to reflect the system dynamics better for small turbines. The sound pressure levels 

were synchronized with the averages of the other data 

channels, and the average wind speed was determined 

for each data point then normalized to standardized 

conditions. 

The wind speed standardization equation takes the wind 

speed measured at any height and roughness length and 

normalizes it to a “standardized” height of 10 m and a 

roughness length of 0.05 m. The formula used for this 

transformation is given in Equation (1). 

Vs = Vz · [ln (10/0.05) ln (H/z0) ÷ ln (H/0.05) ln (z/z0)](1) 

where, 

Vs is the standardized wind speed (m/s) 

Vz is the wind speed (m/s) measured at height z 

H is the rotor center height (m) 

z0 is the roughness length of the test site (m) 

z is the wind speed measurement height (m) 

Noise measurements for the operating wind turbine 

(wind turbine plus background noise) are correlated 

with background-only noise measurements at standardized wind speeds. The noise measurements are then 

corrected for background noise using Equation (2). 

Ls = 10 · log [10(Ls+n/10) – 10(Ln/10)] (2) 

where, 

 

2

 Sound is characterized by small pressure fluctuations overlaying atmospheric pressure, but the human ear does not respond linearly to the amplitude of sound pressure [4]. Doubling the amplitude produces the sensation of louder noise, 

but it seems far less than twice as loud. For this reason, the 

scale used to characterize sound pressure amplitudes is logarithmic, which is an approximation of the actual response of 

the human ear. The definition of sound pressure level Lp is 

Lp = 10 · log [ p2 ÷ p2

ref ] expressed in decibels, dB, 

where p is the root mean square sound pressure and pref has a 

value of 2 · 10-5 Pa corresponding to the weakest audible 

sound – the threshold of human hearing – at a frequency of 

1000 Hz. 
 

3

Ls is the equivalent sound pressure level (dB) of 

the wind turbine operating alone 

Ls+n is the equivalent sound pressure level (dB) of 

wind turbine plus background noise 

Ln is the equivalent sound pressure level (dB) of 

the background noise 

 

The background-corrected sound pressure level of the 

wind turbine is translated into sound power3

 level using 

Equation (3). The 6 dB constant accounts for the approximate doubling of sound pressure that occurs for 

microphone measurements on a ground board [2]. 

LWA = LAeq, c – 6 + 10 · log [4πR1

2

/S0] (3) 

where, 

LWA is the background-corrected A-weighted4

 apparent sound power level of the turbine, dB(A) 

LAeq, c is the background-corrected A-weighted sound 

pressure level determined from analysis of 

multiple data pairs as described below, dB(A) 

R1 is the slant distance, in meters, between the 

microphone and the rotor center 

So is the reference area, So = 1 m2

In practice, Equation (2) is not applied to individual 

data points. Instead, a large amount of data is accumulated and calculations are based on trends or averages. 

A linear regression is used to fit a straight line through 

the measured sound pressure level data for the operating wind turbine between the standardized wind speeds 

of 6 and 10 m/s. The process is repeated for back-

 

3

 Whereas sound pressure level is a property of the observer 

location [4], the total strength of a source of sound is characterized by the sound power emitted by the source. In general, 

the sound power P transmitted through a surface S is the integral of the sound intensity I (energy transmitted per unit time 

and unit area) over the surface. If the surface S encloses the 

source of the sound, then P is the total sound power emitted 

by the source. The definition of sound power level is 

Lw = 10 · log [ P ÷ Pref ] expressed in decibels, dB, 

where Pref =10-12 watts is the standard reference sound power. 

The eardrum can detect incoming sound power as weak as 

one picowatt, and exposure to incoming sound power of more 

than one watt will result in some hearing loss. 

4

 The ear is not equally sensitive to tones of different frequencies. Maximum response occurs between 3000 and 4000 Hz, 

where the hearing threshold is somewhat less than 0 dB. A 

100 Hz tone, however, must have an intensity of 40 dB to be 

heard [4]. Therefore, weighted sound levels have been introduced where lower frequencies are de-emphasized in a manner similar to human hearing. A-weighting is most commonly 

used and is well suited for sound levels that are not too high. 

ground noise measurements. Then, the backgroundcorrected sound pressure level is determined for a particular wind speed by subtracting the two results using 

Equation (2). However, according to the IEC Standard 

[2], if the difference in sound pressure level between 

the operating wind turbine and the background noise is 

less than 6 dB, the data may not be used for determination of the sound power level at that wind speed. If the 

difference is at least 6 dB, the sound power level for the 

turbine is calculated from Equation (3). 

A second method for calculating sound power level was 

used in some cases. All of the acoustic data for the operating turbine and for the background noise were 

sorted and energy averaged in 1-m/s wind speed bins 

centered on integer values. A sound pressure level for 

the operating turbine was calculated for each wind 

speed if there were more than three data points in the 

bin. This process was repeated for background measurements. For each wind speed bin, the operating turbine noise was corrected for background noise using 

Equation (2). If the difference between the two was at 

least 6 dB, the sound power level for the wind turbine 

operating alone was calculated using Equation (3). 

This method was used for the comparisons in Table 1. 

In addition to evaluating the sound power level as described above, it is useful to examine the spectra of 

sound pressure level versus frequency. NREL uses one 

of two approaches, depending upon the availability of 

data. Either two 1-minute spectra or twelve 10-second 

spectra having wind speeds closest to the reporting 

wind speed were energy averaged to obtain one spectrum. These narrow band spectra, so called because 

small incremental frequency bands were used, were 

reported [1] for wind speeds of 6, 8, and 10 m/s. 

In some cases, the narrow band spectra were analyzed 

for the presences of pure tones. That information is not 

reported here but may be found in the individual test 

reports [5, 6, 7, 8]. The spectra were visually checked 

for the presence of possible tones. Similar spectra were 

developed for background noise around the same wind 

speeds to make sure that the peaks did not originate 

from the background noise. If there were no obvious 

tones indicated and nothing was heard during the tests, 

no further analysis was performed. If tones were observed, the Measnet [9] procedure was used to determine tonality. In this procedure, the critical band is 

identified and the tone and masking noise levels are 

calculated. The tonality value is the difference between 

the tone level and the masking noise level. 
 

4

BERGEY EXCEL-S TURBINES

Bergey Windpower Company of Norman, Oklahoma, 

(www.bergey.com) manufactures the Excel-S (shown 

above), which is a three-blade upwind turbine that Bergey rates at 10 kW at a wind speed of 13 m/s. It is connected to a Bergey Gridtek inverter that provides power 

to the NWTC electrical grid. The Excel uses a permanent magnet alternator to produce three-phase variable 

frequency output at a nominal 240-volts. The threephase output is rectified to DC power and then converted to single-phase 240-volt 60 Hz AC power in the 

inverter. The turbine blades are constructed of pultruded fiberglass. In high wind speeds—greater than 

about 16 m/s—the turbine will furl out of the wind to 

protect it from over-speeding. 

The rotor diameter of the machine tested at the NWTC 

was 7 m and its hub height was 36.5 m. The slant distance of the microphone, an important parameter in 

Equation (3), was 54.5 m. To better reflect the dynamics of the turbine, 10-second averages were used instead 

of 1-minute averages. Wind speed was measured at hub 

height and standardized using Equation (1). 

The Bergey Excel operates both loaded and unloaded, a 

condition defined by whether or not it is connected to 

the load. The load in this case was the utility grid. Because the operating condition has a strong influence on 

the noise characteristics, measurements were taken under both conditions. 

Figure 1 shows the measured sound pressure levels for 

an earlier version of the Excel with BW03 airfoils. The 

graph also shows sound pressure levels measured when 

the inverter was offline (turbine was unloaded) for all 

or part of the 10-second averaging period. In this situation, the noise level increases approximately 4 dB(A) to 

5 dB(A) compared to the turbine loaded. The apparent 

sound power level at 8 m/s, a common comparison 

point for wind turbines, was found to be 98.4 dB(A). 

The Excel was also tested with a second blade set that 

had a reduced rotor diameter of 6.17 m, an opposite 

direction of rotation, and a Selig-Hanley SH3052 airfoil. The slant distance from the turbine to the microphone was the same as for the previous BW03 tests. 

Figure 2 shows a dramatic reduction in measured noise 

for this configuration. For example, in the range of 8 – 

14 m/s the sound pressure level of the operating turbine 

was reduced by approximately 10 – 15 dB(A). 

Although the turbine noise could not be separated from 

the background noise for the SH3052 blades (Figure 2), 

the sound pressure level can be compared directly to the 

BW03 blades (Figure 1), because the slant distance was 

identical in both tests, and the background noise levels 

were virtually the same. In high wind conditions, both 

configurations became noisy when the inverter was 

offline and the unloaded rotor increased speed. Thus, it 

is desirable to prevent the inverter from going offline 

under normal operating conditions, a feature that was 

not characteristic of the turbine tested at the NWTC. 

SOUTHWEST WINDPOWER AIR TURBINES

Southwest Windpower, Inc., of Flagstaff, Arizona, 

(www.windenergy.com) produces the AIR 403 (shown 

above), a three-blade upwind turbine with a manufacturer’s rated power of 400 watts at 12.5 m/s. The DC 

output of the turbine was connected to a DC bus that 

was also connected to a battery bank and an Enermaxer. 

This device maintained the DC bus voltage at a constant 13.2 volts to prevent the turbine from shutting 
 

5

down when the batteries were fully charged. The AIR 

403 is a free yaw turbine that employs aero-elastic stall, 

also known as flutter, for over-speed protection. 

The machine tested at the NWTC had a rotor diameter 

of 1.14 m and a hub height of 13.3 m. The anemometer 

was mounted on a boom from the same tower. The microphone at the reference position was located at a slant 

distance of 19.1 m. 

Figure 3 shows the measured sound pressure level for 

the AIR 403. Three patterns are distinguishable. At 

higher wind speeds, the turbine flutters as a means of 

over-speed control. Green triangle markers indicate the 

10-second time periods during which the blades experienced flutter. Small horizontal bars on the markers 

indicate continuous flutter. The 10-second time periods 

during which the blades did not flutter are indicated 

with blue diamond markers. It appears that flutter increases the noise of the turbine approximately 10 – 12 

dB(A). The apparent sound power level at 8 m/s, when 

the blades do not flutter, was found to be 81.2 dB(A). 

We were not able to collect background noise data at 

higher wind speeds nor calculate the sound power level 

when the blades flutter. However, we estimated a background noise level of 65 dB(A) by extrapolating the 

available data to 20 m/s. By binning data between 18 

and 20 m/s, we estimated a sound power level of 112.5 

dB(A) for the blades in flutter, which is quite loud. 

To mitigate the impact of this blade flutter, Southwest 

Windpower developed a new version of the turbine 

called Air X. The Air X controller causes the blades to 

stall if the rotor speed or DC voltage exceed set limits. 

A marine version of this turbine was tested at the 

NWTC. The distinctions from the standard version are 

corrosion protection and sealed electronics. 

The measured sound pressure level of the AIR X is 

shown in Figure 4. During normal operation, when the 

blades are not fluttering, two groups of data can be 

distinguished. One group, which is representative of 

power production mode, is plotted above the background noise level. A second group overlays the background noise level, shown in this plot with open symbols. This lower noise level—sometimes as much as 10 

dB(A) lower—occurs when the turbine is operating in 

stall mode or automatic shutdown mode. This is caused 

by the turbine controller attempting to limit the rotor 

speed. In turbulent winds, which are typical of the 

NWTC test site, rotor speed control is not precise. 

Therefore, the 10-second averages do not always reflect 

the same rotor speed. 

If a curve is fit or a bin analysis is performed using the 

entire set of normal operation data, the resulting sound 

pressures will be mix of normal operation, stall mode, 

and parked data. This procedure would underestimate 

the noise level an observer would experience during the 

normal power production mode. 

Figure 4 exhibits a curious trend between 6 and 10 m/s, 

where the sound pressure level is unexpectedly low. 

Repeated reviews of the test data failed to provide an 

explanation for this behavior, although it is likely to be 

a result of the controller limiting rotor speeds. 

In comparing Figures 3 and 4, it is evident that the control strategy implemented on the AIR X was successful 

in reducing the occurrence of flutter-induced noise. 

BERGEY XL.1 TURBINE

The Bergey XL.1 (shown above) is a three-blade upwind turbine with a manufacturer’s rated power of 1 

kW at a wind speed of 11 m/s. A permanent magnet 

generator produces three-phase variable frequency output that is rectified to 24 volts DC. The turbine uses 

sideways furling for over-speed protection. It has a 

rotor diameter of 2.5 m and a hub height of 9 m. The 

microphone at the reference position was located at a 

slant distance of 13.8 m. 

Figure 5 shows the measured sound pressure level for 

the XL.1. The measured values are quite low and the 

apparent sound power level at 8 m/s cannot be reported 

because the turbine noise level could not be separated 

from the background noise. 
 

6

SOUTHWEST WINDPOWER

WHISPER H40

The Whisper H40 (pictured above) is a three-blade 

upwind turbine with a rated power of 900 watts at a 

wind speed of 12.5 m/s. As tested, the turbine had its 

24-volt DC output grid connected via a Trace SW4024 

inverter. Power and over-speed control are by a patented “angle governor” that combines horizontal and 

vertical furling. 

The Whisper’s rotor diameter was 2.1 m and hub height 

was 9.1 m. The microphone at the reference position 

was located at a slant distance of 13.6 m. Test data 

were averaged over 10-second periods instead of 1-

minute periods to better characterize the noise at higher 

wind speeds when the turbine employs over-speed control. Wind speed measurements, which were obtained 

from a hub-height anemometer located on a compass 

heading of 292o

 from the turbine, were standardized to 

the reference height of 10 m. 

Figure 6 shows the measured sound pressure level for 

the Whisper H40. There was sufficient separation between the turbine and background noise to determine 

the apparent sound power level at 8 m/s. It was found 

to be 84.9 dB(A) 

ATLANTIC ORIENT CORPORATION

AOC 15/50 TURBINE

The Atlantic Orient Corporation, of Norwich, Vermont, 

and Charlottetown, Prince Edward Island, Canada, 

(www.aocwind.net) manufactures the AOC 15/50 wind 

turbine (pictured above). It is a three-blade, downwind, 

free yaw machine with a rated power of 50 kW at 12 

m/s. Its fixed-pitch, constant speed, stall-regulated, 15-

m diameter rotor employs 7.2-m wood-epoxy blades 

manufactured by Aerpac/Merrifield Roberts. The rotor 

is mounted on the gearbox low-speed shaft, and the 

three-phase induction generator is connected to the 

gearbox high-speed shaft. The tower is a 24.4-m high, 

freestanding, three-leg lattice steel structure that provides a hub height of 25 m. 

The turbine employs three independent brake systems. 

Electro-magnetically controlled tip plates are installed 

on the blade tips to provide aerodynamic braking. A 

capacitor/resistor network provides dynamic braking, 

and a mechanical brake is used for parking the rotor. 

Figure 7 shows the measured 1-minute average sound 

pressure levels as a function standardized wind speed. 

The slant distance of the microphone was 41.2 meters. 

The apparent sound power level at 8m/s was found to 

be 101.1 dB(A) [6]. 
 

7

NORTHERN POWER SYSTEMS

NORTH WIND 100 TURBINE

Northern Power Systems of Waitsfield, Vermont, 

(www.northernpower.com) manufactures the North 

Wind 100 (shown above), a three-blade upwind turbine 

with a rated power of 100 kW at 13 m/s. Its fixed pitch, 

variable speed, stall controlled, 19.1-m diameter rotor 

employs modified ERS 0100 blades manufactured by 

TPI Composites. The test turbine was mounted on a 

23.4-m tubular steel tower that provides a hub height of 

25.0 m. The grid-connected turbine uses a direct-drive 

(no gearbox) salient pole synchronous generator and is 

specially designed to operate in very cold climates. 

Figure 8 shows the measured 1-minute average sound 

pressure level for the North Wind 100. The slant distance of the microphone was 42.0 meter. There was no 

difficulty obtaining the 6 dB separation between turbine 

and background noise [8], and the apparent sound 

power level at 8 m/s was found to be 93.8 dB(A). 

Aeroacoustic emissions are a strong function of size. 

With a diameter of 19.1m, the North Wind 100 is larger 

than others in the test group. Comparisons [10] to similar turbines indicate that its sound pressure level is typical for machines of its size. 

COMPARISON OF TESTED TURBINES

We wish to compare the acoustic signatures of all the 

turbines on a common basis, but owing to the difficulty 

of separating wind turbine noise from background noise 

for the quieter machines, a complete database is not 

available for all the turbines tested. For example, as 

noted above, it was not possible to calculate an apparent 

sound power level for the Bergey XL.1. Still, important 

observations may be made from the data that are available. Table 1 and Figure 9 provide this information. 

The AOC 15/50 and the early version of the Excel with 

BW03 blades have the highest noise levels of the turbines tested. Because it was one of the largest turbines 

tested, we expected the AOC 15/50 to be somewhat 

noisier. Test engineers also observed that mechanical 

noise was more prevalent than on other turbines. Furthermore, the AOC 15/50 employs tip plates that are 

likely to add aeroacoustic noise. In support of this hypothesis, we note that tests of an AWT-26 turbine at the 

NWTC measured an increment of almost 2 dB(A) for 

similar tip plates. These tests were conducted with a tip 

plate on one blade and conventional tip on the other, 

thus leaving no question of differences in test conditions or instrumentation. 

Significantly, improvements made to the Excel reduced 

acoustic emissions to the point that turbine noise could 

not be separated from background noise. For this reason, the Excel with SH3052 airfoils does not appear in 

Figure 9, but Figures 1 and 2 corroborate this assertion. 

The Air 403 data do not exhibit the smooth trends of 

the other turbines. By listening to the sound recordings, 

we learned that several of the measurements actually 

captured the noise of the blades in flutter. Figure 3, 

which was discussed previously, clearly illustrates this. 

Considering the difficulties introduced by variations in 

background noise, it is interesting to compare the levels 

from different tests. Several of these are shown in Figure 10, where it can be seen that a range of 10 dB(A) is 

typical for most wind speeds. It appears that the variation in background noise is greater at low wind speeds 

than at high wind speeds where the noise of the wind 

itself masks some of the other constituents of background noise. We also observed that at low wind 

speeds, the highest background noise levels correspond 

to the test sites closest to the concrete plant. This was 

expected because of the relationship between sound 

pressure level and the distance from the source, as seen 

in Equation (3). Recognizing the importance of a quiet 

site for acoustic testing, we are exploring other locations at the NWTC (further from known noise sources) 

for future tests. 
 

8

SUMMARY

A series of field tests were conducted to measure the 

acoustic noise of several small wind turbines. Rigorous 

procedures for both testing and data analyses were followed. Because the NWTC is a turbulent site, the wind 

turbines, some of which have temperamental controls, 

sometimes have different acoustic signatures on different days even at the same wind speed. Particularly vexing is the variation in background noise and the inability to separate it from turbine noise for the quieter machines. This has prompted NREL researchers to seek 

quieter sites that are less susceptible to background 

noise variations. 

In considering individual turbines, we conclude that for 

the Bergey Excel and Southwest Windpower AIR turbines, the manufacturers’ efforts to reduce noise 

through the use of new airfoils or control techniques 

have resulted in quieter turbines. 

In normal operation, the Excel turbine with SH3052 

blades exhibits significantly lower noise than its predecessor with BW03 blades. NREL researchers attribute 

this improvement to the new airfoils and reduced tip 

speed owing to smaller rotor diameter. In high wind 

conditions and unloaded (inverter offline), both turbines 

become much noisier. 

In normal power-production mode, the AIR 403 and the 

AIR X exhibit similar noise characteristics. In high 

wind conditions, when the blades flutter, the AIR 403

becomes much noisier than in normal operation. Control improvements in the AIR X, which stall the blades 

when rotor speed exceeds set limits, reduced the occurrence of this flutter-induced noise. 

For the Excel with SH 3052 blades, the XL.1, and the 

Whisper H40 at virtually all wind speeds above 7 m/s, 

separation between operating turbine and background 

noise levels was less than 6 dB(A). 

ACKNOWLEDGEMENTS

Conducting multiple test campaigns over many years 

while maintaining scientific rigor is a formidable challenge. Arlinda Huskey and Jeroen van Dam spent 

countless hours calibrating instruments; setting up experiments, some in miserable weather; listening to recordings; analyzing and plotting data; and writing reports. It is difficult to overstate their accomplishments 

as documented in this paper and the NREL report [1]. 

REFERENCES

[1] Migliore, P., van Dam, J. and Huskey, A., (2003). 

Acoustic Tests of Small Wind Turbines, NREL SR500-34601. Golden, CO: National Renewable Energy Laboratory 

[2] International Electrotechnical Commission, First 

Edition, (1998). International Standard IEC 61400-

11, “Wind Turbine Generator Systems – Part 11: 

Acoustic Noise Measurement Techniques”. 

[3] NREL National Wind Technology Center, (2001). 

“Test Manual for the Acoustics Testing of Wind 

Turbines per ISO Guide 25, 1990”. 

[4] Wagner, S., Bareiß, R. and Guidati, G, (1996). 

“Wind Turbine Noise,” Springer-Verlag, Berlin, 

pp. 14-21.

[5] Huskey, A. and Meadors, M., (2001). Wind Turbine Generator System, Acoustic Noise Test Report 

for the Whisper H40 Wind Turbine. 

[6] Huskey, A. and van Dam, J., (2003), Wind Turbine 

Generator System, Acoustic Noise Test Report Revision 1 for the AOC 15/50 Wind Turbine, NREL 

Test Report. 

[7] Huskey, A. and Meadors, M., (2003). Wind Turbine Generator System, Acoustic Noise Test Report 

for the Bergey Excel Wind Turbine, NREL Test 

Report. 

[8] Huskey, A. and Meadors, M., (2002). Wind Turbine Generator System, Acoustic Noise Test Report 

for the NW100 Wind Turbine, NREL Test Report. 

[9] “Measnet Acoustic Noise Measurement Procedure”, (1997). www.measnet.org

[10] van Dam, J., (1999).“Trend in de akoestische bronsterktes van windturbines” (Trend in the acoustic 

sound power level of wind turbines), Energieonderzoek Centrum Nederland (Energy Research Center 

of the Netherlands ECN), Proceedings of the Netherlands Renewable Energy Conference. 
 

9

Figure 1. 10-second average sound pressure level for Bergey Excel-S with BW03 airfoils 

Figure 2. 10-second average sound pressure level for Bergey Excel-S with SH3052 airfoils 

30

35

40

45

50

55

60

65

70

75

80

85

90

0 2 4 6 8 10 12 14 16 18 20 22 24

Standardized wind speed - m/s

Sound pressure level - dB(A)

Turbine operating - inverter on line

Turbine operating - inverter off line

Background noise

30

35

40

45

50

55

60

65

70

75

80

85

90

0 2 4 6 8 10 12 14 16 18 20 22 24

Standardized wind speed - m/s

Sound pressure level - dB (A)

Background noise

Turbine operating - inverter on line

Turbine operating - inverter off line
 

10

Figure 3. 10-second-average sound pressure level for Southwest Windpower AIR 403 

Figure 4. 10-second average sound pressure level for Southwest Windpower AIR X 

30

35

40

45

50

55

60

65

70

75

80

85

90

0 2 4 6 8 10 12 14 16 18 20 22 24

Standardized wind speed - m/s

Sound pressure level - dB(A)

Turbine operating

Background noise

Sporadic flutter

Continuous flutter (with bars)

30

35

40

45

50

55

60

65

70

75

80

85

90

0 2 4 6 8 10 12 14 16 18 20 22 24

Standardized wind speed - m/s

Sound pressure level - dB(A)

Turbine operating

Background noise
 

11

Figure 5. 10-second-average sound pressure level for Bergey XL.1 

Figure 6. 10-second-average sound pressure level for Southwest Windpower Whisper H40 

30

35

40

45

50

55

60

65

70

75

80

85

90

0 2 4 6 8 10 12 14 16 18 20 22 24

Standardized wind speed - m/s

Sound pressure level -dBA

Turbine Operating

Background Noise

30

35

40

45

50

55

60

65

70

75

80

85

90

0 2 4 6 8 10 12 14 16 18 20 22 24

Standardized wind speed - m/s

Sound pressure level - dB(A)

Background noise

Turbine operating
 

12

Figure 7. 1-minute-average sound pressure level for Atlantic Orient Corporation AOC 15/50 

Figure 8. 1-minute-average sound pressure level for Northern Power Systems North Wind 100 

30

35

40

45

50

55

60

65

70

75

80

85

90

0 2 4 6 8 10 12 14 16 18 20 22 24

Standardized wind speed - m/s

Sound pressure level - dB(A)

Turbine operating

Background noise

30

35

40

45

50

55

60

65

70

75

80

85

90

0 2 4 6 8 10 12 14 16 18 20 22 24

Standardized wind speed - m/s

Sound pressure level - dB(A)

Turbine operating

Background noise
 

13

Table 1. Apparent sound power level for turbines with at least 3 dB(A) separation from background noise. 

Values were obtained by the bin analysis method described on page 3. 

Figure 9. Apparent sound power level for turbines with at least 6 dB(A) separation from background 

noise. Values were taken from Table 1. 

70

75

80

85

90

95

100

105

110

115

0 2 4 6 8 10 12 14 16 18

Standardized wind speed - m/s

Sound power levle [dB(A)]

Air 403 Air X Excel BW03 NW100 AOC 15/50 Whisper H40

3 4 5 6 7 8 9 10 11 12 13 14 15 16

AIR 403 **   **  ** 80.9 84.2 86.7 92.9 90.5 97.7 98.0 -- --

AIR X 73.1 76.6 78.8 77.7 77.5 ** 81.3 85.2 88.9 90.0 88.8 92.0 94.0 101.6

Whisper H40 -- 82.6 83.8 82.8 83.5 85.3 87.4 91.0 92.4 ** 96.3   ** 

XL.1 -- -- -- **   75.8 78.7 78.0 **  80.8 -- -- --

Excel BW03 -- ** 87.2 91.0 96.1 99.5 102.2 105.4 107.6 109.8 112.2 -- -- --

Excel SH3052 -- -- -- -- 90.7 90.7 92.3 93.4 95.1 96.9 99.0 100.3 101.5 --

AOC 15/50 -- 96.9 96.9 100.1 100.8 -- 101.9 -- -- -- -- -- -- --

North Wind 100 -- -- -- 89.6 91.9 93.9 95.1 97.0 98.1 99.6 100.8 -- -- --

-- Insufficient data are available.

** Separation between operating turbine and background noise is less than 3 dB.

b Numbers in italics have separation between operating turbine and background noise of between 3 and 6 dB.

Wind Speed (m/s)
 

14

Figure 10. Background sound pressure level for several of the turbines tested. 

30

35

40

45

50

55

60

65

70

75

80

85

90

0 2 4 6 8 10 12 14 16 18 20 22 24

Standardized Wind Speed - m/s

Sound Pressure Level - dB(A)

Air X

Air 403

Excel

Northwind 100

AOC 15/50

XL.1
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Acoustic Tests of Small Wind Turbines: Preprint 

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P. Migliore, J. van Dam, A. Huskey 

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13. ABSTRACT (Maximum 200 words)

Eight small wind turbines ranging from 400 watts to 100 kW in rated power were tested for acoustic emissions at 

the U.S. Department of Energy’s National Renewable Energy Laboratory. Rigorous test procedures based on 

international standards were followed for measurements and data analyses. Results are presented in the form of 

sound pressure level versus wind speed, where the sound was recorded downwind of the turbine at a distance equal 

to the hub height plus half the rotor diameter. When there was sufficient separation between wind turbine noise 

and background noise, the apparent sound power level was calculated. In several cases, this was not possible. The 

implications of this problem are discussed briefly. Some of the configurations tested were specifically developed to 

reduce the noise level of their predecessors. Test data for these machines demonstrate marked progress toward 

quieter turbines.

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small wind turbine acoustic tests; wind turbine noise 16. PRICE CODE

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