Annoyance caused by amplitude modulation of wind turbine...

Annoyance caused by amplitude modulation of wind turbine noiseSeunghoon Leea), Kyutae Kima), Wooyoung Choib) and Soogab Leec)

(Received: 6 October 2009; Revised: 3 August 2010; Accepted: 9 August 2010)

A listening test has been performed to investigate the relationship betweenhuman annoyance and the amplitude modulation of wind turbine noise. Toobtain sound samples for the listening test, sound from a 1.5 MW wind turbinein Korea was recorded. The strength of the amplitude modulation of the soundsamples was defined in terms of the modulation depth spectrum, which wasapproximated by assuming that the sound samples are sinusoidally amplitude-modulated. The stimuli for the listening tests were created by reducing themodulation depth spectrum of the sound samples.A total of 30 participants wereinvolved in the listening tests. The results of the listening tests indicate that theequivalent sound level and the amplitude modulation of wind turbine noise bothsignificantly contribute to noise annoyance. © 2011 Institute of Noise ControlEngineering.

Primary subject classification: 14.5.4; Secondary subject classification: 63.2

1 INTRODUCTION

Wind turbines produce sound with levels that fluctu-ate periodically at the blade passing frequency. Thisamplitude modulated sound is clearly perceptible atlocations near the wind turbines, whereas the percep-tion of this sound becomes difficult at large distancesfrom the wind turbines. However, some residents livingnear a wind farm have claimed that in some circum-stances this sound is perceived at a distance of morethan 1 km from wind turbines1. Recent studies2,3 havealso shown that the amplitude modulation can be heardeven at large distances if the background noise level isquite low.

Several previous studies4,5 have argued that theamplitude modulation of wind turbine noise mayincrease noise annoyance. Van den Berg4 reported thatresidents living at 500 m and up to 1900 m from a windfarm expressed annoyance due to wind turbine noise. Theresidents could hear a low pitched thumping soundespecially at night, which is periodic at a blade passingfrequency. The author maintained that this amplitudemodulation may increase annoyance. Moreover, Pedersenand Persson Waye5 performed a field study to evaluate the

a) School of Mechanical and Aerospace Engineering, SeoulNational University, Seoul 151-744, REPUBLIC OF KO-REA.

b) Romax Technology Korea, Yangjae-dong 75-3, Seocho-gu, Seoul 137-889, REPUBLIC OF KOREA

c) Institute of Advanced Aerospace Technology, School ofMechanical and Aerospace Engineering, Seoul NationalUniversity, Seoul 151-744, REPUBLIC OF KOREA;email: [email protected].

38 Noise Control Eng. J. 59 (1), Jan-Feb 2011

prevalence of annoyance caused by wind turbine noise.The result of their study suggested that wind turbine noiseis more annoying than other community noise sourceswith the same A-weighted sound level. They mentionedthat one of the reasons for this result could be the presenceof the amplitude modulation of wind turbine noise.

Indeed, amplitude modulated sound is generallyknown to be more annoying than un-modulatedsound6,7. Kantarelis and Walker6 examined the sourceof difference between the annoyance of diesel andelectric train noises. They suggested that the amplitudemodulation in diesel engine noise is the reason for theextra annoyance. The annoyance caused by the dieseltrain noise decreased as the modulation depth wasreduced from 13 dB to 5 dB. Furthermore, Bradley7

investigated the influence of amplitude modulatedlow-frequency sounds from heating, ventilation, and airconditioning (HVAC) systems on annoyance, finding thatannoyance is correlated with both the sound pressure leveland the amplitude modulation of the noise from HVACsystems. Thus, the amplitude modulation of wind turbinenoise is also thought to increase the annoyance of windturbine noise.

However, for wind turbine noise, only a few experi-ments have been performed on the influence of ampli-tude modulation on annoyance. Persson Waye andŐhrstrőm8 performed a listening test to evaluate annoy-ance from five wind turbine noise sources and analyzedthe relation between noise annoyance and psycho-acoustic parameters such as loudness, sharpness, tonal-ity, fluctuation strength and modulation. The resultsshow that the annoyance ratings were significantlydifferent for the different noise sources, but none of the

parameters, including modulation, could explain thedifferences in annoyance ratings. The reason may bethat this experiment was not designed to evaluateadditional annoyance caused by the amplitude modula-tion of wind turbine noise.

Thus, our study performs a listening test to examinethe annoyance caused by the amplitude modulation ofwind turbine noise. Two kinds of amplitude modulatedsounds from a 1.5 MW wind turbine are used for thelistening tests. The strength of amplitude modulation isquantified by measuring the modulation depth spectrumof the recorded wind turbine sound assuming that thewind turbine sound is sinusoidally amplitude-modulated.The stimuli for the listening test were designed by reduc-ing the modulation depth of the recorded wind turbinesound.

2 METHODS

2.1 Sound Recording

Noise from a 1.5 MW wind turbine was recorded byfive free-field microphones (Brüel & Kjær type 4190)with sound level meters (Brüel & Kjær type 2250) forthree days. This wind turbine manufactured byNEG-Micon is an active-stall fixed-speed type with arotor diameter of 72 m and a hub height of 62 m. Soundrecordings were obtained around the turbine, at distancesof 62, 98, 150, and 200 m from the turbine. The micro-phones were placed on a circular board of 1 m in diameterand connected to the sound level meter with an extensioncable. The sound level meter recorded not only the equiva-lent noise level and frequency spectra, but also thetime-domain signal with a sampling frequency of 24 kHz.In order to minimize wind induced noise, the microphonewas covered with a foam windscreen. During soundrecording, wind speed was also measured at the hubheight. The wind speed varied from 3 m/s to 14 m/sduring the measurements.

Since two different kinds of amplitude modulatedsounds were perceived in the on-axis and crosswinddirection, two samples that represent the two amplitudemodulation characteristics were selected from amongthe sound samples. The recording positions of thesamples are presented in Fig. 1. One sample (Sample I)was recorded at a distance of hub height �62 m� upwindfrom the turbine when the wind speed was approximately4�6 m/s. The other sample (Sample II) was taken at adistance of hub height on the right side of the turbinewhen the wind speed was approximately 10�12 m/s.Figure 2 shows the one-third octave band spectrum ofeach of the two samples. It shows that low- tomid-frequency noise is dominant in Sample I, whereashigh-frequency noise is relatively dominant in Sample II.This is explained by the difference in the directivity

Noise Control Eng. J. 59 (1), Jan-Feb 2011

patterns of trailing edge noise for low and high frequen-cies. For the low frequency noise, its directivity pattern isexpressed as dipole, while the high-frequency noise has acardioid directivity pattern. As a result, high frequencynoise is present only in crosswind direction.

2.2 Measurement of the Modulation Depth

Figure 3 presents A-weighted sound level with timeweighting FAST of the two samples. It is seen that bothsamples are amplitude-modulated at the blade passingfrequency of 0.865 Hz. The overall modulation depths ofthe sample I and the sample II are approximately 4 dBand 5 dB, respectively. However, the overall modulationdepth should not be a parameter for quantifying thestrength of amplitude modulation because it only reflectsthe strength of amplitude modulation in the frequencyrange where the sound pressure level is the highest. Thus,instead of the overall modulation depth, the modulationdepth spectrum is used for quantifying the strength of the

Fig. 1—Recording position of the sound samples.

Fig. 2—One-third octave band spectrum of thetwo samples.

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amplitude modulation in the present study. Since it isdifficult to measure the modulation depth spectrumaccurately, it is approximated by assuming that the soundsignal is sinusoidally amplitude-modulated.

Figure 4 is a schematic of the procedure for measur-ing the modulation depth spectrum of the soundsamples. First of all, a spectrogram is obtained byapplying the Fourier transform to each time step of thesignal. The time step was small enough to resolve theamplitude modulation. Next, the Fourier transform isapplied again for each frequency band, but this time itis applied along the time axis. Figure 4(b), for example,shows the result of the Fourier analysis for thefrequency band of 1 kHz. Since the sound signal isamplitude-modulated at the blade passing frequency, twodominant peaks are visible. One peak at 0 Hz representsthe steady root-mean-square value of the signal. The otherpeak at the blade passing frequency represents thesinusoidal amplitude modulation of the signal. Since thesound signal is assumed as sinusoidally amplitude-modulated, all other values except the two peaks can beneglected, as shown in Fig. 4(c). In Fig. 4(c), p0 and pf

mean the root-mean-square amplitude of the modulationat 0 Hz and the blade passing frequency, respectively.Finally, the inverse Fourier transform is applied to theresult in Fig. 4(c). The modulation depth is defined as thedifference between the maximum and the minimumvalues of the sound pressure level. Hence, the modulationdepth at a frequency band can be obtained as9

Fig. 3—A-weighted sound pressure level with time


�L = 20 logp0 + pf

p0 − pf�1�

This procedure is applied to all frequency bands toobtain the modulation depths at a modulationfrequency of 0.865 Hz for all the frequency bands.Figure 5 presents the modulation depth spectra at amodulation frequency of 0.865 Hz. The frequencyresolution of the modulation spectra in Fig. 5 was set to200 Hz.

2.3 Stimuli

If there are two amplitude modulated sounds whichhave different modulation depth spectra, it is difficult toidentify which amplitude modulation is higher than theother, because no representative is present to determinethe strength of amplitude modulation. Moreover, eventhough the modulation depth spectra of two sounds arethe same, the strengths of the amplitude modulationcan be different, if the frequency spectra of the twosounds are not the same. However, if there are twosounds which have the same frequency spectrum butthe modulation depth spectrum of one sound is clearlyhigher than that of the other sound, it can be said thatthe amplitude modulation of one sound is large thanthat of the other sound. Hence, in order to conduct alistening test to examine the effect of amplitudemodulation on annoyance, it is required that the stimulihave different modulation depth spectrum while thefrequency spectrum of the stimuli remain the same.

hting FAST of (a) Sample I and (b) Sample II.
weig

the m

Since it is impossible to obtain these sounds by soundrecording, the stimuli were created by reducing themodulation depth of the recorded samples.

Figure 6 is a schematic of the procedure for modify-ing the amplitude modulation of each of the soundsamples. First of all, the frequency spectrum of each

Fig. 4—Schematic of the procedure for measuring

Fig. 5—Modulation depth spectra of the twosamples.


sample is obtained by applying the Fourier transform.This frequency spectrum is used to create a filter whosemagnitude is the same as that of the original signal. Theapplication of this filter to white noise creates a modelof un-modulated wind turbine noise. This model isreasonable because the aerodynamic noise, which is thedominant noise source of modern wind turbines, isbroadband in nature10. Finally, by adding the originalsample with the un-modulated wind turbine noise, anew signal whose modulation depth is reduced fromthat of the original signal is obtained. The modulationdepth of the new signal can be adjusted by the soundlevel of the white noise.

Using the procedure illustrated in Fig. 6, a total of 50stimuli (2 base samples�5 equivalent sound levels�5 degrees of modulation) were produced. The equiva-lent sound level was varied in steps of 5 dB from35 to 55 dBA. The degree of amplitude modulation isalso varied in five steps, as shown in Fig. 7, which showsthe modulation depth spectra of the stimuli at an LAeq of35 dBA. Figure 8 presents the narrowband spectra of thestimuli at an LAeq of 35 dBA. Although the stimuli were

odulation depth spectrum.

41

he a

modified from the base samples, it is seen that thefrequency spectra of the stimuli were similar to those ofthe base samples.

The duration of subject exposures in this study wasset to 30 s. In a related laboratory test, wind turbine noiseexposures were 3 and 10 minutes8, which is longer thanthe exposure time in this study. However, Poulsen11 foundthat exposure time did not have a significant effect on theannoyance rating for impulse noise and traffic noise.Although no related study was performed for windturbine noise, it was assumed that the shorter exposuretimes can be applied for wind turbine noise. Thus, theshort exposure time of 30 s was chosen for this study.

2.4 Listening Test

A total of 30 participants, 15 males and 15 females,between 20 and 30 years of age, were involved in thelistening tests. Prior to the listening tests, a screening testwas performed with pure tones, which consisted of the sixoctave band center frequencies from 250 Hz to 8000 Hz.Since none of the participants had a hearing loss greaterthan 20 dB from the reference equivalent threshold soundpressure level (RETSPL)12, all the participants wereregarded as having normal hearing.

Fig. 6—Schematic of the procedure for modifying t


The experiment was conducted in an anechoicchamber �3 m�3 m�2 m� where the backgroundnoise level was between 20 and 25 dBA. The stimuliwere reproduced using a PC (Pentium IV) with a soundcard (RME DIGI 96/8 PAD), and delivered to the listenersthrough a supra-aural headphone (Sennheiser HD25-1)via a power amplifier (NAD C320BEE), as shown in Fig.9. In order to obtain a flat frequency response at theeardrums of the listener, the sound field was calibratedusing a head and torso simulator (Brüel & Kjær Head andTorso Simulator Type 4128) and an audio analyzer (Brüel& Kjær PULSE Type 3560C). Before each test session,the headphone was placed over the ears of the dummyhead, which was connected to the audio analyzer; then thefrequency response obtained by the audio analyzer wascorrected to have a flat frequency response using asoftware equalizer.

The participants were told that they were going to bepresented with two kinds of wind turbine sound. Theywere instructed to record the degree of annoyance aftereach stimulus. The responses were recorded on an11-point numerical scale because it was assumed thatrespondents were more familiar with this scale ratherthan a shorter 7, 9 or 10-point numerical scale13. The

mplitude modulation.

listening test was carried out in two steps. First, the 25stimuli originated from Sample I were randomly deliv-ered to the participants (Test I). After a rest for threeminutes, the 25 stimuli originated from Sample II werepresented in the same manner (Test II). Each stimuluslasted for 30 s and the interval between consecutivestimuli was 10 s. The listening test took approximately40 min for each participant.

3 ANALYSIS AND RESULTS

The annoyance ratings for each modulation depthspectrum in Test I and Test II are plotted in Fig. 10.These values represent average values for all of theparticipants.

In order to test whether the A-weighted equivalentsound level and the modulation depth are significantfactors in determining the annoyance, statistical analy-sis was performed by a two-way analysis of variance

Fig. 7—Modulation depth spectra of the stimulifrom (a) Sample I and (b) Sample II at35 dB LAeq.


(ANOVA). A p-value of less than 0.05 was regarded asstatistically significant. The results of the ANOVA arepresented in Table 1. The annoyance rating increasedsignificantly with A-weighted equivalent sound levelfor both tests [Test I: F�4,725�=114.7, p�0.00001;Test II: F�4,725�=126.2, p�0.00001]. The effect of themodulation depth on the annoyance rating was alsosignificant [Test I: F�4,725�=2.93, p=0.02; Test II:F�4,725�=4.03, p=0.003]. Thus, it can be concluded thatboth the A-weighted equivalent sound level and themodulation depth are significant parameters in the presentstudy.

Moreover, post hoc pair-wise comparison wasperformed using Tukey’s HSD. Table 2 shows theresults of the pair-wise comparison. The mean annoy-ance difference in Table 2 represents the differencebetween the mean annoyance rating for the stimuli

Fig. 8—Narrowband spectra of the stimuli from(a) Sample I and (b) Sample II at 35 dBLAeq.

43

having the modulation depth of (B) and the stimulihaving the modulation depth of (A). It is seen that themean annoyance difference gradually increases as themodulation depth of the stimuli increases for both tests.However, pair-wise comparison showed that not all the

Fig. 9—The listening test setup.

Fig. 10—The annoyance rating for (a) Test I and(b) Test II.


mean annoyance differences are statistically signifi-cant. For Test I, the annoyance rating for the stimulihaving the highest modulation depth was only signifi-cantly higher than the annoyance rating for the stimulifor Case V �p=0.02�, while for Test II, the stimuli of CaseI and Case II were significantly more annoying than thatof Case V [p=0.002; p=0.029].

4 DISCUSSION

The result that the noise annoyance increases withthe amplitude modulation of wind turbine noise isconsistent with the result of the related previous studiescited in the introduction. However, it is not possible tomake a quantitative comparison with the previousstudies because not only the modulation depthspectrum but also the frequency spectrum of the stimuliused in these tests is different from those of the stimuliin the present study.

Although it has been shown that the amplitudemodulation of wind turbine noise increases annoyance,it is still unclear that this amplitude modulation ispresent at large distances from a wind farm whereresidents usually live. In general, there is a buffer zonebetween a wind farm and residents, so if the amplitudemodulation is present only near a wind turbine, or if thestrength of the amplitude modulation at large distancesis too weak to cause additional annoyance, the effect ofthe amplitude modulation on annoyance might beinsignificant. However, as cited in the introduction,there is evidence that in some circumstances the ampli-tude modulation is high enough to causes annoyance toresidents living far from wind turbines. Thus, moreextensive studies are necessary to investigate thepresence of the amplitude modulation at large distancesfrom a wind turbine.

In this study, the amplitude modulation of windturbine noise was quantified by using the modulationdepth spectrum. Since the modulation depth spectrumis not a single value, but a function of frequency, therelation between additional annoyance and the ampli-tude modulation could not be quantified. However, if arepresentative value for the strength of the amplitudemodulation exists, the extra annoyance caused by the

amplitude modulation of wind turbine noise could bepredicted. Therefore, further studies are necessary todetermine the representative value for the strength ofthe amplitude modulation of wind turbine noise.

5 CONCLUSION

In this paper, in order to investigate the noise annoy-ance caused by the amplitude modulation of windturbine noise, a listening test was performed with twosets of stimuli which were modification of recordedwind turbine noise. The results of the listening testshowed that the amplitude modulation of wind turbinenoise has a statistically significant effect on noiseannoyance. Thus, it is concluded that when assessingcommunity responses to wind turbine noise, not onlythe equivalent sound level but also the amplitudemodulation of wind turbine should be considered.

However, it is still a matter of controversy whetherthe amplitude modulation which causes additionalannoyance is also present at large distances from windturbines. Moreover, at present no value is available torepresent the extra annoyance due to the amplitudemodulation. Therefore, further studies are required toinvestigate the characteristics of the amplitude modula-tion of wind turbine noise.

6 ACKNOWLEDGMENTS

This work was supported by the Human ResourcesDevelopment of the Korea Institute of Energy Technol-

Table 1—The results of two-way AN

Listening test FactorTest I Equivalent sound level

Modulation depthTest II Equivalent sound level

Modulation depth

Table 2—The results of pair-wise co

Listening test

Modulationdepth(A)

Modude(B

Test I Case V CasCasCasCa

Test II Case V CasCasCasCa


ogy Evaluation and Planning (KETEP) grant funded bythe Korea government Ministry of KnowledgeEconomy (No. 20094020100060). This work wassupported by the New and Renewable Energy Programof the Korea Institute of Energy Technology Evaluationand Planning (KETEP) grant funded by the Koreagovernment Ministry of Knowledge Economy (No.2009T100100600).

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