Perception of tonal components contained in wind turbine noise

Perception of tonal components contained in wind turbine noise Sakae YOKOYAMA 1 ; Tomohiro KOBAYASHI 2 ; Hideki TACHIBANA 3 1,2 Kobayasi Institute of Physical Research, Japan 3 The University of Tokyo, Japan ABSTRACT Tonal components in wind turbine noise increase psycho-acoustical annoyance in the areas around wind farms. Therefore, the methods to assess the characteristics of this kind of sound should be investigated in both viewpoints, physically and psycho-acoustically. Regarding the latter problem, the authors performed auditory experiments by using a test facility capable of reproducing low frequency sounds including infrasound. As the first experiment on the effect of tonal components in wind turbine noise, the change of auditory impression was examined using artificially synthesized noises containing a tonal component with varying frequencies and levels. For the test stimuli, synthesized noises modelling the frequency characteristics of the general wind turbine noise observed outdoors/indoors in immission areas were used. For the simulation of the transmitted sound from outdoors into a residential room, the proposed house-filter for a single-pane window was applied. As the second experiment, the annoyance sensation due to a tonal component in WTN was examined applying the method of subjective adjustment. For the test stimuli, artificially synthesized sounds modelling outdoor were also used. From the results of these investigations, the method for assessing the tonal components in wind turbine noise in immission areas is discussed. Keywords: Wind turbine noise, Tonal component, Annoyance, Low frequency noise, Auditory experiment I-INCE Classification of Subjects Number(s): 63.2 1. INTRODUCTION Regarding wind turbine noise (WTN) problem, a research project entitled Research on the evaluation of human impact of low frequency noise from wind turbine generators has been conducted over the three years from fiscal year, funded by a grant from the Ministry of the Environment, Japan. In this research project, nationwide field measurement [1,2], social survey [3], and auditory experiments [4-7] were performed. As an experiment regarding the third topic, the authors have investigated the effect of tonal components contained in WTN using artificially synthesized noises containing tonal components with varying frequencies and levels. Tonal components with discrete frequencies are often contained in WTN which are generated by mechanical vibration of rotating mechanism of wind turbine such as gearbox, generator and cooling fans and they are apt to increase psycho-acoustical annoyance. Using recorded data around wind farms in the research project, the authors have been also investigating the method of assessing the effect of tonal components [8]. In this paper, the application of the assessment method of Tonal Audibility (TA) specified in IEC 6-11: 12 to WTNs observed in immission areas around wind farms was reported. The auditory experiment consisted of two subjects: one was to examine the difference of auditory impression by a tonal component in WTN and the other was to investigate the effect of a tonal component on annoyance sensation. In the first experiment, method of paired comparison was applied for both of outdoor/indoor scenario and in second experiment the method of subjective adjustment was used for outdoor scenario. From the results of these investigations, the standard method for assessing the tonality of WTN in immission areas is discussed. 1 sakae@kobayasi-riken.or.jp 2 kobayashi@kobayasi-riken.or.jp 3 pon-t@iis.u-tokyo.ac.jp 2628

2. TONAL COMPONENTS CONTAINED IN WTN 2.1 Analysis Using the Field Measurement Data of WTN TA analysis was performed on the sound pressure data obtained at 164 measurement points at 29 wind farm sites in Japan [1]. In the field measurements, WTN was recorded for 5 days at each measurement point. From the recordings, the data for representative minutes when the wind turbines were under a rated operation condition was chosen. In this study, the assessment method of TA specified in IEC 6-11:12 is mainly applied [8]. In this analysis, the effect of the background noise was eliminated as far as possible. 2.2 Results of Detected Tonal Audibility As a result, Figs. 1(a) and 1(b) show 1/3-octave-band spectra of WTN in emission areas (a) and those in immission areas (b). In the emission areas, TA was detected ( -3 db) at all of the measurement points (29). As for the immission areas, TA was detected at 129 points among the total 164 points. The distributions of frequency (horizontal axis) and TA (vertical axis) for each identified are shown in Fig. 2, in which it is seen that frequency ranged from about Hz to 1, Hz and TA ranged from -3 db up to 14 db. Sound pressure level [db] Sound pressure level [db] 1 2 4 8 1631.563 125 1k 2k 4k 1 2 4 8 1631.563 125 1k 2k 4k (a) In emission areas (29 points) (b) In immission areas (129 points) Figure 1 1/3 octave band spectra of WTNs 15 12 Immission area Emission area 9 6 3-3 Figure 2 Tonal Audibility 2629

3. SUBJECTIVE EXPERIMENTS ON AUDIBILITY OF TONAL COMPONENTS IN WTN As a basic experiment on the effect of a tonal component in WTNs, the change of auditory impression was examined by varying the frequency and TA of a tonal component by using the method of paired comparison. 3.1 Experimental Setting To investigate the difference threshold of auditory impression when any tonal component was included in WTN, the method of paired comparison was applied. In this study, all of test stimuli were presented diotically through an audio interface (RME, Multiface) and an electrodynamic headphone (SONY, MDR-Z7). This system was set in consideration of the possibility of reproducing low frequency sounds. The experimental setting is shown in Fig.3. The volume controller for Comparison stimulus () in Fig.3 was used in the following annoyance matching test. The frequency characteristics of reproduction system through the headphone measured using HATS (B&K 4) is shown in Fig.4. PC Headphone S s Standard stimulus Audio Interface S c Comparison stimulus Volume controller Figure 3 Experimental setting; Volume controller was used in the following annoyance matching test. 5 db Relative S.P.L [db] Figure 4 Frequency characteristics of the reproduction system through a headphone (SONY, MDR-Z7) measured using HATS (B&K4). 3.2 Test Sounds for Outdoor/Indoor scenario In the auditory experiments, both of outdoor and indoor scenarios were assumed. 3.2.1 For Outdoor scenario As the standard stimulus () for the outdoor scenario, an artificially synthesized noise modelling the frequency characteristics of general WTNs observed outdoor (-4 db/octave in band spectrum [1]) was used and its presentation level was fixed at 45 db in terms of A-weighted sound pressure level. It 26

should be noted that the stimuli were steady noises. As for the comparison stimulus (), a pure (,,, or Hz) was superimposed on the model noise so that its TA varies in 7 steps as shown in Table 1 in consideration of results of TAs detected in field measurements. As an example, the FFT spectrum of a test stimulus containing a tonal component at Hz is shown in Fig. 5(a). As the comparison stimuli, 36 sounds in total (5 different frequencies, 7 variations of TA and the same sound as ) were synthesized and they were paired with. In the outdoor scenario, A-weighted sound pressure level of ranged from 45. db (without a tonal component) up to.5 db (a Hz, 15 db TA shown in Fig.5(a)). The duration time of each sound was 5 s for both and. To avoid click sounds, the each sound was gradually risen/fallen with a time of.5 s. 3.2.2 For Indoor scenario In addition, to simulate the situation indoor, all of test sounds mentioned above (36 sounds in total) were convolved with the house filter for the single-pane window proposed in the reference [9]. The 1/3 octave band spectra of the house filter model is shown in Fig.6 (type-b), in which the house filter models for three kinds of window constructions are proposed. As an example, the FFT spectrum of the test sound for indoor scenario containing a tonal component at Hz is shown in Fig. 5(b). Figure 7 shows the comparison between TAs for outdoor/indoor scenarios. In the Fig.7, TAs of test sounds for both of scenarios agreed except for experimental condition of a Hz. TAs of test sounds including a Hz for indoor scenario ranged from. db up to 17.3 db under the experimental condition of TA for outdoor scenario from -3 db up to 15 db. After the convolution with house filter, TAs of test sounds containing a tonal component at Hz became higher than that for the outdoor scenario by about 3 db, respectively. As the standard stimulus () for indoor scenario, the convolved model noise without tonal components was fixed at 35. db in terms of A-weighted sound pressure level. The presentation level of ranged from 35. db (without a tonal component) up to 44.6 db (a Hz, 15 db TA) in terms of A-weighted sound pressure level. 3.3 Experimental Procedure In the experiment, the subject in a test room was asked to judge the difference of the auditory impression between and according to the following four-category system (in Japanese): 1: They are not different at all. 2: They are slightly different. 3: They are considerably different. 4: They are definitely different. 36 pairs of - were randomly arranged and presented to each test subject twice (72 times pair comparisons in total). The total time needed to complete the test on 72 pairs of the test sounds was about three quarters hours including rest times in between. In this experiment, subjects from twenties to forties (5 males and 5 females) with normal hearing abilities participated. Table 1 72 test sounds used in the experiments Stimuli Tonal Audibility [db] Model noise (-4 db/octave band) No tonal components Model noise + Hz -3,, 3, 6, 9, 12, 15 Model noise + Hz -3,, 3, 6, 9, 12, 15 Model noise + Hz -3,, 3, 6, 9, 12, 15 Model noise + Hz -3,, 3, 6, 9, 12, 15 Model noise + Hz -3,, 3, 6, 9, 12, 15 * 16 test sounds enclosed by dotted line in Table 1 were also used in the following annoyance matching test. 2631

Sound pressure level [db] 15dB Sound pressure level [db] 15dB 12dB 9dB 6dB 3dB db -3dB (a) For outdoor scenario (; 45 db) 175 1 185 1 195 5 2 215 2 225 Sound pressure level [db] 15dB Sound pressure level [db] 15dB 12dB 9dB 6dB 3dB db -3dB 175 1 185 1 195 5 2 215 2 225 (b) For indoor scenario (; 35 db) Figure 5 An example of FFT spectrum of the test sound with Hz frequency (right figure; 15 db Tonal Audibility, left figure; 7 variations) SPL difference [db] A: Double-pane window B: Single-pane window C: Single-pane (wooden frame) window - 1 2 4 8 16 31.5 63 125 2 1k 2k 4k Figure 6 House Filter Models for three kinds of window constructions proposed in the reference [9]. 2632

18 15 TA for Indoor scenario[db] 12 9 6 3 Hz Hz Hz -3 Hz Hz -6-6 -3 3 6 9 12 15 18 TA for Outdoor scenario [db] Figure 7 Comparison between TAs for outdoor/indoor scenarios. 3.4 Experimental Results of Audibility Tests From the experimental results, the difference threshold of auditory impression was investigated. In these experiments, (without a component) was also included as a in order to check the reliability of subjects judgment. As a result, only 2.5% of the subjects response of 2 (slightly different) was found. 3.4.1 Subjective response of difference In Figs. 8 and 9, the percentage of the subjective responses in the four categories for outdoor/indoor scenario was compared with TAs of. For all of the frequencies, the tendency that positive response monotonically increased with the increase of level was found. Especially in case of a Hz for indoor condition, subjective response for difference was higher than other cases clearly. This is caused by the fact that TAs in X-axis in Figs. 8 and 9 indicates TAs set for test stimuli assumed outdoor scenario (see; Fig.7). 3.4.2 Difference Threshold To quantitatively assess the experimental results, the ratio of positive subjective response for difference was calculated by applying the logistic regression analysis. In the analysis, two steps of positive response regarding the difference between and were assumed: one is categories 2+3+4 (case-1) and the other is categories 3+4 (case-2). Figures and 11 show the relationship between TAs of and the ratio of the results judged as difference. As for the results for outdoor scenario shown in Fig., it is seen that the subjective response for difference was caused when TA was higher than around -2 db for case-1 and between 6 db and 8 db according to the frequency of a contained tonal component for case-2, respectively. This tendency is consistent with reference [8]. In the results for indoor scenario shown in Fig.11, we can see that the response of difference was caused when TA was higher than between -3 db and db according to the frequency of a tonal component for case-1 and around 8 db for case-2 except for the experimental condition of a Hz because of the reason mentioned above. In the Figs. 11(a) and 11(b) for indoor condition, the difference threshold for Hz frequency was lower than that for other frequency by about 3 db, respectively. The tendency could be explain by the difference of TAs in both scenarios. From the results, the difference threshold of auditory impression is around -2 db in case-1 and 8 db in case-2 in terms of TA. 2633

Hz -3 3 6 9 12 15 Hz -3 3 6 9 12 15 Hz -3 3 6 9 12 15 Hz -3 3 6 9 12 15 Hz -3 3 6 9 12 15 1: They are not different at all 2: They are slightly different 3: They are considerably different 4: They are definitely different Figure 8 Subjective response for difference of auditory impression for outdoor scenario 2634

Hz -3 3 6 9 12 15 Hz -3 3 6 9 12 15 Hz -3 3 6 9 12 15 Hz -3 3 6 9 12 15 Hz -3 3 6 9 12 15 1: They are not different at all 2: They are slightly different 3: They are considerably different 4: They are definitely different Figure 9 Subjective response for difference of auditory impression for indoor scenario 2635

Response of "Difference impression" [%] 75 Hz Hz 25 Hz (a) case-1 Hz (Category: 2, 3, 4) Hz -3 3 6 9 12 15 Hz Hz 25 Hz Hz Hz -3 3 6 9 12 15 Figure Difference threshold calculated from the subjective responses for outdoor scenario Response of "Difference impression" [%] 75 (b) case-2 (Category: 3, 4) Response of "Difference impression" [%] 75 Hz Hz 25 Hz (a) case-1 Hz (Category: 2, 3, 4) Hz -3 3 6 9 12 15 Hz Hz 25 Hz Hz Hz -3 3 6 9 12 15 Figure 11 Difference threshold calculated from the subjective responses for indoor scenario Response of "Difference impression" [%] 75 (b) case-2 (Category: 3, 4) 4. SUBJECTIVE EXPERIMENTS ON ANNOYANCE OF TONAL COMPONENTS IN WTN To examine the effect of a tonal component in WTN on annoyance sensation, auditory experiment was performed applying a method of subjective adjustment. 4.1 Experimental Setting and Test Sounds In order to investigate the relationship between annoyance and the strength of TA, annoyance matching test was performed for outdoor scenario. In this experiment, the reproduction system with a volume controller shown in Fig.3 was used. All test sounds were presented diotically through a headphone system. As for the standard stimulus () in this experiment, the model noise for WTN containing a tonal component used in the previous paired comparison test assumed outdoor scenario where the sounds were used as the comparison stimulus () was again used (see; Table 1). For in the annoyance matching test, 16 test sounds in total including a model noise without tonal component were chosen; a tonal component was set at, or Hz and TA was set at -3,, 3, 6 or 9 db. The reproduction level of test sounds was set at the same level as test sounds for outdoor scenario mentioned above, respectively. A-weighted time-averaged SPLs (L Aeq,5s ) of were listed in Table 2. As the Comparison stimulus (), the model noise for WTN without a tonal component was used. 2636

Table 2 L Aeq of the standard stimuli (). Tonal Audibility [db] Hz Hz Hz No tonal components (45.) (45.) (45.) -3 db 45.1 45.2 45.1 db 45.2 45.3 45.3 3 db 45.5 45.7 45.5 6 db 45.9 46.2 46. 9 db 46.6 47.1 46.8 4.2 Experimental Procedures As the test procedure, the method of adjustment was applied using the experimental system shown in Fig.3. In each condition, the standard stimulus () was firstly presented and secondly the comparison stimulus () was presented. After that, the subject was asked to adjust the annoyance (impression of unpleasant or harsh) of so as to be equal to that of by using a volume controller. For the ascending/descending series for outdoor scenario where the reproduction level of was set at the level shown in Table 2, was firstly set at / db, respectively. The pair of and was repeated until the subject completed the adjustment. For each experimental condition, two trials (ascending and descending) were performed. The total time needed to complete the test of 16 test sounds was about three quarters hours including rest times in between. For this experiment, 9 subjects from twenties to forties (4 males and 5 females) with normal hearing abilities participated. All subjects have also participated in the previous experiment. 4.3 Experimental Results of Annoyance matching Test Figure 12 shows the experimental results of the annoyance matching test. In the figure, X-axis indicates TAs of and Y-axis indicates the each level of test sounds in L Aeq,5s adjusted by all of test subjects. In the figures, the level of which contains a tonal component shown in Table 2 is also represented. In these experiments, (without a component) was also included as a in order to check the reliability of subjects judgment. Looking at the results of investigation for the reliability, we can see that the level difference between and (without a component) was sufficiently low. As an experimental result, it is seen that the level of the adjusted increased as the TA of became higher in all cases of the frequency of a tonal component. The difference between the levels in L Aeq,5s of and in each test condition ranged from.3 db to 3. db. For the subjective response of annoyance, nine test subjects could be divided into the following three groups focusing on the way of adjusting the level of. In this experiment, number of subjects for each group was three for Group A, two for Group B and four for Group C. Group A: most sensitive to tonal components in WTN Group B: the level of was adjusted to that of with a tonal component Group C: the level of was adjusted to that of or model noise with no tonal component The experimental results for each group are shown in Figs.13, 14 and 15, respectively. In the results of Group A shown in Fig.13, the tendency that the level of the adjusted increased as the TA of became higher was clearly seen. The level difference between and in each test condition ranged from.2 db to 6.1 db. Even the case of -3 db TA, the difference ranged from 1.1 db to 2.8 db, though the difference was.2 db in case where was same as with no tonal component. In the results of Group B shown in Fig.14, it is seen that the level difference between and in each test condition was lower than 2 db in almost cases, though the difference was insecure as a whole. Figure 15 shows the results of Group C. In the figure, the level difference was nearly equal db in almost cases. After the experiment, some subjects among Group B commented that it was extremely difficult to adjust the level, because of the difference of sound quality (between with a pure and ) and some subjects among Group C commented that he/she had not yet heard sounds like test stimuli or didn t feel unpleasant for any test stimuli or adjust the level of according to loudness impression. 2637

55 Hz Hz Hz LAeq,5s [db] 45-3 3 6 9-3 3 6 9-3 3 6 9 Figure 12 Experimental results of annoyance matching test by all subjects. 55 Hz Hz Hz LAeq,5s [db] 45-3 3 6 9-3 3 6 9-3 3 6 9 Figure 13 Experimental results of annoyance matching test by subjects of Group A. 55 Hz Hz Hz LAeq,5s [db] 45-3 3 6 9-3 3 6 9-3 3 6 9 Figure 14 Experimental results of annoyance matching test by subjects of Group B. 55 Hz Hz Hz LAeq,5s [db] 45-3 3 6 9-3 3 6 9-3 3 6 9 Figure 15 Experimental results of annoyance matching test by subjects of Group C. 2638

5. CONCLUSIONS Regarding the effect of tonal components contained in wind turbine noise (WTN), two kinds of auditory experiments were performed. As for the first experiment, to investigate the validity of the numerical assessment method regarding the tonality of WTN, a basic auditory experiment on the difference threshold of auditory impression was tried using an artificially synthesized model noise of WTN and that including a tonal component. As a result, the difference threshold for considerably different + definitely different was around 8 db in Tonal Audibility (TA) in all frequency conditions (,,, and Hz) for both of outdoor/indoor scenarios. From the results of the second experiment in which the effect of a tonal component in WTN on annoyance sensation was examined, the tendency has been found that the annoyance impression increases as the increase of tonal audibility (TA) of model noise with a pure at,, Hz on the whole, whereas there are differences among individuals in annoyance sensation for noises with a tonal component. In our study, the effect of tonal components, amplitude/frequency modulation of tonal component(s) and amplitude modulation sounds (swish sounds) in WTNs have not been considered. Further investigation on the penalty for tonal component(s) in WTNs should be continued. REFERENCES 1. H. Tachibana, H. Yano, A. Fukushima and S. Sueoka, Nationwide field measurements of wind turbine noise in Japan, Noise Control Eng. J., 62(2), -1 (14). 2. A. Fukushima, K. Yamamoto, H. Uchida, S. Sueoka, T. Kobayashi and H. Tachibana, Study on the amplitude modulation of wind turbine noise: Part 1 - Physical investigation, Proc INTER-NOISE 13; 15-18 September 13; Innsbruck, Austria 13. Paper 668. 3. S. Kuwano, T. Yano, T. Kageyama, S. Sueoka and H. Tachibana, Social survey on wind turbine noise in Japan, Noise Control Eng. J., 62(5), 3-5 (14) 4. S. Sakamoto, S. Yokoyama and H. Tachibana, Experimental study on hearing thresholds for low frequency pure s, Acoustical ience and Technology, 35(4), 213-218 (14) 5. S. Yokoyama, S. Sakamoto and H. Tachibana, Perception of low frequency components in wind turbine noise, Noise Control Eng. J., 62(5), 295-5 (14). 6. S. Yokoyama, S. Sakamoto, S. Tsujimura, T. Kobayashi and H. Tachibana, Loudness experiment on general environmental noises by considering low-frequency components down to infrasound, Acoustical ience and Technology, 36(1), 24- (15) 7. S. Yokoyama, S. Sakamoto and H. Tachibana, Study on the amplitude modulation of wind turbine noise: part 2- Auditory experiments, Proc INTER-NOISE 13; 15-18 September 13; Innsbruck, Austria 13. Paper 6. 8. T. Kobayashi, S. Yokoyama, A. Fukushima, T. Ohshima, S. Sakamoto and H. Tachibana, Assessment of tonal components contained in wind turbine noise in immission areas, Proc 6th International Meeting on Wind Turbine Noise; -23 April 15; Glasgow, otland 15. 9. H. Tachibana, A. Fukushima and H. Ochiai, Modelling of house filter for wind turbine noise, Proc 6th International Meeting on Wind Turbine Noise; -23 April 15; Glasgow, otland 15. 2639