PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Perception of low-frequency components contained in generalenvironmental noises including wind turbines
Sakae YOKOYAMA1; Tomohiro KOBAYASHI2
1

Kobayasi Institute of Physical Research, Japan

2

Kobayasi Institute of Physical Research, Japan

ABSTRACTThe influence of low frequency components in noises including wind turbine noise has become a serious
problem and various researches are being made in many countries. Also in Japan, synthetic study programs
mainly targeted for wind turbine noise had been performed, in which field measurements, social survey on
the response of nearby residents and laboratory experiments on audibility of low frequency sounds wereconducted. In parallel to these studies, field measurements on general environmental noises includingtransportation noises inside and outside of various vehicles were performed by paying attention to low
frequency components. As a result, it has been found that low frequency components are included not only inwind turbine noise but also in general environmental noises. The environmental sounds recorded on sites
were reproduced by a test facility which can reproduce low frequency components including infrasound andauditory tests were performed. As a result, it has been found that the A-weighted sound pressure level isrobustly applicable to the assessment of loudness for such kinds of environmental sounds and the low
frequency components contained in wind turbine noise around residential area is not audible.
Keywords: Low-frequency, Wind turbine noise, Loudness, Annoyance, auditory experiment

1. INTRODUCTION
To investigate wind turbine noise problem from a psycho-acoustical viewpoint, the authors haveconducted several kinds of subjective tests focusing on the low frequency sound using WTN recorded
on-site and electrically synthesized sounds [1-5]. In addition to WTNs, a variety of environmental
noises such as ambient sounds in natural environments and urban residential areas, transportationnoises inside and outside of various vehicles, machine noises, construction noises, etc. were also
recorded including low frequency components down to 1 Hz. In order to investigate the method ofassessing these general environmental noises including WTN uniformly, the authors have conducted
an auditory experiment on loudness sensation by using an experimental facility which is capable ofreproducing low frequency sounds including infrasound.

2. EXPERIMENTAL FACILITY
To investigate the effects of low-frequency components contained in various environmental noises
including WTN, the experimental facility shown in Figs 1 and 2 was constructed at the Institute ofIndustrial Science, The University of Tokyo [1]. On the partition wall between the two rooms, 16
woofers of 40 cm diameter (Fostex: FW405N) were installed to reproduce low-frequency sounds down
to 4 Hz. In addition, a full-range loudspeaker of 20 cm diameter (Fostex: FE206En) was added at thecenter position of the woofers to cover the middle- and high-frequency sounds up to 16 kHz. The
crossover frequency between the two loudspeaker systems was set at 224 Hz. The listening position
was located 3.5 m from the center point of the loudspeaker system. To co rrect the frequency
characteristic of the total reproduction system, the digital inverse -filtering technique was applied
using a linear-phase FIR filter calculated from the trans-fer function from the input of the amplifier tothe listening point measured in the frequency range from 4 Hz to 16 kHz in 1/12-octave-bands.

12sakae@kobayasi-riken.or.jp
kobayashi@kobayasi-riken.or.jp1463

4.04 m4.21 mSound absorptive
finishing (30 cm)

Listening position

6.86 m

3.5 m

16 woofers+ full range LS.

6.72 mSound absorbing
material

Expanded metal+ carpet (0.9 m height)

Figure 2 – Interior of the receiving room.
Power Amplifier

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Multi-channel Interface(RME, Multiface AE)

PC

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㻞

㻟

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Listener’s response(hp)

Figure 1 – Plan view of the experimental

Figure 4 – 7-step category used in loudness

3. SUBJECTIVE EXPERIMENT ON LOUDNESS
The most significant factor in the assessment of noise is loudness. In this study, therefore, aloudness test was conducted by using 38 different environmental sounds containing low frequency
components down to infrasound and examined the correspondences b etween loudness and various
noise indicators.
3.1 Test Sounds
As test sounds for this study, sounds in the natural environment, ambient sounds in residential areas,interior and exterior noises of transportation systems, and others shown in Table 1 were collected. Allof these sounds can be regarded as stationary and contain no strong tonal components. To record thesesounds, including low-frequency components down to infrasound, a wide-frequency-range sound level
meter (prototype of RION: NL-62) that had a flat frequency response from 1 Hz to 20 kHz was used,
and the sound pressure signals were recorded on an SD card installed in the sound level meter, using
the recording function (48 kHz sampling, 16 bits, WAVE-format). To prevent wind-noise at themeasurement microphone, a newly designed double-skin type wind-screen set [5] was put on the
microphone when recordings outdoors. In addition to the 33 kinds of environmental sounds, anartificial noise with a frequency characteristic of -4 dB/octave in the band spectrum representing thegeneral frequency characteristics of WTNs [5] was used and its level was varied in five steps of 35 dB,
45 dB, 55 dB, 65 dB, and 75 dB in terms of A-weighted SPL. 1/3-octave-band SPLs of the 38 sounds
used in this study are shown in Fig. 3. These data were measured when the respective test sounds werereproduced in the experimental facility mentioned below in the absence of the listener at the position
where the center of the listener’s head would be.
3.2 Test procedure
Each of the test sound was edited to have a duration of 10 s and windowed to incorporate a 1.0 s
ramp-up at the beginning and 1.0 s ramp-down at the end of the presentation in order to eliminate clicksounds.
The loudness test was conducted by applying the rating scale method using seven-step category.
That is, each test sound was presented to the subject and he/she judged the loudness according to the1464seven-step category presented schematically in a panel (see Fig. 4), and orally gave his/her judgmentusing the integer shown in the panel. The 38 test sounds were presented in random order. One seriesof the test took about 30 minutes. To examine the repeatability of the subject’s judgments, the test
was repeated twice for each subject. Between the two judgment tests, a short break was taken to
allow the subject to relax the subject. In this experiment, 20 subjects of the age range from 21 years
to 25 years (18 males and 2 females) with otologically normal hearing ability participated. This
auditory experiment was performed in accordance with the ethical code of The University of Tokyo.
Table 1 – Test sounds used in this study
Categories(a) Environmental
sounds(b) Transportationnoises(c) Cabin noises

No.

(e) Otherenvironmental
noises(f) Artificial noisemodeling WTN

LAeq,10s[dB]

Loudness score

1
2

Environmental sound in a forest
Sound of soughing through pine trees

31
61

Average
1.25
5.25

3

Environmental sound on a seashore : A

61

5.08

0.57

4

Environmental sound on a seashore : B

54

4.08

0.73

5

Environmental noise in residential area : A

43

2.73

0.64

6

Environmental noise in residential area : B

32

1.23

0.42

7

Environmental noise in residential area : C

38

2.03

0.48

8

Noise in an industrial estate

49

3.68

0.62

Std. Dev.
0.54
0.63

9

Chirp of cicada in summer

54

4.25

0.78

10

Chirp of insects in autumn

38

2.00

0.64

11

Railway noise

76

6.40

0.59

12

Road traffic noise (distance: 22 m)

76

6.50

0.60

13

Road traffic noise (distance: 85 m)

63

4.98

0.58

14

Road traffic noise (distance: 85 m, in house)

43

2.83

0.68

15

Aircraft noise

65

5.20

0.65

16

Cabin noise in a super-express train

68

5.80

0.46

17

Cabin noise in a super-express train in a tunnel

71

6.18

0.38

18

Cabin noise in a jet-plane A

73

5.98

0.36

19

Cabin noise in a jet-plane B

81

6.95

0.22

20

Cabin noise in a car running a highway

72

6.05

0.45

21

70

6.05

0.45

70

6.05

0.50

23

Cabin noise in a conventional train
Cabin noise in a conventional train running on at l b idWind turbine noise (near a wind turbine)

56

4.38

0.67

24

Wind turbine noise (residential area: outside)

43

3.00

0.64

25

Wind turbine noise (residential area: inside)

27

1.05

0.22

26

Wind turbine noise (with insects’ sounds)

41

2.65

0.70

27

Wind turbine noise (insects’ sounds were cut off)

37

1.85

0.62

28

Air-conditioning equipment : A

40

2.13

0.56

29

Air-conditioning equipment : B

61

5.15

0.53

30

Air-conditioning equipment : C

66

5.25

0.54

31

Structure-borne sound from a subway truck

45

2.95

0.71

32

Concourse of a railway station

64

5.30

0.56

33

Construction noise (Concrete breaker)

79

6.93

0.27

34

Synthesized noise with -4dB/octave : A

35

1.63

0.59

35

Synthesized noise with -4dB/octave : B

45

3.18

0.59

36

Synthesized noise with -4dB/octave : C

55

4.40

0.55

37

Synthesized noise with -4dB/octave : D

65

5.75

0.54

38

Synthesized noise with --4dB/octave : E

75

7.00

0.00

22

(d) Wind turbine
noisesContents1465

1/3-octave-band sound pressure level [dB]1/3-octave-band sound pressure level [dB]100(a) Environmental sounds

90
80
70

3

60

2

5

50
40
30

9

8

7

4

6

1

10

20
Back ground noise

10
0
4

8

16

31.5

63

125

250

500

1k

2k

4k

8k

100

(b) Transportation noises

90
80
70

11

60

13

50
40

20
10
0

16k

4

8

16

31.5

63

1/3-octave-band sound pressure level [dB]1/3-octave-band sound pressure level [dB]20(c) Cabin noises

80
16

70

17

21

60
50

18

40
22

30
20
10
0
4

8

16

31.5

63

125

250

500

1k

2k

4k

8k

70
29
30

40

32
31

20
10

33

0
4

8

16

31.5

63

125

250

500

Frequency [Hz]

2k

4k

8k

16k

1k

2k

4k

8k

16k

28

80
70

27

25

60
50

26

40

24

23

30
20
10
0
4

1k

2k

4k

8k

16k

1/3-octave-band sound pressure level [dB]1/3-octave-band sound pressure level [dB]

80

30

1k

8

16

31.5

63

125

250

500

Frequency [Hz](e) Wind turbine noises

50

500

(d) Other environmental noises

90

16k

100

60

250

100

Frequency [Hz]90125Frequency [Hz]

100
90

15

14

30

Frequency [Hz]
19

12

100

(f) Artificial noises modeling WTN

90
80

38

70

37
36

60

35

50

34

40
30

-4dB
/Oct.

20
10
0
4

8

16

31.5

63

125

250

500

1k

2k

4k

8k

16k

Frequency [Hz]Figure 4 – Spectral characteristics of the test sounds used in this experiment
3.3 Experimental Results
After completing the loudness tests for the 20 subjects, the correlation between the results of thetwo tests was examined for each subject. As a result, the correlation coefficient ranged be-tween 0.93
and 0.97; indicating that a consider-ably high repeatability was obtained for all of the subjects.
Therefore, all of the experimental results of the twice-repeated tests for the 20 subjects were used in
the data analysis dis-cussed below.
To examine the equal-interval characteristic of the categories used in this experiment, the methodof successive categories was applied, and a correlation coefficient of 0.99 was found between theoriginal categories and those trans-formed into interval scale. Therefore, the arithmetic average of thecategory numbers given by all of the subjects was determined as the loud-ness score for respective test
sounds as shown in Table 1.
Next, the sound pressure measured at the listening position over 10 s (duration of each of the test
sounds) shown in Fig. 4 was evaluated using the following six kinds of psycho-acoustical indicators.

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(1) L A: A-weighted sound pressure level (IEC 61672-1 [6] )(2) LC : C-weighted sound pressure level (IEC 61672-1)(3) L G : G-weighted sound pressure level (ISO 7196)(4) L Z: Z-weighted sound pressure level (IEC 61672-1)(5) LLZ : Zwicker loudness level (ISO 532B [7,8])(6) LLMG : Moore-Glasberg loudness level (ANSI S3.4-2007 [9])L A and LC were obtained using digital IIR filters with the frequency-weighting functions of A andC, respectively, specified in IEC 61672-1 [6]. LG was obtained using that with thefrequency-weighting functions of G specified in ISO 7196. L Z was no frequency-weighting. LLZ wascalculated from the time-averaged 1/3-octave-band SPLs from 25 Hz to 12.5 kHz according to thealgorithm described in Ref. 7 and 8, and LLMG was calculated from the time-averaged
1/3-octave-band SPLs from 50 Hz to 16 kHz according to Ref. 9.
The relationships between the loudness score and the values assessed by the respective indicators
are given in Fig. 5. From these results, it is seen that the loudness score correlated well to all thevalues assessed using L A, LL Z and LL MG exhibiting a high correlation coefficient r larger than 0.99,
whereas the correspondence decreased slightly in the case where LC was applied (r=0.934). In thecases applying LG and L Z, it was much deteriorated, whereas LG .was especially the psycho-acousticalindicator for sounds including low-frequency component.
Here, it should be noted that L A is in no way inferior to LL Z and LL MG .
3.4

Discussion

Regarding the A-weighted SPL, many researchers pointed out its shortcoming in the assessment of theloudness or annoyance under the condition that the test sounds contain strong tonal component(s) or
dominant spectral component(s) in narrow-band(s) [10-14]. In those cases, the A-weighted SPL is too
simple and other sophisticated indicators such as LL Z and LL MG should be applied. However, the result of
this study indicates that the A-weighted SPL is an appropriate indicator of the loudness of generalenvironmental noise even when relatively dominant low-frequency components down to infrasound areincluded.
Another problem of the A-weighted SPL often pointed out when discussing its applicability to a wide
range of sound levels is that the A-weighting function was originally proposed as an inverse characteristicof the 40-phon curve of the equal-loudness contours proposed by Fletcher and Munson [15, 16] (see Fig. 6);
therefore, it is unsuitable when assessing intermediate- or high-level sounds. However, when comparing
the inverse A-weighting curve to the revised equal-loudness contours specified in ISO 226: 2003 [17], itfits around the 60-phon curve as shown in Fig. 7. This finding means that the A-weighting curve is notnecessarily a frequency weighting function for relatively soft sounds.
From the result discussed above, it can be concluded that the A-weighted SPL is a simple and good
estimator of the loudness when assessing general environmental noise including WTN. The applicability of
the A-weighted SPL to WTN with low-frequency components has also been investigated [14, 18, 19].
However, when assessing noise with dominant tonal or narrow-band spectrum components, there would bea limit to the application of this indicator, and precise loudness evaluation and the consideration ofcorrection factors for tonality as specified in ISO 1996 -2 [20] are necessary.

4. CONCLUSIONSIn this study, an auditory experiment on loudness was conducted using a variety of environmental
noises including low frequency components and the experimental results were evaluated by six kindsof psycho-acoustical quantities. As a result, it has been found that A-weighted sound pressure level isthe best indicator among them. In the problem of wind turbine noise, it is often discussed how toevaluate the low frequency components. As shown in the spectral characteristics of the environmental
noises collected in this study, the low frequency components are contained in general environmental
noises, not particularly in wind turbine noise. As a conclusion, it is suggested that A -weighted soundpressure level should be uniformly applied to the assessment of general environmental noises
including wind turbine noise.

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Environmental noise
Cabin noise
Other noise

Transportation noiseWind turbine noise
Synthesized noise

7

6
5
4
3
2
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7

6
5
4
3
2
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7

6
5
4
3
2
1

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Figure 5 – Relationship between loudness score and noise indicators

1468㻝㻜㻜 㻝㻝㻜

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7

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1Environmental noise
Cabin noise
Other noise

Transportation noiseWind turbine noise
Synthesized noise㻝㻜㻜 㻝㻝㻜

130

130

Inverse A-weighting

120

110
100 phons

90
80

80
70

60

60
50

40

40
30

20

20

0
-10
16

63

125

250 500

90
80

80
70

60

60
50

40

40
30

20

20
10

Hearing threshold

31.5

100 phons

100

Sound pressure level [dB]

Sound pressure level [dB]

100

10

Inverse A-weighting

120

110

0
1k

2k

4k

8k

16k

Frequency [Hz]Figure 6 – Inverse A-weighting curve on theequal-loudness contours proposed by Fletcher& Munson.

-10
16

Hearing threshold

31.5

63

125

250 500

1k

2k

4k

8k

16k

Frequency [Hz]Figure 7 – Inverse A-weighting curve on theequal-loudness contours specified in ISO
226: 2003.

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Am., 82(5), 1700-1705 (1987).
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