Amplitude modulation for different weather conditions Conny Larsson Uppsala University, Sweden
EWEA Technology Workshop: Wind Turbine Sound 2014
Economical support from Swedish Energy Agency 2010-2014 Results from two Wind Turbine sites: Forest, flat ground; Forest, heterogeneous terrain Special analyses of AM sound presented here.
Measurements
Rythmic sound heard at distance
• Changes of the sound level occur due to the weather but does the weather also change the character of the sound at an immission point far away?
• Oerlemans demonstrated that mainly the downward movement of the turbine blades, as perceived on the ground, produces the swish sound.
• According to van den Berg, the synchronizing of sound between different WTs during propagation in a stable atmosphere could explain the thumping noise heard at greater distances from WTs.
Measurement sites
Photo: Hans Blomberg
Photo: Svevind
Dragaliden
Ryningsnäs
Dragaliden, 1 km
Ryningsnäs, 400 m
AM study, 1 year at Ryningsnäs and Dragaliden
• Measure 8 times/s (Fast) • Divide into 15 s long periods and analyze. • Observations at the sites and studies of different
amplitude modulation spectra • Our definition: AM if the AMS-value (periodicities
between 0.6 – 1.0 Hz) is ≥ 0.4
Lundmark, Measurement of swish noise, a new method, Fourth International Meeting of Wind Turbine Noise, INCE/Europe, Rome, Italy (2011).
C. Larsson, and O. Öhlund, Amplitude modulation of sound from wind turbines under various meteorological conditions, J. Acoust. Soc. Am. 135(1), 2014, 67-73
Amplitude modulation
Sound level recording from the Dragaliden site from 20 July 2011 at different clock hours. The measurements are taken 3 km from the closest WT. To the right is the amplitude modulation spectra with AMS value for a given 15 s period.
Day
Night
15:42:15 15:42:30
28
30
32
34
Time
Inst
anta
neou
sso
und
leve
l (dB
A)
0 1 2 30
0.2
0.4
0.6
Frequency (Hz)
AMS
valu
e (d
B)
00:01:30 00:01:45
28
30
32
34
Time
Inst
anta
neou
s s
ound
leve
l (dB
A)
0 1 2 30
0.2
0.4
0.6
Frequency (Hz)
AMS
valu
e (d
B)
Amplitude modulation
Sound level recording from the Dragaliden site from 20 July 2011 at different clock hours. The measurements are taken 3 km from the closest WT. To the right is the amplitude modulation spectra with AMS value for a given 15 s period.
Day
Night
15:42:15 15:42:30
28
30
32
34
Time
Inst
anta
neou
sso
und
leve
l (dB
A)
0 1 2 30
0.2
0.4
0.6
Frequency (Hz)
AMS
valu
e (d
B)
00:01:30 00:01:45
28
30
32
34
Time
Inst
anta
neou
s s
ound
leve
l (dB
A)
0 1 2 30
0.2
0.4
0.6
Frequency (Hz)
AMS
valu
e (d
B)
AMS < 0.4
AMS ≥ 0.4
AM AM
No AM No AM
AM heard at distance.
00:27:15 00:28:0440
45
50In
stan
tane
ous
sou
nd le
vel (
dBA)
00:27:15 00:28:0426
28
30
32
34
36
Inst
anta
neou
s s
ound
leve
l (dB
A)
100 101 102 103 104 105−20
−15
−10
−5
0
5
10
15
20
25
30
Soun
d pr
essu
re le
vel (
dBA)
Frequency (Hz)
Emission (upper left) and immission (down left) sound level measurements and immission sound spectrum for the given time frame at Dragaliden site. Distance 2-4 km from the WT.
100 Hz
1000 Hz
Ryningsnäs, Småland Nordex 2500 LS (2,5 MW) hub height 80 and100 m
Ryningsnäs, 400 m
0 100 200 3000
20
40
60
Wind direction (o)
Per
cent
AM
of t
otal
tim
e in
spe
cific
sec
tor (
%)
a)
0 10 20 30 400
20
40
60
Turbulence intensity100m (%)
Per
cent
AM
of t
otal
tim
e in
spe
cific
bin
(%)
b)
−0.1 −0.05 0 0.05 0.10
20
40
60
Sound speed gradient (s−1)calculated between 120−0 m
Per
cent
AM
of t
otal
tim
e in
spe
cific
bin
(%)
c)
−50 0 500
20
40
60
Solar elevation angle (o)
Per
cent
AM
of t
otal
tim
e in
spe
cific
bin
(%)
d)
Percentage of AM in a specific bin for the Ryningsnäs site.
Dragaliden, Enercon-E82 (2 MW) WTs hub height 108 och 138 m
Dragaliden, Enercon-E82 (2 MW) WTs hub height 108 och 138 m
0 100 200 3000
20
40
60
Wind direction (o)
Per
cent
AM
of t
otal
tim
e in
spe
cific
sec
tor (
%)
a)
0 10 20 30 400
20
40
60
Turbulence intensity100m (%)
Per
cent
AM
of t
otal
tim
e in
spe
cific
bin
(%)
b)
−0.1 0 0.10
20
40
60
Sound speed gradient (s−1)calculated between 120−0 m
Per
cent
AM
of t
otal
tim
e in
spe
cific
bin
(%)
c)
−40 −20 0 20 400
20
40
60
Solar elevation angle (o)
Per
cent
AM
of t
otal
tim
e in
spe
cific
bin
(%)
d)
Dragaliden, 19 - 21 July 2011, 2-4 km distance
00:00 00:000
1
2
AM fa
ctor
im
mis
sion
poi
nt (d
B)
a)
00:00 00:0020
40
60
I
nsta
ntan
eous
so
und
leve
l im
mis
sion
poi
nt (d
BA)
b)
00:00 00:0020
30
40
Cal
cula
ted
imm
issi
on s
ound
leve
l (dB
A)
c)
00:00 00:0012141618202224
Tem
pera
ture
(o C)
d)
T3mT100m
00:00 00:00
200
250
WD 11
1m (o )
e)
00:00 00:000
10
20
TI11
9m (%
) f)
00:00 00:000
1
2
AM fa
ctor
e
mis
sion
poi
nt (d
B)
g)
00:00 00:005
10
Win
d sp
eed
(m/s
)
h)
u51mu98m
Ampl i tude modu la t ion (percentage of selected measurements) Evening-night-morning Ryningsnäs (400 m) 33 % Dragaliden (1 km) 19 %
Results – AM during 1 year
AM more common during specific meteorological conditions
Conclusions
• The weather governs the immission sound level even for high elevated sources (WT).
• The weather governs the character of sound from WTs – Amplitude modulation.
• Amplitude modulation 20 - 30 % of time. • Most frequent at evening-night-morning at stable
stratification.
19