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FUNDAMENTAL ACOUSTICS AND WIND TURBINE NOISE ISSUES
1. FUNDAMENTAL ACOUSTICS
AND
WIND TURBINE NOISE ISSUES
Prof. Gopu R. Potty, Ph.D.
Department of Ocean Engineering
University of Rhode Island
Narragansett, RI 02882
potty@egr.uri.edu
4. Sound Waves
Sound is a pressure wave Intensity is the average amount
of sound power transmitted
Sounds have different frequencies through a unit area in a
Human hearing: 20 Hz to 20 kHz specified direction. The unit of
Less than 20 Hz - infrasound intensity is watts per square
meter.
5. Decibel
•The decibel (dB) is a
logarithmic comparison of
intensities.
•Named for Alexander
Graham Bell
⎧ Acoustic Intensity ⎫
Level = 10 log ⎨ ⎬
⎩ Reference Intensity ⎭
Reference acoustic intensity = 1x10 −12 W/m 2
7. Adding decibels
• Let’s say we had 3 sources of sound at 70, 80
and 90 dB each, what is the total level?
• We need to convert the individual levels into
raw intensities and add them
• The sum thus calculated (expressed in dB) in
this case is 90.5 dB
Two turbines produce 3 dB more than one turbine
8. Sound Pressure Level (SPL)
SPL= 20 log Pressure of an acoustic signal
reference pressure
The units of L are dB relative to the
reference pressure.
The reference pressure is
20 micropascals based on
hearing tests of 16 million men
in WW2.
This corresponds to an
Intensity of 1x10-12 W/m2.
9. Source Level SL
(Rogers et al., 2006)
SL is defined to be
20 log Pressure of source at 1 m
reference pressure
The units of SL are dB relative to
Reference pressure of 20 micropascals at 1 meter.
SL referenced at 1 meter
Quantifies the strength of the source !!!!
10. Acoustics at a distance
• We can predict the sound pressure level of an
acoustic signal at a distance.
L = SL - TL
L = Sound Pressure Level
SL = Source Level
TL = Transmission Loss
Nascar fans in the front row are exposed to more intense sound
than the fans in back row due to transmission loss.
11. Transmission Loss
• Transmission Loss TL (aka
propagation loss) describes
the weakening of sound
between a point 1 meter
from the source and a point
at a distance r meters.
• It is the ratio of intensity at
any range ‘r’ to intensity at 1 (Rogers et al., 2006)
m
Intensity at r meters
TL = -10 log
Intensity at 1 meter
12. Transmission Loss Components
Absorption coefft.
• Geometrical spreading expressed in dB/km or
• Absorption dB/m
• Scattering Absorption a function of
– Volumetric scattering, turbulence • Temperature
• Humidity
– Groundcover, trees, structures • Frequency
• Total loss = Geometrical Spreading +Absorption
+Scattering
13. Geometrical Spreading: Spherical
Weakening of the acoustic intensity due
to spreading
Related to the surface areas of spheres (or
hemi-spheres) at two ranges.
Doubling the distance to the
turbine reduces the SPL by 6dB
15. The Hearing Threshold Curve
From: Yost
The range of human hearing is
generally considered to be 20
Hz to 20 kHz, but it is far more
sensitive to sounds between 1
kHz and 4 kHz.
Listeners can detect sounds as low as 0 dB SPL at 3 kHz, but require 40
dB SPL at 60 hertz (an amplitude increase of 100)
16. A and C Weightings
• A weighting filters out the low frequencies
and slightly emphasizes the upper middle
frequencies around 2‐3 kHz. By comparison C
weighting is almost unweighted, or no
filtering at all.
• As a general rule, C weighting is used for
protection against very intense sounds while
A weighting is used for less intense sounds
and predicts annoyance fairly well.
http://www.e-a-r.com/pdf/hearingcons/FAQdba.pdf
17. Wind Noise
Wind turbines differ in several respects from other sources of community noises
Modern wind turbines mainly emit noise from turbulence at the trailing edge
of the rotor blades.
The turbine sound power level varies with the wind speed at hub height.
The sound is amplitude modulated with the rotation rate of the rotor blades, due
to the variation in wind speed with height and the reduction in wind speed near the
tower.
Amplitude-modulated sound is more easily perceived than is constant-level sound
and has been found to be more annoying
Sound that occurs unpredictably and uncontrollably is more annoying than
other sounds
18. Wind Noise
Wind turbines are tall and highly visible, often being placed in open,
rural areas with low levels of background sound.
Consequently, wind turbines are sometimes regarded as visible and
audible intruders in otherwise unspoiled environments.
Furthermore, the moving rotor blades draw attention, possibly
enhancing the perception of sound in a multi-modal effect
19. Wind Turbine Noise Sources
The sources of noise emitted
from operating wind turbines can
be divided into two categories:
• mechanical and
• aerodynamic.
The primary sources of
mechanical noise are the gearbox
and the generator.
The highest contributor to the
total sound power from a turbine
is the aerodynamic noise, which
is produced by the flow of air
over the blades.
20. Portsmouth Wind Turbine
(July/Aug 2009)
Measured at a distance of 65 meters.
Units are dB re 20 μPa2 in a 1/3-octave band
21. Portsmouth Wind Turbine
Science Fair Project (Chitanya Gopu- SK High) At 0.5 km (Heather Rhodes)
Trial 1 Trial 2: Trial 3: Trial 4: Trial 5: Trial 6: 11/30 11/30 11/30 11/30 12/01
6:50 AM 10:31 3:30 8:30 5:30
59.27 59.30 59.40 59.12 59.36 59.41 AM PM PM AM
56.7 54.4 54.7 51.3 49.2
Simple hemispherical propagation model
100
90
80
B
70
SPL dBA
60
50
40
30
20
A
0 100 200 300 400 500 600 700 800 900 1000
distance from tower (m)
sound of the traffic from Rt. 24 was dominant !!!!
23. Low Frequency Noise
• Low frequency noise (20‐100 Hz)
and infrasound (less than 20 Hz)
are issues that are frequently
raised as concerns associated with
wind farm developments
• Usually G‐weighted
• Perceived a mixture of tactile and
auditory sensations
• Threshold of hearing at 10 Hz very
high (~100 dB G)
Sources for low-frequency noise are either
• Low frequency noise generation is of a natural origin, such as air turbulence
generally confined to turbines wind, thunder, ocean waves, volcanic
whose rotors operate downwind eruptions, and earthquakes or of human
origin such as heating, ventilation, air-
of the support tower – a conditioning systems, machinery, cars,
downwind machine. trucks, airplanes, and loudspeaker systems
24. Infrasound Measurements
Note the high
background
noise level
below 5 Hz
10 dB
From: Jorgen Jakobsen, journal of Low Frequency Noise, Vibration and Active Control, 24(3), 2005
26. ‘Swish’ Noise
• Swish‐swish sound is
amplitude modulation at
blade passing frequencies of
higher frequency blade tip
turbulence
• Does not contain low
frequencies
• Diminishes with distance
• Blurs with multiple turbines
Time
28. Wind Noise Regulations
Most international and various states in USA set a base
noise level for low wind speeds.
Many regulations specify a night time level of 35 dBA in a
rural location.
To prevent the adverse impacts from the increased noise of
wind turbine generators at high wind conditions, the
increased noise levels must also be compared to the
corresponding background noise at any location of interest.
For example some codes specify that the wind farm noise
doesn’t exceed the background noise by more than 5 dBA at
higher wind speeds.
29. Typical Guidelines for Pure Tones
A pure tone is defined to exist if the 1/3rd octave band sound pressure
level in the band, including the tone, exceeds the arithmetic average
of the two contiguous 1/3 octave bands by
•5 dBA for center frequencies of 500 Hz and above
•8 dBA for center frequencies between 160 Hz and 400 Hz
•15 dBA for center frequencies less than or equal to 125 Hz
Most of the codes penalize tonals. For example, Huron County, MI,
specifies that when steady pure tone is present, the standard for
audible noise shall be reduced by 5 dBA.
30. ISO 1996-1971 guidelines
Lower
night time
limits !!!
Gabrielson,
Acoustic Today,
2006 A temperature increase
(an “inversion”) with
altitude often occurs at
night and this causes
sound to be refracted
downward
On an expedition to Venezuela in 1899, Baron von Humboldt observed much better
sound transmission from a waterfall on the Orinoco River at night than during the day !!.
35. Study Plan
• Make repeated sound level measurements using sound
level meter (during day and night) near existing turbines
in RI.
• Compare this to sound level models
• Make ambient sound measurements at locations of
interest.
• Develop a weight to reflect noise considerations which
can be incorporated into TDI calculations
• Develop general guidelines on allowable sound level
thresholds
36. Dose-Response Relationship Studies
Dutch and Swedish
studies (Pedersen et
al., J. Acoust. Soc.
Am., Vol. 126, No. 2,
August 2009
Need to account
LDEN dB(A) for perception !!!
Annoyance towards wind turbine sound is enhanced by the
•high visibility of the noise source,
•swishing quality of the sound,
•its unpredictable occurrence,
•continuation of the sound at night.
40. Perception of Sound from Wind Turbines
Annoyance was highest in what was classified as built-up area (mostly
small towns and villages)
Could be interpreted as an effect of place attachment
In this view, new technical devices being deemed not beneficial for the
living environment induce a negative reaction .
This theory cannot, however, be confirmed from the present data set.
42. Perception of Sound from Wind Turbines
Noise from wind turbines was found to be more annoying than other
sources.
Percentage of people annoyed lies between noise from aircraft and from
shunting yards.
Like aircraft, wind turbines are elevated sound sources visible from afar
and hence intrude both visually and aurally into private space
Wind turbine noise (like shunting yard noise) ceases at night
48. Auditory Perception
• A 1 dB change in SPL is below the level of
human perception
• For a sound to double in loudness, an increase
of 10 dB is required
• A 3 dB change in SPL level is minimum level of
human perception (it is just barely noticeable)
• An SPL of 140 dB is the threshold of pain
From: Acoustic Analysis Dartmouth DPW Wind Project (Atlantic Design Engineers, LLC)
