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INFRASOUNDANDLOW-FREQUENCYNOISEFROMWINDTURBINES
RESEARCH·OCTOBER2015
DOI:10.13140/RG.2.1.3826.5049
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50
3AUTHORS:
BrankoZajamsek
UniversityofNewSouthWales
19PUBLICATIONS11CITATIONS
SEEPROFILE
ColinHHansen
UniversityofAdelaide
250PUBLICATIONS3,624CITATIONS
SEEPROFILE
KristyHansen
FlindersUniversity
23PUBLICATIONS59CITATIONS
SEEPROFILE
Availablefrom:BrankoZajamsek
Retrievedon:18November2015
INFRASOUND AND LOW-FREQUENCY NOISE FROM WIND TURBINES
Colin Hansen, Branko Zajamšek and Kristy Hansen
FSSIC2015, Perth, July 2015
OUTLINE● What is it about wind farm noise that distresses rural
residents?o is it imagined?o is it stress and lack of sleep related?o is it a physiological phenomenon that only affects
some people?● What weather conditions exacerbate the problem?● What mechanisms are responsible for generating the
disturbing characteristics?● Propagation effects● Experimental investigations using a model turbine
FSSIC2015, Perth, July 2015 2
Annoying wind farm noise characteristics
● Dominated by low frequencies more than 1 km from a wind farm
● Contains significant levels of periodic (multi-tonal) infrasound due to rotor noise arising from blade tower interaction
● Time varying amplitudeo Random variation (constructive/destructive interference
of blade pass harmonics – infrasound and low-frequency)
o Amplitude modulation (periodic variation)
o Beating
FSSIC2015, Perth, July 2015 3
Various forms of amplitude variation
FSSIC2015, Perth, July 2015 4
Beating
Amplitude modulation
Random amplitudevariation
Annoying wind farm noise characteristics
● Thumping noise (16 – 80 Hz)o Most likely due to part of the blade stalling
when entering a high speed air flow with incorrect angle of attack
o Worse in high wind shear conditions● Rumbling noise (16 – 80 Hz)
o Amplitude modulated gearbox noise?o Blade tower interaction?
● Swishing noise (150 to 800 Hz)o Trailing edge noise variation resulting from
directivity and possibly loading variationso Problems usually below 400 Hz
FSSIC2015, Perth, July 2015 5
What symptoms are experienced by some residents near wind farms?
● Sleep disruption and insomnia● Nausea● Tinnitus (ringing in ears)● Pressure in ears● Headache● Raised pulse rate● Vertigo and dizziness
FSSIC2015, Perth, July 2015 6
What causes symptoms?● Infrasound is below the audibility threshold of normal
hearing minus 2 standard deviationo However, work by Salt shows that the outer hair cells of
the ear can respond to infrasound at levels below the threshold of hearing
● If it’s not inaudible infrasound causing a physiological response, is it audible low-frequency noise?
● Well known that audible low-frequency noise annoys some people, even at low levelso Annoyance can cause a stress response which can
result in sleep disruption, which, over time, can cause symptoms
FSSIC2015, Perth, July 2015 7
Data recorded at Waterloo wind farm
FSSIC2015, Perth, July 2015 8
Measuredat 3.5 km from wind farm
EAR
FSSIC2015, Perth, July 2015 9
Infrasound detection
FSSIC2015, Perth, July 2015 10
What conditions correspond to greatest annoyance?
● When the contrast between background noise and wind farm noise is greatesto Occurs when there is no wind at the residence but
enough wind at turbine height to drive turbineso No wind at residence means no wind noise
In rural areas this can mean very little noise at allo Large difference between wind at turbine rotor and
residence is a result of high wind shear conditions Occurs mainly during night time in stable conditions
● In urban areas, the higher background noise (from traffic) results in turbine noise being less annoying
FSSIC2015, Perth, July 2015 11
Why is audible wind farm noise at residences dominated by low-
frequencies?
● Wind farms produce noise over the audible frequency range from infrasound to a few kHz.
● Noise detected by residents in their homes more than 1.5 km from a wind farm is mainly low frequency (usually less than 160 Hz) as a result ofo Propagation effectso House transmission effects
● Low-frequency noise is more easily detected if mid and high frequency noise is at a low level
FSSIC2015, Perth, July 2015 12
Sound propagation effects● Low-frequency and infrasonic noise domination
o No ground or atmospheric absorptiono Worse when downwind
FSSIC2015, Perth, July 2015 13
Sound propagation● Multiple reflections results in there being a point
beyond which sound propagation results in a less than 6 dB loss per doubling of distanceo Eventually becomes close to 3 dB
FSSIC2015, Perth, July 2015 14
Sound propagation● Sound may decay at less than 3 dB per doubling of
distance due to the arrangement of the wind turbines.● For example, for the Waterloo wind farm, sound decay
is much less than 6 dB per doubling of distance.
FSSIC2015, Perth, July 2015 15
House noise reduction (out – in)
FSSIC2015, Perth, July 2015 16
House 1 House 2
Noise reduction from outside to inside for wind turbine source
House noise reduction (out – in)
● Low-frequency noise and infrasound are much less attenuated by the building structure than mid- and high-frequency soundo At 3-4 Hz there is a resonance associated with the
mass of the walls and roof interacting with the enclosed volume in the house
o Around 8 Hz for a house with windows open, there is a resonance associated with a Helmholtz resonance effect with the window opening acting as the neck and the room as the volume
o Other noise reduction dips are harmonics of the above
FSSIC2015, Perth, July 2015 17
Overall noise reduction probability
FSSIC2015, Perth, July 2015 18
Wind turbine noise characteristics● Aerodynamic noise
o Tonal (rotor noise) Frequency range 1 Hz to 80 Hz Caused by blade tower interaction
o Broadband (trailing edge and leading edge noise) Usual frequency range 160 Hz to 1500 Hz, but mostly
interested in 160 Hz to 500 Hz Frequencies shifted lower when the angle of attack changes
due to variations in the air flow speed – can also result in stall being induced, producing the characteristic “thumping noise”.
● Gear noise radiated by blades and towero Usually amplitude modulated toneo Sometimes heard as a “rumbling noise”
FSSIC2015, Perth, July 2015 19
What produces low-frequency sound and infrasound, and their time variation?● Blade tower interaction● Thickness effect – blade pushing air out of its way● Unsteady blade loading variations due to
o Atmospheric turbulenceo Wind shearo Cross-windo Wakes from upstream turbineso Blades passing the tower
● Blade stall as a result of high wind shear conditionso Blade stalls when entering high speed air flow due
to sub-optimal pitch angle
FSSIC2015, Perth, July 2015 20
Broadband noise generating mechanism● Broadband noise (trailing edge, leading edge and
stall noise) originates from turbulence interaction with the wind turbine airfoilo Strongly affected
by unsteady blade loading
o Leading edge noise can extend to infrasound range when there is in-flow turbulence
FSSIC2015, Perth, July 2015 21
Trailing edge noise● Magnitude proportional to boundary layer
displacement● Can be produced in a smooth air flow due
to the turbulent boundary layer on the blade● Frequency range can extend below 150 Hz
for a large wind turbine in the presence of wind speed variations (atmospheric or wake turbulence)o Causes angles of attack that are too high,
which increases the small-scale turbulence in the flow over the blade, especially if the stall condition occurs
FSSIC2015, Perth, July 2015 22
Atmospheric boundary layer effects● Broadband noise enhanced by unsteady loading on the
blades● Boundary layer has a strong effect on loading unsteadiness
o Stable boundary layer occurs at night and gives rise to strong winds that increase in strength with increasing height producing unsteady loading on a wind turbine rotor
FSSIC2015, Perth, July 2015 23
Mixed layer
Mix
ed la
yer
Stable boundary
Residual layer
layer
Noon NoonSunset Sunrise
Hei
ght,
km
0.5
1
Effect of boundary layer on blade loading● Low level jets can also occur at night when a stable
boundary layer produces a high velocity gradient and low level jets around 200 m high o Cause unsteady loading and increased noise
FSSIC2015, Perth, July 2015 24
Tonal noise generating mechanisms● Rotor noise – 2 components
o Thickness noise Only apparent in the first
few harmonics of the BPFo Steady loading noise
Results from different loading on each side of the blade when angle of attack not zero
Usually much more significant than thickness noise Directivity different to thickness noise
● Blade-tower interaction (BTI) noiseo Caused by the loading excursion shown in the figureo In large turbines, can be seen up to the 60th harmonic of BPF.
FSSIC2015, Perth, July 2015 25
Rotor noise generation
FSSIC2015, Perth, July 2015 26
● A small element in the rotor plane generates sound as a blade passes through it.
● Repeats every blade pass for every element.
● Each element acts as a simple oscillator radiating noise at blade pass frequency and harmonics.
Effect of support tower● Change in air flow speed in front of the tower
o Changes effective angle of attacko Shifts trailing edge noise to lower frequencies
● Resulting change of blade loading produces tonal emissions at BPF and harmonics (up to 60th harmonic)
FSSIC2015, Perth, July 2015 27
Effect of tower wake velocity defect shape on noise spectrum shape
● Very small differences in the wake shape can have a very large effect on the noise spectrum shape
FSSIC2015, Perth, July 2015 28
Large turbine measurement1.3 km from nearest turbineCross-wind
From Thresher, R. W. (1981). Wind turbine dynamics. NASA Technical report.
Experimental work● Aims
o To investigate noise source distribution in the rotor plane, using a microphone array
o To investigate broadband and tonal noise directivity characteristics using point microphones
o To investigate the effect of the support tower on broadband and tonal noise
● Blades – NACA 0012, 70 mm chord, 450 mm length
● Model turbine run as a propellero Similar noise producing
mechanisms as a large turbine
FSSIC2015, Perth, July 2015 29
1. Blade2. Slip ring3.Torque sensor4. Motor5. Support tower
Model vs full size frequency ranges● Different frequency ranges but same noise producing mechanisms● Rotor noise extends to 60th harmonic of blade pass frequency in a full
size turbine (3 MW)● Larger turbines produce lower frequency TE noise due to larger chord● Frequency range between TE and rotor noise dominated by leading
edge noise and stall noise.
FSSIC2015, Perth, July 2015 30
Measurement set-up● Support tower – rotor spacing 70 mm for model represents
approximate large wind turbine spacing (6-7 m)● Microphone array distance from rotor plane
o 100 mm for statistically optimised nearfield acoustic holography (SONAH)
o 1.5 m for beam forming
FSSIC2015, Perth, July 2015 31
Microphone array● Used for acoustic holography and beam forming● 64 mics and 1.5 m diameter
FSSIC2015, Perth, July 2015 32
Beamforming● Used to locate and quantify broadband noise source
in the rotor plane● Sound sources are identified by steering the beam
using the standard delay and sum approach
FSSIC2015, Perth, July 2015 33
Beamforming (and SONAH) processing methods
● Time averaged● Phase averaged
FSSIC2015, Perth, July 2015 34
Time averaged beamforming
FSSIC2015, Perth, July 2015 35
● 0º angle of attack and 3.15 kHz 1/3 octave band (scaled from 150 Hz for full size turbine)
● Figure shows reflection from support tower and uniform blade noise generation independent of blade location
Phase averaged beamforming
FSSIC2015, Perth, July 2015 36
P2
P1
● 4 kHz 1/3 octave band, 10º angle of attack
● SPL greater at P1 than at P2● reduction of SPL at P2 is due to a
combination of acoustic destructive interference (between direct and tower-reflected waves) and scattering
Leading edge
Directivity, 4 kHz 1/3 octave band
FSSIC2015, Perth, July 2015 37
● 10º angle of attack● Decrease of 3 dB
compared to the BPM model at 90°, is most likely due to destructive interference between direct and tower-reflected waveso Less for bottom mics
● Possible contributor to enhanced amplitude modulation
Mics at rotor centre height
Mics at rotor bottom height
Effect of blade-tower spacing
FSSIC2015, Perth, July 2015 38
● Tonal amplitude is a function of blade-tower spacing● Relative amplitudes similar to full size turbine
o Corresponds to 70 mm spacing in model
Measured infrasound● 2 locations, Houses 1 and 2● Blade-tower spacing 6 – 7 m
o Corresponds to 70 mm in model
FSSIC2015, Perth, July 2015 39
Near field holography
FSSIC2015, Perth, July 2015 40
● Sound visualisation and sound localisation technique● Capture of an evanescent wave is essential, so
hologram plane must be close to source plane (100 mm in our case)
Time averaged holography● Tower has a significant effect on
blade pass harmonics● Higher SPL occurs to the left of the
tower as it takes a finite time for the noise to develop and radiate
FSSIC2015, Perth, July 2015 41
Phase averaged holography
● 4th blade pass frequency (180 Hz)
FSSIC2015, Perth, July 2015 42
Directivity for blade pass harmonics● Sound pressure level for 180º, 1.5 m in front of rotor plane
– near field for 1st and 3rd harmonics (45 Hz and 135 Hz)o In far field, lower order harmonics become more like the
dipole shape of the 10th harmonic (450 Hz)o First harmonic of industrial wind turbine has a wavelength
of 400 m
FSSIC2015, Perth, July 2015 43
Control of noise● BTI produces infrasound and low-frequency tones that
can be detected by sensitive people at large distances from a wind farm
● Possible means to reduce BTI noiseo Blade phase desynchronisation and randomisation to vary
times when blades from different turbines pass the tower Will shift around the areas of max. sound reinforcement
o Blade pitch change as it passes the tower o Use of swept back blades to control higher harmonics by
causing the sound generated along the blade span to be out of phase
o Adding strakes to the tower to change the wake characteristics
o Active noise cancellation system in residences
FSSIC2015, Perth, July 2015 44
Control of noise
● Thumping noise – most likely caused by fluctuating loads on the blade could be controlled byo continuously varying the blade pitch to prevent stallo Fixing the blade pitch sufficiently below the stall angle
for the mean wind speed so that turbulence or wind shear does not cause stall
o Use of random blade pitch variation
● Infrasound and low-frequency noise caused by aero-acoustic excitation of bladeso Use more efficient blade designs
FSSIC2015, Perth, July 2015 45
Conclusions
● Wind farm generated infrasound and low frequency noise is a problem for some nearby residents
● Some people have been adversely affected at distances of 5 km from the nearest turbine in a large wind farm
● BTI noise is an important source in the infrasonic and low-frequency range
● It is possible to reduce turbine low-frequency noise and infrasound but guidelines for acceptable noise levels need to be revised if all residents are to be protected from adverse effects
FSSIC2015, Perth, July 2015 46