FUNDAMENTAL ACOUSTICS AND WIND TURBINE NOISE ISSUES
FUNDAMENTAL ACOUSTICS ANDWIND TURBINE NOISE ISSUES Prof. Gopu R. Potty, Ph.D. Department of Ocean Engineering University of Rhode Island Narragansett, RI 02882 firstname.lastname@example.org
Major TaskDevelop general guidelines onallowable sound level thresholdsand appropriate setbacks
Outline• Acoustic fundamentals• Wind turbine as a noise source• Review of noise regulations
Sound WavesSound is a pressure wave Intensity is the average amount of sound power transmittedSounds 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.
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
Decibel level of some soundshttp://www.redferret.net/?p=9346
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 dBTwo turbines produce 3 dB more than one turbine
Sound Pressure Level (SPL)SPL= 20 log Pressure of an acoustic signal reference pressureThe units of L are dB relative to thereference pressure.The reference pressure is20 micropascals based onhearing tests of 16 million menin WW2.This corresponds to anIntensity of 1x10-12 W/m2.
Source Level SL (Rogers et al., 2006)SL is defined to be20 log Pressure of source at 1 m reference pressureThe units of SL are dB relative toReference pressure of 20 micropascals at 1 meter. SL referenced at 1 meter Quantifies the strength of the source !!!!
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.
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
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
Geometrical Spreading: SphericalWeakening of the acoustic intensity dueto spreadingRelated to the surface areas of spheres (orhemi-spheres) at two ranges. Doubling the distance to the turbine reduces the SPL by 6dB
Closer is Usually LouderNascar carwith a sourcelevel of 130 dBat 10 meters, the level would be 110 dB at 100 meters, the level would be 90 dB
The Hearing Threshold CurveFrom: 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 40dB SPL at 60 hertz (an amplitude increase of 100)
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
Wind NoiseWind turbines differ in several respects from other sources of community noisesModern wind turbines mainly emit noise from turbulence at the trailing edgeof 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, dueto the variation in wind speed with height and the reduction in wind speed near thetower.Amplitude-modulated sound is more easily perceived than is constant-level soundand has been found to be more annoyingSound that occurs unpredictably and uncontrollably is more annoying thanother sounds
Wind NoiseWind 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 andaudible intruders in otherwise unspoiled environments. Furthermore, the moving rotor blades draw attention, possiblyenhancing the perception of sound in a multi-modal effect
Wind Turbine Noise SourcesThe sources of noise emittedfrom operating wind turbines canbe divided into two categories:• mechanical and• aerodynamic.The primary sources ofmechanical noise are the gearboxand the generator.The highest contributor to thetotal sound power from a turbineis the aerodynamic noise, whichis produced by the flow of airover the blades.
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
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 !!!!
Vestas V‐52 850 kW Wind Turbine; 10 m/s wind speed; 80 m from the turbine (Leventhal) tonalsLevel (dB) Frequency (Hz)
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
Infrasound Measurements Note the high background noise level below 5 Hz 10 dBFrom: Jorgen Jakobsen, journal of Low Frequency Noise, Vibration and Active Control, 24(3), 2005
‘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
Noise Varies with Wind Speed Note the higher noise levels at low wind speedsDownwind at 34.5 meters
Wind Noise RegulationsMost international and various states in USA set a basenoise level for low wind speeds.Many regulations specify a night time level of 35 dBA in arural location.To prevent the adverse impacts from the increased noise ofwind turbine generators at high wind conditions, theincreased noise levels must also be compared to thecorresponding background noise at any location of interest.For example some codes specify that the wind farm noisedoesn’t exceed the background noise by more than 5 dBA athigher wind speeds.
Typical Guidelines for Pure TonesA pure tone is defined to exist if the 1/3rd octave band sound pressurelevel in the band, including the tone, exceeds the arithmetic averageof 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 HzMost of the codes penalize tonals. For example, Huron County, MI,specifies that when steady pure tone is present, the standard foraudible noise shall be reduced by 5 dBA.
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 downwardOn an expedition to Venezuela in 1899, Baron von Humboldt observed much bettersound transmission from a waterfall on the Orinoco River at night than during the day !!.
Massachusetts Dept. Env. Protection Criteria A noise source will be considered in violation if the source results in:• An increase in the broadband sound pressure level of more than 10 dBA above ambient or• Pure tone condition: when any octave band level exceeds two adjacent band levels by 3 dB
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
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.
QuestionsAudio: winds 20-30 mph, 50 ft tower, 50 ft from tower, wind slows down then speeds up
Perception of Sound from Wind TurbinesAnnoyance 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.
Perception of Sound from Wind TurbinesAnnoyance was highest in what was classified as built-up area (mostlysmall towns and villages)Could be interpreted as an effect of place attachmentIn this view, new technical devices being deemed not beneficial for theliving environment induce a negative reaction .This theory cannot, however, be confirmed from the present data set.
Perception of Sound from Wind TurbinesAnnoyance was found to be significantly higher in the Dutch study in the35–40-dBA interval.The perceived difference could be due to the larger wind turbinesincluded in the Dutch study.
Perception of Sound from Wind TurbinesNoise from wind turbines was found to be more annoying than othersources.Percentage of people annoyed lies between noise from aircraft and fromshunting yards.Like aircraft, wind turbines are elevated sound sources visible from afarand hence intrude both visually and aurally into private spaceWind turbine noise (like shunting yard noise) ceases at night
Directivity (Wei Jun Zhu) Single turbine at the center Receiver positions range from 60 to 200 mWind direction
Summary of Infrasound MeasurementsFrom: Jorgen Jakobsen, journal of Low Frequency Noise, Vibration and Active Control, 24(3), 2005
Air Absorption of SoundAbsorption coefft.expressed in dB/km Absorption a function of • Temperature • Humidity • Frequency Harris, Handbook of Acoustical Measurements and Noise Control, 1998.
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 painFrom: Acoustic Analysis Dartmouth DPW Wind Project (Atlantic Design Engineers, LLC)