FUNDAMENTAL ACOUSTICS AND WIND TURBINE NOISE ISSUES

FUNDAMENTAL ACOUSTICSAND

WIND TURBINE NOISE ISSUES

Prof. Gopu R. Potty, Ph.D.Department of Ocean Engineering

University of Rhode IslandNarragansett, RI 02882

potty@egr.uri.edu

Develop general guidelines on allowable sound level thresholds and appropriate setbacks

Major Task

Outline

• Acoustic fundamentals

• Wind turbine as a noise source

• Review of noise regulations

Sound Waves

Sound is a pressure wave

Sounds have different frequenciesHuman hearing: 20 Hz to 20 kHzLess than 20 Hz - infrasound

Intensity is the average amountof sound power transmitted through a unit area in a specified direction. The unit of intensity is watts per square meter.

Decibel

•The decibel (dB) is a logarithmic comparison of intensities.•Named for Alexander Graham Bell

212 W/m101intensity acoustic Reference −= x⎭⎬⎫

⎩⎨⎧

Intensity ReferenceIntensity Acousticlog10Level =

Decibel level of some sounds

http://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 dB

Two turbines produce 3 dB more than one turbine

SPL= 20 log Pressure of an acoustic signalreference pressure

The units of L are dB relative to the reference pressure.

The reference pressure is20 micropascals based onhearing tests of 16 million menin WW2.

This corresponds to an Intensity of 1x10-12 W/m2.

Sound Pressure Level (SPL)

Source Level SL

SL is defined to be

20 log Pressure of source at 1 mreference 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 !!!!

(Rogers et al., 2006)

Acoustics at a distance

• We can predict the sound pressure level of an acoustic signal at a distance.

L = SL - TL

L = Sound Pressure LevelSL = Source LevelTL = Transmission Loss

Nascar fans in the front row are exposed to more intense soundthan 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 m 

TL = -10 log Intensity at r meters

Intensity at 1 meter

(Rogers et al., 2006)

• Geometrical spreading• Absorption• Scattering

– Volumetric scattering, turbulence– Groundcover, trees, structures

• Total loss = Geometrical Spreading +Absorption +Scattering

Transmission Loss Components

Absorption coefft. expressed in dB/km or dB/m

Absorption a function of• Temperature• Humidity• Frequency

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

Closer is Usually Louder

Nascar car with a sourcelevel of 130 dB

at 10 meters, the level would be 110 dB

at 100 meters, the level would be 90 dB

The Hearing Threshold CurveFrom: Yost

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)

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.

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 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

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

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.

Wind Turbine Noise Sources

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

Trial 1 Trial 2: Trial 3: Trial 4: Trial 5: Trial 6:

59.27 59.30 59.40 59.12 59.36 59.41

Science Fair Project (Chitanya Gopu- SK High)

0 100 200 300 400 500 600 700 800 900 100020

30

40

50

60

70

80

90

100

SPL

dBA

distance from tower (m)

Simple hemispherical propagation model

11/30

6:50 AM

11/30

10:31

AM

11/30

3:30

PM

11/30

8:30

PM

12/01

5:30

AM

56.7 54.4 54.7 51.3 49.2

At 0.5 km (Heather Rhodes)

sound of the traffic from Rt. 24 was dominant !!!!

A

B

Vestas V‐52 850 kW Wind Turbine; 10 m/s wind speed; 80 m from the turbine (Leventhal)

Frequency (Hz)

Leve

l (dB

) tonals

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)• Low frequency noise generation is 

generally confined to turbines whose rotors operate downwind of the support tower – a downwind machine.

Sources for low-frequency noise are either of a natural origin, such as air turbulence wind, thunder, ocean waves, volcanic eruptions, and earthquakes or of human origin such as heating, ventilation, air-conditioning systems, machinery, cars, trucks, airplanes, and loudspeaker systems

Infrasound Measurements

From: Jorgen Jakobsen, journal of Low Frequency Noise, Vibration and Active Control, 24(3), 2005

Note the high background noise level below 5 Hz

10 dB

Low frequency sound (10‐160 Hz)

‘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

Downwind at 34.5 meters

Note the higher noise levels at low wind speeds

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.

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.

ISO 1996-1971 guidelines

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 !!.

Gabrielson, Acoustic Today, 2006

Lower night time limits !!!

* LAeq

WHO guidelines

U.K, France, South Australia and Canada

Canada

Denmark

Lp= Sound Pressure level

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

Need to account 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.

Dutch and Swedish studies (Pedersen et al., J. Acoust. Soc. Am., Vol. 126, No. 2, August 2009

LDEN dB(A)

Dose-Response Relationship Studies

Questions

Audio: winds 20-30 mph, 50 ft tower, 50 ft from tower, wind slows down then speeds up

Extra slides

Perception of Sound from Wind Turbines

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.

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.

Perception of Sound from Wind Turbines

Annoyance was found to be significantly higher in the Dutch study in the 35–40-dBA interval.The perceived difference could be due to the larger wind turbines included in the Dutch study.

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 spaceWind turbine noise (like shunting yard noise) ceases at night

Background Wind Noise

•Masks wind turbine noise•Increases with wind speed•Typical levels 30-45 dBA

wind speed (m/s)

Sound Pressure Levels

Directivity(Wei Jun Zhu)

Single turbine at the centerReceiver positions range from 60 to 200 m

Wind direction

Summary of Infrasound Measurements

From: Jorgen Jakobsen, journal of Low Frequency Noise, Vibration and Active Control, 24(3), 2005

Harris, Handbook of Acoustical Measurements and Noise Control, 1998.

Air Absorption of Sound

Absorption a function of• Temperature• Humidity• Frequency

Absorption coefft. expressed in dB/km

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)