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Sound propagation through Australian forest land - With special regards to noise generated by wind turbines

Grönberg, Alexandra LU (2015) MMK920 20142
Product Development
Abstract
Wind has become an increasingly accepted source of renewable energy and is currently exploited worldwide. Wind energy has the advantage of close to zero green-house gas emissions, however one of the key concerns the public has about wind farm development is noise emissions. To address these local governments have introduced noise regulations, which limit noise levels at residential buildings located near wind farms. Failure to comply with these limits can lead to turbines being forced to be shut off, with associated loss of revenue. There is therefore a strong motivation to ensure compliance with current noise regulations. This is typically checked in the development stage via noise modelling. Australia has seen an escalation of wind... (More)
Wind has become an increasingly accepted source of renewable energy and is currently exploited worldwide. Wind energy has the advantage of close to zero green-house gas emissions, however one of the key concerns the public has about wind farm development is noise emissions. To address these local governments have introduced noise regulations, which limit noise levels at residential buildings located near wind farms. Failure to comply with these limits can lead to turbines being forced to be shut off, with associated loss of revenue. There is therefore a strong motivation to ensure compliance with current noise regulations. This is typically checked in the development stage via noise modelling. Australia has seen an escalation of wind energy projects in recent years and as a result many of the best sites for wind farm development have been utilised, thus forcing companies into more complex regions where noise modelling can be difficult. Such areas commonly comprise a complex topography or surrounding barriers, such as vegetation. The effects of forestry on sound propagation is difficult to predict, trees and shrubs generally have a dampening effect on noise, however the sound reduction rate often depends on the forest type and characteristics. Attenuation by vegetation is commonly divided into two parts; foliage as well as trunks and branches. The sound attenuation caused by foliage is largely dependent on the tree canopy and leaf characteristics, furthermore the attenuation rate generally increases with frequency. Sound attenuation by trunks and branches is also frequency dependent. Sound with wavelengths that are large in comparison with the tree diameter, i.e. low frequency sound, will be transmitted through tree trunks during interference, whereas sound within the high frequency range will scatter at the surface.

Many of the noise prediction models employed by wind farm developers have been designed for sound sources close to the ground surface. Although these models are often capable of incorporating the effects of forestry in noise predictions, they assume that the sound source is positioned below the tree canopy, which is generally not the case for wind turbines. A series of noise measurements have been conducted to study the difference in attenuation caused by vegetation for a sound source positioned below and above the canopy of an Australian forest. The measurements were performed at three separate occasions, during which a loudspeaker was employed to emit white noise which was recorded with three sound pressure level meters located at equal distance from each other. In total 21 noise measurements were recorded, three of which were performed with the loudspeaker elevated to 26 m, thus exceeding the average tree height. The results indicated that the attenuation by vegetation curve with frequency is comparable in both situations, with a low attenuation within the low frequency range and high attenuation with increasing frequency, which coincided with the expected behaviour of the curve. A portion of the sound energy was reflected at the forest edge when the sound source was positioned close to the ground surface, this sound level decline was however not observed when the source was elevated above the tree canopy.

A sound level calculation model was also constructed with the use of Microsoft Excel, which is an established software within engineering as well as other business sectors. The model comprises attenuation by geometrical divergence, atmospheric absorption, ground interaction and vegetation. It also incorporates the ambient weather conditions present at the particular site and enables implementation of noise restrictions. The particular wind turbine used at the site may be selected from a range provided by GE Power & Water or entered manually. Sound level prediction are made in 1/3 octaves for the frequency range 25 Hz to 20,000 Hz. The results are presented as the A-weighted noise level detected at the receiver point for mean wind speeds ranging from 3 m/s to 10 m/s. (Less)
Abstract (Swedish)
Vind har blivit en alltmer accepterad källa till förnyelsebar energi och utnyttjas idag världen över. Vindenergi har fördelen av nästintill inga utsläpp av växthusgaser, men allmänheten har i vissa uttryckt oro över det buller som associeras med vindkraftverk. For att möta detta missnöje har kommuner infört föreskrifter som begränsar ljudnivåerna vid bostadshus belägna i närheten av vindkraftverk. Underlåtelse av att uppfylla dessa krav kan leda till att vindturbiner tvingas stängas av, med tillhörande förlust av intäkter. Det finns därför en stark motivation för att säkerställa att gällande ljud restriktioner uppfylls. Detta kontrolleras normalt i utvecklingsstadiet med hjälp av ljudmodellering. Australien har sett en upptrappning av... (More)
Vind har blivit en alltmer accepterad källa till förnyelsebar energi och utnyttjas idag världen över. Vindenergi har fördelen av nästintill inga utsläpp av växthusgaser, men allmänheten har i vissa uttryckt oro över det buller som associeras med vindkraftverk. For att möta detta missnöje har kommuner infört föreskrifter som begränsar ljudnivåerna vid bostadshus belägna i närheten av vindkraftverk. Underlåtelse av att uppfylla dessa krav kan leda till att vindturbiner tvingas stängas av, med tillhörande förlust av intäkter. Det finns därför en stark motivation för att säkerställa att gällande ljud restriktioner uppfylls. Detta kontrolleras normalt i utvecklingsstadiet med hjälp av ljudmodellering. Australien har sett en upptrappning av vindenergiprojekt under de senaste åren och många av de områden som anses lämpliga for utveckling av vindkraft har således blivit upptagna, vilket har tvingat företag in i regioner där ljudmodellering kan vara besvärligt. Sådana områden omfattar vanligtvis en oregelbunden topografi eller omgivande barriärer, såsom vegetation. Effekterna av ljudutbredning genom skogsområden är svåra att förutspå, träd och buskar har generellt en dämpande effekt på ljud, men reduktionsgraden beror ofta på vilken typ av skog som avses. Ljuddämpning på grund av växtlighet delas allmänt upp i två delar; bladverk samt stammar och grenar. Den ljuddämpning som orsakas av bladverk är till stor del beroende på trädkronans samt bladens dimensioner, dessutom ökar dämpningsgraden generellt med ljudets frekvens. Även ljuddämpning på grund av skogens stammar och grenar är beroende av frekvensen. Ljud med våglängder som är stora i jämförelse med trädets diameter, det vill säga lågfrekvent ljud, kommer transmitteras genom trädstammarna, medan ljud inom högfrekvensområdet kommer reflekteras vid ytan.

Många av de beräkningsmodeller som används inom vindkraftsindustrin har utformats för ljudkällor placerade nära markytan. Även om dessa modeller ofta har förmågan att integrera effekterna av skog på ljudutbredning vid beräkningar av vindkraftsbuller, antar dessa att ljudkällan är placerad nedanför trädkronorna, vilket ofta inte är fallet för vindkraftverk. En serie ljudmätningar har genomförts för att studera skillnaden i dämpning orsakad av vegetation för en ljudkälla placerad ovanför respektive nedanför trädkronorna i en australiensisk skog. Mätningarna utfördes vid tre skilda tillfallen och en högtalare användes for att avge vitt ljud, vilket spelades in med tre ljudnivåmätare placerade på lika avstånd från varandra. Totalt registrerades 21 ljudmätningar, varav tre utfördes med högtalaren förhöjd till 26 m ovanför marken, vilket översteg den genomsnittliga trädhöjden. Resultaten klargjorde att den frekvensberoende ljuddämpning som uppstår på grund av vegetation är jämförbar i båda situationerna, med låg dämpning inom det låga frekvensområdet och hög ljuddämpning for höga frekvenser, vilket överensstämde med det förväntade beteendet av reduktionskurvan. En del av ljud energin reflekterades vid skogens rand då ljudkällan var placerad nära markytan, denna ljudnivå minskning observerades dock inte i fallet då källan höjdes ovanför trädkronorna.

En beräkningsmodell konstruerades även med hjälp av Microsoft Excel, vilket är en etablerad programvara inom både ingenjörsbranschen och andra verksamhetssektorer. Modellen innefattar dämpning genom geometrisk divergens, atmosfärisk absorption, markeffekt samt vegetation. Den inbegriper även väderförhållandena i området samt möjliggör tillämpning av ljudrestriktioner. De vindkraftverk som används i området kan väljas från ett antal modeller tillhandahållna ifrån GE Power & Water eller införas manuellt. Ljudnivån vid en mottagarpunkt beräknas i 1/3 oktaver för frekvensområdet 25 Hz till 20 000 Hz. Resultatet presenteras som den upplevda A-viktade ljudnivån hos mottagaren för gensnittliga vindhastigheter från 3 m/s till 10 m/s. (Less)
Please use this url to cite or link to this publication:
author
Grönberg, Alexandra LU
supervisor
organization
alternative title
Ljudutbredning genom Australiensisk skogsmark - Med särskild betoning på buller genererat av vindkraftverk
course
MMK920 20142
year
type
H2 - Master's Degree (Two Years)
subject
report number
TVBA-5044
ISSN
0281-8477
language
English
id
5240160
date added to LUP
2015-06-23 11:22:54
date last changed
2015-06-23 11:22:54
@misc{5240160,
  abstract     = {Wind has become an increasingly accepted source of renewable energy and is currently exploited worldwide. Wind energy has the advantage of close to zero green-house gas emissions, however one of the key concerns the public has about wind farm development is noise emissions. To address these local governments have introduced noise regulations, which limit noise levels at residential buildings located near wind farms. Failure to comply with these limits can lead to turbines being forced to be shut off, with associated loss of revenue. There is therefore a strong motivation to ensure compliance with current noise regulations. This is typically checked in the development stage via noise modelling. Australia has seen an escalation of wind energy projects in recent years and as a result many of the best sites for wind farm development have been utilised, thus forcing companies into more complex regions where noise modelling can be difficult. Such areas commonly comprise a complex topography or surrounding barriers, such as vegetation. The effects of forestry on sound propagation is difficult to predict, trees and shrubs generally have a dampening effect on noise, however the sound reduction rate often depends on the forest type and characteristics. Attenuation by vegetation is commonly divided into two parts; foliage as well as trunks and branches. The sound attenuation caused by foliage is largely dependent on the tree canopy and leaf characteristics, furthermore the attenuation rate generally increases with frequency. Sound attenuation by trunks and branches is also frequency dependent. Sound with wavelengths that are large in comparison with the tree diameter, i.e. low frequency sound, will be transmitted through tree trunks during interference, whereas sound within the high frequency range will scatter at the surface.

Many of the noise prediction models employed by wind farm developers have been designed for sound sources close to the ground surface. Although these models are often capable of incorporating the effects of forestry in noise predictions, they assume that the sound source is positioned below the tree canopy, which is generally not the case for wind turbines. A series of noise measurements have been conducted to study the difference in attenuation caused by vegetation for a sound source positioned below and above the canopy of an Australian forest. The measurements were performed at three separate occasions, during which a loudspeaker was employed to emit white noise which was recorded with three sound pressure level meters located at equal distance from each other. In total 21 noise measurements were recorded, three of which were performed with the loudspeaker elevated to 26 m, thus exceeding the average tree height. The results indicated that the attenuation by vegetation curve with frequency is comparable in both situations, with a low attenuation within the low frequency range and high attenuation with increasing frequency, which coincided with the expected behaviour of the curve. A portion of the sound energy was reflected at the forest edge when the sound source was positioned close to the ground surface, this sound level decline was however not observed when the source was elevated above the tree canopy. 

A sound level calculation model was also constructed with the use of Microsoft Excel, which is an established software within engineering as well as other business sectors. The model comprises attenuation by geometrical divergence, atmospheric absorption, ground interaction and vegetation. It also incorporates the ambient weather conditions present at the particular site and enables implementation of noise restrictions. The particular wind turbine used at the site may be selected from a range provided by GE Power & Water or entered manually. Sound level prediction are made in 1/3 octaves for the frequency range 25 Hz to 20,000 Hz. The results are presented as the A-weighted noise level detected at the receiver point for mean wind speeds ranging from 3 m/s to 10 m/s.},
  author       = {Grönberg, Alexandra},
  issn         = {0281-8477},
  language     = {eng},
  note         = {Student Paper},
  title        = {Sound propagation through Australian forest land - With special regards to noise generated by wind turbines},
  year         = {2015},
}