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Why sound research will make a big splash

Modelling sound from wind turbines and drones to combat noise pollution

Olivier Doaré, Professor in fluid mechanics at ENSTA Paris (IP Paris) and Benjamin Cotté, Associate Professor at ENSTA Paris (IP Paris)
On March 6th, 2024 |
4 min reading time
Olivier Doaré
Professor in fluid mechanics at ENSTA Paris (IP Paris)
Benjamin Cotté
Benjamin Cotté
Associate Professor at ENSTA Paris (IP Paris)
Key takeaways
  • Sources of aerodynamic noise are multiplying and can disturb local residents and biodiversity.
  • To anticipate the noise generated by these sources (drones, wind farms, etc.), researchers are attempting to characterise and model the sound they emit.
  • The difficulty is that the intensity of the sound from a wind farm varies enormously depending on several parameters (type of ground, meteorological fluctuations, shape of the blades, etc.).
  • Numerous technical solutions for reducing the decibel level of wind turbines are under study or have already been implemented.
  • Sometimes, limiting noise pollution from a device goes hand in hand with optimising its performance, such as the speed of a drone or the output of wind turbines.

Recent years have seen an explo­sion in the num­ber of drones used for leisure or pro­fes­sion­al pur­pos­es, and the pro­lif­er­a­tion of wind farms on land and at sea. As such, there are more and more sources of aero­dy­nam­ic noise. And these are like­ly to dis­turb the pub­lic along with ani­mal bio­di­ver­si­ty. Reg­u­la­tions are becom­ing stricter and, gen­er­al­ly at a Euro­pean lev­el, noise lim­its are being imposed that must not be exceed­ed. But noise lev­els are not easy to define, and the per­cep­tion of cer­tain sounds is some­times irrational.

Sound modelling using “sound models”

In 2017, researchers asked around 40 peo­ple to lis­ten to 103 sounds made by drones fly­ing at dif­fer­ent alti­tudes and the nois­es made by dif­fer­ent four-wheeled vehi­cles. The lis­ten­ers were then asked to rate them from “not at all annoy­ing” to “very annoy­ing”. The result was that, even at the same vol­ume, the sound of drones is much more annoy­ing to the human ear than that of cars or lor­ries1. The authors of the exper­i­ment sug­gest­ed that this was prob­a­bly because we are used to the noise of these vehi­cles. The same applies to the noise gen­er­at­ed by wind turbines.

Com­pared with many oth­er envi­ron­men­tal noise sources, whether man-made or nat­ur­al, the noise lev­els gen­er­at­ed by a wind farm are mod­er­ate. Accord­ing to a study by Céré­ma2, the sound rarely exceeds 40 dBA out­side a resident’s home. In com­par­i­son, traf­fic pass­ing in front of a resident’s home can emit more than 70 dBA, which is a noise lev­el that by law must not be exceed­ed. Wind farms locat­ed in pre­dom­i­nant­ly rur­al envi­ron­ments gen­er­ate a rel­a­tive­ly low lev­el of local back­ground noise, but this can con­tribute to the per­cep­tion of noise. In addi­tion, wind noise con­tains low fre­quen­cies (20–200 Hz, audi­ble) or infra­sound fre­quen­cies (below 20 Hz, gen­er­al­ly con­sid­ered inaudi­ble) which can trav­el fur­ther than high­er fre­quen­cy sounds.

“While the annoy­ance caused by noise increas­es with the lev­el of noise expo­sure, its devel­op­ment does not gen­er­al­ly fol­low a sim­ple law and depends on each noise source and its char­ac­ter­is­tics (con­tin­u­ous noise vs. impulse noise, low-pitched vs. high-pitched noise, etc.),” explains Olivi­er Doaré, pro­fes­sor at the Mechan­ics Unit (UME) at ENSTA Paris. His lab­o­ra­to­ry is con­duct­ing stud­ies into the mod­el­ling of dif­fer­ent noise sources, includ­ing one on wind tur­bines. “For these devices, the noise lev­el in deci­bels is not sig­nif­i­cant, because the noise is not uni­form. It is char­ac­terised by tem­po­ral fluc­tu­a­tions in ampli­tude, poten­tial­ly linked to the weath­er, the shape of the blades, etc.”

The lab­o­ra­to­ry has been work­ing on mod­el­ling the sources of aero­dy­nam­ic noise from wind tur­bines and the prop­a­ga­tion of sound in the atmos­phere. The aim is to be able to sim­u­late the result­ing ambi­ent noise before a wind farm is installed. “The idea is to be able to hear the farm before it is built, as if we were cre­at­ing a sound mod­el rather than a visu­al one”, explains Ben­jamin Cot­té, a lec­tur­er at ENSTA (IP Paris) and co-author of the study, along with David Mas­caren­has, now an engi­neer at Capgem­i­ni. The researchers hypoth­e­sised that var­i­ous fac­tors were being over­looked in the mea­sure­ment of wind tur­bine noise, such as the height and qual­i­ty of the ground (whether it is more or less absorbent) on which they are installed, or the effect of wind direc­tion and pro­files on the prop­a­ga­tion of sound.

By devel­op­ing a tool for syn­the­sis­ing wind noise, using phys­i­cal mod­el­ling rather than sam­pling, they can analyse the sen­si­tiv­i­ty of sounds. This can be done accord­ing to a num­ber of fac­tors: blade geom­e­try, wind or tem­per­a­ture pro­file, upstream tur­bu­lence rate. This also makes it pos­si­ble to pre­dict the pow­er of the noise for each seg­ment of the wind tur­bine blade. Sound syn­the­ses are thus pro­duced and pre­sent­ed for dif­fer­ent mete­o­ro­log­i­cal conditions.

These process­es, which aim to recre­ate a sound envi­ron­ment (known as “aural­i­sa­tion” of wind tur­bine noise), can be used in vir­tu­al real­i­ty appli­ca­tions. The study is part of the Euro­pean Vir­tu­al Real­i­ty Audio for Cyber Envi­ron­ments (VRACE) network.

Practical technical solutions

In prac­tice, the chal­lenge fac­ing the design­ers and oper­a­tors of wind farms is to reduce the noise gen­er­at­ed by the machines, to avoid com­plaints from local res­i­dents. Dif­fer­ent approach­es are being stud­ied or have already been imple­ment­ed. “It is pos­si­ble to influ­ence the speed of rota­tion or the angle of pitch of the blades,” explains Ben­jamin Cot­té. “Some teams are work­ing on intel­li­gent tools that auto­mat­i­cal­ly reg­u­late wind tur­bines to min­imise noise while opti­mis­ing pro­duc­tion. One of the lat­est advances is the use of “combs” or “ser­ra­tions”. These are elon­gat­ed “saw-toothed” parts, direct­ly inspired by the wings of noc­tur­nal birds of prey, which can fly in per­fect silence thanks to the spread of their wingtip feath­ers, which allow air to pass through, thus reduc­ing tur­bu­lence.” These combs are attached to the trail­ing edge of the blades. They reduce aero­dy­nam­ic noise by an aver­age of 2 to 3 deci­bels, by reduc­ing the tur­bu­lence cre­at­ed by air fric­tion at the tip of the blade. 

As far as drones are con­cerned, more research is under­way. The “Aero-acoustics of mul­ti-propul­sion sys­tems for UAVs” (APRO) project, in part­ner­ship with the Com­put­er Sci­ence and Sys­tems Engi­neer­ing Unit (U2IS), aims to improve under­stand­ing and mod­el­ling of noise gen­er­a­tion in mul­ti-propul­sion sys­tems. The aim is to lim­it noise pol­lu­tion. Doc­tor­al stu­dent Car­o­line Pas­cal is look­ing to auto­mate the analy­sis of acoustic fields using robot­ic mea­sure­ments. Today, her work enables the pre­cise char­ac­ter­i­sa­tion of pro­peller radi­a­tion, and will make it pos­si­ble to opti­mise the tra­jec­to­ry, speed, and accel­er­a­tion of a drone, in order to lim­it noise pollution.

Marina Julienne

Ref­er­ences :

Mas­caren­has, D., Cot­té, B., & Doaré, O. (2023). Prop­a­ga­tion effects in the syn­the­sis of wind tur­bine aero­dy­nam­ic noise. Acta Acus­ti­ca, 7, 23.

Mas­caren­has, D., Cot­té, B., & Doaré, O. (2022). Syn­the­sis of wind tur­bine trail­ing edge noise in free field. JASA Express Let­ters, 2(3).

2Cen­tre d’études et d’expertise sur les risques, la mobil­ité et l’aménagement https://www.mdpi.com/1660–4601/19/1/23

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