Modelling sound from wind turbines and drones to combat noise pollution

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.

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.

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).