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).