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Loudspeakers: new materials in acoustics

Corinne Rouby
teacher-researcher in mechanics at ENSTA (IP Paris)
Olivier Doaré
Professor in fluid mechanics at ENSTA Paris (IP Paris)
Key takeaways
  • Current loudspeakers use a magnet coupled with the movement of a copper coil to vibrate a membrane.
  • In the future these heavy, bulky, and expensive components could be replaced by a dielectric elastomer membrane.
  • This membrane would make it possible to lighten the object and meet three essential criteria: efficiency, spectral balance, and linearity.
  • Several constraints still prevent the generalisation of this new method, such as the fragility of the membrane.
  • Once these obstacles have been overcome, this type of loudspeaker, which is less expensive and lighter, could be industrialised on a large scale.

Today there are many mod­els of loud­speak­ers, pro­duced in dif­fer­ent sizes. These mod­els gen­er­al­ly use the clas­sic method intro­duced by Wern­er von Siemens, founder of Siemens, in the 19th Cen­tu­ry. This method con­sists of cou­pling a mag­net with the move­ment of a cop­per coil to vibrate a cone-shaped membrane. 

These first two ele­ments are already heavy, space-con­sum­ing, and expen­sive to pro­duce. But they could well be replaced, in addi­tion to the cone-shaped mem­brane, by a sim­ple dielec­tric elas­tomer mem­brane, and some sci­en­tif­ic magic.

Elas­tomer, more com­mon­ly known as rub­ber, is an extreme­ly flex­i­ble mate­r­i­al. Its dielec­tric prop­er­ties means that this new mem­brane con­ducts very lit­tle elec­tri­cal cur­rent. By adding a con­duc­tive grease (form­ing an elec­trode) to each side of the mem­brane, a trans­mit­ted elec­tri­cal sig­nal will cause the flex­i­ble mate­r­i­al to react, caus­ing the vibra­tions nec­es­sary to send sound waves.

Dia­gram of the oper­a­tion of a loud­speak­er using a dielec­tric elas­tomer mem­brane1.

This method could make it pos­si­ble to cre­ate a new gen­er­a­tion of loud­speak­ers. Corinne Rou­by, a lec­tur­er in mechan­ics at ENSTA Paris (IP Paris), co-direct­ed Emil Gar­nel­l’s the­sis with Olivi­er Doaré, a pro­fes­sor in mechan­ics, aimed at opti­mis­ing this new tech­nique23.  

Improving the materials used

“To pro­duce a per­fect loud­speak­er, three cri­te­ria must be met: effi­cien­cy, spec­tral bal­ance, and lin­ear­i­ty,” explains Olivi­er Doaré. The aim is to emit as much acoustic ener­gy as pos­si­ble with as lit­tle elec­tri­cal ener­gy as pos­si­ble (effi­cien­cy). It is also nec­es­sary to repro­duce the trans­mit­ted elec­tri­cal sig­nal as faith­ful­ly as pos­si­ble in the form of an acoustic wave (spec­tral bal­ance). And this is true regard­less of the desired sound pow­er (lin­ear­i­ty).

To pro­duce a per­fect loud­speak­er, three cri­te­ria must be respect­ed: effi­cien­cy, spec­tral bal­ance, and linearity.

Espe­cial­ly since the role of loud­speak­ers is not nec­es­sar­i­ly to emit the loud­est sound, but rather to remain faith­ful to the sound it reflects. The choice of a dielec­tric elas­tomer mem­brane could not only light­en the object, but also pro­duce a sound that is just as cor­rect, if a few con­di­tions are met. All this, through a much less expen­sive production.

“Research inter­est in dielec­tric elas­tomers has been grow­ing since the 2000s4,” states Corinne Rou­by. “But the appli­ca­tions were not direct­ly linked to loud­speak­ers.” How­ev­er, the char­ac­ter­is­tics of this type of mate­r­i­al quick­ly placed it in the acoustic domain. “Con­ven­tion­al loud­speak­ers are heavy and quite expen­sive to pro­duce. This is due to the use of mag­nets, which are not nec­es­sary,” she says.

The idea of the dielec­tric elas­tomer mem­brane could there­fore replace both the coil and the mag­net. Lighter in weight, this new design looks promis­ing for the loud­speak­er indus­try, but is still in an exper­i­men­tal phase. “This the­sis, although com­plet­ed, is still intend­ed to enrich the research,” explains the researcher. The next step is in the hands of a post-doc­tor­al chemist whose objec­tive is to improve the cou­pling between the var­i­ous materials. 

Constraints to overcome 

This type of loud­speak­er is still only at the exper­i­men­tal stage, and its indus­tri­al­i­sa­tion will not be for tomor­row. The results are promis­ing enough to mer­it fur­ther inves­ti­ga­tion, but they do reveal sev­er­al con­straints that still need to be overcome.

“First, the elas­tomer mem­brane is flex­i­ble, but also very frag­ile. Too much volt­age can cause an elec­tric arc and make the mem­brane unus­able,” explains Corinne Rou­by. This is par­tic­u­lar­ly true for low fre­quen­cies, which require a lot of ener­gy to trans­mit, and which lead to greater move­ments that make the mem­brane more fragile. 

For this new type of loud­speak­er, the researchers worked on the shape to be giv­en to the elec­trodes (through the con­duc­tive grease), to pro­duce any type of fre­quen­cy. “Each mode of vibra­tion can cause res­o­nances in the object,” explains the researcher. It is there­fore nec­es­sary to con­trol them so that the speak­er does not favour cer­tain fre­quen­cies5. Fre­quen­cy bal­ance can also be achieved by fil­ter­ing the elec­tri­cal sig­nal sent to the loud­speak­er6.

A major con­straint was also iden­ti­fied: “in the lab­o­ra­to­ry, our loud­speak­er was accom­pa­nied by a pres­sure con­troller that made it pos­si­ble to man­age the var­i­ous leaks with­in the cav­i­ty. To imag­ine such a mech­a­nism in a liv­ing room is not yet pos­si­ble,” she con­cedes. Although this her­met­ic prob­lem is real, it is a tech­ni­cal con­straint that Corinne Rou­by does not con­sid­er insurmountable.

Once these obsta­cles have been over­come, this type of loud­speak­er can be mas­sive­ly indus­tri­alised. Less expen­sive and lighter, its appli­ca­tions can be dreamed of. Olivi­er Doaré, co-direc­tor of this the­sis, is cur­rent­ly work­ing on a sim­i­lar sys­tem for head­phones. By using mem­branes, this time piezo­elec­tric, this sci­en­tif­ic advance could soon be in our ears. 

Pablo Andres
1E. Gar­nell, Dielec­tric elas­tomer loud­speak­ers : mod­els, exper­i­ments and opti­miza­tion, Doc­tor­al dis­ser­ta­tion, Insti­tut Poly­tech­nique de Paris / Unité de Mécanique de l’EN­S­TA Paris, 2020.
2E. Gar­nell, C. Rou­by and O. Doaré, Dynam­ics and sound radi­a­tion of a dielec­tric elas­tomer mem­brane, Jour­nal of Sound and Vibra­tion, 459, 114836, 2019.
3E. Gar­nell, O. Doaré and C. Rou­by, Cou­pled vibro-acoustic mod­el­ing of a dielec­tric elas­tomer loud­speak­er, The Jour­nal of the Acousti­cal Soci­ety of Amer­i­ca, 147(3), 1812–1821, 2020.
4R. Pel­rine, R. Korn­bluh, Q. Pei and J. Joseph, High-Speed Elec­tri­cal­ly Actu­at­ed Elas­tomers with Strain Greater Than 100%, Sci­ence, 287(5454), 836–839, 2000
5E. Gar­nell, B. Aksoy, C. Rou­by, H. Shea and O. Doaré, Geo­met­ric opti­miza­tion of dielec­tric elas­tomer elec­trodes for dynam­ic appli­ca­tions, Applied Acoustics, 181, 108120, 2021.
6E. Gar­nell, O. Doaré and C. Rou­by, Mod­el-Based Adap­tive Fil­ter­ing of Dielec­tric Elas­tomer Loud­speak­ers, Jour­nal of the Audio Engi­neer­ing Soci­ety, 69(6), 389–397, 2021.

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