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Metamaterial Absorption Technology - KEFs new material

Werner said:
I read a similar announcement a couple of years ago. That one was indeed very narrow-band.
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AMG are claiming to have got round the narrow-band constraint such that they are able to build an anechoic chamber exploiting the metamaterial.

“Abstract of the METARoom project:
The extraordinary functionalities of acoustic metamaterials have led to the realization of wave manipulation techniques previously regarded as impossible with deep subwavelength structures. Unfortunately, since much of metamaterials’ properties originate from the resonance phenomenon, the novel functionalities are necessarily restricted to narrow frequency ranges; yet broadband is usually a necessity in practical applications. Recently, breakthroughs in the designed integration strategy have overcome the narrow frequency limitation and showed that some functionalities, such as sound absorption, can be made to be tunable in accordance with the target absorption spectrum. Such designed integration scheme has already led to the formation of a Hong Kong startup company, Acoustic Metamaterials Group (AMG), which has achieved mass production capability of the designed prototypes, and of a French startup company, Metacoustic, which proposes acoustic and vibration solutions consisting in metaporous and metaporoelastic layers. In this project, we would like to extend the previous success in acoustic absorption to open a new frontier in room acoustics, by constructing walls that can passively switch from a totally absorbing to a spatially modulated reflection phase, by utilizing resonances to tune the impedance of the walls. Such change can significantly alter the audio experience of a room, from anechoic-like to the audio feel of a larger room than it is in reality. Traditionally, acoustic wall constructs are static and achieve only one functionality. Moreover, they are efficient at high frequencies but results in bulky and heavy structures at low frequencies. This project aims at designing deep subwavelength re-configurable acoustic metamaterials for altering the room acoustics. We intend to draw on the combined expertise of the Hong Kong and French teams to prove experimentally the effectiveness of such a system in two demonstration rooms, one in Hong Kong and one in Le Mans. The two demonstration rooms will use different integration”

The full quote is on page 11 of this euroacoustics.org newsletter
https://euracoustics.org/news/eaa-newsletter/2019/May/April_2019_wapp.pdf
 
AndyU said:
Here is a paper by some of the people involved in the Acoustic Metamaterials Group.

Does this help make sense of the image at the beginning of the KEF video? A “membrane decorated with platelets”.

edited to add: “almost unity absorption .. where the relevant sound wavelength in air is three orders of magnitude larger than the membrane thickness.“ implies they can soak up 100Hz with a membrane 4mm thick. You would need a lot of foam to do that.
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Thanks for the link which answers some of the question but not all.

It is a membrane absorber which are driven by pressure rather than particle velocity, pressure is a maximum at the wall and so on-wall is the optimum location unlike conventional foam/stuffing.

It is a tuned absorber with high absorption over a narrow range of frequencies at least in the form presented. The need for absorbers of this kind in a speaker is limited. It could be used for the walls of the port to absorb the unwanted compressible pipe resonances (as often seen in Stereophile measurements of the port response) but I think KEF are already doing something along these lines according to the LS50 marketing but perhaps things could be improved. It could be used in a quarter wave transmission line speaker to eliminate the unwanted higher order compressible pipe resonances without affecting the wanted fundamental resonance. KEF don't make TL speakers and are unlikely to adopt them given the existing brand (i.e. not pure audiophile but more consumer/audiophile) and it being a poor configuration in terms of size and cost for the performance. Anything else?

So what might KEF be doing? The second chap in the video talked about "mathematically optimised to absorb the maximum of sound" which I perhaps incorrectly took to mean PML but maybe it was more marketing speak rather than acoustics speak. Nonetheless it suggests they might be constructing a more broadband absorber from tuned absorbers. An application that would seem able to cover the almost certain substantial increase in cost compared to conventional stuffing are shallower on wall speakers to place either side of TVs which can extend low enough in frequency to hand over to subwoofers. Many people clearly place significant value on thinness in phones, laptops and TVs. This would be interesting and a significant step forward but then again it might be activated carbon part two. We will see.
 
And following on from the above, I’ve just found this from one of the companies involved, who seem to have somewhat different absorber:

https://metacoustic.com/

There is a video of a simulation of a concert hall with and without their absorbers. If this is remotely true or feasible it could be quite extraordinary.
 
h.g. said:
Thanks for the link which answers some of the question but not all.

It is a membrane absorber which are driven by pressure rather than particle velocity, pressure is a maximum at the wall and so on-wall is the optimum location unlike conventional foam/stuffing.

It is a tuned absorber with high absorption over a narrow range of frequencies at least in the form presented. The need for absorbers of this kind in a speaker is limited. It could be used for the walls of the port to absorb the unwanted compressible pipe resonances (as often seen in Stereophile measurements of the port response) but I think KEF are already doing something along these lines according to the LS50 marketing but perhaps things could be improved. It could be used in a quarter wave transmission line speaker to eliminate the unwanted higher order compressible pipe resonances without affecting the wanted fundamental resonance. KEF don't make TL speakers and are unlikely to adopt them given the existing brand (i.e. not pure audiophile but more consumer/audiophile) and it being a poor configuration in terms of size and cost for the performance. Anything else?

So what might KEF be doing? The second chap in the video talked about "mathematically optimised to absorb the maximum of sound" which I perhaps incorrectly took to mean PML but maybe it was more marketing speak rather than acoustics speak. Nonetheless it suggests they might be constructing a more broadband absorber from tuned absorbers. An application that would seem able to cover the almost certain substantial increase in cost compared to conventional stuffing are shallower on wall speakers to place either side of TVs which can extend low enough in frequency to hand over to subwoofers. Many people clearly place significant value on thinness in phones, laptops and TVs. This would be interesting and a significant step forward but then again it might be activated carbon part two. We will see.
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See my previous two posts for more info on the potential broadband nature of this technology. There seem to be many different applications.

PS. Here is the website of AMG. If you click on the Publications tab you will get a list of references which you can track down if you are interested.
https://acousticmetamaterials.org/
 
AndyU said:
See my previous two posts for more info on the potential broadband nature of this technology.
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Thanks for your efforts but I am still wondering about the KEF application. Perhaps it is room treatment rather than speakers which would be less interesting at least to me. Small cell-like absorbers have been around for many decades in the aerospace industry but they are expensive because of the way they are manufactured. I presume the current approach will address this cost albeit not to the extent of bringing it down to the level stuffing.

Smart materials have received a lot of funding in the area of acoustics over the last couple of decades particularly with a view to combining sound reduction with other functions like carrying load so that future smaller lighter devices required for reasons of efficiency are not noisier. Most of this funding won't lead directly to products but it is steadily raising the acoustical performances of devices with less need for acoustical add-ons. This looks to be part of this movement/trend.
 
h.g. said:
Thanks for your efforts but I am still wondering about the KEF application. Perhaps it is room treatment rather than speakers which would be less interesting at least to me.
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The way I understood the video it is something to use inside a speaker - the French guy at around 00:35 says they want to “absorb the sound behind the drivers because we want to hear the music and not any artefacts created by the driver in itself or the cabinet”. Though certainly there also seem to be applications in room acoustics, as you can see on the metacoustic site.
 
Here’s some more info. I had misunderstood the material originally, I thought it was some kind of fancy absorption, but I was evidently wrong, It seems to be some magic “about the size of a hockey puck” fixed to the back of the drive unit to absorb sound above 600Hz coming from the back of the tweeter. But there seem to be many other changes too, so who knows what the meta stuff contributes.
https://www.whathifi.com/reviews/kef-ls50-meta
 
Mr Pig said:
Is it just me or are modern speakers with less colouration less fun than speakers used to be?
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You have a point, I think. We may have traded one form of distortion/coloration for another, less obvious one. We somehow seem to have decided that harmonic distortion is the only enemy of fidelity. And in pursuit of harmonic truth, we sometimes accept compromises in timing, or dynamics. Some loved, but flawed loudspeakers of old, like Epos ES14s maybe, went the other way and focussed on the time domain and dynamic abilities at the expense of strict harmonic fidelity. I suspect that’s what we miss with ‘correct’ modern designs. If you’re going to rely on measurements, best be sure you’re measuring what’s most important, perhaps.
 
Sue Pertwee-Tyr said:
We somehow seem to have decided that harmonic distortion is the only enemy of fidelity. And in pursuit of harmonic truth, we sometimes accept compromises in timing, or dynamics.
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Exactly. Last year I talked my son into buying a pair of Linn Saras, because they're awesome. Being a sound tech, he plugged them into an oscilloscope and was horrified to discover how much harmonic distortion they produced. My answer was 'so what?' That's probably why they are awesome.

I'm struggling to think of a modern speaker that stirs the soul in the way many old ones did.
 
Mr Pig said:
Exactly. Last year I talked my son into buying a pair of Linn Saras, because they're awesome. Being a sound tech, he plugged them into an oscilloscope and was horrified to discover how much harmonic distortion they produced. My answer was 'so what?' That's probably why they are awesome.

I'm struggling to think of a modern speaker that stirs the soul in the way many old ones did.
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Why don't you just accept that you prefer a distorted sound and move on?
 
So its 30 helmholtz resonator cavities in a single disc. Nothing to do with material spec, everything to do with shape
 
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