Metamaterial Absorption Technology

[Geoff Millar]

A review without a single measurement is as useful as mammary glands on a male bovine. I'd be quite surprised if Guttenberg's technical knowledge extends any further than what he reads in advertising copy. And to suggest that a speaker can remove the echo and reverb from John Lennon's voice on a recording is just plain ludicrous. Next up: Which tastes better, Coke or Pepsi?

The 'raw mixes' of "Imagine" aren't treated to the reverb, so the KEFs (or any speakers for that matter) couldn't have removed that. It almost sounded to me like maybe Steve hadn't played that disc before and was so surprised at the raw vocals, he's perhaps attributing the differences in the sound to the speakers.....

I liked the 'raw mixes', but after the initial double-take I prefer the final versions. John apparently hated his voice and was always on at George Martin and later Phil Spector to modify it. John's posthumous 'Acoustic' collection - NLA, I think - features his raw voice and it's great.

These speakers are $2500 pr in Oz, plus another $800 pr for the stands - so they would want to be pretty good.

Geoff

[billfitzmaurice]

John didn't like his voice, but he did like the sound of double tracked vocals. Playing live, before they could afford an Echolette tape echo, John and Paul would often sing the lead vocal in unison to try to duplicate double tracking. They still did that in the studio on many tracks. There are quite a few songs where it's hard to tell if it's John or Paul singing the lead vocal, the reason why being that it's both of them, usually with John being a bit louder than Paul. 'Eight Days a Week' is one example. John and Paul sang the first part of each verse in unison, with John also double tracking his own part, with Paul shifting up to a higher harmony part at the end of the verse. George adds a third part in the chorus.

[djg]

Beach Boys blended "pretty good".

[djg]

Paul sings lead here, you can see his lips moving.

https://www.youtube.com/watch?v=yYvkICbTZIQ

[djg]

Bill F., you play bass. Do you like this bass line?

https://www.youtube.com/watch?v=M4vbJQ-MrKo

[wavefunction]

Steve Guttenberg directly compared the LS50 to the new Meta version and reported a marked improvement.

KEF was quite benevolent to throw in the Meta technology for the same price as the original!

My guess is that the tweaks to the drivers slightly altered their response, so they did a redo of the crossover, yielding a flatter response. And that improved FR is what Guttenberg was hearing. As I recall, someone posted a measured FR on this forum a year or so ago for the LS50, and it had a bit of excess upper midrange. It would be interesting to see a comparison between the FR of the old vs. new model.

[mcargill]

On the KEF website about the LS50 Meta, they provide a link to their white paper about the "metamaterial" technology--an AES convention paper dated June 2020, by Sebastien Degraeve and Jack Oclee-Brown.
https://www.aes.org/tmpFiles/elib/20200926/20758.pdf

In that paper, they cite some Hong Kong physicists, Min Yang and Ping Sheng, as the source for their use of the term and concept "metamaterial." They rely most heavily on this paper by Yang and Sheng, in the Annual Review of Materials Research, 2017:
http://sheng.people.ust.hk/wp-conten...Absorption.pdf

In that paper, Yang and Sheng cite several other articles using the word "metamaterial" in their titles, dating back as far as 2006 (Fang, et al., in Nat. Mater.), along with similar terms such as "metasurface." So, it turns out, the word is not marketing hype. At least it did not originate as marketing hype. It is instead physics technical lingo. That doesn't prevent a marketing department from turning it into hype. But the concept has a legit technical origin.

Here is how I understand it: When talking about acoustic absorption, there is something like a law that the more absorption you want, the thicker the material has to be. Each kind of sound-absorbing material of a given density, whether gypsum board or fiberglass batts or polyester stuffing, has its coefficient of absorption, and you can calculate total absorption by multiplying that coefficient by the thickness. The idea of a meta-material is a sheet-like structure that has absorbing qualities out of proportion to the thickness of the structure, so you get very thin sheets that absorb as if they were very thick, or absorb in specific wavelengths unpredictable by their density or their thickness.

The familiar instance of this effect is in treating rooms to reduce standing waves. We can install layers of fiberglass, and calculate broadband absorption according to the thickness and density of that material, or we can install a Helmholtz resonator tuned to damp one specific frequency that is the biggest problem. The absorption properties of a Helmholtz resonator make it a "metamaterial" in the sense that its absorption properties are not calculated from its thickness or what it is made of.

We are also familiar with Helmholtz resonators from building bass-reflex speaker cabinets using calculated internal volumes and port sizes. They are typically designed to "absorb" the speaker's rear wave (in a very narrow band) by resonating at the same frequency as the speaker's electro-mechanical resonance. The same effect can be achieved by another physical principle, using a quarter-wave resonating pipe instead of a Helmholtz resonating volume of air. A big quarter-wave pipe would also work, I imagine, when treating a room for a troublesome standing wave. An set or array of such resonators could be used to damp a number of different specific frequencies, until all of the main room modes were under control. Thus, the sound-absorption characteristics of a room's walls can be altered by adding some of these "meta-materials" to the construction materials.

What KEF has done is build a "sheet" material that is 11mm thick, but absorbs the rear wave of the tweeter disproportionately to that thickness. The thickness doesn't matter to how and why it absorbs the energy from the back side of the tweeter diaphragm, because it absorbs by a different principle--resonators, instead of a coefficient of absorption native to the material it is made of. It is an array of thirty tiny quarter-wave pipes, folded to fit into a neat circle. The longest pipe resonates at 620Hz, and also at 1860Hz, 3100Hz, 4340Hz, etc. By using an array of thirty such pipes, each a different length, these resonating frequencies effectively cover the entire bandwidth of sound coming from the tweeter. The tweeter crossover is at 2100Hz, so this absorption is designed to cover the crossover band as well as the pass band. As a result, all of the acoustic energy reaching this structure meets with a resonator that cancels it or absorbs it so that it does not reflect back to the tweeter and out through its membrane. Also, no reflections inside the tweeter enclosure means no standing waves in there, either.

This is to some extent a solution to a problem KEF introduces by choosing to mount their tweeter in the center of the mid-woofer. This construction limits how they can treat the rear chamber of the tweeter. It is quite possible that stand-alone tweeters can be given sophisticated rear chambers that effectively damp all of the internal energy without resorting to an array of thirty resonators. To be specific, KEF admits that it is possible to achieve the same effect using a rear chamber that is an exponential horn. This is the solution developed by Laurence Dickie for the B&W Nautilus, and Dickie continues to use it in his Vivid Audio Giya series speakers. That horn, KEF claims, would have to be .5 meters long to be as effective as their solution, but it would be a comparatively simple build, and I am guessing that horn could be coiled to fit inside an ordinary box, if you don't want the Nautilus look. There is probably also a solution using sound diffraction--chaotic reflection instead of absorption--just as diffraction can be used to control room reflections.

But I hate that name, "metamaterial." Not that I doubt the physicists. I see what they are after by it. But when taken out of the context of a specific scientific problem, where "material" has a very specific referent (namely acoustic absorbing materials), speaking of a "meta" material makes no sense whatsoever. What it is is a resonator array, and the name should reflect that. Those physics papers speak of a "broadband optimal metamaterial absorber," which they abbreviate as BOMA. That is closer to a fair description, but still does not admit it is an array.

I'd call it a BROADBAND ACOUSTIC RESONATOR ARRAY. For our purposes as speaker designers, it would probably be enough to say we are building a cabinet that is a RESONATOR ARRAY, as the alternative to a ported box or open baffle or transmission line.

Putting it in these terms makes it sound like it is feasible for lower frequencies, which may not be the case. There are hints in the technical papers that the idea may work only for high frequencies, though I suspect that means only that it cannot be implemented at lower frequencies in commercially-marketable-sized enclosures. Some of us may take that as a DIY challenge. Some of us don't think in terms of commercially marketable enclosures.

-Meredith

[fpitas]

Thanks for wading through that for us.

[billfitzmaurice]

I said the same thing, succinctly, in Post #4. What I didn't address there was the use of the term 'meta material'. In the simplest of terms it refers to a non-naturally occurring composite. That makes the KEF doo-dad a muti-frequency Helmholtz resonator constructed from a composite material. That's all an engineer has to know to deduce how it works. Whether it works better than a traditional damping material would be seen in SPL and waterfall charts of the same speaker using both technologies. Anyone can create those with OmniMic V2. One would think an outfit like KEF would have at the very least that level of technology at their disposal. Instead they resort to advertising piffle that's as informative as this: https://www.youtube.com/watch?v=Ac7G7xOG2Ag

[djg]

Should I add these to my bucket list?

[fpitas]

I said the same thing, succinctly, in Post #4. What I didn't address there was the use of the term 'meta material'. In the simplest of terms it refers to a non-naturally occurring composite. That makes the KEF doo-dad a muti-frequency Helmholtz resonator constructed from a composite material. That's all an engineer has to know to deduce how it works. Whether it works better than a traditional damping material would be seen in SPL and waterfall charts of the same speaker using both technologies. Anyone can create those with OmniMic V2. One would think an outfit like KEF would have at the very least that level of technology at their disposal. Instead they resort to advertising piffle that's as informative as this: https://www.youtube.com/watch?v=Ac7G7xOG2Ag

Yeah, it would have been more effective advertising for me at least if they demonstrated a measurable improvement. Well, once again, I doubt you or I are the target audience.

[tvrgeek]

38 posts. No one has heard them. Expensive little speakers so we should expect they are tweaking the smallest of details.

[Geoff Millar]

I think the thing is that it might be a good product, but why do marketing people have to drown everything in bollock - speak?

I know it's their job, but we pay for that in the price of what we buy.

I get really sick of this sort of drivel about anything:

• Mercedes Benz C-Class 'it changes everything' (everything?!)
• 'new and improved' whatever: either it's new, or old and improved...
• 'new Ford Falcon, "the supreme road car" (oh really?)
• 'may help to relieve symptoms in certain circumstances' i.e. it does nothing......
• ad nauseum......

Geoff

[fpitas]

38 posts. No one has heard them. Expensive little speakers so we should expect they are tweaking the smallest of details.

Why bother? It features a coaxial horn tweeter. I have it on good authority that horns sound like crap.

[djg]

I'm going to make some speakers just like these Kefs for next to nothing. I'm thinking papier mache.

[mcargill]

When I said "diffraction," I meant diffusion:

There is probably also a solution using sound diffusion--chaotic reflection instead of absorption--just as diffusion can be used to control room reflections.

[mcargill]

DJG, those of us who take fabrication seriously know that you must want one desperately and immediately, without delay. Designs without one are now pretty much obsolete. The decisive issue is the "Gee!" forces involved in accelerating technology.

We talk about how important a driver is; we also must pay attention to the whole drivetrain behind it, from the motor and the bass manifold all the way back to the exhaust port in the rear, including the resonator and muffler. The transmission line between the motor and the axial response unquestionably needs a turbo encabulator, though one might limp on without the turbo (with a gain in efficiency offsetting the loss in punch). We therefore all need a supply of meta-materials and trans-substances on hand when fabricating, with unobtanium being the only such that always produces measurable benefits.

[djg]

My best friend and I made a papier mache Box Turtle in 4th grade. We layed up the shell on a balloon.

[mcargill]

A wisp of melancholy hope still slips my grasp
The summer dusk slides the day into the morrow
Between then and now, and now, and now
Translucent and fragile, like wasp paper

[billfitzmaurice]

There is probably also a solution using sound diffusion--chaotic reflection instead of absorption--just as diffusion can be used to control room reflections.

I've seen B&W utilize that technique.

[fpitas]

djg, paper mache won't be sturdy enough. People will only buy these for PA use. Roadies will be tossing them off a truck. There's no way someone would put this in their living room.

[djg]

With that shape they'll just roll and be fine, I really do believe.