Odd. It worked when I checked it immediately after posting.
Perhaps going this route to get to the paper, which is what I did originally, will work better:
AES Convention Papers Forum >> Metamaterial Absorber for Loudspeaker Enclosures
Few
Perhaps going this route to get to the paper, which is what I did originally, will work better:
AES Convention Papers Forum >> Metamaterial Absorber for Loudspeaker Enclosures
Few
LS50 Meta reviews have started to appear
LS50 Meta | KEF
Inside the KEF LS50 Meta - SoundStage! InSight (October 2020) - YouTube from about 3 mins shows the meta absorber. That looks 3D printable!
LS50 Meta | KEF
Inside the KEF LS50 Meta - SoundStage! InSight (October 2020) - YouTube from about 3 mins shows the meta absorber. That looks 3D printable!
Agreed: 3D print, or CNC...
It seems it's very important to control the wavefront impinging on the metamaterial and arrange impedances appropriately. Not a quick fake-it project.
There's been a lot of metamaterial research in the optics world for the last decade or two. Lenses and other optical elements are replaced by thin devices with subwavelength structures embedded. Presumably the directivity of speakers could be similarly controlled with the right device---and acoustics works on a much more accessible length scale!
By the way, I just noticed this started as a diffuser thread but the KEF devices are absorbers. I hope I didn't take things off track.
Few
It seems it's very important to control the wavefront impinging on the metamaterial and arrange impedances appropriately. Not a quick fake-it project.
There's been a lot of metamaterial research in the optics world for the last decade or two. Lenses and other optical elements are replaced by thin devices with subwavelength structures embedded. Presumably the directivity of speakers could be similarly controlled with the right device---and acoustics works on a much more accessible length scale!
By the way, I just noticed this started as a diffuser thread but the KEF devices are absorbers. I hope I didn't take things off track.
Few
darko also has an article;
Podcast: Jack Oclee-Brown talks MAT, LS50 Meta & LS50 Wireless II | Darko.Audio
When i saw this i was reminded of the hegeman 1. This looks to be based on the same idea but at a much smaller scale. Similar to what i have imagined 3D printed for damping material in a 3D printed speaker box ("chaotic open cel rigid foam"). You can enter but you cannot leave.
That they are using it behind the speaker suggests a bandwidth limit on any build and perhaps a size limit.
dave
Podcast: Jack Oclee-Brown talks MAT, LS50 Meta & LS50 Wireless II | Darko.Audio
When i saw this i was reminded of the hegeman 1. This looks to be based on the same idea but at a much smaller scale. Similar to what i have imagined 3D printed for damping material in a 3D printed speaker box ("chaotic open cel rigid foam"). You can enter but you cannot leave.
That they are using it behind the speaker suggests a bandwidth limit on any build and perhaps a size limit.
dave
Kef has come out with a new version of the LS50. The biggest "innovation" in the new design, is the inclusion of metamaterials in the design.
Over on Facebook, I hypothesized that the metamaterials in the KEF design are basically a quadratic diffusor that's been folded into a transmission line.
This is a quadratic diffusor. If you put it on your wall, it scatters and absorbs sound. As I understand it, the way that it works, is that sound enters the wells in the diffusor, and the sound is nullified due to the resonant frequency of the well.
This is a transmission line. Normally it's used to amplify that back wave of the sound that's entering the line. But there's nothing stopping you from using the same technology to nullify the sound entering the line.
Don't quote me on this, but as I understand it, this is similar to the 'notch' that you see in transmission lines and tapped horns.
In a nutshell, you can take a diffusor and fold it into a transmission line.
I said this on Facebook, and Andrew Jones, (who used to work for Kef) said that I was wrong.
And he's probably right.
But that won't stop me from exploring this one further
Which is what I'll do in this thread.
Here's some reading material, in the mean time:
https://www.diyaudio.com/forums/room-acoustics-and-mods/321644-magic-metamaterial-diffusers.html
Inside the KEF LS50 Meta - SoundStage! InSight (October 2020) - YouTube
Over on Facebook, I hypothesized that the metamaterials in the KEF design are basically a quadratic diffusor that's been folded into a transmission line.
This is a quadratic diffusor. If you put it on your wall, it scatters and absorbs sound. As I understand it, the way that it works, is that sound enters the wells in the diffusor, and the sound is nullified due to the resonant frequency of the well.
This is a transmission line. Normally it's used to amplify that back wave of the sound that's entering the line. But there's nothing stopping you from using the same technology to nullify the sound entering the line.
Don't quote me on this, but as I understand it, this is similar to the 'notch' that you see in transmission lines and tapped horns.
In a nutshell, you can take a diffusor and fold it into a transmission line.
I said this on Facebook, and Andrew Jones, (who used to work for Kef) said that I was wrong.
And he's probably right.
But that won't stop me from exploring this one further
Which is what I'll do in this thread.
Here's some reading material, in the mean time:
https://www.diyaudio.com/forums/room-acoustics-and-mods/321644-magic-metamaterial-diffusers.html
Inside the KEF LS50 Meta - SoundStage! InSight (October 2020) - YouTube
According to the Kef interview, the tubes in their "metamaterial" are as long as 25mm at their longest.
Each tube measures 2mm x 2mm:
Jack Oclee-Brown discusses KEF LS50 Meta & LS50 Wireless II development by Darko.Audio podcast | Free Listening on SoundCloud
According to my calculator here:
QRD Diffuser Well Depth Calculator
A quadratic diffusor using seventeen wells, which measure the same as the Kef device, would work from 6400Hz to 80khz(!)
This seems to indicate that maybe this isn't a quadratic diffusor folded into a transmission line, but I'm still charging down that path...
Each tube measures 2mm x 2mm:
Jack Oclee-Brown discusses KEF LS50 Meta & LS50 Wireless II development by Darko.Audio podcast | Free Listening on SoundCloud
According to my calculator here:
QRD Diffuser Well Depth Calculator
A quadratic diffusor using seventeen wells, which measure the same as the Kef device, would work from 6400Hz to 80khz(!)
This seems to indicate that maybe this isn't a quadratic diffusor folded into a transmission line, but I'm still charging down that path...
Hi Patrick, I made a mistake in the Darko interview. I meant to say 250mm, not 25mm. I didn't realise I'd made an error until you mentioned it but listening again I did say 25mm, whoops.
In fact, even 250mm is not right. The absorber works by using the combined effect of the resonances of the individual tubes. The longest tube is a quarter-wavelength at 620Hz, so it's about 137mm.
Anyhow, the best info is from the AES paper (free download):
AES E-Library >> Metamaterial Absorber for Loudspeaker Enclosures
All the best, Jack.
In fact, even 250mm is not right. The absorber works by using the combined effect of the resonances of the individual tubes. The longest tube is a quarter-wavelength at 620Hz, so it's about 137mm.
Anyhow, the best info is from the AES paper (free download):
AES E-Library >> Metamaterial Absorber for Loudspeaker Enclosures
All the best, Jack.
a simulation of a tweeter with 12.8cm long back chamber - YouTube
Here's a hornresp sim of a one inch tweeter playing 2khz on a flat baffle. The tweeter is loaded in a cylinder that's 13.2cm in depth (5.2" deep.)
Not gonna lie, I thought this "metamaterial" technology seemed like a marketing gimmick, but even in a basic wave pool sim, we can see that waves from the tube are impacting the response of the tweeter.
Whoah.
This has some fairly significant repercussions. For instance, if you have a tweeter with a back chamber that's an inch deep, it may be adding coloration to the FRONT wave at 3375Hz.
It's also hideously complex, because there will also be higher order modes generated in the back chamber.
I saw the Youtube videos, where reviewers of the LS50 Meta were saying that it sounds noticeably better, and I was a bit skeptical, but these sims are changing my mind...
Also, how did we miss this?
Here's a hornresp sim of a one inch tweeter playing 2khz on a flat baffle. The tweeter is loaded in a cylinder that's 13.2cm in depth (5.2" deep.)
Not gonna lie, I thought this "metamaterial" technology seemed like a marketing gimmick, but even in a basic wave pool sim, we can see that waves from the tube are impacting the response of the tweeter.
Whoah.
This has some fairly significant repercussions. For instance, if you have a tweeter with a back chamber that's an inch deep, it may be adding coloration to the FRONT wave at 3375Hz.
It's also hideously complex, because there will also be higher order modes generated in the back chamber.
I saw the Youtube videos, where reviewers of the LS50 Meta were saying that it sounds noticeably better, and I was a bit skeptical, but these sims are changing my mind...
Also, how did we miss this?
Thanks, Jack, for participating here! Very generous of you! Can I impose by asking a basic question?
Can you explain where the loss comes from in the metamaterial? I can see how adjusting dimensions tunes each tube, but I don't have an intuitive feel for the mechanism for thermalizing the acoustic energy. Is it just air viscosity in the small diameter (~2 mm?) channels?
And I understand there to be two disks of metamaterial. Are there two identical/similar disks doubling up to increase the absorption, or are their roles more distinct? (I guess that's two questions....)
Great to see some real innovation in the speaker design world. Congratulations on such a successful implementation!
Few
Can you explain where the loss comes from in the metamaterial? I can see how adjusting dimensions tunes each tube, but I don't have an intuitive feel for the mechanism for thermalizing the acoustic energy. Is it just air viscosity in the small diameter (~2 mm?) channels?
And I understand there to be two disks of metamaterial. Are there two identical/similar disks doubling up to increase the absorption, or are their roles more distinct? (I guess that's two questions....)
Great to see some real innovation in the speaker design world. Congratulations on such a successful implementation!
Few
Here's a hornresp sim of a one inch tweeter playing 2khz on a flat baffle. The tweeter is loaded in an enclosure that's 13.2cm in depth (5.2" deep.) The enclosure includes a quadratic diffuser calculated using this:
QRD Diffuser Well Depth Calculator
The diffuser seems to make a noticeable, though subtle, improvement in the polar response of the tweeter.
Something I've begun to wonder, is if the quadratic diffuser would work better if it included some bends. I think bends in the diffuser would tend to destroy sound INSIDE the diffuser.
With straight channels, a lot of the sound that ENTERS the diffuser LEAVES the diffuser; we could fix this by adding some bends to it.
Anyways, check out the video!
In particular, notice that the waves radiated into the back chamber seem to impact the waves radiated from the front. I'm note entirely sure how Hornresp is able to sim this. When you think about it, you'd think that Hornresp would treat the loudspeaker as if it's infinitely rigid. Of course, in the real world, it's not. And due to that, paper and silk diaphragms will let allow a lot of sound in the back chamber to radiate out of the FRONT of the loudspeaker.
a simulation of a tweeter with a quadratic diffusor back chamber - YouTube
QRD Diffuser Well Depth Calculator
The diffuser seems to make a noticeable, though subtle, improvement in the polar response of the tweeter.
Something I've begun to wonder, is if the quadratic diffuser would work better if it included some bends. I think bends in the diffuser would tend to destroy sound INSIDE the diffuser.
With straight channels, a lot of the sound that ENTERS the diffuser LEAVES the diffuser; we could fix this by adding some bends to it.
Anyways, check out the video!
In particular, notice that the waves radiated into the back chamber seem to impact the waves radiated from the front. I'm note entirely sure how Hornresp is able to sim this. When you think about it, you'd think that Hornresp would treat the loudspeaker as if it's infinitely rigid. Of course, in the real world, it's not. And due to that, paper and silk diaphragms will let allow a lot of sound in the back chamber to radiate out of the FRONT of the loudspeaker.
a simulation of a tweeter with a quadratic diffusor back chamber - YouTube
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Okay, three questions (sorry).
If there is a 2 mm x 2 mm entrance to a given tube, how does it manage to almost completely convert the energy at that tube's tuned frequency to heat when most of the sound goes elsewhere? I don't doubt your results, I'm just trying to update my understanding of acoustics. How do you achieve 99% absorption of 1 kHz sound with a tube whose cross section is a few mm^2? Most of the 1 kHz sound impinges on tubes not tuned to that frequency. Even if one tube achieves 100% dissipation, the area of its input seems tiny. What am I missing?
Few
If there is a 2 mm x 2 mm entrance to a given tube, how does it manage to almost completely convert the energy at that tube's tuned frequency to heat when most of the sound goes elsewhere? I don't doubt your results, I'm just trying to update my understanding of acoustics. How do you achieve 99% absorption of 1 kHz sound with a tube whose cross section is a few mm^2? Most of the 1 kHz sound impinges on tubes not tuned to that frequency. Even if one tube achieves 100% dissipation, the area of its input seems tiny. What am I missing?
Few
I'm still trying to figure this out myself.
So take this with a grain of salt:
Let's say that you have a back loaded horn, and you drive that horn with a loudspeaker at it's throat. The horn is 56" deep. 240Hz is 56" long.
Due to that, you're going to get a quarter wave resonance at 60Hz. (240Hz divided by four.)
OK, all of us understand that, right? It's basic quarter wave theory.
Now:
What if you added another tube that's 128" long?
That would lead to a REALLY deep notch, nearly infinite, at 60Hz. Because the back wave isn't 90 degrees out of phase, it's 180 degrees out of phase.
Does that make sense?
Now repeat that process for the entire audible spectrum.
You would have a series of frequencies that are AUGMENTED but you'd also have a series of frequencies that are NULLIFIED.
Arguably, the key to this entire thing is that:
1) when you combine two sounds, you get an increase of output of 6dB
2) when you combine two sounds that are out of phase, you get a decrease of output that's infinite.
IE, there's going to be frequencies when you get a standing wave in the wells that's CONSTRUCTIVE. But when the combined waves are DESTRUCTIVE it's way more efficient.
It's a lot easier to nullify sound than it is to combined sound.
So take this with a grain of salt:
Let's say that you have a back loaded horn, and you drive that horn with a loudspeaker at it's throat. The horn is 56" deep. 240Hz is 56" long.
Due to that, you're going to get a quarter wave resonance at 60Hz. (240Hz divided by four.)
OK, all of us understand that, right? It's basic quarter wave theory.
Now:
What if you added another tube that's 128" long?
That would lead to a REALLY deep notch, nearly infinite, at 60Hz. Because the back wave isn't 90 degrees out of phase, it's 180 degrees out of phase.
Does that make sense?
Now repeat that process for the entire audible spectrum.
You would have a series of frequencies that are AUGMENTED but you'd also have a series of frequencies that are NULLIFIED.
Arguably, the key to this entire thing is that:
1) when you combine two sounds, you get an increase of output of 6dB
2) when you combine two sounds that are out of phase, you get a decrease of output that's infinite.
IE, there's going to be frequencies when you get a standing wave in the wells that's CONSTRUCTIVE. But when the combined waves are DESTRUCTIVE it's way more efficient.
It's a lot easier to nullify sound than it is to combined sound.
