Where the heck are my futuristic deep subwavelength meta-diffusers / absorbers?

[kemmler3D]

We were promised flying cars!!

Hang on, wrong thread. OK.

There have been a few papers describing acoustic treatments that are very thin by current standards that can work well into the bass region.

Metadiffusers for quasi-perfect and broadband sound diffusion

Sound diffusion refers to the ability of a surface to evenly scatter sound energy in both time and space. However, omni-directional radiation of sound, or perfe

pubs.aip.org

Hybrid acoustic metamaterial as super absorber for broadband low-frequency sound - Scientific Reports

A hybrid acoustic metamaterial is proposed as a new class of sound absorber, which exhibits superior broadband low-frequency sound absorption as well as excellent mechanical stiffness/strength. Based on the honeycomb-corrugation hybrid core (H-C hybrid core), we introduce perforations on both...

www.nature.com

https://www.sciencedirect.com/science/article/pii/S0360132324000921

etc etc.

Apparently the manufacture of most of these is pretty straightforward with a consumer-grade 3D printer. Looking at the pictures of the panels in the papers, I could certainly whip some of these up if I had a design to use.

The problem is - I don't have the designs. And I don't have the math to design one myself. And there's no calculators out there for these things, as far as I can tell. I understand that serious FEM/BEM modeling is required to validate these designs, but that just means it would take maybe a day per design instead of 5 minutes.

Meanwhile we're still in threads wondering and arguing about 1970s diffusion tech (BADs and QRDs) and how much fiberglass to screw to our walls. What gives??

1) Why aren't the commercial acoustics outfits manufacturing these?

2) If they're not selling them, why isn't there any way for me to make them myself?

I think QRDs look cool which is why I'm making one. But for god's sake, why can't I make something invented this century? Does anyone know?

If my understanding of these papers is correct, the metamaterial panels would blow the doors off everything on the market and can be manufactured using whatever 3D printer you happen to have at hand. So...? Is it a patent problem, are the papers bogus, or what?

 

[DVDdoug]

As far as I know, physics doesn't allow for bass diffusion from small diffusers or in small rooms.

But there are thin bass absorbers: Build A Better Bass Trap

 

[kemmler3D]

As far as I know, physics doesn't allow for bass diffusion from small diffusers or in small rooms.

But there are thin bass absorbers: Build A Better Bass Trap

Take a look at the papers. I think it might be more absorption than diffusion at low frequencies, but they claim an order of magnitude improvement over traditional solutions by leveraging viscosity as well as a series of helmholtz resonators in a systematic way. This is actually pretty well established from what I can tell.

e: This explains it pretty clearly with photos and experimental data. I'm currently building a diffuser with a higher low frequency limit and it's about 20x thicker than this thing. Not for nothing, the panel in the photo would be a lot easier to make at home than what I'm attempting in the QRD thread.

These sound diffusers are rigidly-backed slotted panels based on slow-sound metamaterials, i.e., each slit is loaded by an array of Helmholtz resonators. In essence, strong dispersion is introduced and the effective sound speed inside each slit is drastically reduced in the low frequency regime 13,14 due to the loading HRs. In this way, the quarter-wavelength resonance is shifted to the deep-subwavelength regime and, therefore, the effective thickness of the panel can be strongly reduced15–17.

This is not far off, conceptually, from what KEF uses in their Meta line, but I believe it's taken a step or two further. Bottom line, according to these simulations and experiments, you can absorb 80hz without having several feet of crap piled up around your room, or giant sheets of rubber hanging in closets, or whatever.

I'm well aware of Winer's stuff, but this is exactly what I'm talking about. The science has moved on QUITE A BIT from that kind of thing, but literally nobody seems to be making use of it, at least in the home / studio listening world. We're stuck with 50-year old tools when 5-year old tools are apparently (not surprisingly) vastly superior.

It "only" allows us to accomplish similar goals with thinner panels and less material, but in the home environment, as far as acoustic treatment goes, I'd say that's a big deal.

Why isn't anyone commercializing this stuff?

Can we crowdfund a campaign to get a Physics + Comp Sci PhD to code us a calculator for these panels?

 

[Bjorn]

Take a look here:

Lyd & Akustikk | Schroeder diffuser using a metasurface: new idea?

www.lydogakustikk.no

 

[kemmler3D]

Take a look here:

Lyd & Akustikk | Schroeder diffuser using a metasurface: new idea?

www.lydogakustikk.no

Certainly relevant and I don't disagree with the overall thrust that thin diffusers are nothing FUNDAMENTALLY new. However, they've also apparently been able to tune the resonators to create absorption at "deep sub-wavelength" frequency, which is potentially much more useful.

I am also not sure whether this answers my question - why aren't these things (more) commercially available, if they actually work as described? Is it just that RPG took out a patent on a broad concept, or what?

 

[kemmler3D]

Following up on this thread because I'm still really wondering about this.

Plenty of examples of 3D printed treatments that show effective absorption at much smaller sizes than anything you can buy today. Things any of us can make at home with a little DIY spirit, or that a manufacturer could crank out at similar cost to existing treatments, examples below:

To me the basic principle seems fairly straightforward, although varied in approach. Use a helmholtz-like resonator, but folded to make things compact, or with a restricted neck, to make them more compact and/or slow sound down to shift the effective frequency downwards, or both. Put a bunch of them with different sizes together to get effective broadband treatment.

So my question stands - these things seem to work, at least the in the lab, they seem to solve the biggest problem with low frequency treatment (size), but nobody is selling them... why?

Edit: I guess I could have answered this. Patents... https://patents.google.com/patent/US11037543B2/en?q=~patent/US9076429B2 ... but why patent it and not commercialize? I'm going to get in touch with MIT...

And furthermore, am I really such a dork that I'm the only one who finds this stuff exciting? This is real audio science here, people!

Three-Dimensional Single-Port Labyrinthine Acoustic Metamaterial: Perfect Absorption with Large Bandwidth and Tunability

Sound absorption is a significant issue in acoustics and relevant to many applications, but current approaches suffer from either bulky dimensions or narrow bandwidth. The authors create an acoustic metamaterial consisting of curled, hollow, dead-end channels, which can totally absorb sound in a...

journals.aps.org

https://arxiv.org/pdf/2002.05325

 

[Keith_W]

@kemmler3D I am with you, I find this exciting.

I have a general view on reflections, I think that it needs to be spectrally correct. The problem is that speakers may not be spectrally correct, and ordinary room furnishings may not make it spectrally correct (e.g. soft furnishings selectively absorb high frequencies). What is needed is measurement and correction of the speaker to be spectrally correct, followed by another measurement of the room to see if the reflections are now spectrally correct. If they are not, then parts of the frequency response need to be treated to restore the correct spectrum.

The problem with existing acoustic treatments is that velocity absorbers like foam are broadband and contribute to spectral distortion by selectively absorbing high frequencies. Pressure absorbers like membranes and Helmholtz are narrowband and mostly work in bass frequencies. Ideally a combination of the two types should be guided by measurement and needs to be tailored for each individual room.

From what I know of these metamaterials, they work like miniature Helmholtz resonators and the effective frequency can be tuned by changing the design. They are narrowband by nature, but can be designed to be broadband. Typically they are used in speakers like the KEF Uni-Q Meta series or headphones like Dan Clark designs. I imagine that the drivers were measured, and a metamaterial absorber specifically designed to absorb resonances for that particular driver, in that particular chamber.

A metamaterial absorber for your room should be designed with the same goal - it should work for your particular room. Which means a "one size fits all" solution would not be appropriate - it would contribute to spectral distortion instead of restore it. I would like to see metamaterials sold as tiles designed to absorb different frequency bands. You then measure and determine how many of each particular tile that you need, and assemble your own array. What I don't know - how much it would cost to develop and sell such a product, how broadband/narrowband they can be, and what the lower frequency range is.

 

[Bjorn]

@kemmler3D I am with you, I find this exciting.
The problem with existing acoustic treatments is that velocity absorbers like foam are broadband and contribute to spectral distortion by selectively absorbing high frequencies.

A product that I've been involved in developing absorbs very well from about 110 Hz and all the way they way up with 13 cm thickness. It has some effect below 100 Hz as well, but not with high effiency. For reflection control that's good enough. Sure it would be nice to get lower but not very important since most room modes terminate at other places than at reflection points. Product looks like this and is called Broadsorbor Plus:

Pressure absorbers like membranes and Helmholtz are narrowband and mostly work in bass frequencies. Ideally a combination of the two types should be guided by measurement and needs to be tailored for each individual room.

It's possible to make Helmhotz quite broadband, but with a bit lower efficiency. And we have the VPR bass trap like the Modex Plate and Modex Broadband. It''s also possible to combine Modex Plate with a panel on the outside to make it even more broadband, something I regular do in my designs.