Thought this was quite interesting.
"Researchers have designed coiled metasurfaces that not only completely absorb low frequency sounds, but are a tiny fraction of the size of traditional sound-absorbing systems(Credit: Assouar/CNRS)"
http://newatlas.com/acoustic-metasurfac ... 5/#gallery (Feb 21, 2016)
I am building a studio and was looking for build information on Helmholtz Resinator's when I came across this.
This research was recently published in the journal Applied Physics Letters (AIP).
Article by Colin Jeffrey @ newatlas.com
Coplanar Chamber to Absorb Low Frequency
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Re: Coplanar Chamber to Absorb Low Frequency
Hi there "diaphony", and Welcome! 
I downloaded the original paper, titled "Acoustic metasurface-based perfect absorber with deep subwavelength
thickness", by Li and Assouar", and took a look at what it actually says about this device.
Firstly, it is tightly tuned. Each such chamber is tuned to one specific frequency, with high Q and very narrow bandwidth (much like a Helmholtz resonator), so you would need a series of many such chambers to cover the entire low end of the spectrum. Let's say that you wanted to cover 20 Hz to 200 Hz: you'd need about 50 different tunings to do that smoothly, although you might be able to get away with as few as 10 or 15 different tunings if you don't mind patchy coverage.
Secondly, each chamber is only as effective as the size of entry hole. Only the area of the actual hole itself is absorbing sound. The rest of the area used by the chamber is reflective at all other frequencies. Therefore, you need a very large area in your room covered with a very large number of such chambers, to get a useful effect, and even then the total effect is limited to the ratio of "hole area vs. frontal area", similar to the problem with perforated panels.
Third, the manufacturing tolerances are tight. Each chamber needs to be built accurately, in order to ensure perfect impedance matching.
Forth, they never actually built anything to test! It's just a theoretical prediction and computer simulation of how such a device would work, but nothing physical was built or tested. There's not even enough information in the paper to actually build a device to test, since there's no specification of what the material properties would need to be, other than to say that they are "hard". But there's nothing in the equations that would suggest what type of mass, hardness, rigidity, elasticity, thickness, or any other physical property would be needed.
So basically you'd need to build a very, very large number of such chambers (hundreds at least, probably thousands), in a broad range of different tunings, each built very accurately, but from unknown materials with unknown characteristics, in order to have any useful effect, and with no guarantee at all that it would even work in the real world...
It's just an interesting acoustical curiosity at present.
Hopefully some company or acoustician will take this concept further, and test it with real materials in the real world, to derive real results, but until then it looks like we are stuck with traditional approaches, that actually have been tested and are understood.
Most acousticians stay away from Helmholtz resonators for the above reasons, especially in small rooms where space is already at a premium. Broadband porous absorption is far more useful, and if you do happen to end up with a really stubborn modal issue, then a membrane trap or panel trap might be effective at dealing with it.
Take a look at this thread for example: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=20471 The results speak for themselves, but there are no Helmholtz resonators in that studio at all! I did consider it at one point in the design and build, but discarded the idea as they are just too finicky. However, there are some perforated panel devices in there, as well as slot walls, which are both modified forms of Helmholtz resonator, but they are all tuned to certain ranges of frequencies, rather than to specific individual frequencies, and the Q is reasonably low. They are also not targeted at low frequencies: All of the bass trapping in that room is accomplished with porous absorption, except for one membrane trap that deals with a small set of issues. Trying to deal with very low frequencies using Helmholtz resonators is not nearly as easy some text books make out.
Please start a thread about your build, and post the details of where you are right now in the process, along with photos, diagrams, plans, etc., so forum members can help you out with suggestions.
I look forward to seeing your thread!
- Stuart -

It's a fascinating concept, but there's a few things that are not immediately obvious from the article, that make this far less attractive than it appears at first glance.Researchers have designed coiled metasurfaces ...
I downloaded the original paper, titled "Acoustic metasurface-based perfect absorber with deep subwavelength
thickness", by Li and Assouar", and took a look at what it actually says about this device.
Firstly, it is tightly tuned. Each such chamber is tuned to one specific frequency, with high Q and very narrow bandwidth (much like a Helmholtz resonator), so you would need a series of many such chambers to cover the entire low end of the spectrum. Let's say that you wanted to cover 20 Hz to 200 Hz: you'd need about 50 different tunings to do that smoothly, although you might be able to get away with as few as 10 or 15 different tunings if you don't mind patchy coverage.
Secondly, each chamber is only as effective as the size of entry hole. Only the area of the actual hole itself is absorbing sound. The rest of the area used by the chamber is reflective at all other frequencies. Therefore, you need a very large area in your room covered with a very large number of such chambers, to get a useful effect, and even then the total effect is limited to the ratio of "hole area vs. frontal area", similar to the problem with perforated panels.
Third, the manufacturing tolerances are tight. Each chamber needs to be built accurately, in order to ensure perfect impedance matching.
Forth, they never actually built anything to test! It's just a theoretical prediction and computer simulation of how such a device would work, but nothing physical was built or tested. There's not even enough information in the paper to actually build a device to test, since there's no specification of what the material properties would need to be, other than to say that they are "hard". But there's nothing in the equations that would suggest what type of mass, hardness, rigidity, elasticity, thickness, or any other physical property would be needed.
So basically you'd need to build a very, very large number of such chambers (hundreds at least, probably thousands), in a broad range of different tunings, each built very accurately, but from unknown materials with unknown characteristics, in order to have any useful effect, and with no guarantee at all that it would even work in the real world...

It's just an interesting acoustical curiosity at present.
Hopefully some company or acoustician will take this concept further, and test it with real materials in the real world, to derive real results, but until then it looks like we are stuck with traditional approaches, that actually have been tested and are understood.
I'm wondering why you think you will need Helmholtz resonators in your studio? You might not be aware that they are notoriously hard to tune correctly, they take up a huge amount of space, and they need to positioned in strange locations around the room. For example, there's no point in placing a Helmholtz resonator tuned for the 0.0.3 mode in the middle of the rear wall! It would do nothing at all there, even if it were perfectly tuned to that mode. It would have to either hang from the ceiling or poke up from the floor, and that would be problematic for obvious reasons...I am building a studio and was looking for build information on Helmholtz Resinator's...
Most acousticians stay away from Helmholtz resonators for the above reasons, especially in small rooms where space is already at a premium. Broadband porous absorption is far more useful, and if you do happen to end up with a really stubborn modal issue, then a membrane trap or panel trap might be effective at dealing with it.
Take a look at this thread for example: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=20471 The results speak for themselves, but there are no Helmholtz resonators in that studio at all! I did consider it at one point in the design and build, but discarded the idea as they are just too finicky. However, there are some perforated panel devices in there, as well as slot walls, which are both modified forms of Helmholtz resonator, but they are all tuned to certain ranges of frequencies, rather than to specific individual frequencies, and the Q is reasonably low. They are also not targeted at low frequencies: All of the bass trapping in that room is accomplished with porous absorption, except for one membrane trap that deals with a small set of issues. Trying to deal with very low frequencies using Helmholtz resonators is not nearly as easy some text books make out.
Great! Then you are most certainly in the right place!I am building a studio...

I look forward to seeing your thread!
- Stuart -
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Re: Coplanar Chamber to Absorb Low Frequency
Stuart,
Awesome studio build you have there. Very nice. Thank for your suggestion, I will start a new thread with some pics. I will get it started (the post) so you can take a look.
Its a small studio, but at least I will have a few rooms.
Jonathan
California
Awesome studio build you have there. Very nice. Thank for your suggestion, I will start a new thread with some pics. I will get it started (the post) so you can take a look.
Its a small studio, but at least I will have a few rooms.
Jonathan
California