My Experiment with a Metal Panel Absorber

[audiothings]

I built this a few weeks ago, but at the time, the guy I built it for didn't want me to share the the details... Finally managed to convince him...

The room was a tiny 11' 6" x 8' 6" x 9' 6". For various reasons, I decided to go with one large 4' x 8' trap with a 1 mm steel sheet. At the time, I was not able to source Basotect, so I went with open cell PU form of 24 Kg/m3.

Thing is... I couldn't take the risk of of the sheet falling... serious risk to life and gear. So heres what I did... I took 2 nos. 8' x 4' x .75" sheets of plywood and nailed them together to create a boundary of as much mass as I could afford. I then put a 6" deep frame around it and vented the middle 2". I sourced the spray glue that the guy in the hardware shop recommended, and the machine to spray the glue was hired. 6" of the PU foam was stuck to the aforementioned double layer of plywood and the steel sheet was stuck on the foam. Then a beading was nailed in all around the frame... as such, the beading was not touching the metal sheet... but I don't know really if the steel sheet has come unglued and is now resting on the beading.

I put the speaker (Sonodyne 8") in one corner and the trap on the other side of the room, in three positions... against the wall, on the floor, and diagonally, between the wall and the floor. I measured in 5 positions around the "sweet spot"... 38% from the front wall I know is pretty arbitrary... but it was what made sense in the room, anyway...

The most encouraging result is with the trap against the wall... which has reduced the decay time @ 50 Hz from ~1750 ms to ~730 ms. 50 Hz corresponds with the length of the room (11' 6"). With the trap placed diagonally, the mode has moved up by a few Hz, this is almost certainly because the trap has significantly reduced the length of the room...

Anyway, the pics below should give us a clue of what the traps are doing.

What would I do differently, if I were to do it again?

* Get rid of the frame, but for this, I'd have to be confident that the metal panel will not come unglued.

* Use a 2.5 mm steel sheet, which is far more effective around 50 Hz.

* Reduce the thickness of the foam from 6" to 4", so I can get that much closer to the wall... In this case, I used 6", so that I could put a frame around it and still be able to vent 2" in the middle...

* Test using corners only... mic, speaker and trap. This is what G.E. did, and I think it is the better way...

Let me know what you guys think...

[avare]

Thank you for sharing the project and results with us!

Andre

[DanDan]

Great stuff audio. Thank you for sharing this experiment, there are quite a few of us waiting for this. Could you send me the FM file plse, email address on my site below.
I want to play with the graph parameters and examine the Decays.
Some comments.
PU foam insulation that I see around here is very rigid. Was your like this?
I would guess that Basotect and Caruso Isobond are both quite soft.
The Fraunhofer patent and RPG's mounting instructions both suggest mounting on a solid backing, i.e. brick or concrete wall. However yours and GE's scenarios do not include that. I do not know how or why Gernot's unit seems to stand in the corner as shown. I would have thought steel would bend down.
Perhaps his struts are all there is. In any case, I have been looking for steel and Basotect myself and considering support. I have found self adhesive Basotect in the UK which is convenient. I was thinking of finding some ceiling beams and hanging the unit from it which avoids the frame, with any damping or constriction it might cause, and the ungluing issue.


DD

[Adhoc]

Interesting! I read Fraunhofer´s patent file some years ago, don´t remember much of it now though.

I believe the Basotec you mentioned is an open cell structure, melamin based.

Polyurethane based cell structures can be open or closed. If it is used as damper only, providing energy conversion to heat as the metal sheet vibrates it might not matter much which type you choose. Compression set and springback would be two things to look for in data sheets. Low compressoin set is usually good, springback is measured in % from the materials compressed state, so a high figure here might still mean a little in mm if the compression set has a large figure.

Info from some suppliers I have come across in work:
Gasketing and Sealing - DAFA A/S - seals, absorbs, protects

Sylomer or Sylodyne may do the trick as well: Getzner Werkstoffe: Sylomer

Good luck with your experiments!

[audiothings]

Adhoc, thank you for your post.

PU foam insulation that I see around here is very rigid. Was your like this?

No it was very soft, and could be rolled. 24 Kg/m3 was the lowest density they had. I wanted 10 Kg/m3 like Basotect G.

The Fraunhofer patent and RPG's mounting instructions both suggest mounting on a solid backing, i.e. brick or concrete wall.

Well, the best I could do to mimic the solid backing was two layers of 19mm plywood... If I had not planned to lay the trap diagonally, I would have opted for a different solution. The room was 8' 6" wide, and the trap was 8' wide... almost spanning the entire width of the room. RPG's data indicates that with the 1mm sheet, absorption coefficient at 50 Hz is around .5, while the 2.5mm sheet yields a coefficient of .9 at the said frequency... it seems to have worked despite this...

Another thing... according to RPG's site "Sound (1) strikes the steel plate (2) which pistonically vibrates (3) against the porous absorption spring (4), mounted on a rigid backing (8). The porous absorption also damps plate bending modes (5) and absorbs higher frequencies which diffract around the plate (6) through a perforated (7) metal frame."



The "diffraction" component is supposed to take care of the 150-500 Hz range, and despite my bastardizing the original plan with frames, vents et al., it seems to have worked to some degree, all the way up to 500 Hz...

BTW, I wonder if there is anything illegal in this? We are trying to reverse engineer a patented design in public!

[DanDan]

Thanks for the details audiothings. The Fraunhofer patent is probably registered in one country only. As it is so broad, even generic, and may have some false notions (note G.E.'s panel did not appear to have any solid backing) I doubt that it is sustainable. Anyway they invented MP3 so......
Basotect seems extraordinarily expensive, I am moving on to a search for a supplier of Caruso Isobond.
DD

[hsal]

The Fraeunhofer patent is developed by Renz sollutions Gmbh, why not contact them and find out?

[John White]

Why steel for the panel? How does the steel perform differently than say a sheet of plywood of equal mass per area? I can only think that its "vibrational properties" would be more consistent from piece to piece than wood due to it's inherent nature.

[DanDan]

It think it worth noting that the EMT 140 reverb plate, designed very carefully to mimic a concert hall, is steel.
audiothings sent me the FM file so that I could play around with it. Here's another view of what has been achieved.
This is with a 2000mS Window and the graph settings equalised.



DD

[scoring4films]

Wow! I can't wait to give this a try. So tired of suffering in a small, almost square control room.
VPRs look like they beat the pants of Helmholtz and 1/2 wave tube traps.
Thanks audiothings. In time I hope I can also contribute to this growing pool of knowledge/experience with DIY VPRs.

[silverdisk]

Looks intertesting, where did you source the steel sheet? was it pricey?

[scoring4films]

My guess is that the glueing concern has to do with the metal surface, yes?

Turns out there are a bunch of ways to prep a metal surface to improve the performance of an adhesive. Detergents, chemical etching, etc. Here's a relatively brief overview of some the options...

http://www.adhesives.org/Portals/0/a...esives.org.pdf


And here's another possibly useful bit of research...

WEST SYSTEM - Projects - Effects of surface treatments on adhesion to metals

Their data chart list "wet sand" as an effective way to get metal to stick to stuff. If you pop open the PDF version of the chart, you'll see that "wet sand" means "sanding the fresh epoxy into the surface with 100-grit paper." Sounds like a typical kind of DIY-er tactic

[scoring4films]

Originally Posted by audiothings

BTW, I wonder if there is anything illegal in this? We are trying to reverse engineer a patented design in public!

I'd be surprised if it was illegal to talk about glueing a piece of metal to a piece of foam, even in a public forum. I'm not a lawyer, but it's my understanding that the purpose of a patent is to give the holder the exclusive right to exploit their intellectual property commercially. As long as no one here is planning on selling VRPs, we're probably OK.

[scoring4films]

Instead of building a frame as a safety measure, maybe it would be easier/more effective to drill a couple of small holes in the top corners of the metal sheet and attach a couple of short lengths of light gauge, loose hanging chain, fixed securely to the mounting surface. That way if the glue joint failed, the metal sheet wouldn't go far. Obviously using a smaller sheet, like the ~5'X 3' one that RPG uses, would also help.

[scoring4films]

According to RPG...

"The backing material shall be 2.5 pcf density sound dampening material."

[see pg. 3, section 2.02
http://www.rpginc.com/products/modex...ateCSISpec.pdf]

Doesn't that come out to ~40 kg/m3? Basotect G is ~10 Kg/m3.

[audiothings]

Thank you Tyler for your ideas... certainly things to look into, when I build again.

With regard to the density of the foam, I did see conflicting suggestions... Please see attached formula taken from the attached document. It seems to suggest that a denser porous material will lead to a higher resonant frequency... but I am mathematically challenged



There was also somebody on this board who did an informal German to English translation of excerpts of the original paper (if I remember correctly) and it was suggested that the lower the density of the foam, the better it is for our purposes.

[scoring4films]

This glue will supposedly work on melamine and metal...

http://www.grainger.com/Grainger/TITEBOND-Glue-4EMN7



This is probably a dead end, but...
Steel-framed office furniture panels with a melamine core. Not too expensive. Maybe a way to mod them by glueing a steel panel on one side. maybe not. Unfortunately they're only 1.25" thick.


Cubicle Accessories - Furniture - Furniture and Furnishings - Grainger Industrial Supplies

-panels come in a variety of L/W dimensions

[gullfo]

its also useful to remember the plate has modal properties and the relative mode distribution on the plate has benefits in aligning with the room modes...

[audiothings]

Originally Posted by gullfo

its also useful to remember the plate has modal properties and the relative mode distribution on the plate has benefits in aligning with the room modes...

Yes... it is given by this formula...



The thing I'm finding difficult to work out is:

Young's Modulus, E, for stainless steel is 200 GPa as per this page, but our formula needs it to be in N/m2... 200 GPa translates to 2e+11 N/m2 as per this page.

I have reached my mathematical brickwall at this point and have requested a couple of my educated friends to try and make this usable.

[DanDan]

Just a reminder with some more detail. The EMT 140 reverb plate, designed very carefully to mimic a concert hall, uses a steel plate about 1x2 meters and half a millimeter in thickness. The plate was suspended on springs and carefully tensioned.
DD

[scoring4films]

Originally Posted by gullfo

its also useful to remember the plate has modal properties and the relative mode distribution on the plate has benefits in aligning with the room modes...

Does this mean the absorber will work better if the plate's modes have some sort of relationship to the room's modes?

If so, then how can RPG get away with selling VPRs with standard dimensions?

[DanDan]

I think EMT chose steel plate because it is like air on steroids. Because sound travels faster in steel, a small area of steel with boundaries, can simulate a much larger area of air, with boundaries. Now the EMT thing is interesting but different from here. It is thin and tensioned. It has a very heavy metal armature/voice coil attached to drive it. However, they did chose steel. I presume they considered other metals and materials.
I would place a bet that steel's behaviour is a multiple of air's across a broad spectrum of parameters.
DD

[gullfo]

Matching modes is different than not being effective at all. Based on the research work audio things has referenced, coupling to room modes simple adds to the effect. A generic size VPR absorber is still going to be helpful.

[audiothings]

What I can gather from the formula is that the length and the width of the panel DO make a difference to the absorption characteristics, although how much of a difference they make, i don't know... i want to bear in mind that this is a "shelving absorber", as opposed to the "tuned" absorbers that helmholtz and panel based absorbers are... of course i am referring to things i have read, and not necessarily experienced...

[scoring4films]

If I have to get my head around that equation before attempting a DIY VPR, I'm screwed. I hope your math pals come through for us audiothings!

[AwwDeOhh]

Originally Posted by scoring4films

If I have to get my head around that equation before attempting a DIY VPR, I'm screwed. I hope your math pals come through for us audiothings!

LoL,
Just Looking at the equation(s) there gave me a massive headache.. & i haven't even plugged in any numbers yet!

Thanks for this thread, very interesting discussion so far.

[avare]

Where is the patent? I have looked through USPTO.GOV with no luck.

Patents have to include parameters necesary for the product to work.

On the echo plate comparison, echo plates are designed to have sonic character. Abosrbers are not. Like comparing live room to control room acoustics.

Least any one misinterpret the tone of this post, my first post in this thread rules!

Andre

[daxliniere]

Originally Posted by avare

echo plates are designed to have sonic character. Abosrbers are not. Like comparing live room to control room acoustics.

Yes, this is absolutely correct.

Originally Posted by scoring4films

According to RPG...
"The backing material shall be 2.5 pcf density sound dampening material."
Doesn't that come out to ~40 kg/m3? Basotect G is ~10 Kg/m3.

I'm surprised to hear that Basotect (G?) is that density. When I was speaking to the sole Australian importer, he said there is only one density of Basotect and from my experience with the product (as "Magic Erasers"), I know it to be quite dense, more dense than the ordinary types of open cell foam we see in Australia.

The cell structures appear to be smaller (more dense).

I will get a mathematician friend of mine to take a look at that formula and I'll see what they say and get back to you.

[daxliniere]

Originally Posted by audiothings

It seems to suggest that a denser porous material will lead to a higher resonant frequency.

Yes, that sounds about right. Think about the resonant frequency of, say, plasterboard vs MDF.

If you tap a sheet of plasterboard, it resonantes much lower than a sheet of MDF.

So, low density could be a good thing, but remember that if the density is too low, the sheer strength of the material will become a problem.
A 1500*1000*2.5mm mild steel sheet is 29.4kg, which is not too bad, considering the surface area, but still worth keeping in mind.


All the best,
Dax.

[DanDan]

The patent was filed in Germany by the Fraunhofer Institute I believe.

DD