My Experiment with a Metal Panel Absorber

[Absorbentbobs]

Do we have some sense of agreement on what kind of glue/adhesive should be used for securing the steel plate to foam???

Thanks in advance.

AB

[boggy]

hunecke.de | Panel absorbers

It is good explanation about panel absorbers functioning, with regards on difference between bending type only and mass-spring movements of metal plate.

EDIT: I didn't find this anywhere at gearslutz.... so if it is reposting, sorry.

[PaulRain]

just ordered 2 Isobond panels, in the afternoon I will call my brother to arrange the two steel plates.

[felixocean]

Dan, what's the CBAv version you mentioned? 70mm foam back, 30mm front?

I'm about to order 3 plates and get them powder-coated with a nice copper vein finish for ambience as well as sound performance, but if sandwiching it will make it perform better and make powder coating unnecessary, I'll save 50-100 bucks per panel.

Anyone know if sandwiching improves performance? Anyone tested this yet? (if not, I'll give it a shot. I have two 4" thick foam mattress pads that should do the trick for testing, one sandwiched version, one not.)

[DanDan]

Compound Baffle Absorber. I have no idea how the LF of such a sandwich stacks up against 100mm at the back only. Check the RPG, and other manufacturers specs. However the surface layer should take care of HF. Depends what you want.
DD

[felixocean]

Just before calling in the order to powder coat, I realized that two shiny steel plates in the front corners become major new first reflection points. Oops. Most people doing RFZ calculations aren't putting reflective metal sheets angled in the front corners! Anyway, that's a good argument for sandwiching at least the front two to kill early reflections, the alternative being sticking a monitor-sized piece of absorber over the reflection point on both panels.

Going to call in the metal order this morning, and I'll take measurements with both backing-only and sandwiched versions to put this issue to rest.

[felixocean]

The question now is how to support the sandwiched version straddled against the corner without gluing the foam to the metal. Don't have any convenient telescoping arms laying around. Anyone have any ideas?

[PaulRain]

Originally Posted by felixocean

Just before calling in the order to powder coat, I realized that two shiny steel plates in the front corners become major new first reflection points. Oops. Most people doing RFZ calculations aren't putting reflective metal sheets angled in the front corners! Anyway, that's a good argument for sandwiching at least the front two to kill early reflections, the alternative being sticking a monitor-sized piece of absorber over the reflection point on both panels.

in my opinion the two front corners are going to be more likely out of the RFZ equation and there is a good chance that a little bit of reflection back in the room is not so bad if the room is on the HF dead side.
But I guess that glueing some HF absorber on top of the plate won't change it function.

[DrFrankencopter]

If someone has built a VPR, can you turn it around and test if it works with the metal plate facing (but not touching)the wall?

If so, then there is no need to build broadband versions.

I can't think of a reason why it wouldn't work, except if there is a speed of sound issue for the part of the pressure wave that passes through the porous absorber first, or a pressure build up between the wall and plate. But then again I'm not an acoustics expert. I just figure it should be an easy thing to test and compare to the usual orientation of the VPR.

Cheers

Kris

[scoring4films]

Originally Posted by felixocean

...Most people doing RFZ calculations aren't putting reflective metal sheets angled in the front corners! Anyway, that's a good argument for sandwiching at least the front two to kill early reflections, the alternative being sticking a monitor-sized piece of absorber over the reflection point on both panels.

or use a diffusor

[felixocean]

Q:

Would I get better broadband absorption if I made the foam backing 1-2" inset from the plate edges (in other words, make a baffle like the rpg ones)? I'm considering ordering some isobond, need to figure out what size. Would still be straddling corners with a 4" foam backing.

[felixocean]

Okay, got my panels. Pretty amazing results so far.

Here's a before/after. First image is before (room with no treatment at all, 12'x10'x9' roughly), and second image is after placing 2x 40"x60" panels straddling the two front corners. 4" foam backing. They're just leaning up against the foam + corners, no adhesive or supports used. The foam is just some polyurethane mattress foam I had as my bed -- I think it's 1.8 pcf. Not sure the firmness -- pretty squishy. Probably 40.

With no treatment:



After treatment (just 2 metal panels + foam, nothing else):

[scoring4films]

Looks great. How thick was the steel?

[boggy]

Originally Posted by scoring4films

Looks great. How thick was the steel?

Yes.... looks great, and I'm interested for knowing steel thickness too.

[G. E.]

Originally Posted by felixocean

Okay, got my panels. ...

I think the most remarkable change is that the dip just below 100Hz vanished. I've seen this behavior quite often with straddled metal panels behind the monitors.

[johnbomb]

Originally Posted by boggy

hunecke.de | Panel absorbers

It is good explanation about panel absorbers functioning, with regards on difference between bending type only and mass-spring movements of metal plate.

EDIT: I didn't find this anywhere at gearslutz.... so if it is reposting, sorry.

This article gave me an idea. Since laterally clamping the steel plate will restrict its vibration, I wonder if the floor is similarly clamping the plates that are leaned up against walls. Obviously, as we have seen, the VPRs built in this manner which straddle corners do a fantastic job, but I wonder if we can do even better.

Specifically, I would really like to see someone take a large steel plate, drill a single hole in its center, and suspend it from the ceiling, straddling a corner as it rests against a foam backer, as shown here:

John

[DrFrankencopter]

Hanging the plate will change its mode distribution. Similar to tuning a plate reverb by adjusting the retaining clip tensions.

Cheers

Kris

[johnbomb]

Originally Posted by DrFrankencopter

Hanging the plate will change its mode distribution.

Perhaps in a way that's advantageous compared to resting on the floor? Doesn't the floor "tethering" also change its modal distribution, with respect to a freely vibrating plate?

John

[scoring4films]

When considering mounting options, keep in mind that the 2.5mm thick plate at 1.5m X 1.0m is over 60lbs. It's large, heavy and cumbersome.

Any mounting approach needs to be very strong and secure. Likewise, testing it in a variety of locations/configurations isn't easy.

After some preliminary tests, I can understand why leaning it in the corner is becoming a popular way to go.

[felixocean]

The metal is 14 gauge. .064 inches or 1.628mm. Interesting thoughts on hanging the thing. Seems like the 50hz mode didn't get improved nearly as much as higher frequencies--maybe that has something to do with the fact that it's resting on the ground and dampening/preventing that range of modal resonance from happening. Certainly worth testing! Though hanging the thing from one eyehook is a little iffy considering the weight. Gonna see if I can try this.

I really want to see that 50hz mode come down and the 60ish null come up. I have enough fiberglass for about 18 2'x4'x4" absorbers, so that should help a bit.

[felixocean]

Also, the null just below 100hz was almost completely nuked with just one panel. I'll try and dig up that waterfall, as well as post an mdat with about 15 various tests.

[felixocean]

Originally Posted by scoring4films

Likewise, testing it in a variety of locations/configurations isn't easy.

Yup. And mine aren't even that heavy! Navigating these metal panels around a small room with speakers and a mixing desk is fun indeed.

The funny thing is, after getting a pretty dang good low-end response to the room on paper, it still sounds like crap because not only have I not treated the RFZ at all, but also the corner-straddling metal panels create huge first reflections themselves! You can see here in the photo of my test setup (purply walls are because this used to be my sister's room... haven't gotten around to repainting ):



Here's my MDAT with about 15 different test scenarios. Only look at the ones with "no sub" in the label. My subwoofer was causing some major constructive/destructive interference. Interesting to look at, I guess, but beside the point for measuring the response of the panels. BTW the room is approximately 12'x10'x9'. Speakers are Focal CMS65, microphone is an OktavaMod MK-012 with omni head.

http://www.filedropper.com/feb-11-steel-testing-6

Per the suggestion to hang the metal, I did the next best thing and raised it up on just one wooden shim. Bout a 2" wide piece of wood right in the middle of the bottom of the plate, so most of it is still free to move. I know next to nothing about the actual geometry of plate resonance, so it's tough to say whether this would actually help. The result? Zero difference from the plate resting on the floor. Still have yet to test actual hanging. Have to find some beefy eyehooks and solid studs for that shenanigan.

Also, here's a test showing the metal plate with NO foam vs. with foam. That null just below 100Hz is still pretty gone even with no foam, with the panel resting on the floor and leaning into the corner. Here's a before/after1/after2 (before=no treatment, after1=one panel with no foam, straddling front left corner, after2=same panel but WITH foam):

no treatment:



one panel, no foam. straddling front left corner:



one panel, WITH foam. straddling front left corner. (4" thick crappy mattress foam):



So the foam definitely helps, but the panel isn't USELESS without it.

Gets me thinking that maybe my foam is garbage, and I need to get some ISO-BOND. Has anyone had ISO-BOND shipped to the states yet?

[DanDan]

Great stuff, another positive result. Seems to me one cannot go wrong with these devices.
Caruso are fine for shipping and communication within the EU, but seems to be a problem elsewhere. Perhaps someone who speaks German would have a word with them.

Google Polyester Insulation. There are others.
Polyester Insulation Specifications | T-Max
And an Italian one called Termobond
A Dutch one Akotherm at http://www.acousticshop.eu/akotherm_absorption.html
EDIT, A USA one. Density looks low but maybe they can be persuaded to do denser batts for acoustics.
http://enguardinsulation.com/overview.cfm

DD

[PaulRain]

For sure Caruso will ship it t you the problem is the cost of the shipment, from Germany to Italy for two panels is 50€, I'm afraid to the US will be a lot more.
I have a piece of foam like the one you used at home, I think I will get more steel and use that one too, it's smaller than 1 x 1.5 m.... something like 80cm x 140cm (11cm thick) do you guys believe it will be too small to be effective?

[PaulRain]

looking for Polyester matress give more results actually, I could have done that before, I found two companies in Italy, even if I think the cost both for the material and the shipment won't be much different from what I've got from Caruso.
Looking at the tests looks like at 50Hz it's maybe too low for the panels, the null at 60Hz probably have nothing to do with modes but more with the position of the speakers or of the mic, did you try to move the speakers? maybe it could be the next step for test leaving the panels in the corners

[boggy]

Originally Posted by felixocean

......
Gets me thinking that maybe my foam is garbage, and I need to get some ISO-BOND. Has anyone had ISO-BOND shipped to the states yet?

Can you try to measure again with your steel plates/foams mounted to your side walls (in line with your measurement microphone)... and to front and back wall...

EDIT: You have something similar in your REW meas. file, I saw this now, but have you tried to use two steel plates, one over another (simulate one thicker plate), in both corners with or without foam

[felixocean]

What kind of density should I be looking for? Caruso doesn't post that info on their ISO-BOND site. I found a USA one called EnGuard that looks to have a pretty high-density option. Says it's good for acoustics.

[DanDan]

The CIB seen in the tests is 40KG/m^3
Plse link to the Engard high density one you found.
DD

[Absorbentbobs]

Although the thread seems to be gravitating towards a non-adhesive implementation, the following has been recommended to me by a couple of US Basotect suppliers as the appropriate glue to use for a metal plate:

3M Scotch Grip 4550

Any opinions out there about this solvent based product??

Thanks in advance.

AB

[NLP]

Low Frequency Absorbers
and Comparisons
http://www.aes.org/tmpFiles/elib/20120213/12403.pdf
not directly compared to dr. Fuchs pistonic devices but interesting.