My Experiment with a Metal Panel Absorber

[DanDan]

bastawa, that is disappointing.
It is hard to imagine that something would work really well in one room and not another. Something must be different.
Did you have strong LF modes to begin with? Did you try the corners?
Could you post your measurements plse.
DD

[bastawa]

i tried 3 different walls, 3different corners and combinations of those. i glued the isobond to plates and i used perforated steel like in VPR to mount them close to the wall. on the walls i tried different positions. i measured with RoomEq wizard. the was almost no difference. actually there was bigger difference with just unpacked isobond stacked in the corner.

[kasmira]

Originally Posted by bastawa

hi guys, just to let you know to me my DIY metal panel absorbers didn't work. i built them with isobond, and 2,5mm metal plates 1mx1.5m. i built 6 units. tried various positions on the walls and in the corner. there were some minor results but really small. i've built one with 1mm and this one worked very little aswell.

Hey Bastawa,

That is a bummer. Could you provide us with the placement details (as already asked) as well as the thickness of the Isobond and the density of the Isobond? Was the Isobond only behind the metal or both in front of and behind the metal? Was it glued, or resting on the metal? Was the metal touching anything else (floor, frame, etc?) Was the metal bent/curved/bowed in any way? Did you use stainless steel, or galvanized, or mild steel? Or did you use a different type of metal all together?

[DanDan]

i used perforated steel like in VPR to mount them close to the wall.

That is different to audiothings or G.E.'s experiments.
Something about your mounting arrangement is probably preventing the action.
Again though, did you have strong modes in the suitable frequency range?
Did you stimulate the modes by placing a speaker on the floor in a corner?
Do you still have the REW file?
DD

[bastawa]

i will look for rew files but i don't think i kept them it was 3months ago and i was so dissapointed with results that ditched the whole idea. not to mention i'm not renting that room anymore. i still have metal plates and CARUSO-ISO-BOND®-WLG 035 - 100 mm but now i keep them at my friends place. regarding mounting i also tried them just laying on the wall with my friend keeping them close to the walls. i built 6 of them and i tried all available positions (only walls and corners) in my room. the room was around 4,6x6,8x3,0m. there was no other treatment. i only tried measurmets with left or right speakers and mic in mixing position. and in this room there were quite few resonating modes under 100hz.

kasmira:

the isobond was only behind the metal (100mm) it was isobond wg35. it was galvanized steel 2,5mm thick. one absorber was galvanized steel 1mm thick.
some of plates were slightly bowed like they use huge steel rolls and cut them into 1mx1,5m plates. some plates were almost totally flat some bowed in the middle 5mm. i actually thought that this could be the problem but 2 of them were flat and they didn't perform any different from bowed ones so i discarded that idea.
i glued the isobond to the metal.

[DanDan]

bastawa, I note a couple of things.
I have a memory, probably from way back in this thread, that the 2.5MM plates are not as successful as the 1MM. Gernot, did you find that?
Your steel is galvanised, I haven't seen that mentioned before.
Don't know if it matters or not.
Holding the device might damp it, but in any case we have seen much better effect in corners. As with most LF devices.

I believe it would be worth your while testing again. Modes are best explored with speaker(s) on a floor corner, mic at opposite ceiling corner.
A mic and or speaker at another postion may be at modal nulls, not sending or receiving if you will. Try the Signal Generator in REW, with Frequency Follows Cursor checked. Slide the cursor over a mode, then find out where it is loudest. Usually a corner or whole wall. The loudest spot would be the best place to trap.


DD

[bastawa]

i tried the method with signal generator in REW and put plates in loudest spots. but i always had monitors in mixing position + mic in sitting position. i really tried many different positions. with my friend we while testing we haven't dampened plates he held them on the edges not in the middle. i might try them in the future but i don't have new room to test them. the place i'm working now sounds ok enough and i don't want to mess with acoustic there plus each of plates weight around 35kg and they are big so moving them requires 2 persons.

[OpusOfTrolls]

The rolls might be the problem here. If you are using cold rolled steel, it will have a difference in flexural tension because of the work hardening that the plate undergoes in the flattening process. The long side of the roll is made short, while the short side is made long.

I mentioned this a few posts back, but I will mention it again. The modal distribution of the plate cannot be guaranteed by any ideal values. If there are tensions in the plate itself, they will change the speed of vibration and therefore any modal condition. A tension in the plate could be a sheared edge, a dent, the flexed part of the plate whilst in transport, the list goes on. The more surface area the flexural tension affects, the more it will affect the plate's lowest modes.

Now, while this makes VPRs a stubborn thing to design, I don't think the root of your problem is exclusively the metal you are using. The plates should be resonating without any damping/absorbent mounted on them. When you locate the plate inside your room, you want the most vibration transferred to it from the sound. Once this is found, you should add the absorbent to the plate, and measure from there. Another factor is possibly SPL. Take two measurements at low and high volume.

[Tim Farrant]

It's a pity that some people who post in this forum are intent on simply disproving everyone else instead of exchanging ideas and learning together.

[Tim Farrant]

Regarding the bass trap, it does not make sense to me that 2.5mm thick steel would be effective in this design, it's too heavy and rigid? Surely the panel needs to be a lot more flexible, 0.8mm thickness? Or perhaps aluminium is a better material?

[DanDan]

Thank you Tim. Here it is from the horses mouth.
IBP - Compound Baffle Absorbers
The VPR was invented at the Fraunhofer Institute by Dr. Fuchs, I believe. Yes;-)
I would assume they tried all sorts of materials.
Ended up with 1MM and 2.5MM Steel and Basotect, but later Isobond was found to be better.
Absorption peaks at 70 and 40 Hz, but as you suspect there is less absolute absorption from the 2.5MM.
bastawa may not have had 40Hz issues, thus his lack of apparent success.

Note that both types are commercially available from RPG and Renz. Afaik 38 of em were used in the Blackbird Studio.

Here's a reminder of how good they can be. This is only one, but large and in a corner. See Post 46 first. Perforated Panel with Porous Absorber trap


DD

[kasmira]

Originally Posted by Tim Farrant

Regarding the bass trap, it does not make sense to me that 2.5mm thick steel would be effective in this design, it's too heavy and rigid? Surely the panel needs to be a lot more flexible, 0.8mm thickness? Or perhaps aluminium is a better material?

Aluminum has got to be at least 5 times more expensive than steel, and it is highly conductive which I don't see as being a good idea in multiple panels in a room. Perhaps this is why it hasn't been tested? I see your point about the thickness of 2.5mm being extensive, it only leads us to wonder why other thicknesses inbetween haven't been used? I imagine larger sheets of 2.5mm to be still flexible, just 2.5mm would require more attenuation to be effective. Perhaps that is why it is used though, imagine if you had a sort of threshold on absorbers - only absorb peaks over 85dB or something. Just a thought, it might be overly ridiculous though.

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[AwwDeOhh]

Originally Posted by Tim Farrant

Regarding the bass trap, it does not make sense to me that 2.5mm thick steel would be effective in this design, it's too heavy and rigid? Surely the panel needs to be a lot more flexible, 0.8mm thickness? Or perhaps aluminium is a better material?

It might have been mentioned, but was there a formula to approximate the design frequency (ie resonance) of the metal for different sizes? (ie surface area)
Was it determined what a good stock of steel would be? (ie, galavnized? manufacture process?)

I would assume smaller dimentions would ring higher in frequency [assuming same plate thickness]. Just wondering if there is a way to 'tune' these to my problem frequency(s), assuming i can make a successful build in the first place.

I am interested in trying out various designs here for my home, but just wondering if anyone can suggest a ballpark guesstimate. time is precious, and my back burns out easy.

I see the success was had in a corner.. i'm wondering if it could be used at a floor-wall junction (ie, on it's side) with the same or similar success. . I have the perfect space for such a device, but i'm wondering if having the length of the panel resting on the floor would significantly change it's resonance.

[DanDan]

Perhaps that is why it is used though, imagine if you had a sort of threshold on absorbers - only absorb peaks over 85dB or something. Just a thought, it might be overly ridiculous though.

Resonant devices and absorption for that matter are normally linear.
They do exactly the same job at 30dBSPL as they do at 90.
There is no 'waking up', or threshold, or gating effect.
None that I have heard of in any case.
At extremely high levels the air itself may do a bit of audio compression, but we wouldn't hear it due to burst eardrums.

2.5MM does not appear to absorb as much 'area under the curve' in the link provided. However, we are talking about a 40Hz peak.

Looking at the data, and the commercial versions, the choices are bang on the money for real rooms. They are not high Q devices, I wouldn't attempt to tune them. Note G.E. successful one is about 2.5M x1M.

Again I suspect bastawa's lack of success may be that he did not have a strong 40Hz issue to begin with. Or didn't place the traps where it peaked, Or perhaps measurement anomalies. There is a reason. These things generally work.

DD

[bastawa]

Again I suspect bastawa's lack of success may be that he did not have a strong 40Hz issue to begin with. Or didn't place the traps where it peaked, Or perhaps measurement anomalies. There is a reason. These things generally work.

I had strong modes below 50hz, thats why i built 2,5mm. i built 5 of them and tried many placements. I think as someone pointed out it might be the problem with steel plates being not perfectly flat. I'm really curious how the commercial VPRs perform.

[OpusOfTrolls]

I have been trying to say it, in two posts already.

Just because your VPR panel is the same size, does not mean is has the same modes OR absorption.

But, the adhesive and porous absorbent is supposed to damp the plate, which will lower the Q and make it less focused on these modes.

I am unsure as to why you see no absorption, but have you noticed any change at all? What does the decay plot do? Do peak/null changes look very low?

Maybe the 2.5mm plates are tensioned in a less useful way. Have you tapped your finger to the plate to hear the resonance? These sounds would be the absorption peaks. Every plate will be different but mostly regular. Don't bother calculating anything, just get your hands on the steel and let it ring.

One more important thing. Make sure you rotate the plate around a high pressure area, to check for angle of incidence. IT could be that this was the problem all along.

[bastawa]

OpusOfTrolls, i've read your post thats why i said that bend might have been a problem.

when i touched them they were resonating but really they didn't work. maybe after installing 5 of them there was 2db of improvement and time wise from 700ms to 650ms at 45hz. or something like that. i figured out that to make it working i would have to use all walls and ceiling and still improvement would be small. thats why i gave up.

for now i'm done with VPR, i'm not renting that room anymore and i store plates at my friend's house.

just to mention again i built one 1mm VPR and this one worked a little bit better for 100hz area. but still you could spend the same money on more efficient bass traps.

[visa.the.dog]

Would this PolyMax product be a viable alternative to IsoBond? Density is listed as 7.5 lb/ft^3, which is definitely higher than IsoBond. Maybe too rigid to be a good "spring"?

http://www.acousticalsurfaces.com/ac...ll-panels.html

[soundofdcm]

Have it been established which type of steel is the best for this kind of absorber? galvanized? mild? stainless?

[Yannick]

I have about the same question:
cold welded or not ?
Cold welded steel is more solid, it will bend less.
That must mean it is stiffer and resonances will be higher ?

Can anyone confirm if it is better to use hot welded steel ?
I can find every thickness between 1 mm and 3 mm locally, 0.5mm increments.
In both qualities.

[OpusOfTrolls]

I bought Hot Rolled sheets. One reason for this was price. But I could have gotten Cold Rolled or Galvanized. I do think because of the slight annealing that Hot Rolled steel undergoes might make it resonate in a better way. I am in the process of testing some panels resting on the floor on a layer of 4" Thermafiber SAFB. Using a DB Meter, I saw a sound reduction of about 2 dB in front of the panel, compared to only mineral wool and then only bare floor. I didn't have my microphone or REW hooked up, but I will do some tests soon.

[kasmira]

Awesome! I can source 4" Thermafiber for a great price, very curious to your test results Opus (what is your name if you don't mind me asking?). Could you test straddling a corner as GE has tested as well as against a wall? And how large is your steel sheet?

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[OpusOfTrolls]

Originally Posted by kasmira

Awesome! I can source 4" Thermafiber for a great price, very curious to your test results Opus (what is your name if you don't mind me asking?). Could you test straddling a corner as GE has tested as well as against a wall? And how large is your steel sheet?

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I didn't choose Thermafiber SAFB for this, it chose me. I have some laying around and figured I might as well use it. I don't expect to see huge results, I kinda feel the plate is too heavy to work how I planned. It is 1.7mm Hot Rolled at 48" wide with 3 different lengths to choose from. I will be testing all 3 for comparison. But each plate has a bow in it and resting them on the mineral wool isn't creating a even weight distribution. I still have some things to work out before I start measuring.

[kasmira]

Well I hope despite the problems you may find some use for your materials at hand.

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[OpusOfTrolls]

Originally Posted by kasmira

Well I hope despite the problems you may find some use for your materials at hand.

Sent from my Nexus S 4G using Gearslutz App

I tried to find hot rolled in a 1mm thickness, but to no avail. It seems like it would be a specialty item. Anyway, the hot rolled steel is more omnidirectional with it's vibration, which is a property I am sure we want.

[BriHar]

Seems to me that brass would be an interesting material to try - being a very musical metal (used in many instruments from xylophones and piano soundboards to horns to church organ pipes) - might even be tunable. No idea of the costs involved though - probably expensive.

[OpusOfTrolls]

Okay, despite having compatibility issues with windows and my interface, i was able to test a new layout today. i am mainly interested in treating a 70hz null as well as its harmonics. by laying the plate on mineral wool and running 70hz through the monitor, i saw a 2db change at the mix position as well as the surface of the panel, versus only the mineral wool. this enhanced porous absorber as GE calls it is worth a shot. later i will test it with two and three panels of mineral wool underneath the panel.

[kasmira]

Originally Posted by OpusOfTrolls

Okay, despite having compatibility issues with windows and my interface, i was able to test a new layout today. i am mainly interested in treating a 70hz null as well as its harmonics. by laying the plate on mineral wool and running 70hz through the monitor, i saw a 2db change at the mix position as well as the surface of the panel, versus only the mineral wool. this enhanced porous absorber as GE calls it is worth a shot. later i will test it with two and three panels of mineral wool underneath the panel.

Just to add - his 'epa' is absorbent-plate-absorbent. Absorbent on only one side of the plate is considered a VPR. You may want to try different configurations. According to GE thicker absorption behind (under in your case) should lower the resonant frequency. Hopefully, if you add absorption on top, you should get similar results as your test with only wool in addition to the plate and absorber. Perhaps even better, since the wool is velocity based and the velocity is 0 at the boundary

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[Syncamorea]

I've been following this thread since Audiothings kicked it off and I've been hoping to see more clarity emerge. Once again, I'm reading an acoustic treatment thread where one of my nagging concerns revolves around just how sensitive room measurement science is to the little specifics, like mic and monitor placement.

Regardless, I'm encouraged that this tech is worth digging into and giving it a shot. The mix room I'm about to treat is ~3.9 x 3.5 x 2.4m and considering that those dimensions suck, the room in totally untreated condition doesn't sound so bad. Measurement-wise, below about 350Hz, it looks worse than it sounds, put it that way. I've got another month or so before I rip out the carpet and put down a hardwood floor - and then the real fun will begin. Thanks to all that have contributed here - I hope to add something worthwhile this fall/winter.

[Plutonia]

I have read thru about 150 posts here and then skipped to the end to see how it all turned out.

I don't have experience yet in building room treatments. But I do have a couple of ideas that perhaps a more experienced person may be able to evaluate. I will state my ideas below, but know that I do not know if what I am proposing really makes sense or not.

First, it seems to me that tuned pistonic motion is only a small part of how these devices are intended to work. I think that there need not be any tuning needed for a usefully-wide band of low frequency absorbtion.

What is the purpose of the steel plate? Well, low freq resonance comes to mind at first. But maybe that really isn't the main thing. I think that the plate simply acts as a high-impedance acoustical barrier at low frequencies. A plate presents a single, unified structure that even low frequncies cannot 'slip through'. Foam is not actually a single, unified structure. It is an amalgamation of millions of tiny structures, each of which is connected to the others by far less rigid and far more permeable bondings than steel atoms. So foam and fabric do not have the structural 'self-connectedness' that steel does.

This is important because low frequencies do not reflect off materials that do not have this high degree of 'self-connectedness'. They pass through them, due to their higher total power and lower angular momentum. Also, I think that the surface area of a would-be low freq reflector must be higher, because the sound waves themselves occupy (?) a larger physical space, when thought of as vibrating rays of energy. The physical dimensions of the 'sound ray' is much larger at low frequencies, so if you want to reflect it, you must present an obstable to it that is much larger or you are only affecting a portion of the total physical waveform/ray.

This 'larger size' is the size of the 'self-connected' portion of the proposed reflector; NOT THE TOTAL SIZE OF MILLIONS OF TINY FOAM CELLS which are not really a SINGLE reflector.

So the reason the steel plate is used is because it makes a good low frequency reflector. Obviously, you cannot have pistonic action if the sound waves pass thru the piston. They have to hit it, and impart a physical force to it. Any bass management must, in some way, CAPTURE the low frequency waveform. It can't let low freqs slip thru.

Basically, all that I am saying is that the only way to capture low frequency sounds so as to absorb some of their energy is to create a proper low frequency reflector, and then attach that reflector to a physical mechanism which can damp the motion of the reflector, turning the sound energy into kinetic energy and then into heat.

Notice how none of this requires a system that is specifically or intentionally 'tuned' to any resonant frequency. All that is required is a way to convert low freq sounds to motion, and then damp that motion. The way to capture is the steel reflector. The resonant characteristics of the plate itself are of lesser importance, as long as the plate is large enough to catch large waveforms. So I believe devices like this should be as large as practicable. I suspect that smaller plates, even several of them, would be less effective at the lowest frequencies. And this is because of the contiguous surface area factor; not because of the fact that larger plates tend to have a lower resonant frequency. You could have ten devices covering 20 square feet and I think they would be less effective than a single device covering the same area.

So I tend to think that very large plate/foam arrays are needed, probably with the smallest dimension being about 6 ft / 2m.

I do not think that thicker plates help much. They only need to be thick enough to actually reflect the low freqs. Less than 1mm may work even better than 1mm, because it will be more flexible. The optimal design point for the plate should be as thin as possible while still maintaining full reflection of low freqs.

The resonant frequency of the plate is not that important either. It does not even have to move in a fully sinusoidal, resonant manner. It merely has to deflect and rebound in some fashion that need not be either periodic or symmetrical.

I do not think that the plate needs to be completely free-floating on the foam. It just needs to be able to move in as many degrees of freedom as possible.

I propose making a 'Sandwich Trap'. Size approx 6' X 9'. Use 2 layers of a low-density (low weight), high strength foam to create a 'shell' around the plate. These 2 foam layers should be slightly larger than the steel plate. Physically connect these 2 'shell' foam layers together all around their perimeter, sealing in the steel plate inside. Between the foam shell and the plate, there should be 2 sections of something like shag carpet, one on either side of the plate. The soft part (shaggy part) of these should both face and LIGHTLY touch the plate. The plate should not fit completely snug inside. There should be only slight tension holding all the parts together in contact, so the shell needs to be big enough and loose enough around the plate. Adjust the thickness of the inner carpet layers to adjust 'plate tension'.

Mounting is accomplished by any of various ways, but the mounting point on the Sandwich Trap will be THE SHELL ONLY, at the extreme margins where only the shell/foam is physically inhibited by the mount. So you could drill thru the shell (and not thru the plate) and use nylon ropes, or even bolts could be used connected to wall-mounted brackets.

The plate will hang inside the sandwich, and will be encumbered only on 1 side, the side that is down. But even that point of encumbrance will be on foam only, not any hard surface, so there will still be some freedom of motion.

Wall mount, or simply lean against the wall on the floor.

Low freq waves pass thru the outer shell and reflect off the plate, moving the plate. The plate motion is damped by the carpet and the surrounding foam. Any time the plate moves and is damped at all, low frequencies are being absorbed. The high freq reflections off the plate are handled by the foam shell. For a more live (mid & high) room, use a thinner shell. More dead room, use a thicker shell.

No foam/glue/plate connection required. No need for a wood enclosure, except for aesthetic purposes. No headache in how to get the plate mounted and yet be relatively free.

Ok that is definitely enough for now! Let's hear from some more experienced folks now.