Projection Screen Absorber?

[Lonely Raven]

OK, so I have a problem, and I was flipping through a stack of acoustics books skimming for ideas. Tell me what you think might be a solution to this problem.

I have a projection screen in my home theater, it's a 10' diagonal, and made from white counter top laminate (about 1/16" thick), held in a wood frame, with one brace down the middle.

Now, the whole thing resonates at somewhere around 15hz to 20Hz, I've not pinned down the exact frequency, but it's in there. I originally thought to use some 3M 90 strength spray glue and glue some 8pcf Rockwool to the back of the laminate material thinking this would make a giant bass trap and stop the resonating all at once.

But after skimming through some acoustics books, they mention that a panel absorber should NOT have the insulation touching the panel, but be separated by at least 1/4" (if I remember correctly). Which then made me realize, if I pile up some 8pcf insulation on the screen, I'm changing the frequency the screen resonates at...

So, if I make this projection screen into a panel absorber, would simply mounting the Rockwool to the wall behind the screen (screen floats on a french cleat) work, or would it have to be a sealed environment? The book I skimmed didn't mention anything about panel absorbers needing to be sealed, just had some calculations for figuring out your own panels.

Suggestions?

[Brainchild]

To work as a panel absorber like every panel absorber I've ever seen, the screen would have to be the front face of a sealed enclosure.

Mounting some kind of damping material (substantially more substantial than fiberglass) would lower the resonance frequency because of the added mass, but if you got the mass and the damping factor just right it could totally conteract the screen's natural resonance. I'd be curious to see the results of that excruciating process of trial and error.

[Jeffrey Hedback]

Lonely Raven,

First, quite impressive you can identify a resonance at 15-20Hz.

It's unlikely that any rigid fiberglass or minerwool will affect this range.

Best bet is to damp the material...in this case (and it would be trial and error...don't think you'll find anything in a book to assist):
- glue and otherwise properly secure a layer of a 2#/ sq ft, 1/4" Limp Mass Barrier (note, not the more common 1#, 1/8")

Look at: Soundproofing Material, Noise Control Curtains, Acoustical Panels, Soundproof Walls, Foam

that should damp the material to the point any "ring" is less or non-problematic.

WOW...I'm guessing your sub is setting this off???

[Lonely Raven]

Originally Posted by Brainchild

I'd be curious to see the results of that excruciating process of trial and error.

I sometimes have trouble reading your posts...I can't figure if you're just making fun in general (as I do) or poking at me specifically because your annoyed with my questions. But I have to say, I giggled when I read this.

Originally Posted by Jeffrey Hedback

Lonely Raven,

First, quite impressive you can identify a resonance at 15-20Hz.

It's unlikely that any rigid fiberglass or minerwool will affect this range.

Best bet is to damp the material...in this case (and it would be trial and error...don't think you'll find anything in a book to assist):
- glue and otherwise properly secure a layer of a 2#/ sq ft, 1/4" Limp Mass Barrier (note, not the more common 1#, 1/8")

Look at: Soundproofing Material, Noise Control Curtains, Acoustical Panels, Soundproof Walls, Foam

that should damp the material to the point any "ring" is less or non-problematic.

WOW...I'm guessing your sub is setting this off???

Yes, I was thinking of car audio damping material as is used for loading down doors and the cabin of a car. But then, if I'm spending that kind of money on this, I might as well just build an Acoustically Transparent screen and be done with it.

Oh, and it's easy to discern the 15-20hz resonance...it's when the screen starts flapping like a bird about to take off. Not hard at all to see. A 21" sub with 2400 watts driving it tends to excite objects in the house.

You can hear the room rattling in this video. Around 15 Hz is the screen, 30Hz is the dogs kennel, and the whole kitchen excites between 22Hz and 27Hz.

YouTube - Maelstrom 21 Opus Dei Excursion Video.MPG

[Brainchild]

I sometimes have trouble reading your posts...I can't figure if you're just making fun in general (as I do) or poking at me specifically because your annoyed with my questions.

Just for the record, definitely not making fun of you! I just wouldn't want to be the one to have to go through the process of sticking a bunch of different materials to the back of a big screen and taking all the measurements. But I would like to know the results!

[Lonely Raven]

Originally Posted by Brainchild

Just for the record, definitely not making fun of you! I just wouldn't want to be the one to have to go through the process of sticking a bunch of different materials to the back of a big screen and taking all the measurements. But I would like to know the results!

I'm glad to hear that...I find your posts illuminating (when I can follow them), and I'd hate for you to not chime in because you don't feel my questions warrant a response.

And yes, I agree with you completely that trial and error would be fun (assuming it's not you doing the work!), but the results are only as good as the person performing them...and I'm not fully confident in my skills of measurement to want to post them online. Others seem to read things into my measurements that I just don't see. When I can trust and understand my measurements as well as others, then I'd probably enjoy the painful (and often lacking any exciting results) process of attempt and measure, attempt and measure, attempt and measure.

Right now I have a 45Hz hump that magically appeared in my sub measurements that I can not explain. If I can't figure out what I did to induce this hump and resolve it myself, then I clearly shouldn't be posting my results online! LOL

[PaulP]

Originally Posted by Lonely Raven

But after skimming through some acoustics books, they mention that a panel absorber should NOT have the insulation touching the panel, but be separated by at least 1/4" (if I remember correctly). Which then made me realize, if I pile up some 8pcf insulation on the screen, I'm changing the frequency the screen resonates at...

The BBC panel absorbers were like this but I recently came across this site
(pointed at by someone here) which recommends glueing the insulation to
the panel : Low Frequency Absorbers (go to the bottom of the page).

Paul P

[PaulP]

Originally Posted by Lonely Raven

...and I'm not fully confident in my skills of measurement to want to post them online. Others seem to read things into my measurements that I just don't see. When I can trust and understand my measurements as well as others, then I'd probably enjoy the painful (and often lacking any exciting results) process of attempt and measure, attempt and measure, attempt and measure.

Right now I have a 45Hz hump that magically appeared in my sub measurements that I can not explain. If I can't figure out what I did to induce this hump and resolve it myself, then I clearly shouldn't be posting my results online! LOL

I think measuring and posting your results will not only help you in gaining
more experience with measurements, but it'll help us a well. The effort we
expend in trying to help you gives us experience too. At this point I'm far
from confident using REW myself, but watching other people struggle with
it is enlightening.

Have you tried plugging your values into the membrane absorber formula
so see what you could get from a ten foot sealed bass trap with a countertop
membrane ?

Paul P

[Lonely Raven]

Originally Posted by PaulP

I think measuring and posting your results will not only help you in gaining
more experience with measurements, but it'll help us a well. The effort we
expend in trying to help you gives us experience too. At this point I'm far
from confident using REW myself, but watching other people struggle with
it is enlightening.

Have you tried plugging your values into the membrane absorber formula
so see what you could get from a ten foot sealed bass trap with a countertop
membrane ?

Paul P

I'm ashamed to say that math class was about 20 years ago for me, and though I was really good at it, I was busy chasing girls rather than paying attention. If there is an online calculator I'll hit it, but I really need to go back to college for a refresher on some of this Acoustic Math I'm running into in these books.

Thanks for the vote of confidence Paul, I will post some results...it's just, I'm really not seeing much if any change with all the work I'm doing. That makes me wonder if I'm barking up the wrong tree, or taking my measurements incorrectly. I'd hate to clutter up my already long winded threat with REW floundering.

I'll get some measurements this week, and maybe the gurus here can help me dial in my home theater. I love sharing what I learn, so I always make sure to pay it forward by helping and teaching others what I learn. Especially if I can prove it with measurements!

[Lonely Raven]

Originally Posted by PaulP

The BBC panel absorbers were like this but I recently came across this site
(pointed at by someone here) which recommends glueing the insulation to
the panel : Low Frequency Absorbers (go to the bottom of the page).

Paul P

Oh wow! That's what I was thinking about in my head...these make sense to me! I was thinking about making early reflection "flags" for my ceiling and I figured hanging panels, maybe some with facing on them would also work as a really large, long bass absorber.

So maybe some of my weird ideas do have merit. I just need to prove it with measurements.

ETA: Thanks Paul!

[surflounge]

here's some screen fabrics etc

AcousticPro1080 Woven Screen Material

Screen Research: acoustically transparent screens

and then you can just paint it on...
Paint on Screen - Projector Screen Paint

[PaulP]

Originally Posted by Lonely Raven

I'm ashamed to say that math class was about 20 years ago for me, and though I was really good at it, I was busy chasing girls rather than paying attention. If there is an online calculator I'll hit it, but I really need to go back to college for a refresher on some of this Acoustic Math I'm running into in these books.

The formula I was thinking about is actually pretty simple, perhaps too simple
to be worthwhile as it seems to leave a few things out. Found in the Master
Handbook of Acoustics here (open the box and scroll up a bit). You'd have
to figure out the surface density of you membrane by dividing its weight by
its surface area.

Paul P

[Jeffrey Hedback]

Lonely Raven,

If it's the structureborne aspect of your sub exciting these room features...simply pick up the Auralex GRAMMA to decouple the sub. You'd at least know at that point what is airborne and what is structureborne.

[Lonely Raven]

Originally Posted by Jeffrey Hedback

Lonely Raven,

If it's the structureborne aspect of your sub exciting these room features...simply pick up the Auralex GRAMMA to decouple the sub. You'd at least know at that point what is airborne and what is structureborne.

I've thought about that a lot...but the box is 41" X 37" X 23" and over 100lbs. Heck, the speaker alone is 56lbs. I'm not sure the GRAMMAs will work.

I've thought about trying some insulation as an isolator for the sub, but just haven't gotten around to testing that.

Plus, the drawback is that I won't be able to feel it in the seat of my pants like I can now, though I know it could improve the sound a bit.

[euphonic]

You could DIY something like this:
RPG Diffusor Systems - Modex Plate
(they even show you how to build it )

The frauenhofer institute specificaly promote the use of compound baffle absorbers as a screen or chalkboard.

source: IBP - Compound Baffle Absorbers

Though gearslutz is all about the "broadband bass trapping approach", those are the state of art and do work excepional well even in the lower bass range with being only 4" deep.

[Lonely Raven]

Originally Posted by euphonic

You could DIY something like this:
RPG Diffusor Systems - Modex Plate
(they even show you how to build it )

The frauenhofer institute specificaly promote the use of compound baffle absorbers as a screen or chalkboard.

source: IBP - Compound Baffle Absorbers

Though gearslutz is all about the "broadband bass trapping approach", those are the state of art and do work excepional well even in the lower bass range with being only 4" deep.

That's exactly what I'm thinking about building this screen into. I'm just not sure what approach will work best.

It's looking more and more like I have to guess and test.

[Jeffrey Hedback]

The RPG Modex Plates are fantastic and can truly help a 45Hz issue when placed proper.

Back to thge GRAMMA, full discloser...I worked at Auralex. I designed the GRAMMA and I have patent on it...however I have no financial benefit from their sales....all that junk being said...try it. One will support your sub, two would be ideal. I suspect you'll not only clear up the sympathetic vibrations, but enjoy a tighter, punchier bass response.

Many dealers have return policies, so it's a simple no risk effort.

Back to the tuned absorber topic...be leary of expecting such devices to be miracle workers below 50Hz...it's more likely to create an "after ring" or not address the issue than be successful. They can and do work...I like to use them in conjunction with proper broadband trapping for extra muscle.

[Lonely Raven]

Thanks for your honesty, Jeff, I appreciate that.

I still think there is some potential for making my projection screen an absorber. I mean, it's a 10' panel!

[Jeffrey Hedback]

You're welcome.

And, absolutely you should multi-purpose that screen.

Seems you have three different issues on the table (15Hz vibration, 45Hz resonance and turning screen into effective absorber).

I just finished a high end audiophile room where I designed a "floating" front wall that was tuned to the "Y" first order axial mode in the center and the SBIR behind the speakers (speaker location was determined). It worked very well with the broadband corner trapping.

You could certainly do a similar application tuned to specific needs with your screen.

All the best!

[RKrizman]

Originally Posted by euphonic

You could DIY something like this:
RPG Diffusor Systems - Modex Plate
(they even show you how to build it )

The frauenhofer institute specificaly promote the use of compound baffle absorbers as a screen or chalkboard.

source: IBP - Compound Baffle Absorbers

Though gearslutz is all about the "broadband bass trapping approach", those are the state of art and do work excepional well even in the lower bass range with being only 4" deep.

The Modex plate looks interesting, but I'm not sure there's enough info for a DIY. How thick is the steel plate, and how is it attached to the backing? I assume something like 703 could be the backing.

-R

[dykstraster@gmai]

Off topic...sorry.

RE: Sub isolation.

You may want to look into some appliance isolation feet. I got a set of 4 from Amazon for my HE washer I put on my second floor. Before the feet, (they're simply a dense hockey puck type thing, with an extra layer of far less dense material touching the floor) you could clearly feel the washer during spin cycles. Afterward, the vibration is greatly reduced. I wanna say I paid about $50 for a set of 4 of these.

It's my understanding, that for isolation decoupling type devices, that a certain density of material must be used to effectively "shock absorb" whatever weight it is that they are carrying. A 100 lb. subwoofer could likely be compared to the weight of a running washing machine.

Then again, I have a Wedgie, and it works really well for bass amps that weigh quite a bit.

Ah well, back to the screen thing.