My Bass Trap Design.

[darnold]

Well i guess i should have come here first to ask about the designs affectiveness, but basically what i realized is its either this or buying extremely expensive foam because ive looked everywhere for a vendor of rigid fiberglass around here, and even tried to get some special orders. First of all, no one around here even knows what im talking about, even if i call it all the other names it has.

So ive basically given up on that. So ive decided to use R19 batt insulation instead.

As i understand it, the absorbtion works a bit differently with this kind of stuff (similiar to foam) as compared to the 705 rigid fiberglass. Plus since its alot thicker its really narrowed my options on what i can build.

But when i bought the new studio i noticed the previous owner had similiar problems. But he had designed a bass trap in the Surround mixing room that i thought should be pretty effective.

Hopefully, because i pretty much copied the design even though i built mine alot smaller.

Basically it consists of a frame in the corner, much like Johns corner slot resonators, only its not an air tight box. It consists of 3 layers of fiberglass sheets. Two going adjacent to each other in the corner and one going across the corner. This makes a triangle with a bit of an airpocket in between.

They way i understand it, and i hope im correct, is this makes a very thick broadband absorber. Unlike a slot resonator or Ethan Winers bass traps that really on a resonating chamber that is tuned to certain frequencies. I just really hope that what i did created efficient bass trapping, maybe not as good as a resonating trap, but maybe still probably good? Id rather hear that i did overkill with it then not enough .

Since i didnt want it to suck up all the highs so im going to put (havnt yet) put 1x2" furring strips down the front to reflect some of the highs back into the room. Im unsure exactly on how far i should space these however. Any suggestions? I could also alternate them like 1x2" and a 1x4" because i havnt cut the 1x4"s in half yet. So any suggestions would be great. So far they are looking really cool but i hope i just didnt waiste my time and precious money doing these if i could have done it a better way with what i have.

Guess i should have asked the pros here first what they thought. Also, its not too late to fix a few things or add a few things if it will make a big improvement so let me know.

Thanks a ton.

Danny

 

[DDev]

Danny,

I did something that is kind of similar with one exception. I actually put a sealed membrane panel across the corner after stuffing it full of insulation like you have done. I didn't do any measurements to determine the exact effect but it really did change the audible characteristics of the bass buildup in the room.

Darryl.....

 

[DDev]

I forgot to mention that I used 1/8" luan plywood (like what is on the face of cheap hollow core doors) for the membrane panel.

Darryl.....

 

[Ethan Winer]

Danny,

Any type of fiberglass in a corner is good. Rigid fiberglass is better than the fluffy stuff because it packs more material into a smaller space. But thick batts of fluffy fiberglass work well too.

--Ethan

 

[darnold]

Thanks DDev and Ethan.

I basically spent the entire night before, and the entire morning stirring my brain wondering if i understood acoustics enough to make a decision. Since the membrane abosorbers and slot resonators are so popular around here, its kinda difficult to get my self to do something different .

Ill let you know if i think it made a difference. Thats basically the theory i had myself is that the Rigid Fiberglass is more effective because of space saving, so you can do a couple more things with it. But one question i have, is it possible to make a slot resonator with the fluffy pink stuff?

Ive got the bass traps in each of the corners of the wall, and the west wall is completely covered with 3" acoustic foam. I have 6 RPG Protraps (i think thats what they call them) on my 25 ft ceiling corners, because i have them and i know it will be alot more difficult to build a fiberglass bass trap up there. The 3" foam was at the studio when i bought it, so i decided it would be cheaper to make use of it instead of buying all new materials especially since it should do the same thing. So now ive got the bass traps, and mid/high absorbtion on the 25 ft wall.

Oh btw my room is 16x25 ft with a 7'8 ft ceiling that slopes up to 8'11. Im trying to decide the best diffusion to put on the 16 ft wall (to diffuse the longer length of the room). I think its recommended with what i have already to use a slot resonator for that wall, but im not sure if i can build those with the materials i can get, nor do i know exactly what kind of frequency span it should absorb. Any tips here would be awesome.

Thanks a ton for the help guys, its nice to finally get the acoustics done in this room.

Danny

 

[RICK FITZPATRICK]

Hey Ethan, since the thickness of the panel has a bearing on the frequency, as well as the size, and depth, I was thinking of using a sheet of plastic laminate. It is VERY thin. But also fairly rigid. Although it will bend, in one plane. What do you think? To seal it, I would silicone it to the edge for the box, and use an aluminum angle, with a layer of weatherstripping or caulk, on both flanges, and then screws to really pull up the laminate tight to the box. Seems like this would be a very effecient panel. Trouble is, WHAT frequency to go after Ha! Maybe 5 or 6 different size boxes. Hmmm, wonder where that comes from...eh?.
fitZ

 

[DDev]

Rick - If you place it in a corner you kind of get the benefit of variable depth so in theory I think it will broaden the frequency range a bit. I don't know how to calculate it, though. I think your membrane will work as long as it is stiff (are you talking about plastic laminate like what is applied to counter tops? That should work).

Danny - I wish I knew enough about slot resonators to give you a good answer, but I don't know yet. I think your insulation will be fine, however. It will probably make the box for the resonator a little deeper, but I don't think that should be an issue.

Have fun

Darryl.....

 

[Ethan Winer]

Rick,

> I was thinking of using a sheet of plastic laminate. <

For what? As a front panel for a panel trap? If so, I have no idea how that will affect the performance, whether better or worse. I do know that wood works.

--Ethan

 

[RICK FITZPATRICK]

Hello Ethan, the only reason I was thinking about using the laminate, was because I read that the thinner the panel, the lower the frequency it will absorb. There is NO wood panel other than veneer that is as strong yet thin as laminate. Hence the idea.
fitZ

 

[knightfly]

Rick, it's the MASS of the panel, not the thickness - and the higher the mass, the lower the frequency center.

Danny, Ethan's right about the rigid fiberglas - 703 rigid fiberglas is roughly 50 TIMES as dense as the "fluffy stuff" - and since absorption is partially proportional to the number of "interstices", or fibers, it would take several FEET of the fluffy stuff to equal one inch of the rigid board.

On slat resonators - the fiberglas inside widens the response curve (less peaky) and the heavy cloth right behind the slots is what makes the design work correctly. There's an online calculator here -

http://www.saecollege.de/reference_material/index.html

Click on "helmholz calculator" on the left sidebar... Steve

 

[RICK FITZPATRICK]

it's the MASS of the panel, not the thickness - and the higher the mass, the lower the frequency center.

Hello Steve, they must have let you out for good behavior huh? Ok Steve, but why would the formula specify the thickness of the panel, not the mass? At least from my memory of it. My Everest book is packed away, otherwise I'd look at it. Sometimes, these things are confusing. So what you are saying, is a 3/4" MDF panel will absorb a LOWER frequency than a 1/8" sheet of ply Wow. I would have thought the exact opposite. OK, thanks Steve. hmmm, damn, have to revise my drawings, AGAIN One of these days I'm going to have these finished....and correctly.



fitZ

 

[knightfly]

Panel trap theory - front panel is set into vibration by sound waves' PRESSURE. Panel resonates at one main frequency, the heavier the panel the slower it moves.

Moving panel causes the air INSIDE the trap to move, which then attempts to move through the rigid fiberglas (or rockwool) filling located some distance behind the front panel. The air movement is restricted in both directions through this absorbent.

The two ways a panel trap works: first, the flexing of the front panel converts some of the energy into heat. Second, the air moving through the inner absorbent also converts some of the moving air into heat.

Cool thing is, by working on pressure (as far as the EXTERNAL world is concerned) the traps can be relatively thin and be placed against a wall (no air gap necessary behind the absorber) , and they still work at lower frequencies. Then, if you want absorption in higher ranges at the same time, you can just mount an inch or two of rigid fiberglas in FRONT of the panels, with effectively no loss in efficiency at the lower frequencies... Steve

Oh, the Everest thing - I think he mentioned thickness instead of mass, but the formula uses mass. There's a CHART though, and that uses thickness of plywood. For other materials, you just weigh a sheet and divide by the area to get weight per square foot for use in the formula...

 

[darnold]

Steve - Thanks for the help. I dont really understand how 1" rigid fiberglass could pack that many more fibers then R19 insulation. And 50 times more dense? I thought Rigid fiberglass was the same stuff just compressed. My questions about the slat resonator is whether or not i can use R19 insulation to build them. I know the big factor is the vibration, so if i have the cavity stuffed with R19, there probably isnt much room to vibrate? But maybe im wrong about that? Maybe it can still be done? If Rigid Fiberglass wasnt so expensive to get around here i would definately go that route, but i need alot of it. So is 3 full strips of R19 doing a pretty good job? It sound like its working because the room is definately much tighter then before.

My next project is the North wall. I will definately need a little help on chosing what to do there.

I wish i could draw this out for you, but i dont have space to put a picture on the web right now.

My room is 16x25. The North and south wall are 16 ft and the east and west wall are 25 ft long. The ceiling is 7'11 along the east wall, then slopes up to 8'9 to the west wall. Ive got the bass traps finished now. The west wall is completely covered with 3" aurelex foam, yes i know its not the best, but its what i have right now and need it to keep costs down. I am planning on keeping the east wall bare until i think it needs some panel absorbers or not. The design is basically how they have it in most of the rooms on the www.sae.edu site, with the bass traps, mid/high absorbers along one wall. But i need to build something on the north wall that doesnt absorb the highs but reflects the parellel walls. Its recommended at SAE to use slat resonators, but im uncertain of whether i can build those or not yet. And if so, what frequencies should i have them resonate at? (250-500hz maybe?). But i was also looking for some alternatives. Maybe put some real diffusion in there?

I dont think i will need anything on the east and south wall, but unless somebody highly recommends it or if i hear something later, i will do something with them.

Please let me know what you think.

Thanks a ton,
Danny

 

[RICK FITZPATRICK]

it's the MASS of the panel, not the thickness - and the higher the mass, the lower the frequency center.

Hello Steve, man, been a while since we heard from ya.. You get abducted or something Hahaha! Ok Steve, but why would the formula specify the thickness of the panel, not the mass? At least from my memory of it. My Everest book is packed away, otherwise I'd look at it. Sometimes, these things are confusing. So what you are saying, is a 3/4" MDF panel will absorb a LOWER frequency than a 1/8" sheet of ply Wow. I would have thought the exact opposite. OK, thanks Steve. hmmm, damn, have to revise my drawings, AGAIN One of these days I'm going to have these finished....and correctly. er....I hope.

Wait a minute. This does it....I'll be right back.....

Ok, I finally got off my ass and dug out my Everest book. Been 2 1/2 years since I looked at it. Hurray!. Alright, lets see......ah.. Wow, this is wierd. I actually understand SOME of this stuff now, thanks to you and others here Steve. Hey, maybe I can make some sense of this now, compared to the first time I read it. Ha! Hmmmm, maybe I won't even have to bug you anymore with my moronic questions!. But heres one anyway Steve. What do make of this?
http://www.smr-archive.com/forum_3b/messages/1486.shtml

hmmmmm....well, I'm really feeling let down now.

fitZ

 

[knightfly]

Danny, the way they get 50 times the density is to SQUISH it a lot. Seriously. Think large industrial rolling mill, huge rolls that can squish a car into a 1" pancake... The slat absorbers could get away with using fluffy insulation, but will probably be "peakier" - IOW a narrower range of frequencies for any given set of dimensions.

Your room sounds tighter because it is - any treatment beats no treatment (unless you overdo one type of absorbent and suck too much of a particular range of frequencies out of the room) - as to target frequencies, I'll have to get back to you - room dimensions will mostly decide that.

Rick - now that you've found your Everest book, look up the formula for panel resonators and refresh your memory - you'll see that the chart shows panel thickness, but also gives the mass of each. The formula itself only cares about mass, the chart is just an example to make it easier to visualize.

As to the article on non-parallel walls - I have all Dr. Toole's papers on my hard drive, and that particular set of articles is more aimed at LISTENING rooms, as Harmon sells consumer stereo gear and audiophile level gear, other than JBL stuff.

Mr. Toole is only one of several PhD's with opinions - Dave Moulton and he would probably have some interesting discussions on that subject. Consider this - if your room is already dead enough, yet still has flutter echoes, how much more absorption would you add? Or would you rather keep it live and still get rid of the flutter? Angle the walls a bit. Want a control room that's live but without smearing of the stereo image? Angle the walls to re-direct those early reflections back to a common absorber - the reflections get taken care of, but the room is still not dull.

I totally agree with the premise that angled walls don't get rid of modes, and also that they make it nearly impossible to calculate - however, I'd rather have a symmetrical CR that I decide how dead I make it, than one where you have NO CHOICE but to hang a bunch of crap on ALL the walls if you don't want everything you hear to sound like Elvis.

Once the recording is mixed/mastered with the degree of ambience you want as part of the recording, it's not as big a deal if the listening room is relatively dead.

Live rooms - non-parallel walls = no "boingies", you place mics with your ears anyway, they don't have to be symmetrical because you're re-building your own "stereo image" in the mix anyway.

One possible analogy - a Ferrari, an SUV and a Semi will all get a person from point A to point B - The Ferrari will get you there quicker, The SUV will get you there even if it's muddy or snowed in, and the Semi will let you take your entire household along. Different vehicles for (only similar) functions. Same with sound - different characteristics depending on whether you're recording, mixing, mastering, or listening to the finished result.

Who the fuck is this josh dude, anyway? Did you ask him if Dave Moulton is a hack? How about Tom Holman; another hack? Tell Dolby labs that... Steve

 

[RICK FITZPATRICK]

Who the fuck is this josh dude, anyway? Did you ask him if Dave Moulton is a hack? How about Tom Holman; another hack? Tell Dolby

DOLBY? Why, do they sell snake oil too? I wonder if Bose and Dolby know each other?

I don't have a clue who "josh" is Steve. And I didn't ask anyone anything except here and Johns site. Too many forums, and that is exactly what you get. I just happened to run across that page in a search for small room diffusion. Which also has me bugged lately. But when I read it, it was another example of the difference of opinion that exists out there regarding many aspects of this non exact science. Too many camps. They should hoist flags so you know who's who.
In fact, this wasn't the only site that expresses this same opinion. I found a site for an international acoustics firm, which I won't link, cause its pointless, but they say the same thing as this engineer. And they were talking about studios. Hmmmm. Of course, they had a proprietary ceiling diffuser/ absorber for sale. Amongst other things. Which brings up another thing. Tex mentioned that diffusers have to be rigid, like stone or thick wood, to work. What about all these diffusion products that are thin plastic, that you pin up on a wall. I guess thats another snake oil sales pitch. Or do they work? How bout "skylines". Or any number of diffuser products on the market. Its bewildering. And yet, from what I've read, true diffusion tests and analysis are still being developed. Of course, wtf am I talking about. I'm just a homerecording enthusiast who happens to be interested in acoustics from a standpoint of "bedroom" size studio wannabe perspective, and got no business questioning any of this shit. Does as much good as critiqueing the engineers at Mercedes. Ha!

Anyway, thanks for the clarification S. I can always count on you to put things into perspective. Like I've said many times Steve, I trust your judgement on these matters.

You'd think by this time I'd done as so many people do and just get the damn studio done, right or wrong and live with it. But money and time is so hard to come by, once I get a little, I can't stand the thought of squandering either on crap. I squander enough of the time already on the bbs's, but where else am I going to waste it. At the local bar or indian casino's. I think not. So, I keep asking stuff to keep my interest alive untill such time as I have a hunk of change, and a month of time to shoot my arrow. After all, I'm a Saggitarius, so what does that tell ya. In the event that ever finally happens, I want to be prepared. Ha! Well, thanks again Steve, time for work so I better go. Or I'll never get this damn thing done.

Hey darnold, sorry your thread went this direction. Shit happens. Especially here.

fitZ

 

[knightfly]

Hey Fitz, we're cool - I'm just like you in many ways, tired of everybody thinking they have the ONLY answer - and just because someone has a PhD, doesn't mean (necessarily) that they know what they're talking about, or that it applies to everything.

There are a few things that acoustics people agree on (not many) but just because I don't agree with them (or they me) doesn't make either person a "hack" - name calling is for idiots (regardless of their degrees) who can't prove their point any other way IMO. Whether our friend Josh knows anything or not, he's an idiot if he thinks there's only one way to accomplish ANYTHING.

As to stiffness of diffusors - I would think that one with deeper wells (lower frequencies, NOT the plastic ones) should be as stiff as possible, to avoid flexing (distortion) of the wells. However, the plastic ones don't go that low in freq, so I doubt it would matter.

And I still wouldn't use much (if any) diffusion in a room that's smaller than 25-30 feet, because closer than about 10 feet from most diffusors you get too much comb filter effect with small movements, since the "diffusion" hasn't really "kicked in" yet.

Guess opinions really ARE like a-holes... Steve

 

[darnold]

Cool im glad your talking about diffusion cause ive been interested in it.

My room has a 25ft deep wall, is it worth doing some sort of diffusion or is it really not worth it? And if so, should i keep everything i record at least 10 ft away from the diffusion? This is the same wall im talking about that im having a hard time deciding what to do with.

I was under the impression there is no comb filtering involved with diffusion, thats why its as good or better than absorbtion because it doesnt suck anything out. But i remember reading about that its useless on small rooms too. Does diffusion cause more comb filtering then regular reflection then?

Dudley came to the studio yesterday and suggested i use a little diffusion on that wall which is why i decided to bring it up. Also by that wall and that half of the room, i was going to build a 15x16 ft wood platform on the floor. This way i can have a section of the room that will be a little more brighter and live sounding than the other half. But would this be too close?

I wish i do a cad of the room, and post it up here so you could have a better visual perspective of what i want to do.

Thanks,
Please let me know what you think.

Danny

 

[RICK FITZPATRICK]

Hey darnold, I've been interested in this diffusion thing for a long time. Ever since I saw a few pictures in Alton Everests book called the Master Handbook of Acoustics. I figure he knows what he is talking about. Which is why I am confused about this thing of diffusion in a small room.
Steve, look on pages 313, 314, 315, and there in black and white, is a studio, built by another acoustician, with diffusers, overhead, sidewalls, and rear walls, with a dead front end. Typical LEDE I think. Very small room. So why would an acoustition use this if it DOESN"T WORK?
Man, that is what I want to do, IF IT WORKS, cause I have a ton of MDF, and other stuff. I love that design. EXCEPT, I'd put a slat absorber at my rear closet, which is very wide, and symetrical. But I won't do this overhead and sidewall diffuser thing, IF it doesn't do anything other than lots of comb filtering. Instead, what I HAVE designed, above my console, is a 3 way MEMBRANE absorber, with some regular bass traps. You know, just regular ole 703 with fabric over the whole shebang. Here is a reflected ceiling plan of my room. On the front, is angled MDF soffets for my farfields, with a double angled MDF panel, with slots machined into them. 703 behind. This will take the place of a studio window, and will be removable, so I can still use my sliding glass door to go out onto the deck when not recording.

Listen Steve, I'm about done with my plan. I'm ready to purchase a bunch of 3 or 4" 703, and my building materials, and get on with this before I get to old to even use the MOFO> Sooo, I'm going to finally post this here plan, and elevations, and sections, and details, so you can REALLY see what I am planning on. I'll post it on a new thread, so I won't interfere with someone elses.

Steve, I would post this on Johns site, but for some reason, the attatchment option won't work. I've tried 5 times. So I'll do it here. What I was wondering, is if you will critique my plan, and advise me what you would do if it were you. If that is ok with you. I do have some unusual things in there, that may not be like some people would do, but its my room, so what the hell!!

So, let me know about that "diffuser" overhead, sidewall thing in the book. Cause I really like that. SEEMS like it would diffuse EVERYTHING around you. But what do I know. Ha! Ok, darnold, I'm done interfering on your thread. However, I will post a section drawing of this diffuser thingy just for the practice of drawing it. OK. I'm done.

fitZ

 

[darnold]

Rick - Thanks for the info on the diffusion. Yah ive also seen alot of studios with diffusion is quite small rooms, and seen quite a few with diffusion in rooms the same size as mine, so this has been very confusing to me.

Oh and dont worry at all for postin in this thread, it hasnt bugged me at all. Infact i think it puts more added material here.

Steve - Any ideas yet?

Danny