question about panel absorbers

[hollywood_steve]

I finally got the major furniture moved into my new editing space earlier today. Now that the room is pretty much the way its going to be, I'm ready to get serious about acoustical treatment. I had already run the numbers and the calcs indicated a few probable problem areas. I didn't bother testing the room until the big furniture and equipment were moved in, so that's the next step. If my testing confirms the calculations, I can expect to find a serious problem around 70hz and that means bass trapping. Space is at a premium in this very small room (120SF), so I can't give up large amounts of space to treatment; but I do have one useless corner (due to a veritcal drain pipe) where I plan on installing a corner panel absorber, similar to Fig.3 on this link: http://www.customaudiodesigns.co.uk/...membranes.html

Today's question: what is required at the top and bottom of this type of absorber? This design has been part of almost all of F. Alton Everest's books, going back to the early 60s; but he never indicated how to close off the top/bottom. I am unable to run the absorber all the way to the ceiling or the floor due to strange construction details at both the floor and ceiling. So I intend to make the absorber stop about one foot short of both the floor and ceiling. Does this need to be a sealed system with a triangular cap screwed into place and then sealed with caulk? These designs have been published in each of Everest's many books, so many of you guys must have built them at some point over the years - how did you handle the end caps?

Thanks.

[Ethan Winer]

Steve,

I suspect you'll be better off with four inch thick rigid fiberglass, two feet wide, and without a front panel or sealed enclosure.

When you add a membrane and seal the system it becomes a resonant trap that works over a narrower range of frequencies. Yes, you can target a lower center frequency this way, but at the expense of not absorbing all the other low frequencies that must be tamed too.

--Ethan

[dbluefield]

Hi Steve,

I concur with Ethan.

Ideally you could do both panels and rigid fiberglass.

Why don't you post a drawing and ask for some suggestions over at John's forum?

Best,

Dave

[jajjguy]

If you fill the sealed airspace behind a panel with fiberglass or other absorbing material, you "damp" the resonance of the system, which means you widen the effective frequency range, possibly as wide as one octave. So if you target 70Hz, you could damp the resonance and achieve absorption in the range of say 50 to 100Hz.

Of course Ethan is right that a resonant chamber acts on a narrower range of frequencies, but this isn't necessarily a bad thing. If your aim is to absorb certain bass freqs, you may prefer not to absorb high freqs at the same time. Not everyone likes the "dead" sound that can create.

[Ethan Winer]

Jonah,

Another cellist, eh?

> If your aim is to absorb certain bass freqs, you may prefer not to absorb high freqs at the same time. <

Absolutely agreed. Though with such a small room, Steve needs as much broadband absorption as possible. Small rooms also need absorption at all low frequencies, not just those related to the room's dimensions.

--Ethan

[hollywood_steve]

Small rooms also need absorption at all low frequencies, not just those related to the room's dimensions.

That was my goal with the corner panel trap described in my original post above. According to everything that I have read, the variable depth of a corner trap makes them the best solution for broadband absorbtion.

I may also add a standard rectangular panel trap on the rear wall of the room, next to the doorway (that's the only wall space left except for the area on either side of the nearfields, where I am in need of some serious diffusion to kill a classic example of flutter echo caused by two parallel walls in close proximity.) I was initially thinking of a deep (6 to 8") trap with 1/2" plywood and at least 4" of 703/705 board for the panel next to the door.

This brings up another question. All of the panel trap performance charts in the Everest books always show 1/2" plywood as the thickest panel material and the 1/2" thickness always results in the ability to trap much lower frequencies than traps with 1/4" or 1/8" panels. But I have never seen a panel design utilizing 1/2" plywood. At some thickness, the plywood has to stop acting as a membrane (given a fixed 2ft x 4ft panel size). Is 1/4" the thickest practical size or is it 1/2" or more? In theory, there is no easier way to increase the traps ability to handle lower frequencies than to increase the thickness of the front panel; but where does practicality take over from theory? (assuming the added weight is not an issue)

[Ethan Winer]

Steve,

> According to everything that I have read, the variable depth of a corner trap makes them the best solution for broadband absorbtion. <

I've never paid to have a variable-depth panel trap tested in an official lab, so I can't say for sure how low a Q such a design will be. My gut feeling is it will not be as broadband as thick dense rigid fiberglass mounted straddling the corner.

> I have never seen a panel design utilizing 1/2" plywood. At some thickness, the plywood has to stop acting as a membrane <

Yes, if you use 1/2 inch plywood you may need to make it 3x6 or 4x8 feet so the plywood will have enough expanse to flex sufficiently. I suppose you could try 3/8 inch plywood and use a larger air gap to achieve a low frequency.

Too bad you're not closer to me. I have four experimental panel traps we built and never sold. They use 1/2 inch plywood and were calculated to have a center frequency of about 50-60 Hz. But they're way too big and heavy to ship. If you're willing to drive to Connecticut to pick them up, you are welcome borrow them and try them out.

--Ethan

[hollywood_steve]

If you're willing to drive to Connecticut to pick them up

Thanks, but Connecticut was a long drive when I was living in NYC or Boston; from LA it might as well be Mars. The 6 months I lived in Groton were the longest year of my life...

Seriously, what made them "experimental"? Would you be willing to post the plans for these traps?


My gut feeling is it will not be as broadband as thick dense rigid fiberglass mounted straddling the corner.

So, you are suggesting that I place the 703/705 boards in the corner all by themselves, with nothing but an acoustically transparent fabric cover (to catch the loose fibers and make them somewhat presentable)? When you say "mounted" I'm guessing that you mean that I can't just stand the board up in the corner, it must be firmly mounted in place - correct? Please explain whether the board needs to be caulked in place or just leaned up against the wall (or some third method?)

Why do you believe that the bare rigid insulation will be better than the corner trap? Is this based on experience with a similar situation, mathematical modelling or ??? Its not that I don't believe you or that I'm questioning your position regarding the matter, I'm just curious what led you to this point of view. (Most of us just read the same handful of books that all include that same sketch of the corner panel absorber and the associated text describing it as an ideal broadband absorber for low frequencies.)

Thanks.

[Ethan Winer]

Steve,

> what made them "experimental"? <

We built them but never got around to testing them in a lab. By the time we could have done that, we had discontinued panel traps altogether in favor of MiniTraps which are a sort of hybrid between rigid fiberglass and a membrane trap.

> Would you be willing to post the plans for these traps? <

They're similar to the panel trap plans on my web site, but use 1/2 inch plywood and a deeper box. As I recall the box was 5-1/2 inches deep on one side and 6-1/2 inches deep on the other.

> So, you are suggesting that I place the 703/705 boards in the corner all by themselves <

Yep.

> When you say "mounted" I'm guessing that you mean that I can't just stand the board up in the corner <

Yes, you can just stand it up in the corner. It might look a little funky, but it will work just as well as if it were secured.

> Please explain whether the board needs to be caulked <

Rigid fiberglass is a porous paterial, so the notion of sealing anything is not applicable.

> Why do you believe that the bare rigid insulation will be better than the corner trap? <

Again, I've never built or tested a corner shaped panel trap so I can't say for sure. But all panel traps are resonant and have a center frequency. As opposed to rigid fiberglass that's broadband.

--Ethan

[hollywood_steve]

Ethan, thanks for all of the information and guidance.


regarding the corner traps:

I've never built or tested a corner shaped panel trap so I can't say for sure. But all panel traps are resonant and have a center frequency. As opposed to rigid fiberglass that's broadband.

I guess I should question whether anyone ever tested the corner traps. Because they are proposed as being broadband due to their variable depth. Unlike a typical panel trap that has a fixed depth and a fixed resonance frequency, the depth of a corner trap varies over its length and so does its resonant frequency (in theory). I wonder if this is just a concept or proven fact?

Any body have experience with these things?

[Tim Farrant]

Steve, don't use plywood or such a rigid material, use something "limp". The efficiency of a limp mass bass absorber is much better than that of a rigid one. The frequency to which you tune it is also more accurate as the formula does not take into account the "stiffness" of the panel, only the "mass".

You can get various types of material for the job. In NZ here we have a product called Acoustop which is a barium loaded rubber matting used for lining the inside of boat hulls and trucks to reduce LF sound transmission. It's flexiable but heavy. I think Auralex make a similar product. You can also use vinyl flooring type materials or butynol sheet, but you need to know (or measure) the weight (mass).

Use the same formula they give on the Custom Audio Designs page. The other very important thing is that the area of the the trap has to be much smaller than the wavelength of the frequency you are trying to absorb. Make boxes of varying depth to absorb across the band, but generally I would work out the room modes and design several absorbers based on those frequencies, in this way you will cover the bass band with the peak of absorbtion at your problem frequencies It is important that the box/cavity behind is airtight too.

This type of panel absorber is only good up to 200Hz max, because the size begins to match the wavelength of sound, plus in the average size room (not sure what your size is) the modes start become quite dense above this frequency anyway.

I have treated many rooms with limp mass absorbers with excellent results. Great for pulling out a real problem freq. Good luck with it.

Tim

[7rojo7]

even just changing the geometry of the room will help, I would look to evade having any 90° angles except for at the junction from the wall to the floor. You can hang these panels you'll be making from the ceiling about a foot or so from the wall and pull them towards the wall and attach them there, the attachment wouldn't have to be rigid. You could even hang your corner traps from the ceiling, it hypothetically would improve their performance and it shouldn't take away much space at all. I think that Buzz is right on with the material choice, and rigid fiberglass works well by its self, with chicken wire you can do wonders, the fabric is essential because the fiberglass is strange with the high end.
I have a whole back wall in 1/8" luan covered with 1" cork that acts as a 8'x12' panel absorber (well it has a door in the middle) inside the framing I've hung many PVC tubes of different lengths and diameters and sprayed with this stuff they use for car undercoating to help dampen resonances inside the frame. I have another wall that is all triple pane glass which is about 15'x8' which does an nice job of absorbing bass, it has glass doors in it too. I killed all the corners in my place starting from about 4' high and left the construction open at the top, on my ceiling I have cork pannelling or my own version of a prime root diffusor which just long pieces of different sized timbers cut in a wavy pattern I developped myself which completely kills any flutter or hard reflections with out having that dead dry sound. All the wall to ceiling corners angled out at 60° and have cork or the diffusors applied to them
I went to survey a space that will one day be a studio that I'm designing for someone and by all logical reasons it would have had to have a very lively slap and boomines to it, but it didn't. It was a picture frame workshop and it had long bars of picture framing materials lined up in a chaotic manner against the walls and this helped to kill the slap. Interesting concept with very little mass. I've tried it now on a virgin live space with a problem, I put up a lot of sticks against the wall and it helped alot. the I-Ching method. Hass kickers with a diffusing surface could be in order too, in small spaces you may not want to kill everything from behind as this is an unnatural listening experience for humans and animals and can cause listening fatigue. your mission is to turn sound into heat for absorption but there are other things to consider too. Creating alot of differently timed early reflections around the speakers can help the hi end more than just killing it, it's more about how the brain percieves sound than what an RTA of the space says. In my space I find that I don't have to keep reaching for the trouble knob on my mixes, which is something that can happen in a too dead of a space. break a leg

[Ethan Winer]

Steve,

> I guess I should question whether anyone ever tested the corner traps. <

This is a huge problem with DIY acoustic projects in general. I see all kinds of stuff ranging from great to incredibly lame, but nobody ever tests their plans because it's so difficult and expensive. When I published my panel trap plans in 1995 I had no clue about the absorption specs, though I knew they worked well because I'd built so many in a lot of different studios. Since then I paid to test panel traps, when I used to sell them, so now I know for certain how well they work.

> they are proposed as being broadband due to their variable depth. <

As far as I can tell the variable depth is irrelevant, and just makes it more difficult to calculate the center frequency. A panel trap is basically a shock absorber for low frequency sound waves. It has a mass (the panel) and a spring (the air inside). So I assume the air has some amount of springiness, but the shape of the cavity should not be a factor. That is, there can be only one sum total of springiness. Versus corner mounted fiberglass or foam where the varying depth does broaden the absorption curve. But porous absorbers work on an entirely different principle than panel traps.

Again, this is all "how it seems to me," and it's not based on anything more scholarly than that!

--Ethan

[jajjguy]

Originally posted by Ethan Winer
As far as I can tell the variable depth is irrelevant, and just makes it more difficult to calculate the center frequency. ... So I assume the air has some amount of springiness, but the shape of the cavity should not be a factor. That is, there can be only one sum total of springiness. Versus corner mounted fiberglass or foam where the varying depth does broaden the absorption curve. But porous absorbers work on an entirely different principle than panel traps.

Quite right. With a resonant absorber, it is the total volume of the trapped airspace that matters, not the shape. With porous absorbers, the actual distance from the wall matters.