Panel Absorbers

So, just a quick question and a half: 1) With panel absorbers there appears to be the Ethan way and the John way (that is, insulation positioned slightly away from the panel and insulation attached to the panel, respectively). Is there a difference in effectiveness or is mounting the insulation to the panel just a step to eliminate extraneous framing for the insulation inside the sealed box? 1.5) Has anyone seen the formula for dominate panel resonance, I think it is Frequency / [mass per meter squared x cavity depth (in meters)]^.5 Does anyone know if that is correct? Just ran across it randomly on the web, and we all know if it is on the internet it must be true, ha. Thanks for all the help.

Good question. I've always glued the insulation to the panel, Ethan doesn't. Mine is easier to construct for sure. I learnt how to build mine from the guys at the acoustic laboratories at Sydney University. We divided a wall up into 2' x 2' squares on center using 4" x 2"s on end. (i.e 4" depth) We then cut 2' x 2' panels from various thickness plywoods and masonites, glued insulation to the back and attached them to the frames. BTW - I then got an artists to paint it all as a mural. ;) I've also observed where you attach the panels to the frame with a silicon seal between the frame and the panel which is then screwed lightly to the frame. You then attach a contact mike to the panel which is displayed on a 1/3 octave RTA. You then place a speaker in front of the panel and generate a sine wave, as you sweep the sine wave you will observe the resonant frequency of the panel appear in the RTA when the sine wave equals the resonant frequency of the panel. Loosening and tightening the attachment screws will alter the resonant frequency. I have no idea of what the difference between Ethan's method and mine would be, perhaps Eric has an idea here. The formula I have is thus: f = 170/sqrt(m*d) where f = center frequency of absorption, m = mass of front panel per square foot , and d = depth of cavity behind the front panel. i hope this helps. cheers john

John, I don't know for sure. What's clear to me, and confirmed by your explanation, is that you indeed can glue the absorption directly to the panel, which is often in groups referred to as to be avoided (no contact). Your adjust ability sounds attractive. Theoretically I should think that yours give a bit a less higher Q, since (depending on mass ratios) your wool will damp the panel somewhat. But that can be as well an advantage. I used the idea of dampening the panel in several manners. A very low damped panel resonator, can cause trouble in itself when having an internal decay longer than the decay (RT60) in the room itself, causing an overlapping secondary reverb tale (but starting at lower level depending on TL of panel). Such an effect can be disguised by a reverb lab measurement. Can you remember (roughly) how large the range was you could adjust the frequency, and did tightening lower or increase frequency?

John Sayers wrote:

I have no idea of what the difference between Ethan's method and mine would be

Yes, and I wish someone would test this! I was told by the acousticians who showed me how to make panel traps that the fiberglass must not touch the wood. But obviously this is not a firm rule since I'm sure your way works too! :) But maybe that shifts the frequencies somewhat? Or makes the bandwidth better (or worse)? Who the hell knows! Again, I wish someone would test a dozen panels traps both ways at high resolution (not 1/3 octave in a lab) and report the results publicly. --Ethan

well - I bought a ticket in lotto yesterday and if I win I'll get us all together and we can do the testing together. :D Eric - I wasn't there when they did the testing so I'm not sure whether tightening raises or lowers the frequency. These were big panels in a Sydney studio that were 8' x 4'. BTW the room sounded pretty awful - but that's anther story ;) cheers john

well - I bought a ticket in lotto yesterday and if I win I'll get us all together and we can do the testing together. :D Eric - I wasn't there when they did the testing, it wasn't my design, so I'm not sure whether tightening raises or lowers the frequency. These were big panels in a Sydney studio that were 8' x 4'. cheers john

John, In the equation above, is "M" measured in kilograms or lbs? Also is "D" measured in feet or meters? Thanks

f =170/sqrt(m*d) f = the center frequency m = the mass of the panel in lbs per square foot d = the depth of the air space in inches.

I really don't understand how the panel size doesn't enter the equation. Once you brace a trap I'm sure it affects all parameters, because there must be a relationship between size and corresponding wavelength. Probably the width and length should have direct or harmonic relationship to the frequency we're targeting. Furthermore, about the subject whether the wool should touch the panel- I could imagine that the broadband absorbing nature of the wool would affect the Q of the foundamental and harmonics of the product's NRC, maybe flatter when it's touching the panel. Does this make sense, guys?

Ethan Winer wrote:

I was told by the acousticians who showed me how to make panel traps that the fiberglass must not touch the wood. But obviously this is not a firm rule since I'm sure your way works too! :) --Ethan

It probably depends to what degree it limit the motion of the wood..

Got an Idea. How about the wool on the outside rather then the inside? I guess it will be less effective to stop panel vibration, but to what degree? That way upper frequencies will be absorbed as well... And by the way, if the insulation's roll is only to damp the panel's movement, why use wool anyway? The "spacer" can almost reach the wall and then have something to dump the motion, as long as it's not too tight. In the sketch you can be 3 spacers- maybe it could be less... Is this idea worthwhile, and if so, how would you layout the spacers? Maybe there's no need in spacers at all, if there's wool on the outside? I remember once I was into building speakers - there was an issue with how you brace the speakers- you should try to avoid multiples in the dimetions you create by the divisions of the panel, in order to avoid a dominating resonant frequency- in our application, it seems the other way around- maybe if the total panel size doesnt correspond to the targeted center frequency, perhaps the bracing could at least target it or from the first harmony? Food for thought, einsteins... I'm just brainstorming, but I don't really know how to calculate these sort of things :shock:

Another issue- is is ok to use a 12mm particle board and increase the depth? Or would you think it will become too resonant? (and again, the panel size issue comes into mind... darn!)

i think panel size comes into play for efficiency of the trap depending on sound levels. in theory a small 2'x2' trap could trap the same frequencies as the 8'x4' trap given the same panel density and depth (per Eric's calculation), but I would think that the 8'x4' trap would tend to be excited at far lower sound levels than the 2'x'2 panel given its area and relative "limpness" (due to have far apart the edges are) and therefore be more effective at absorbing the target frequency. maybe there's something about in the Everest's Master Handbook of Acoustics? as far as putting the insulation on the outside, you could probably add a layer of insulation over the top or even spaced out from it, but you'll still need something inside. maybe building the trap normally and use an exterior layer of absorption to control the damping of the panel?

gullfo wrote:

as far as putting the insulation on the outside, you could probably add a layer of insulation over the top or even spaced out from it, but you'll still need something inside. maybe building the trap normally and use an exterior layer of absorption to control the damping of the panel?

I don't understand what you are saying, it seems that the second sentance (which is my original idea) goes against your first... :?: Anyway, why do I need the insulation inside, if I can put it outside and damp it that way? I can also damp the panel from the inside with a bit of soft styrofoam, how about that?

gullfo wrote:

I would think that the 8'x4' trap would tend to be excited at far lower sound levels than the 2'x'2 panel

A wood panel trap is a purely linear device, unless the SPL is so extreme that the wood bottoms out causing distortion. I just addressed this in another thread, but here's an elaboration: A wood panel bass trap is a "shock absorber" for sound that's based on a mass-spring resonance. The mass is the front panel and the spring is the air trapped inside. It might seem that the total panel size is a factor, but it's not. At least not until you get to secondary effects. --Ethan

Ethan Winer wrote:

The acoustic treatment experts

I thought this type of self-advertisement was not compatible with proper discussion forum usage ?

Ethan Winer wrote:

gullfo wrote:

I would think that the 8'x4' trap would tend to be excited at far lower sound levels than the 2'x'2 panel

A wood panel trap is a purely linear device, unless the SPL is so extreme that the wood bottoms out causing distortion. I just addressed this in another thread, but here's an elaboration: A wood panel bass trap is a "shock absorber" for sound that's based on a mass-spring resonance. The mass is the front panel and the spring is the air trapped inside. It might seem that the total panel size is a factor, but it's not. At least not until you get to secondary effects. --Ethan

Ethan, Can you please elboate on what determines the depth? Another thing- you say that thr size doesn't matter- So why is there a "recommended width/height" in Acoustic Calculator? And also, there must be a minimum size. What are the secondary effects worth thinking about? And what do you think about the idea of damping from the front with insulation, and the back with styrofoam spacers, between the panel and the wall? I don't yet understand the whole "suspention" concept to the detail- for example, tightening the suspention would decrease the "release" time of the panel- I don't yet understand the time frame in which the vibration of the panel is positive and when it becomes negative. Help me sort this out (-: Also, I don't exactly follow on how the insulation (if mounted inside, like in the regular version) widens the Q or lower the center frequency. Thanks for your time, I hope I can get these points right once and for all 8)

AdamZuf wrote:

I don't understand what you are saying, it seems that the second sentance (which is my original idea) goes against your first... :?: Anyway, why do I need the insulation inside, if I can put it outside and damp it that way? I can also damp the panel from the inside with a bit of soft styrofoam, how about that?

the insulation inside is part of what causes the exchange of energy within the trap. the panel and air cavity itself does the bulk of work but without the internal damping, you can end up with ringing...

AdamZuf wrote:

Can you please elboate on what determines the depth?

The distance between the rear of the front panel and the wall the trap is mounted on.

you say that thr size doesn't matter- So why is there a "recommended width/height" in Acoustic Calculator?

The wood must be large enough to flex, but not so large that it flops around.

What are the secondary effects worth thinking about?

Being too small to vibrate freely or too large to vibrate as a single piston.

And what do you think about the idea of damping from the front with insulation, and the back with styrofoam spacers, between the panel and the wall?

The only way I've built these panels is as described in my article. If you build them that way I guarantee they'll work. Anything else and you're on your own!

I don't exactly follow on how the insulation (if mounted inside, like in the regular version) widens the Q or lower the center frequency.

This is just what happens with absorption and damping. The frequency is also shifted lower due to sound traveling more slowly through the absorption. Maybe one of the more physics-minded people here can elaborate. --Ethan

msikio wrote:

I thought this type of self-advertisement was not compatible with proper discussion forum usage ?

LOL, I like this guy! 8) --Ethan

Ethan Winer wrote:

LOL, I like this guy! 8)

You might like my new signature then....

msikio wrote:

You might like my new signature then....

Excellent. But are you really an expert, or just a legend in your mind? :oops: :) :P :twisted: Of course, this means you'll be providing lots of expert advice here now, yes? --Ethan

msikio wrote:

Ethan Winer wrote:

The acoustic treatment experts

I thought this type of self-advertisement was not compatible with proper discussion forum usage ?

Actually, John encourages product promotion on these (his) forums.

Ethan, I know what depth is, thanks :-) I was thinking 60x120cm panels. How did yo reach your size? What's the logic behind ratios? I'm thinking of using drywall panels. How about that?

lovecow wrote:

Actually, John encourages product promotion on these (his) forums.

Right, though in case it wasn't clear I was just kidding around. Everyone in the know here considers me an acoustics buffoon not to be taken seriously. --Ethan Me an expert? Hah! Not on your life!