The line between an absorbing panel to an extra membrane

Hello guys There is something I'm still confused about, in regard to panel traps for the low frequencies. There is the line between a good vibrating panel that absorbs to something like a window that vibrates but acts like a membrane. Where is that line drawn? Is it dependant of size? (is a gybsom wall with porous material inside bad, but a sealed gybsom panel with porous material inside is good?) Is it SPL dependant? (for example, would you use the panel trapping for both a studio and a rehersal room?) Does every material that vibrates in a room and has porous damping (in order not to project back) is effetive for the frequencies it vibrates with? Is it certain that if you don't use porous damping inside the trap, then it will act as a membrane instead? Thanks guys, I appreciate it. Adam

the panel is a membrane, the window (depending on how its constructed) is effectively an opening where sound leaves and never returns. gypsum (GWB) is generally not a good panel for this purpose - plywood or MDF in thinner depths is likely a better choice, unless perhaps you're using 5mm GWB... check on another thread here as there was quite a discussion a couple of weeks back regarding membrane/panel traps.

The panel trap formula does not care for material, but for the mass and air volume. I have compared the reaction of a 60x120 gybsom panel trap to a chipboard panel trap, both has almost identical weight, and both vibrate almost at the same manner. I've made gybsom traps and I can say that the bass response has evened out much to the better and spreads more evenly through the room. From that experience, I have just the slightest doubt I went wrong with doing it, but still I have these other questions about the other parameters. PS, when I said window, I spoke of a glass window my friend, I can't vibrate the opening of the window :D

gullfo wrote:

the panel is a membrane, the window (depending on how its constructed) is effectively an opening where sound leaves and never returns. gypsum (GWB) is generally not a good panel for this purpose - plywood or MDF in thinner depths is likely a better choice, unless perhaps you're using 5mm GWB... check on another thread here as there was quite a discussion a couple of weeks back regarding membrane/panel traps.

Just some addition, Sorry Glenn, but I find this a bit confusing. Every membrane damps, a window pane is a membrane, and the panel of a paneltrap is a membrane. They vibrate in themselves which is related to panel modes which are reflections in the panel itself. If you speak about a window you must distinguish between a single pane or a double leave pane (2 panes as in thermal glazing). A panel trap is meanly used in front of a reflective backing, which causes another resonance (the MSM, mass-spring-mass resonance, which is used for absorption). A double leaf pane will cause exact the same kind of behavior. If you see it as a single pane, what goes out is defined by the TL (transmission loss or sound insulation). When sound falls on that pane it starts vibrating, and will transform this vibration back into sound and radiate it out and inwards. This means that that window pane will radiate as much sound back into your room as it will radiate outwards. This is not some kind of 1-directional thing. The way you can use both (panel/pane) for absorption depends on the TL and internal damping of the panels. When the TL is high, a lot of the sound is reflected back. The sound that goes out (window pane), expressed in ratio is very little. If that pane shows a TL of 20 dB only 1% of that sound goes out. With 30 dB only 0.1%. The rest is absorbed or reflected back. Also a panel trap will radiate sound back into the room. Hence the main difference in your question between both, assuming you speak about a single leaf window is that the panel from the trap is mostly used in front of a backing, triggering another absorption principle (MSM) than a window pane which you use free in space. But once a double pane it becomes a typical panel trap as well. If you should use such a thin window pane in front of a wall it should also become a panel trap. If a panel vibrates it will radiate as much to both sides, whether that's a window pane or a wooden panel. How thick or heavy a panel should be, or can be to absorp as a resonator depends on the targeted frequency and on its TL. By definition a high TL will make the panel less easy vibrate (mass-law).

Let me get it straight, Eric. A transmission loss is how much the sound is not transferred to the panel or passed on to what's behind it, but reflected back into the room, which corresponds to a STC value? What are ways to achieve low TL value? Suppose the panel vibrates easily, the moment you have the mass-spring-mass principal working, there's no risk of harming the sound, instead of helping? How does using (or not using) porous absorbing enter this equation? Suppose you got a material that vibrates easily with a wide range of frequencies, even without the mass-spring-mass idea of a panel trap, would you use it that way?

Adam: What has been confusing me in this thread is your writing "good" and "bad" or "harmful" effects. All surfaces do something. Whether it is good or bad depends on what you want to achieve and if you have taken into account the acoustic effects of the surfaces. Would you please expand on what you mean by good and bad effects? Mostly good: Andre

AdamZuf wrote:

1) Let me get it straight, Eric. A transmission loss is how much the sound is not transferred to the panel or passed on to what's behind it, but reflected back into the room, which corresponds to a STC value? 2) What are ways to achieve low TL value? 3) Suppose the panel vibrates easily, the moment you have the mass-spring-mass principal working, there's no risk of harming the sound, instead of helping? How does using (or not using) porous absorbing enter this equation? 4) Suppose you got a material that vibrates easily with a wide range of frequencies, even without the mass-spring-mass idea of a panel trap, would you use it that way?

1) You must be careful using the notion STC. STC is a WEIGTED single number Transmission Loss rating. Different walls having the same STC can show quite different frequency related TL values. Hence when speaking about the physical side of things don't use a general notion as STC which is based on a pragmatic defined weighting curve and calculation method. How to calculate STC: http://forum.studiotips.com/files/STC_MTC_OITC_RW.doc 2) Lower mass, lower internal damping. But this then can be contradictory with the mass needed to obtain a certain low frequent MSM. Hence it doesn't become better just by using lower mass, but it's a combination of interacting parameters. But it should be clear that a heavy brick wall is a poor solution as panel in a panel trap (:) unless it acts as the reflective backing of course). For the diffusers in a reverb room, in order to be reflective enough, and not influence measurements one defines the mass/m2 at >= 5 kg/m2. This then is based on the standardized measurement range of >= 100 Hz (1/3 oct. band). 3) There is a risk of harming the sound.

1. It can be too narrowbanded, wrongly tuned, too broadbanded, ..... Hence whatever you make must be targeted to obtain a goal. If what you make does something else than what you intended it to do it's no good.
2. If the internal damping is too low and the TL is low, hence it does vibrate strongly with little internal damping, it can cause a secundary reverb given back to the room and can cause coloration. Strange is that often is assumed that since things are measured in a lab (as e.g. panel traps) and they get official absorption reports everything should be OK. But that idea isn't necessarily true since in a lab this phenomenon can be hidden, since the lab is build to have high reverb time (reverb room), while in practice it will be used in spaces with very short reverb time. Hence the reverb in the panel itself can be larger than the reverb of the e.g. control room, causing this secondary reverb tale, which indeed can be disturbing. I've seen, on more than one occasion referred to light single window panes as being good for low frequencies since that sound should go out. In my previous post I made clear that isn't really or hardly the case (is caused by not being used to logarithmic reasoning). Yet such a lightweight pane if poorly mounted can often cause such disturbing secondary reverb tale.

4) That's a hard to control property, with mostly relative low efficiency. But in fact one uses this in practice (in large quantities). See the RT curve in a drywall room versus the same in a concrete room. Check the RT in standard steel buildings (with this lightweight steel paneling) where often/mostly the low frequent RT is lower than the mid frequent RT.

AVare wrote:

Adam: What has been confusing me in this thread is your writing "good" and "bad" or "harmful" effects. All surfaces do something. Whether it is good or bad depends on what you want to achieve and if you have taken into account the acoustic effects of the surfaces. Would you please expand on what you mean by good and bad effects? Mostly good: Andre

The goal is to even out standing waves and to flaten the response.

Eric, I have found a certain kind of cardboard in the hardware shop that vibrates very easily to a very wide range of frequencies, when put against porous material. It also vibrates differently on different parts of the panel, responding to the peaks at that specific area. I also found that if I make a sealed gybsom panel trap and use the cardboard as backing (insulation doesn't touch the gybsom), it makes the gybsom vibrate as effective as if there was a wall behind it, and at the same frequencies, with mild deviation which I believe is more because the comparison was not very prcise, location wize, and the response in a room change if you just move the mic a few centimeters. While the gybsom vibrates to more narrow band, according to the panel depth, the cardboard covers the other frequencies. This trap can be used from both sides: The side facing the room vibrates more effectively then the other, then it's a question what you want to achieve. The cardboard starts to reflect just above 5kHz back into the room, making such a trap (with cardboard facing room) also avoids a too dead sounding studio. Does this make sense? You talked about the low efficiency of such a low TL, broadband panel, which is clearly what the cardboard is, however, the porous material is in contact with it, damping the motion immediatly. Please refer to that in your answer :) Thanks for the advice Adam

AdamZuf wrote:

[quote="AVare" Would you please expand on what you mean by good and bad effects? Mostly good: Andre

The goal is to even out standing waves and to flaten the response.[/quote] Describe the room and the concerns about isolation! Andre

Andre, Although my questions came up when designing my studio, take them as global questions- they don't ask for a complete solution, just to understand how a single modular trap acts.

Eric_Desart wrote:

Sorry Glenn, but I find this a bit confusing. Every membrane damps, a window pane is a membrane, and the panel of a panel trap is a membrane. They vibrate in themselves which is related to panel modes which are reflections in the panel itself. If you speak about a window you must distinguish between a single pane or a double leave pane (2 panes as in thermal glazing).

didn't mean to confuse you :shock: yes understood the window is a panel/membrane, but compared to a massive wall its effectively a hole (unless of course you take measures such as you describe with multiple glazing, heavy windows etc). I'll try to be more precise going forward :oops:

Albert Einstein wrote:

In the matter of physics, the first lessons should contain nothing but what is experimental and interesting to see. A pretty experiment is in itself often more valuable than twenty formulae extracted from our minds.