Questions for Ethan(and anyone else for that matter)

Last week I sent a mail to Ethan with some questions regarding panel absorbers, and he suggested I put up a public thread about it for anyone else to participate in, so here goes!

Hello Ethan, I've read some stuff about panel absorbers but need clarification regarding some of it. According to Rod Gervais you could remove the layer of insulation behind the panel trap to decrease the Q value of the trap, but increase isolation around the center frequency, but quoating Knightfly: "Generally; the air gap between the front panel and the absorbent is mandatory for the unit to act as a panel absorber(un-perforated, that is) - behind that, doing a complete fill front to back tends to broaden the traps response, but also lessen the absorption by lowering the Q of the trap." What's up with this? Am I always supposed to have one layer of insulation close to the front face for the panel to function properly, or could I remove it for better attenuation around the center freq on behalf off more narrow Q-value? Also, I wonder about density and weight of the front panel and the depth of the trap, which would yield the highest amount of attenuation? Lower density/thinner front panel together with larger air-gap or the opposite? I read somewhere on this forum about panel traps needing a lot of energy to start to move and correction to take effect, which would mean that I have to have a lot of volume for the trap to start to vibrate and the attenuation to take effect, what do you think about this? simply put: What yields best attenuation, large air-gap with lower weight panel compared to small air-gap with high-weight panel? Also, I wonder if I could replace the MDF/plywood panel with drywall panel, or if wooden products is needed for the best effect?

Osse wrote:

What yields best attenuation, large air-gap with lower weight panel compared to small air-gap with high-weight panel?

The best way to build those panel traps is exactly as shown in my plans. 8) Seriously, there may be improvements one could make, but the plans as shown are known to work well. Doing anything else risks having worse performance and not even knowing it. I can tell you that removing the insulation from behind the front panel is not correct. That will raise the Q (bad) and also reduce the absorption (worse).

I wonder if I could replace the MDF/plywood panel with drywall panel, or if wooden products is needed for the best effect?

Again, the only way to know if that will work is to build at least four traps and compare them (measure the room) to four other traps made with wood. If you do that, let me know what happens. :D --Ethan

Ethan Winer wrote:

Osse wrote:

What yields best attenuation, large air-gap with lower weight panel compared to small air-gap with high-weight panel?

The best way to build those panel traps is exactly as shown in my plans. 8) Seriously, there may be improvements one could make, but the plans as shown are known to work well. Doing anything else risks having worse performance and not even knowing it. I can tell you that removing the insulation from behind the front panel is not correct. That will raise the Q (bad) and also reduce the absorption (worse).

I wonder if I could replace the MDF/plywood panel with drywall panel, or if wooden products is needed for the best effect?

Again, the only way to know if that will work is to build at least four traps and compare them (measure the room) to four other traps made with wood. If you do that, let me know what happens. :D --Ethan

excuse me if I'm being blind, but I'm searching your site for the product, but I can't find any. Where is the plans you are talking about? Is it availiable to the public? Would be cool to take part of! Thanks

Osse wrote:

Ethan Winer wrote:

excuse me if I'm being blind, but I'm searching your site for the product, but I can't find any. Where is the plans you are talking about? Is it availiable to the public?

I have two web sites, my personal site which has DIY stuff, and my company's site which sells commercial products. The plans for wood panel bass traps are in this article: Build a Better Bass Trap There's a lot of other information in my Acoustics FAQ and Articles pages. My company is RealTraps, and there's a lot of non-sales acoustics advice on that site too: RealTraps Articles RealTraps Videos --Ethan

Cool Ethan... thank you very much for your effort! I see you use 1/4" polywood, which means approx 0,63 cm thick, the sheets i'm looking at is about double the thickness, as the frequency I'm intending to hit is 50 hz, this would decrease the depth of the trap. I wonder, will this increased thickness also bring more resistance from the trap, not being as effective as a 1/4 sheet? It's more mass to move... what do you think? Should I increase the depth and use 1/4 plywood or am i good to go?

Osse wrote:

will this increased thickness also bring more resistance from the trap, not being as effective as a 1/4 sheet? It's more mass to move... what do you think?

The problem isn't the mass, which is required for the mass/spring system to absorb. But the stiffness may be an issue when a panel that thick is only two feet wide. Maybe if it were 4 by 8 foot sheet? I honestly don't know because I never tried thicker panels.

Should I increase the depth and use 1/4 plywood

That seems like a better choice, but I never tried that either! --Ethan

Thanks for your effort Ethan! according to your "build a better basstrap" article you seem to use 1/4" thick plywood for your installations, and the example in rods book uses 1/4 thick. 1/4" thick, 5.7" deep = approx 50 hz center frequency 1/2" thick, 2.75" deep = approx 50 hz center frequency What would yield best absorption coefficient? I also think 1/4 feel a little light for this duty, 50 hz is a lot of energy... What could I expect as absorption coefficient of a properly built trap? I read the SAE site and they had a sheet about absorption coefficient, but only down to 125 hz. In one example it was a 3/16" over a 2" deep airspace, and IF the plywood density would be as same as mine, it would yield a center frequency around 120 hz, at 125 hz, it had an effectivity of 0,42. Is this the kind of effectivity I could expect from a properly built trap? On the other hand, they seem to glue the insulation directly towards the panel, and together with some other obviously faulty statements one could believe the test was faulty... I've got a rather big peak in the 50 hz register, so I'm thinking about building a pretty big trap with two sheets together, making it like 8x8'. Here is a plan, at the perimeter(red line) i'll screw it to studs, but what about the green line, where the sheets meet each other? I'm thinking about just gluing them together with a tight seal, which I guess would yield greater effectivity due to more moving mass, as if I would screw them to a stud in the middle too, it would create more static area which aren't moving. What do you think? Thanks!

Osse wrote:

What would yield best absorption coefficient? I also think 1/4 feel a little light for this duty, 50 hz is a lot of energy..

They will yield about the same absorption. The absorption is result of the sum of the mass and the depth of the air. Either will work the same. Andre

AVare wrote:

They will yield about the same absorption. The absorption is result of the sum of the mass and the depth of the air. Either will work the same.

I have to add a qualifier to that advice, though I don't know the best way to optimize the mass / spring ratio. Maybe you do? There is an ideal balance between the mass and spring. I believe it's when both have the same impedance at the center frequency. The low-bass panel trap on my web site uses 1/4 inch plywood with a 4-inch gap to give a center frequency around 100 Hz. When I tested traps of that design at IBM's lab, the absorption peaked at 100 percent. So we can take that as known to work efficiently. You can lower the center frequency one octave by using a gap twice as large. But the tuned system will not be as efficient. As an extreme example, imagine 1/4 plywood with a 40-inch gap. It should be intuitively obvious that this will not absorb 10 Hz efficiently. :D A tuned LC (inductor / capacitor) circuit is very similar to a mass / spring absorber. You can tune the circuit to a wide range of frequencies by changing either the inductor, the capacitor, or both. But the Q and efficiency also vary. With LC electronic circuits it's easier (for me) to determine when the inductor and capacitor impedances are the same. I'm sure there's a way to do this with panel traps! I just don't know how. --Ethan

Thanks guys, now things is getting interesting! Ethan, if the 1/4 inch plywood together with a 4" airgap produces a center frequency at 100 hz, it must be rather low-weight plywood right? I'm using the formula from Rods book... F = 170/sqrt(m * d) Do you use the same? I'll get a center frequency of about 60 when using the same with the plywood from around here... 17.6 kg/sheet... 12x1200x2440 mm I wonder what the perfect relationship between air gap and panel density would be... I'll have to build a 5,7" airgap if i'll use half the density of the plywood compared to the sheet described above... which actually feels rather good, don't you think? compared to about 3 cm with the 17.6 kg sheets. The trap that you measured 100% effectivity, what was the spring depth? All these decisions... :lol:

I've just been at the place where they sell rockwool. they only have 50 mm and 100 mm thickness of both 65 kg/m3 and 45 kg/m3 alternatives, but if I go up in density, to 100 kg/m3 or even 140 kg/m3 they have thinner alternatives. Now I wonder, if i'm about to capture a 50 hz wave, could I use and even benifit from a very heavy panel? What is the recomended thickness of the internal absorber in this kind of trap? If i'll use 2/4" thickness, I'll only have 76 mm trap depth, but I've read around that you should keep some air-space inside the trap for better performance, rather than filling it totally with insulation, what do you think about this Ethan?

Ethan Winer wrote:

It should be intuitively obvious that this will not absorb 10 Hz efficiently. :D

Intuition and acoustics do not go together. :P Andre

Osse wrote:

Ethan, if the 1/4 inch plywood together with a 4" airgap produces a center frequency at 100 hz, it must be rather low-weight plywood right? I'm using the formula from Rods book... F = 170/sqrt(m * d) Do you use the same? I'll get a center frequency of about 60 when using the same with the plywood from around here... 17.6 kg/sheet... 12x1200x2440 mm

When I built my first wood panel bass traps I didn't do any calculating. This was in the mid 1970s when I hired an acoustic consulting firm to advise me on a large pro studio I was building. They showed me how to build those traps, and that's how we built them. It wasn't until years later that I decided to get into this a little deeper. In 2003 I built some "portable" traps using the same dimensions, and tested them in a lab. That's how I know how well they absorb and at what center frequency.

The trap that you measured 100% effectivity, what was the spring depth?

That was for the traps in my wood panel bass traps article (LINK). So the 1/4 panel was used with a cavity depth of 4 inches (slightly less due to wood dimensions), and the 1/8 inch plywood had a 2-inch depth. --Ethan

Osse wrote:

Last week I sent a mail to Ethan with some questions regarding panel absorbers, and he suggested I put up a public thread about it for anyone else to participate in, so here goes! Hello Ethan, I've read some stuff about panel absorbers but need clarification regarding some of it. According to Rod Gervais you could remove the layer of insulation behind the panel trap to decrease the Q value of the trap, but increase isolation around the center frequency, but quoating Knightfly: "Generally; the air gap between the front panel and the absorbent is mandatory for the unit to act as a panel absorber(un-perforated, that is) - behind that, doing a complete fill front to back tends to broaden the traps response, but also lessen the absorption by lowering the Q of the trap."

Osse, if you are going to say I said something - please quote what I said the same way you quoted Steve? In my book I clearly state the following - I will include the entire paragraph and add underline for the most meaningful part.

The panel shown previously in Figure 9.9 has a center frequency of 88.64 Hz, based on the densities calculated in Figure 9.11. If you were to deepen the trap by 2" (substitute 1x8 for the 1x6 frame), the center frequency would drop to 75.84 Hz. Likewise, if you were to leave the frame as is and substitute 1/8" plywood for the 1/4" plywood, the center frequency would raise to 126.18 Hz. Also, understand that the placement of the insulation behind the panel gives you attenuation that covers about one octave around the center frequency (half above—half below). If you do not install the insulation, the panel frequency tightens to roughly one half octave (one quarter above and below).

[/quote] That is taken from page 178, when speaking specifically about panel traps. The narrower the band - the higher the Q value.......... thus I stated the exact opposite of what you claim I said. Also - nowhere did I state that you increase the isolation - only that you increase the Q. Please don't tell people your interpretations of what I or anyone else (for that matter) say - just quote us- the people who are going to answer your questions will understand what we mean even if you don't. Sincerely, Rod

I'm sorry Rod... two out of two mistakes by me :oops: I thought that you wrote this... should have checked it up before writing tho. http://www.johnlsayers.com/phpBB2/viewtopic.php?f=3&t=408&hilit=panel+trap Also, I mixed up the q-values, upside down it went in my head..! :roll:

rod gervais wrote:

Osse wrote:

Last week I sent a mail to Ethan with some questions regarding panel absorbers, and he suggested I put up a public thread about it for anyone else to participate in, so here goes! Hello Ethan, I've read some stuff about panel absorbers but need clarification regarding some of it. According to Rod Gervais you could remove the layer of insulation behind the panel trap to decrease the Q value of the trap, but increase isolation around the center frequency, but quoating Knightfly: "Generally; the air gap between the front panel and the absorbent is mandatory for the unit to act as a panel absorber(un-perforated, that is) - behind that, doing a complete fill front to back tends to broaden the traps response, but also lessen the absorption by lowering the Q of the trap."

Osse, if you are going to say I said something - please quote what I said the same way you quoted Steve? In my book I clearly state the following - I will include the entire paragraph and add underline for the most meaningful part.

The panel shown previously in Figure 9.9 has a center frequency of 88.64 Hz, based on the densities calculated in Figure 9.11. If you were to deepen the trap by 2" (substitute 1x8 for the 1x6 frame), the center frequency would drop to 75.84 Hz. Likewise, if you were to leave the frame as is and substitute 1/8" plywood for the 1/4" plywood, the center frequency would raise to 126.18 Hz. Also, understand that the placement of the insulation behind the panel gives you attenuation that covers about one octave around the center frequency (half above—half below). If you do not install the insulation, the panel frequency tightens to roughly one half octave (one quarter above and below).

That is taken from page 178, when speaking specifically about panel traps. The narrower the band - the higher the Q value.......... thus I stated the exact opposite of what you claim I said. Also - nowhere did I state that you increase the isolation - only that you increase the Q. Please don't tell people your interpretations of what I or anyone else (for that matter) say - just quote us- the people who are going to answer your questions will understand what we mean even if you don't. Sincerely, Rod[/quote]

Ethan Winer wrote:

AVare wrote:

They will yield about the same absorption. The absorption is result of the sum of the mass and the depth of the air. Either will work the same.

I have to add a qualifier to that advice, though I don't know the best way to optimize the mass / spring ratio. Maybe you do? There is an ideal balance between the mass and spring. I believe it's when both have the same impedance at the center frequency. The low-bass panel trap on my web site uses 1/4 inch plywood with a 4-inch gap to give a center frequency around 100 Hz. When I tested traps of that design at IBM's lab, the absorption peaked at 100 percent. So we can take that as known to work efficiently. You can lower the center frequency one octave by using a gap twice as large. But the tuned system will not be as efficient. As an extreme example, imagine 1/4 plywood with a 40-inch gap. It should be intuitively obvious that this will not absorb 10 Hz efficiently. :D A tuned LC (inductor / capacitor) circuit is very similar to a mass / spring absorber. You can tune the circuit to a wide range of frequencies by changing either the inductor, the capacitor, or both. But the Q and efficiency also vary. With LC electronic circuits it's easier (for me) to determine when the inductor and capacitor impedances are the same. I'm sure there's a way to do this with panel traps! I just don't know how. --Ethan

Ethan, I've been thinking. If the 1/4" thick plywood panel together with a 4" deep spring yields approx 100 hz center frequency according to your tests, it doesn't hold up to the formula presented in Rods book.(F = 170/sqrt (m x d)). I'm calculating on a 7 mm(approx 1/4" thickness) together with 5" airgap, and i'm achieving around 50 hz, how could that 1" depth yield a lowering of the center frequency of 50 hz? must be something strange going on here.

Osse wrote:

how could that 1" depth yield a lowering of the center frequency of 50 hz?

I have no idea, but 1) I'm confident in my empirical testing, and 2) in theory you double the cavity depth to lower the frequency by one octave. Maybe the disparity is calculating the weight of plywood? --Ethan