Size/Tuning a Panel Resonator

My room is in pretty good shape but I've got a large hump at 135Hz. I'd like to build a panel resonator to deal with this hump. Could someone direct me to where I'd find resources to build the appropriate-sized panel resonator, using Ethan Winer's design? From my mix position, I'm looking at an 8' wide wall, 7'9" high. My left wall is 11' long. My right wall is 15' long. The back wall is parallel with the front wall, and is about 20' long. Here is where I've positioned my traps.

I have corner traps, 24" wide--ceiling height--and 4" thick of OC703, in each corner. 3' from the front wall, 2" from the ceiling, are 2'x4'x4" OC703 traps. I have a 2'x6'x4" OC703 trap suspended 6" from the ceiling, above the studio desk, and 30" from the front wall. I have a 2'x4'x4" OC703 trap in each wall-ceiling corner, along the size walls, starting 7' from the front wall. In the right back corner I have another 2'x7'8"x4" OC703 corner trap. On the back wall, running horizontal and covering the wall-ceiling corner, I have a 2'X6'X4" OC 703 trap. Finally, running lengthwise and suspended 6" from the ceiling, I have a 2'X4'x4" OC703 trap, mounted 7' from the front wall.

Using Ethan Winer's directions, I measured my room's bass response from the mix position, using a small-diaphram condenser measurement mic. I've attached the chart. Thanks a lot. Nick

Last week most of us were certain how to build a membrane absorber. This week we're a little nervous on the topic. How do you feel about helmholtz? http://www.bobgolds.com/absorberLowPass.gif http://www.bobgolds.com/absorberContraCarpet.gif You could do an RT60 calc to figure out how much surface area.

z60611 wrote:

Last week most of us were certain how to build a membrane absorber. This week we're a little nervous on the topic. How do you feel about helmholtz? http://www.bobgolds.com/absorberLowPass.gif http://www.bobgolds.com/absorberContraCarpet.gif You could do an RT60 calc to figure out how much surface area.

Heck, I'll do whatever works! Now, how to figure out the surface area using an RT60 calculation? I'll search for it, but if anyone has a link, please pass it on. Thanks! Nick

I remember when they put some panel absorbers into Rhinoceros Studios in Sydney back in the early 80s. They were large plywood panels, maybe 6' x 3' x 3/8"and were attached over a rubber seal around the perimeter of the frame and held with securing screws at around 18"' spacing. They then attached a contact mike to the panel and set up an oscillator through a speaker. As they changed the frequency they hit the resonant frequency of the panel and the contact mike mike registered it. Apparently the frequency could be tuned by changing the tightness of the securing screws. cheers john

This week we're a little nervous on the topic.

I wonder why? :shock:

Now, how to figure out the surface area using an RT60 calculation

The best book on doing RT60 calcs is How to Build A Small Budget Recording Studio From Scratch : With 12 Tested Designs by Michael Shea and F Everest It's an incredibly easy read. There's a relationship between RT60, and square footage of absorbers. (an RT60 calc). If you're building a control room (not a recording room), then your target RT60 should be based on room volume. Just look it up on this chart. V in ft^3, V in m^3, ITU RT60 63 1.8 0.07 (shower stall - waterproof acoustical treatment required?) 1000 28.3 0.16 1250 35.4 0.18 1500 42.5 0.19 1750 49.6 0.20 (approximately 16' x 16' x 7') 2000 56.6 0.21 2250 63.7 0.22 2500 70.8 0.22 2750 77.9 0.23 3000 85.0 0.24 3250 92.0 0.24 3500 99.1 0.25 3750 106.2 0.26 4000 113.3 0.26 4250 120.3 0.27 4500 127.4 0.27 4750 134.5 0.28 5000 141.6 0.28 5250 148.7 0.29 5500 155.7 0.29 5750 162.8 0.29 6000 169.9 0.30 6250 177.0 0.30 6500 184.1 0.31 6750 191.1 0.31 7000 198.2 0.31 7250 205.3 0.32 7500 212.4 0.32 7750 219.5 0.32 8000 226.5 0.33 8250 233.6 0.33 8500 240.7 0.34 8750 247.8 0.34 9000 254.9 0.34 9250 261.9 0.34 9500 269.0 0.35 9750 276.1 0.35 10000 283.2 0.35 11000 311.5 0.37 12000 339.8 0.38 13000 368.1 0.39 14000 396.4 0.40 15000 424.8 0.40 16000 453.1 0.41 17000 481.4 0.42 18000 509.7 0.43 19000 538.0 0.44 20000 566.3 0.45 22500 637.1 0.46 25000 707.9 0.48 27500 778.7 0.50 30000 849.5 0.51 32500 920.3 0.52 35000 991.1 0.54 37500 1061.9 0.55 40000 1132.7 0.56 50000 1415.8 0.60 60000 1699.0 0.64 70000 1982.2 0.68 80000 2265.3 0.71 90000 2548.5 0.74 100000 2831.7 0.76 125000 3539.6 0.82 150000 4247.5 0.87 175000 4955.4 0.92 200000 5663.4 0.96 225000 6371.3 1.00 (approximately 100' x 100' x 22')

Thanks so much. I really appreciate the help. My school's library has the book you referenced, but I think that when I tried to read it, it'd only show me the first page of each chapter (it's only available electronically). Technology: sometimes it gets the better of me! I downloaded a bunch of Excel spreadsheet calculators, and I've been reading for the past few days about panel resonators, Helmholz resonators, and so forth. I'm not sure I have things figured out. Controversies over equations, and stuff like that, make me a little uneasy, especially since I don't have the math background to work some of it out for myself. I'd like to make some resonator panels to deal with the axial modes and place them on my rear wall. I'd like to target 70-75Hz and 140-150Hz. I've got about 8' of rear wall that is 7'9" high. My hope is to find out that if I build four panels (slat resonators) and build them to target these two frequency ranges--2 each--would I achieve satisfactory results and bring these frequencies into balance with the other low frequencies. Thanks again for the help. Nick

Be really careful there; one of the FEW helmholtz/slat calculators on the web that does NOT use the WRONG formula is posted here in the acoustics forum - and even THAT one has been questioned by greater math minds than mine. Also, I believe that the formula I used (same as Everest's books) is likely to be based on OC 703 insulation, since that was THE absorption material when it was written - I have no proof of that other than time/circumstances and "gut feel", but I think there's a good chance it's so. For more accurate calculations, you also need to know flow resistivity, etc, and this isn't covered with that calculator - others that may even LOOK identical are wrong, because they use the wrong operand in one place. Some possibly helpful previous things - http://www.johnlsayers.com/phpBB2/viewtopic.php?t=6564 And some possibly NOT so helpful :? http://www.johnlsayers.com/phpBB2/viewtopic.php?t=4252 Something else to consider - axial modes alone don't tell the whole story of a room, even a simple rectangular one; there are better calculators out there for this, one I use very regularly is modesV2, written by Jeff Szymanski and posted in the calculation tools area of studiotips - URL is http://forum.studiotips.com You need to join in order to get to the calculation tools IIRC, but do it... Steve

knightfly: His room isn't rectangular. It's L shaped. So modesV2 and http://www.bobgolds.com/Mode/RoomModes.htm and http://www.hunecke.de/english/calculato ... modes.html aren't going to help. That's why I posted the RT60 volume list above, rather than simply linking to http://www.bobgolds.com/Mode/RoomModes.htm which calculates it. BTW, those other two links were a fun re-read. :) Interesting that Eric mentioned 1DOF even in Feb 2004 -- of course I look at it with different eyes now. And will undoubtedly see with different eyes again in the future. http://www.johnlsayers.com/phpBB2/viewt ... =9966#9966 <- trap joke !

Oops, missed those extra dimensions - good catch Bob. Still, I think a VERY rough Idea of what to expect from an L shaped room might be gotten by "assuming" the extra wall (twice, once for each way) and running the modes using that assumption of two independent rectangular rooms - just a thought, with little time spent analyzing how dumb it might be :lol: Trap joke; he he, I'd forgotten that one :lol:

BTW, wavelength of 140hz is 8'. I'm thinking a floor/ceiling axial mode. Room volume = (7.75 * 8 * 15) + (7.75 * 12 * 4) = 1130 ft^3 RT60 for control room around 0.18 seconds Absorbtion to achieve ITU RT60: 323 sabins

Thanks for the help! I've joined the studiotips.com site and have the calculators mentioned. Yes, I'm going to proceed cautiously, and be sure I have a good idea of what I can accomplish before I start building. I appreciate the links, too, and will check those out soon! Nick

z60611 wrote:

External image, not preserved — original: http://www.bobgolds.com/nickdahl/nickdahl1.gif

BTW, wavelength of 140hz is 8'. I'm thinking a floor/ceiling axial mode. Room volume = (7.75 * 8 * 15) + (7.75 * 12 * 4) = 1130 ft^3 RT60 for control room around 0.18 seconds

Wow, this picture is great! Let me provide some more data to enhance the accuracy: First, if you spin the two absorbers drawn next to the ceiling absorber above the desk 90 degrees, and move them against the left and right wall, you'd have it right! Second, my rear wall is 16' long. My "room" uses 8' of the wall, and the other 8' extends into the second half of my basement living space. The connecting space is 7'4" long (parallel to the 8' extension of my rear wall) and 4'4" wide, and 7'9" high. Third, the other "room" in my basement living space is 11' x 22' x 7'9". Probably the easiest way to give better measurements is to walk you around the perimeter of the room.

My rear wall extends 16' from the corner of my right wall and the rear wall. The extension is on my left, as we make a left turn. The next dimension is 8'. We then make a right turn. The next dimension is 11'. The width of the other room is 11'. We'll now make a right turn. The next dimension is 22'. This is the length of the other room. We'll make another right turn. This dimension is 11'. Same as the other width measurement of this room. We'll make one more right turn. This dimension is 9'9". Then, let's make one final left turn. The final dimension is 7'4". We now connect to the left wall of my music room.

Again, I really appreciate all your help, and hope that the information provided here can help you learn more about acoustics as you really help me a lot! Thanks! Nick

nickdhal

First, if you spin the two absorbers drawn next to the ceiling absorber above the desk 90 degrees, and move them against the left and right wall, you'd have it right! Second, my rear wall is 16' long.

I think it's time you drew your own diagrams. You can upload your pictures to websites that you can upload pictures to for free. e.g. http://www.imagevenue.com/ http://www.imageshack.us/ And then you can use the [ img ] tag around the URL to that image when posting a message here, and it will show. Or post them as attachments here. Please try to keep them about 600pixels wide. Anyway, how about if you add 210 ft^2 of 4" 703 flushmount or air-spaced, and 20 ft^2 of absorber low pass. Placement to be determined based on: room dimensions giving axial mode lengths the same as your problem frequencies, SBIR and early reflections issues. http://www.bobgolds.com/nickdahl/RT60_nickdhal.xls The spreadsheet was done at the 1130 ft^3 room volume estimate -- which seems to be wrong now. But I haven't a clue how big your room is. Extending the spreadsheet down to 63 hz might help. I used the Absorber Low Pass because I think it'll do nice things between 63hz and 100hz particularly if it's placed on the surfaces that are giving axial mode problems, whereas leaving it out and just using 220 ft^2 4" 703 air mount would do a reasonably nice job from 125hz to 4khz without any absorber low pass. However, I gave you the spreadsheet so you could fiddle with the surface area, and include the AC/Frequency results from other things you might have in your room (carpet, slat absorbers, membrane absorbers, diffusers that also absorb, whatever), or be willing to try. BTW, the absorption coefficients (AC/frequency) that I used for the walls/floor/ceiling assume that you're on the 2nd floor with gypsum walls and a wood floor over joists. If you're in a basement with a concrete slab for a floor and bare concrete walls then those absorption coefficients that I used are WAY TOO HIGH.

Thanks for the help. Hope I haven't been too much trouble. I'll keep working with this and see what happens. Nick

nickdahl:

Hope I haven't been too much trouble

Not yet. :) Actually I had planned on doing the spreadsheet the day you asked about RT60, but it took me a while to get around to it. Don't bother surfing for RT60 instructions. Last time I checked there weren't any worth reading on the www. On the other hand if you buy that book I mentioned, they're trivial. It's easy, clear, reasonably complete, lots of examples.

I've been reading the How to build a small budget recording studio from scratch, 3rd ed. This is a good book! I learned a lot in just a couple of days. Still trying to figure these things out: Contracarpet is used to mitigate the negative effects of carpet on reverberation time. So, ceilings are the place to locate contracarpet? I'm still trying to figure out absorption coefficients for my walls. The front wall is 1/2" gypsum, in front of unpainted concrete blocks, and the front yard. The right wall is 1/2" gypsum, with a 2x4 wall space, and another 1/2" gypsum, which separate my room from the garage. The rear wall is 1/2" gypsum with a 2x4 wall space. This wall separates my room from the laundry room (8'x16'). My left wall is 1/2" gypsum, with a 2x4 wall space, which separates my room from my house's stairwell. How would I calculate absorption? When I use my absorbers (4" OC703) at a 45-degree angle, diagonally across the wall and ceiling, how do I allocate Sabins between the wall and ceiling? My room is small, but my gear is big (amp head + 4x12cab, 88-key MIDI controller, desks, computers, pedalboard, and so forth). Do I adjust RT and absorption calculations to compensate for this stuff? A lot of questions! Thanks again, Nick

nickdahl

Contracarpet is used to mitigate the negative effects of carpet on reverberation time. So, ceilings are the place to locate contracarpet?

Contracarpet can be used in the same room as carpet, but you can use other things, and you can put them anywhere in the room you want. I'd prefer to put them in a place to defeat the loudest axial modes rather than opposite the carpet. In your case, it might be that the loudest axial mode is the floor-ceiling, and if you also have carpet on the floor, then the ceiling is good place for contracarpet. Remember my rule a) use RT60 to estimate how much absorption and what kind of absorption to put in the room b) and then use other acoustic ideas (SBIR, comb filtering, axial modes, early reflections, desk reflections, spaciousness, etc) to figure out where to place them

I'm still trying to figure out absorption coefficients for my walls. The front wall is 1/2" gypsum, in front of unpainted concrete blocks, and the front yard. The right wall is 1/2" gypsum, with a 2x4 wall space, and another 1/2" gypsum, which separate my room from the garage. The rear wall is 1/2" gypsum with a 2x4 wall space. This wall separates my room from the laundry room (8'x16'). My left wall is 1/2" gypsum, with a 2x4 wall space, which separates my room from my house's stairwell. How would I calculate absorption?

The book has two examples for different kinds of walls. A single layer of gypsum wall, and a double layer of gypsum wall. Use those two make an educated guess about what values you want to use. When in doubt, lower the absorption coefficients to play it safe.

When I use my absorbers (4" OC703) at a 45-degree angle, diagonally across the wall and ceiling, how do I allocate Sabins between the wall and ceiling?

Don't do it that way. Ignore the wall percentage. Think of it as sabins in the room. You can also ignore absorption coefficients and use sabins per unit directly (unit = 4'x8'). That's what I did in my spreadsheet for you. Here's some more absorption coefficients: http://www.bobgolds.com/Sabin.htm (start with "Sabin Data text file") http://www.bobgolds.com/AbsorptionCoefficients.htm and of course, for DIY Theoretical values you can try: http://www.whealy.com/acoustics/Porous.html BTW, your target RT60 doesn't have to be flat at that one value I mentioned. There's an acceptable range by frequency. If you tell me the volume of your room, I can give you the acceptable ranges. Flat RT60 is ok too (i.e. same RT60 from 30hz to 20khz). Also BTW, corner traps are great in an otherwise untreated room. But now that you're doing RT60 calcs, you have more options.

Great! Thanks for the help. Here's what I'm working with, so far as room volume goes, and I'll follow along to see how you work the problem. My music area, where I do recording and a little mixing, is right at 950 cubic feet (15.33' x 8' x 7.75'). This area is connected to another small room (1875ft^3 = 22' x 11' x 7.75') by a 240ft^3 hall (7' x 4.33' x 7.75'). So, the actual room I'm in is 3065ft^3, but the space is split into two separate spaces, connected by a small hallway. What I've been wondering is whether I have to do additional measurements and treatment for the larger space. My music area is almost rectangular, except for the 4.33' x 7.75' opening in the rectangle, which empties into the other room. The entire floor (395ft^2) is carpeted. Hope the info helps. Nick

z60611 wrote:

nickdahl

Contracarpet is used to mitigate the negative effects of carpet on reverberation time. So, ceilings are the place to locate contracarpet?

Contracarpet can be used in the same room as carpet, but you can use other things, and you can put them anywhere in the room you want. I'd prefer to put them in a place to defeat the loudest axial modes rather than opposite the carpet. In your case, it might be that the loudest axial mode is the floor-ceiling, and if you also have carpet on the floor, then the ceiling is good place for contracarpet. Remember my rule a) use RT60 to estimate how much absorption and what kind of absorption to put in the room b) and then use other acoustic ideas (SBIR, comb filtering, axial modes, early reflections, desk reflections, spaciousness, etc) to figure out where to place them

I'm still trying to figure out absorption coefficients for my walls. The front wall is 1/2" gypsum, in front of unpainted concrete blocks, and the front yard. The right wall is 1/2" gypsum, with a 2x4 wall space, and another 1/2" gypsum, which separate my room from the garage. The rear wall is 1/2" gypsum with a 2x4 wall space. This wall separates my room from the laundry room (8'x16'). My left wall is 1/2" gypsum, with a 2x4 wall space, which separates my room from my house's stairwell. How would I calculate absorption?

The book has two examples for different kinds of walls. A single layer of gypsum wall, and a double layer of gypsum wall. Use those two make an educated guess about what values you want to use. When in doubt, lower the absorption coefficients to play it safe.

When I use my absorbers (4" OC703) at a 45-degree angle, diagonally across the wall and ceiling, how do I allocate Sabins between the wall and ceiling?

Don't do it that way. Ignore the wall percentage. Think of it as sabins in the room. You can also ignore absorption coefficients and use sabins per unit directly (unit = 4'x8'). That's what I did in my spreadsheet for you. Here's some more absorption coefficients: http://www.bobgolds.com/Sabin.htm (start with "Sabin Data text file") http://www.bobgolds.com/AbsorptionCoefficients.htm and of course, for DIY Theoretical values you can try: http://www.whealy.com/acoustics/Porous.html BTW, your target RT60 doesn't have to be flat at that one value I mentioned. There's an acceptable range by frequency. If you tell me the volume of your room, I can give you the acceptable ranges. Flat RT60 is ok too (i.e. same RT60 from 30hz to 20khz). Also BTW, corner traps are great in an otherwise untreated room. But now that you're doing RT60 calcs, you have more options.

Nickdahl

So, the actual room I'm in is 3065ft^3, but the space is split into two separate spaces, connected by a small hallway

I don't know. But there's a thread about two rooms with a doorway/hallway over at studiotips and what happens. I doubt there's any easy rules for that relative to RT60 targets. You could try the smaller room volume for 500hz and up, and the grand total big room volume for 200hz and down --> does this make sense based on the response curves you are getting? i.e. do you think the other room is a LF reverberation chamber, and your little room is well treated at HF, based on what you measure and hear? On an unrelated note, have you considered the aurolex SpaceCoupler filling the hallway as a door dividing the two rooms? http://www.auralex.com/partscience/spacecoupler.asp There's a better link for this somewhere that shows how it sits between a lower room and an upper room.

I'll check out the sites you mentioned. I did some measurements using Ethan Winer's tones. I used my calibration mic in the larger room, pointing into it, and placed at the boundary between between the larger room and the "hallway." Here's what I found:

z60611 wrote:

Nickdahl

So, the actual room I'm in is 3065ft^3, but the space is split into two separate spaces, connected by a small hallway

I don't know. But there's a thread about two rooms with a doorway/hallway over at studiotips and what happens. I doubt there's any easy rules for that relative to RT60 targets. You could try the smaller room volume for 500hz and up, and the grand total big room volume for 200hz and down --> does this make sense based on the response curves you are getting? i.e. do you think the other room is a LF reverberation chamber, and your little room is well treated at HF, based on what you measure and hear? On an unrelated note, have you considered the aurolex SpaceCoupler filling the hallway as a door dividing the two rooms? http://www.auralex.com/partscience/spacecoupler.asp There's a better link for this somewhere that shows how it sits between a lower room and an upper room.

I made a better sketch of my room. Here's another question... Is there a formula for entering the dimensions of my entire room into some of the mode calculation spreadsheets I've been using? The room seems to contribute quite a bit in terms of LF activity, given my measurements from yesterday, so I'm wondering if I should make calculations as one "big" room? Or should I calculate modes for each of the two "smaller" rooms (ignore the hallway)? The measurements I took yesterday involved turning up my monitors to 85dB and measuring LF activity in the larger room, about 15' away from my mix position. Thanks, Nick