Building panel corner traps... help please!

Hi I'm building a paneled corner trap and would really love some advice please.. I might be doing a mistake, since I can barely find any info about this... but I've already bought the materials and I quite like the look of these veneered panels, although the use is more important than the look, I know... so here we go.. Some details about the room: The room is roughly about 3.50 x 3.57m. The ceiling is slighlty slanted. On one side it's 2.48m high and on the other side 2.45m high. There's a big window on one side (I can't remember the exact dimensions but maybe about 2.80 x 1.30 m). For the 2 corner traps I will be using mineral wool Knauf Universal Slabs RS45 100mm thick, cut in triangles and stacked on top of each other, and a 6mm veneered plywood panel, on the whole height of the room. The trap will be sealed with silicone. RS45 NRC: Absorption co - efficients - S = Solid Backing - C = Cavity Thickness RS45 125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz NRC 25S 0.05 0.25 0.55 0.75 0.9 1.00 0.61 40S 0.14 0.40 0.87 1.00 1.00 1.00 0.82 50S 0.25 0.65 1.05 1.10 1.05 0.95 0.96 75S 0.50 1.05 1.20 1.15 1.10 0.95 1.13 100S 0.80 1.15 1.20 1.15 1.15 1.00 1.1 I've been reading quite a bit about bass traps/broadband absorbers but my knowledge is still very basic. The reason why I've opted for paneled corner traps is that they look nice with the veneer but also as far as I understand, sealed traps are good at absorbing bass freq. I haven't made tests to check the room response yet, but I'm planning to do them asap. I know that some high freq will be reflected by the panel but I'm not too worried about that for now... I will also be adding more 100mm thick absorbers on the walls and probably ceiling. I understand that for bass traps to work best they need an air gap. The veneered panel is 60 cm wide (2'), 2.4m high, and 6mm thick. It seems quite flexible to me... According to my calculation the triangle with the panel against the corner would be 42.4 x 42.4 x 60 cm. I can cut each slab in 16 triangles with these dimensions 30 x 30 x 42.40 cm, so that there will be an air gap all around the mineral wool triangles (see drawing). Also, I've calculated that inside the trap, it would be 50% mineral wool 50% air. So my question are: Is this a good idea? Is the ratio mineral wool/air ok? or should I have more mineral wool inside the trap? Also, so to fix the panels to the walls, I'm planning to use wood quadrants with straight edge, is that fine or is there anything better so that the panel keeps flexibility? I will post pictures once I've done it, with tests results.. Thanks a lot for any help!

What you are planning to build is not really a broadband bass trap (which is what small rooms need), but rather a tuned membrane trap (or panel trap): the panel will resonate at a specific frequency, and the trap will absorb that frequency. There will be several things going on with that trap. First, is the set of resonances of the panel itself, which is given by the equation: F = 0.45 * Vl * t * [ (r/w)^2 + (r/h)^2] Where Vl is the longitudinal velocity of sound in the partition (in meters per second), t is the thickness of the panel (in meters), w and h are the width and height of the panel (in meters) and r is harmonic number (1 for the fundamental frequency, 2 for the first harmonic, etc.). Then there's the tuning of the trap as a whole, given by the surface density of the panel and the depth of the cavity, with the equation: F = 60 / SQRT ( m * D), Where m is the surface density of the plywood (kg/m2) and D is the depth of the cavity (meters). However, in your case the cavity is not a fixed depth, but rather triangular: so the depth varies from a minimum at the edges to the maximum at the center. So it will cover that range, theoretically. You actually get a bell-shaped absorption curve, centered on the average cavity depth in the middle of the panel, and tapering off to either side. There's a paper by Kimihiro Sakagami and Masayuki Morimoto that gives more details about the theory of panel traps with non-uniform depth. Now, to design your trap properly, you first need to know where your bass issues are: what frequencies will your modal resonance issue be at? Then you tune your trap to cover those frequencies, centered on the worst offender. That would probably work, but then you'd have the problem of reflections. While that trap would absorb bass pretty well in the range set by the above equations, it would be a darn good reflector for mid and high frequencies. And since the trap will be placed diagonally across the rear corners, you'd have to angle the panel very carefully to make sure that you don't reflect sound from the speakers directly back to the listening position. It can be done, if you do all your calculations and ray-tracing carefully, but it might just be easier to do a standard "superchunk" style bass trap, which is very broadband and has proven to work very well.

I understand that for bass traps to work best they need an air gap.

I think you are confusing bass traps with porous absorbers. Porous absorbers achieve greater effectiveness and work down to lower frequencies when there is an air gap behind them, but they are even more effective if that air gap is also filled with absorption, especially for randomly incident sound. When you do that, then you have a superchunk bass-trap, which is a rather special case of a porous absorber.

According to my calculation the triangle with the panel against the corner would be 42.4 x 42.4 x 60 cm. I can cut each slab in 16 triangles with these dimensions 30 x 30 x 42.40 cm,

The recommended size for a superchunk trap is at least 90cm across the diagonal face, although you can go down to 60cm if you have to. 42 cm is pretty small: it will have some effect, yes, but "bigger is better" when it comes to bass traps. The triangle also does not have to be symmetrical: One side can be longer than the other, if you need to do that to fit it in to the corner well.

as far as I understand, sealed traps are good at absorbing bass freq.

So are un-sealed ones. :) It all depends on what you are trying to accomplish. If it is a tuned trap, then it must be sealed. There won't be good resonance if the cavity isn't sealer But if it is a broadband trap, then it is better to just be totally open. Sealing a broadband trap automatically makes if into a tuned trap, in which case you have to tune it yourself to hit the right frequencies. - Stuart -

Hi Stuart, Thanks so much for your reply. The equations are like chinese to me, but I will try to understand. I need to find out the panels density first. I'll ask the shop where I've bought them if they know..

Where Vl is the longitudinal velocity of sound in the partition (in meters per second

Could you please explain this? what is the partition? How do I find out the longitudinal velocity of sound? Thanks

Then you tune your trap to cover those frequencies

How can I tune it? Do I have to cut the panel and/or add more insulation? I guess relying on the equations?

That would probably work, but then you'd have the problem of reflections. While that trap would absorb bass pretty well in the range set by the above equations, it would be a darn good reflector for mid and high frequencies. And since the trap will be placed diagonally across the rear corners, you'd have to angle the panel very carefully to make sure that you don't reflect sound from the speakers directly back to the listening position.

The traps would be on the corners "behind" the speakers. Due to the configuration of the room, I can't have corner traps in the corners behind the listening position, so I suppose that it should be ok re reflections? Also, bearing in mind that I'm planning to add broadband absorbers on some walls and on the ceiling.

It can be done, if you do all your calculations and ray-tracing carefully, but it might just be easier to do a standard "superchunk" style bass trap, which is very broadband and has proven to work very well.

I know I'm probably doing something stupid, but I've already bought the panels, so maybe I'll just give it a go.. I wanted to do the room tests today but then I've realised that my large diaphragm mic wasn't gonna work..., so I've bought a measuring mic today. When I've received it, I'll do some tests and post them here. Then if it doesn't work you can use this post as STICKY for what NOT to do... :) Thanks a lot for your help!

ah and also..

I think you are confusing bass traps with porous absorbers. Porous absorbers achieve greater effectiveness and work down to lower frequencies when there is an air gap behind them, but they are even more effective if that air gap is also filled with absorption, especially for randomly incident sound. When you do that, then you have a superchunk bass-trap, which is a rather special case of a porous absorber.

Maybe I'm confused indeed. What's the difference between bass traps and broadband absorbers? in terms of materials used to absorb the sounds? I thought that both used mineral wool for greater effectiveness? I've also read everywhere that broadband absorbers mounted on walls needed an air gap, for them to work best, between them and the walls, so I suppose I just thought the same would apply to superchunks or tuned corner traps?

With your slanted roof, how do you have the room set up? Also, with a room this small I would just take care of the bass traps (ie, superchunks style) then check if you need to add tuned traps.

What's the difference between bass traps and broadband absorbers?

A broad band absorber is typically a panel of rigid fiberglass or rockwool that is 5cm to 10cm (or more) thick and will usually absorb effectively down to about 125hz and everything above. You can increase the effectiveness of these by creating an airspace from moving them out from the wall. Bass traps are typically designed to absorb the frequencies below 100hz (and will act as a broadband absorber above) and require much thicker. Depending on the familiarity that you have with electronics, the absorber will act as an impedance device. Think about the crossover in a speaker. It uses coils to impede the flow of electrons. It is easy to stop high frequency signals because they don't have a lot of energy so if you only want to filter high frequency you just need a small coil (inductor), but if you want to take low frequencies, you need a much larger inductor. The same works for sound, thin absorbers will impede high frequency sound waves, but you need a lot of thickness for the lower frequencies. The hangers that are used for bass traps are also impedance devices. Tuned absorbers also have their electronic equivalent circuits. They are all just sound filters. Resistance would be like STC and the voltage drop over a resistor would be the equivalent to the amount of sound reduction on each side of a wall. (I don't know what would be the equivalent of a capacitor :) ) (sorry about the ramblings, I hope they help, if not just ignore them) Also, don't forget that there are corners where the ceiling meets the walls and the floor meets a wall. So there is always space for a bass trap. edit: the main point, is forget the panel.

A couple more questions, what wall is the window on, and which way does the ceiling slant?

Hi Eric, Thanks for your replies. I'm not familiar with electronics but I kind of get what you mean... When you enter the room, the window is on the opposite wall. The speakers will be on the right hand side. The ceiling is slanted on the wall opposite the window (the wall where the door is)

Is the window on the 3.5m wall or the 3.57m wall? Is it centered? Is the door on the other side centered?

The Window is on the 3.57m wall. It's not centered. It goes almost to the edge of the wall on the left when entering the room. The door is not centered, I haven't got the figures with me, but it must be at about 1-1.5m from the left wall when entering the room.

Is this approximately what your room looks like? I don't think 3mm slope on the the ceiling will make any difference.

Yes, except that the window is a bit more to the right from this angle, and the window is a bit more higher up on the wall. I've done a sketchup file as well and can post it if you need. No I don't think either that 3 cm slope will make a difference in the ceiling.

I moved the bottom of the window up to about .80m (I am actually doing all of the measurements in imperial units because that is what I'm comfortable with. Just did a modal analysis on your room, it doesn't look too bad. You will have some problem points around 98hz, 85hz and 70hz. That should be taken care of with all of the bass trapping that i'm putting in the design. If there are some modal problems after measurement, the you can put a tuned trap at the back of the room with fiberglass panels set off from the front to prevent reflections, but still be targeted for the frequencies that you want.

Ah thanks so much for that! :) Can I just ask what's on the left wall when entering the room? Are there broadband absorbers underneath this white worktop or whatever that is? thanks Also. can I just ask, if you don't mind, what you think of that: My original plan: _ I've already fixed my worktop (not a desk) to the right wall with battens (not the best I know..), so obviously my speakers are rather close to the wall, and on the worktop. As they are quite close to the wall my plan was to have an 1.80m x 1.20m (10cm thick) broadband absorber on the wall behind the speakers (with a 5 cam gap behind). _ On the right wall when entering, one 1.20m x 1.20m broadband absorbers. _ On the ceiling, one 1.2m x 0.60m absorber (put horizontally) just above the desk, and three 1.20 x 0.60m absorbers on the rest of the ceiling. _ On the left wall, I was planning to have a skyline diffusor, as well as some shelves by the window. I would also have a sofa against that wall. What would you think of that setup? I've now done room measurements. I hope I've done them correctly.. Thanks a lot for your help. Really appreciated!

_ On the right wall when entering, one 1.20m x 1.20m broadband absorbers. _ On the left wall, I was planning to have a skyline diffusor, as well as some shelves by the window.

That would be a bad idea for two reasons. First, it destroys the symmetry of the room. Symmetry is critical, if you want an accurate stereo image and balanced soundstage. Second, the room is way too small to be able to use numeric-based diffusers anyway, and certainly not so close to your ears. I would just go with Eric's plan of absorbers on both sides, arranged symmetrically.

I've now done room measurements. I hope I've done them correctly..

You'll have to post the actual MDAT file: there isn't useful information in the two graphs you show! :)

_ I've already fixed my worktop (not a desk) to the right wall with battens

That also sounds like you are creating an asymmetrical room: The room MUST be symmetrical. Imagine a line drawn down the middle of the room: the left half must be a mirror image of the right half.

so obviously my speakers are rather close to the wall,

That's fine. And for most small rooms, it is the only option.

and on the worktop.

That's not so fine: Speakers should never be on the worktop, desk, console, meter bridge, or any place else like that. They should only ever be on stands behind the desk, or even better, soffit-mounted. If they are on stands, then the stands must be massive (very heavy), and set to the correct height so that the acoustic axis of the speaker ends up at 1.2m above the floor (47-1/4").

As they are quite close to the wall my plan was to have an 1.80m x 1.20m (10cm thick) broadband absorber on the wall behind the speakers (with a 5 cam gap behind).

That's fine too.

I would also have a sofa against that wall.

Once again, that would upset the room symmetry. If you have a sofa then it should be placed symmetrically in the room, half to the left of the center line, and half to the right. - Stuart -

Under the shelf are acoustic hangers for additional bass trapping. Since you have the window there, you can't really do a corner trap, so I figured the corner between the floor and the back wall would be a good place for a trap, and you also have a shelf.

Ok Eric thanks for the explanation :) I've got a radiator there though, on the "left wall" where you're suggesting to have additional traps. Stuart, thanks for your reply. Sorry I didn't know that there JPG files wouldn't be enough... :-/ It doesn't allow me to attach a MDAT file, should I post it somewhere and give a link to download it? I've also attached my sketchup images, to show what I originally had in mind...

Quote: I would also have a sofa against that wall. Once again, that would upset the room symmetry. If you have a sofa then it should be placed symmetrically in the room, half to the left of the center line, and half to the right.

I'm not sure I understand what you mean. Should I cut the sofa in half? :D all joking aside.. is that not good location for the sofa as on the pictures?

Quote: and on the worktop. That's not so fine: Speakers should never be on the worktop, desk, console, meter bridge, or any place else like that. They should only ever be on stands behind the desk, or even better, soffit-mounted. If they are on stands, then the stands must be massive (very heavy), and set to the correct height so that the acoustic axis of the speaker ends up at 1.2m above the floor (47-1/4").

I didn't mention it before but the speakers are sitting on foam pads, on an additional shelf on top of the worktop. So they are sitting at about 1m above the floor. Is that not fine? So I've got a few more questions: 1) Is it better to have a bigger cloud over the worktop as you've done, Eric, or is it fine to have a smaller cloud and other broadband absorbers on the rest of the ceiling as on my sketchup pic? 2) Would it work how I've positioned the absorbers (and the amount)? and I guess that to respect the symmetry I should have at least one movable broadband absorber by the window, opposite to the 1.20 x 1.20m absorber on the wall which is on the right of the desk? 3) I have have 2 absorbers on the wall behind the sofa instead of 3, and just a 1.20m x 0.60m cloud over the worktop, instead of absorbers on the whole ceiling, would that be enough treatment? I suppose that's a matter of trying out as well.. Again, thanks a lot for your help guys!

The mdat file can be downloaded here http://we.tl/jtMAI5NCIK Thanks! :)

I'm not sure I understand what you mean. Should I cut the sofa in half? :D all joking aside.. is that not good location for the sofa as on the pictures?

Originally you said it was on the left wall ("_ On the left wall, I was planning to have a skyline diffusor, as well as some shelves by the window. I would also have a sofa against that wall. "), but the pictures show that in reality it is on the rear wall, which is fine.

I didn't mention it before but the speakers are sitting on foam pads, on an additional shelf on top of the worktop. So they are sitting at about 1m above the floor. Is that not fine?

Not so good, no. The foam pads isolate some frequencies but not all, and that position means they are too close to the desk surface, so there will be reflections from that, and therefore also comb filtering and other issues. The best location is on stands behind the desk, and the stands need to be very heavy. Some people build or buy hollow stands, then fill them with dry sand to make them heavy enough. Other people just stack up a few concrete bricks to make their stands.

So they are sitting at about 1m above the floor.

The acoustic axis of the speaker should be 1.2m above the floor, which is the average height of your ears when seated. NOTE: "acoustic axis"! Not the top or bottom of the speaker cabinet. The acoustic axis is the point that the sound appears to emanate from, and is different for each speaker. The manufacturer of your speakers can tell you where that point is on you speakers.

1) Is it better to have a bigger cloud over the worktop as you've done, Eric, or is it fine to have a smaller cloud and other broadband absorbers on the rest of the ceiling as on my sketchup pic?

It's hard to figure out what your images are showing, since you are using parallel projection: Turn that off and use "perspective" instead: it gives a much more realistic view. But from what I can see, you have your desk way too close to the front wall, and you don't actually have a cloud on the ceiling first reflection points at all. I would go with Eric's plan: pull the desk away from the wall until you get your ears in the correct position, leave the speakers against the wall on their stands, and place the cloud so it is centered on the first reflection point.

2) Would it work how I've positioned the absorbers (and the amount)? and I guess that to respect the symmetry I should have at least one movable broadband absorber by the window, opposite to the 1.20 x 1.20m absorber on the wall which is on the right of the desk?

Once again, it's hard to get a clear picture of what goes where, due to the parallel projection. However, it does seem that there is not enough bass trapping in that room. Once again, I would go with Eric's plan for that.

3) I have have 2 absorbers on the wall behind the sofa instead of 3, and just a 1.20m x 0.60m cloud over the worktop, instead of absorbers on the whole ceiling, would that be enough treatment?

Probably not enough...

The mdat file can be downloaded here

Excellent! OK, that room has waaaaay too little bass trapping. In fact, it is more like an echo chamber than a control room! :shock: The decay time is way up around 600 ms, and it should be down around 200 to 300 ms for a room that size. You need a huge amount of trapping. There are also a lot of early reflections clearly visible, and the definitely appear to be coming from the desk surface: the path lengths are mostly just a couple of feet different. There is also a strong refection from the rear wall, so you definitely need a lot of absorption back there: perhaps even more than Eric suggested. So I would suggest doing all that Eric suggested first, then measure again and post that new MDAT file here, with those results, so we can suggest what the next step should be. - Stuart -

Hi Stuart, Thanks a lot again for your reply! I've added new pictures with the "perspective" turned on.

Originally you said it was on the left wall ("_ On the left wall, I was planning to have a skyline diffusor, as well as some shelves by the window. I would also have a sofa against that wall. "), but the pictures show that in reality it is on the rear wall, which is fine.

Sorry I guess that wasn't clear. I meant "on the left wall when entering the room" but I understand why this is misleading..

Not so good, no. The foam pads isolate some frequencies but not all, and that position means they are too close to the desk surface, so there will be reflections from that, and therefore also comb filtering and other issues. The best location is on stands behind the desk, and the stands need to be very heavy. Some people build or buy hollow stands, then fill them with dry sand to make them heavy enough. Other people just stack up a few concrete bricks to make their stands.

Yes I understand but I have fixed the worktop to battens on the front wall so I can't move it away from the wall, and I don't think that I'd have enough space to move the worktop to the ideal location in the room :( so I'm stuck with having the speakers on the worktop. Is there no alternative solution? like putting the speakers on something that has the least sound transmission to the worktop?

But from what I can see, you have your desk way too close to the front wall, and you don't actually have a cloud on the ceiling first reflection points at all. I would go with Eric's plan: pull the desk away from the wall until you get your ears in the correct position, leave the speakers against the wall on their stands, and place the cloud so it is centered on the first reflection point.

I can't move the worktop to the ideal location :( ... Re the first reflection point, please correct me if I'm wrong but I thought that the cloud just between the listening position and the speakers was at the 1st reflection point? since the speakers are very close to my ears?

OK, that room has waaaaay too little bass trapping. In fact, it is more like an echo chamber than a control room! :shock: The decay time is way up around 600 ms, and it should be down around 200 to 300 ms for a room that size. You need a huge amount of trapping.

This room measuring is without any bass trapping. There aren't any absorbers installed yet.. I will re-post the mdat file after I install the traps. One more question please: I've been researching on that but I still don't know what's the best option re mounting absorbers on the ceiling? Thanks!

I've added new pictures with the "perspective" turned on.

Then there is something wrong with the way you are using SketchUp, since those are NOT perspective views: they are parallel projections. On the "Camera" menu, makes sure that "Parallel Projection" is NOT ticked, and that "Perspective" is ticked. Nothing else should be ticked. Then select "Field of View" and make sure it is set to a reasonable angles, such as 40 for example. - Stuart -