Bass trap design in a specific case

hello out there first- sorry, my english isn't perfect. i've a question regarding to the design of a bass trap in a specific case. i already did measure a lot and know which reflections and room modes have to be absorbed at which positions (walls, corners) in my room. in this case it's a quite strong 39hz mode and a bit less strong 57hz mode. additionally, frequencies around 90, 125 and 160-300hz should be absorbed aswell at this place. those latter frequencies are either room modes aswell or frequency ranges in which exists to much energy (due to the nature of the walls which are partly based on a solid plaster board construction). i've got two ideas to catch - more or less - all those frequencies: A) filling up the whole region with mineral wool (approx. 60kg/m3), probably with an air space between each slabs (to save some dollars). this would be a volume of around 15m3. B) placing panel absorbers with metal and wood panels (depending on the target frequency) which are tuned on 39/57/90hz and distribute them at the positions where the most sound pressure exists. that will be on the floor, ceiling, back wall and side walls in the same area where the minearal wool would be. to catch the higher frequencies also i would combine some of the tuned panel traps with slightly spaced minear wool slabs on the front. to figure out, how that should look like i did some drawings. the relevant data are kept in red: https://www.dropbox.com/sh/wv9mbkf5ig2l4hm/jVnDH2-PF9 those considerations I did to this topic: the basic question is which type of design A or B will perform better to absorb those frequencies? as porous absorber will be only very effective at lambda/4 I assume that design A should work very well down to approx 40hz as the depth will be bit more than 2 meters and 1.6 meters high. but, I did read once that porous absorber will be less performing at very low frequencies as the sound particle velocity is lower there. furthermore I thought that design B could be better performing as there would be more absorbing surface (panels on every boundary). opposite to that is the fact that in this room the axial modes aren't spread in the common way only (between each parallel boundaries), they are partly bent throught the long dimension as the room shape is not cuboid. hence I assume that also design A should work as good (or better?) as the the sound waves and room modes will reach the spot not only in a 90 degree angle. basically i tend to design A but i'm very excited to read what you experts are thinking about this issue. many thanks! jakob

Hi Jakob, and welcome to the forum!!! :)

i already did measure a lot and know which reflections and room modes have to be absorbed at which positions (walls, corners) in my room. in this case it's a quite strong 39hz mode and a bit less strong 57hz mode. additionally, frequencies around 90, 125 and 160-300hz should be absorbed aswell

It would help if you could post the REW data file, and also the dimensions of the room itself, so we can check that. Measuring 39 Hz is a major challenge: even most acoustic labs are not certified to measure such low frequencies accurately.

A) filling up the whole region with mineral wool (approx. 60kg/m3), probably with an air space between each slabs (to save some dollars). this would be a volume of around 15m3.

Which "region" do you mean? I tried to download your diagrams at the link you posted, but that doesn't seem to work.

B) placing panel absorbers with metal and wood panels (depending on the target frequency) which are tuned on 39/57/90hz

I think you are talking about panel traps? That's a possibility, but tuning them to the right frequency isn't easy.

distribute them at the positions where the most sound pressure exists. that will be on the floor, ceiling, back wall and side walls

Actually, pressure is highest in the room corners, for all modes. Against the boundary surfaces is a good place too, but corners are best.

as porous absorber will be only very effective at lambda/4 I assume

Actually, porous absorbers are effective down to much lower frequencies than that. Andre Vare has shown that porous absorbers are still very effective when their thickness is only 5% to 7% of the wavelength, especially if there is an air gap behind as well.

design A should work very well down to approx 40hz

For frequencies that low, I would use hangers. Absorption by itself will still work down that low to a certain extent, but not very effectively.

furthermore I thought that design B could be better performing as there would be more absorbing surface (panels on every boundary).

Yes, but panel traps are tuned to specific frequencies, whereas porous absorbers and hangers are broad-band.

opposite to that is the fact that in this room the axial modes aren't spread in the common way only (between each parallel boundaries), they are partly bent throught the long dimension as the room shape is not cuboid.

If the walls are significantly non-parallel, then you do not have axial modes! You have tangentials and obliques, but no axials. By definition, an axial mode can only form between parallel surfaces. If one of the surfaces is angled by a significant amount, then there will still be a mode close to where the axial mode was, but it will now be tangential or oblique. Also, when you seem to be implying that your room has several non-parallel walls, and that it is asymmetric: That does not sound like a good shape for a control room.

hence I assume that also design A should work as good (or better?) as the the sound waves and room modes will reach the spot not only in a 90 degree angle.

I would probably use a combination of hangers on the rear wall, superchunks in several corners, and perhaps panel traps if there are still problematic modal issues remaining after doing that. - Stuart -

Hey Stuart Thanks for your time to get into this topic. I appologize being offline that long- I had an acoustic planing break of 3 weeks. Strange, that you can't open the dropbox link I did provide. At my place I can open it problem-free. But anyway, I made JPGs of the most important drawings. Here we go: ControlRoom_TopView_BassTrapDesign-A.jpg ControlRoom_ViewToSideWall-Right_BassTrapDesign-A.jpg ControlRoom_TopView_BassTrapDesign-B.jpg ControlRoom_ViewToSideWall-Right_BassTrapDesign-B.jpg The REW datafile is here: http://www.audiolager.ch/exchange/johnsayers/ControlRoom_SpeakerPosB_NO-BASSTRAPING.mdat.zip As you can see, in Basstrap Design A the whole area/region will be stuffed with mineral wool while in Design B only the surface areas of the same region will be covered with panel absorbers at the target frequencies. Yes, targeting those low frequencies isn't easy. I would try to get those panel absorbers as broadband as possible (i.e. not exactly 39hz, better 30-50). But the only way I know to reach that is filling the panel absorbers with mineral wool. Hence I tend to design A. Regarding hangers. As much as I know the hangers have to be quite big (height) to target very low frequencies at around 40hz. I assume that the height in this area isn't enough to hit the goal with hangers. Or am I wrong? The goal should be to absorb the low frequeny range below 300hz as much as possible in this area. Especially around 40hz as there is really much energy due to room modes. Of course I plan to treat also the other aeras of the room- some superchunks in corners, where it is possible. Panels on the rear wall, ceiling and side walls behind the listening position- everywhere as much as I can. I did also measure (using SPL measuring) at which spot of walls/corners/ceiling which frequencies are very strong. At those spots I try to absorb the corresponding frequencies. I plan to do reach this using superchunks (porous absorbers), panel absorbers (panel traps with a membrane built of plywood/metal, airtight cavity, and partly stuffed with mineral wool) and slat/perforated absorbers. If you have any suggestions- that would be very appreciated. Thanks again jakob

Wow! Now that I see the shape of the room, I'm understanding why you are having such a hard time figuring this out. That's a really, really tough shape to deal with! Then I downloaded the REW file, and I've been looking at it. My fist impression is: :shock: :!: :? :?: :?: That's pretty wild! That room must sound like a huge cave, lined with carpet! That's about the only way I can think to describe it... You seem to have modal decay times of over 1300ms in the lows, and around 500 in the highs, with overal RT of well over one second! Ummm.... There's major problems in there, far beyond what a couple of bass traps will accomplish. But first, there seems to be a calibration problem somewhere: There are 3 data sets in that file, one for the left speaker, one for the right, and one for both together, but there's a big issue there: the one for "both together" sows the exact same level as the other two! That's impossible. The combined output of both speakers must be about 6 dB higher than either one by itself, for this test. In fact, the "L+R" test actually shows a level about 6 db LOWER than the individual tests, in the high mids and parts of the high end. That's hard to explain! So something went wrong with either your calibration or you test methodology. But apart from that, from looking at your diagrams it seems there's an even more basic issue here, which is the speaker / mix position geometry. From what I can see, you have your listening position closer to the rear of the room than the front, and it seems to be rather close to the 50% too. I realize that the steeply sloping ceiling is a major issue governing where you can sit, but it still looks like you can get quite a bit further forward without too much hassle. At lest another 150 cm, maybe even 200 cm. That would put you much closer to the ideal location for the room, and get your head further away from the back wall. You still have more than 2, of head room at that point, so you should be fine. You also have your speakers mount too high, and tilted down. They should be mounted so that the acoustic axis is 1.2m above the floor, and not tilted at all. By putting them so close to the ceiling you are introducing major SBIR issues, and by tilting them down you are creating potential refection issues, comb filtering, and psycho-acoustic perception issues. There doesn't seem to be any need to tilt them, or at least not one big enough to justify such penalties. They are about the right distance from the side walls and from each other for room that large, if it were rectangular, but it isn't rectangular. Far from it. So I would pull the speakers closer together, so they are maybe just 1.7m apart (which you need to do anyway, to get your chair closer to the front wall). What speakers are those, by the way? (Brand and model number). And how do you have the rear panel controls set?

I would try to get those panel absorbers as broadband as possible (i.e. not exactly 39hz, better 30-50). But the only way I know to reach that is filling the panel absorbers with mineral wool. Hence I tend to design A.

Yes, but that will only have an effect on modes related to the front wall, and since that isn't a very large area, I doubt it will have a very big effect. For example, it will have no effect at all on the lateral axial modes, or on any tangential modes that do not involve the front wall.

Regarding hangers. As much as I know the hangers have to be quite big (height) to target very low frequencies at around 40hz. I assume that the height in this area isn't enough to hit the goal with hangers. Or am I wrong?

It's more about depth than height. The deeper the cavity, the lower coverage goes. You normally need them high as well in order to get them in the most important places in the room: both the top and bottom corners. But in your case, you'd be doing that anyway, since your hangers can still run all the way from the ceiling to the floor, even though that is not very "high". But it is very deep, which is great for low frequencies.

The goal should be to absorb the low frequeny range below 300hz as much as possible in this area

Not just that area: You are going to need a LOT more bass trapping in other parts of the room, to deal with the modes that do not involve the front wall. I would suggest superchunks in the rear corners, very thick absorption across the rest of the rear wall, and hangers way up the top there, near the peak if the roof, coming down as low as you can, plus diagonal panels (or superchunks) on the side walls, in the wall/ceiling corners, and also some major absorption on the side walls and the entire ceiling. That room is way, way to reverberant right now. You need to get the decay times down to about one fifth of what they are now, across the entire spectrum.

Especially around 40hz as there is really much energy due to room modes.

I plan to do reach this using superchunks (porous absorbers), panel absorbers (panel traps with a membrane built of plywood/metal, airtight cavity, and partly stuffed with mineral wool) and slat/perforated absorbers. I would stay away from reflective surfaces for the time being, and just go with pure absorption until you get the decay times down. Then think about reflective surfaces, to add back some of the highs, selectively, and probably also some diffusion too. My approach would be to basically kill the room enough to get the modes under control, then afterwards to use side slot walls, plastic, diffusers, etc. to return the missing mids and highs to the room. That would be my suggestion. - Stuart -