Measurements Interpretation & Bass Trap Choice

I recently put together a new mix room and I'm about to add some more acoustic treatment over what is already in place. There are lots of people on this forum with far more knowledge and experience than me, so it would be great to get some advice. I've read lots of similar threads and I've tried to save time by gathering as much relevant information as possible. So apologies for the long post, but hopefully it should save some of the normal starter questions. Please chip in with any ideas, thanks! The biggest problem I can easily hear in the listening position is a bass null, in and around 100Hz, this is the main area that I'm hoping to improve. I'm realistic about what's possible in a room of these dimensions, and the compromises that have to be made to maintain the ergonomics and aesthetic of the room... The ergonomics of the room work well, so the furniture will stay in place, but there's lots of bare wall space to work with, so I should be able to make some improvements. I'll be doing the work myself, I'm experienced with carpentry so construction with wood will be no problem, and I can have metal cut for me locally if perforated steel sheets etc are an option. What I need to achieve to increase bass in listening position: - Choose the most suitable type of traps to place around the room (perforated panel / broadband / membrane etc) - Choose the most suitable dimensions of traps (I can potentially have deeper traps than shown on the proposed diagram below) My initial thoughts - perforated panels seem like a sensible option, membranes seem to have a too small margin for error for a DIY setting and broadband traps will compromise higher frequencies. I have been reading the BBC documents on low frequency absorbers and they seem to have had good success with perforated panel designs... I will outline details of the room and the testing I have done so far... The dimensions of the room are 5.5m long x 4.1m wide x 2.6m high. The layout of the furniture in the room is shown in the floor plan below. The treatment already in place is a 10cm layer of rockwall across all the front wall, a big superchunk bridging the ceiling and front wall, and a corner trap from table to ceiling in the rear left corner. These are covered in acoustic fabric, and will be staying, but I can remove fabric to add a membrane or perforated panel between the rockwool and fabric if needed. The current treatment is shown in green on the floor plan. The available positions currently free for further treatment are shown in yellow: Here is a waterfall plot at the listening position using both speakers: This was after lots of experimenting with moving the speakers around to find the best position and minimise SBIR issues. It shows approximately a 15db null around 100hz and around 60hz. The modes of the room are shown below: I calculated the first cancellation frequency for each surface using right speaker only: I moved the test mic in different directions around the listening position, one axis at a time, to try to understand which directions are causing the cancellations. I plotted graphs of the best & worst responses for each axis. Here is the graph when moving the mic in the up / down axis: (showing a big change at problem frequencies) Here is the graph when moving the mic in the front / back axis: (showing not much change at problem frequencies) Here is the graph when moving the mic in the left / right axis: (showing a big change at problem frequencies) My interpretation of the above graphs would suggest that treatment on the faces of the front/back axis would have little effect in comparison to treatment on the faces of the left/right and up/down axis - which goes against the common solution of bass trapping on the back wall... anyone have any other interpretations? If this is true then the side traps at the first reflection points and the cloud above the listening position are the traps that could have the greatest effect on improving the bass nulls. Any ideas of types of traps and dimensions of traps for these locations? Any help, thoughts or ideas are really appreciated - thanks!

Hi marmaduke. Welcome! Please re-size your pictures to comply with the forum rules. Thanks! - Stuart -

The dimensions of your room are producing, width=62Hz and multiple and length/width/height=99.9Hz and multiples, these are your trouble spots. Your treatments should be placed from floor to ceiling (horizontal EDIT=VERTICAL---sorry I had somebody else on my mind :)) in order to approach a better ability to trap these frequencies. They should also be designed to be as tall as the floor to ceiling measurement where you will have a better opportunity for the treatments to actually work as designed rather than trying to pin point specific areas, when the pin pointing is required yet :) You should pull all the furniture out of the room and do your testing...you are not getting an accurate reading with all these items in the room, especially if they are hard surfaces, but in any event, they should be out of the room for the testing, this will give you a more accurate reading.

@ Soundman2020 - Thanks, I did my best to make sure they all comply. Only one image is above 750 pixels wide, I tried making it smaller but it became too small to read the text within the picture, hope this is ok. @ xSpace - Good points, thanks. I did the original testing a few months back with the minimum amount of furniture possible (the racking is fixed into the floor and walls so had to stay put). The extra furniture has actually made only a little difference to the low end response, the sofas improved things to a small degree, but not much change. So perhaps side traps targeting 60hz and a ceiling cloud targeting 100hz would be a good start? Does anyone have a good program for calculating the design of a perforated panel trap to target specific frequencies?

Side traps tuned to 60hz and a ceiling cloud tuned to 100hz - am I interpreting the results correctly? The choice is whether to make panel traps, perforated panel traps, slats or membrane traps... any opinions? If anyone has any ideas to chip in with, especially if anyone has any relevant diagrams or construction details, I'd really appreciate the help, thanks.

For low frequencies like that, I think I'd go with panels or maybe slats. You have some pretty major nulls there, so you are going to need large area of treatment. My 0.02. (Other opinions may be different. Your mileage may vary. Void were prohibited by law. etc. ... :) ) - Stuart -

@ Soundman2020 - Thanks Stuart, that's what I'm thinking as well - although I'm trying to incorporate some rockwool and fabric on the outer layer of the traps to deal with high frequencies, and I think trying to do this on a slatted trap would hurt the LF efficiency. The results in the BBC papers suggest that a panel design would be most efficient dealing with 60hz, but a perforated panel design would be most efficient dealing with the 100hz null. Does anyone have any design details for panel traps tuned to 60 and 100hz - drawings, construction details, materials, rockwool density etc... Any help would be really appreciated, thanks.

bump Does anyone have any design details for panel traps tuned to 60 and 100hz? drawings, construction details, materials, rockwool density etc...

Does anyone have any design details for panel traps tuned to 60 and 100hz? drawings, construction details, materials, rockwool density etc...

Basically, it's just a large panel of some type of material (usually wood) mounted over a sealed cavity with insulation inside, and a small air gap between the panel and the insulation. The material and cavity are chosen specifically for the resonant frequency that you need. For example, a sheet of 1/2" plywood over a 6" cavity would be about right for a 60 Hz. problem (not exact, but close). 1/4" plywood over a 4" cavity would be right for 100Hz. Etc. The position of the trap in the room is also important: unlike absorption, which is velocity based, a panel trap is pressure based, so it should go right up against the wall, in the position where the pressure peak is highest for that particular resonance. Check out the chapter on those devices and concepts in MHoA. Chapter nine is what you want. Page 205 and following. - Stuart -

@ Soundman2020 - Thanks Stuart, is there an online version of that book? Also, when you say a 6 inch cavity for example, how much of that cavity is filled with insulation? I've tried out a few designs on the Whealy calculator but I was wondering... does the overall dimensions of the trap affect the target frequency (the BBC papers suggest yes due to the resonance of the sealed air cavity in a panel design) - will a 2x2 ft trap (with the same panel thickness, depth, construction materials etc) target the same frequency as a 8x8 ft trap?

Thanks Stuart, is there an online version of that book?

Well, you can get it on-line, but I'm not aware of an on-line version! :) http://www.amazon.com/Master-Handbook-A ... 676&sr=8-1 The book is worth its weight in gold! Strongly recommended...

I've tried out a few designs on the Whealy calculator but I was wondering... does the overall dimensions of the trap affect the target frequency

Not according to Everest! The frequency is set only by the surface density of the panel itself, and the depth of the cavity below it. It is a simple one-degree-of-freedom resonant system. Making it bigger will make it more effective, but it shouldn't change the resonant frequency, as far as I'm aware. Maybe if you built a really small one, there might be an issue due to the inability of the panel to vibrate, but if you build it large enough that most of the panel is free to vibrate, then you should be fine. (IIRC, theoretically the panel is presumed to act like a piston, without considering that the edges are constrained to the frame, so I guess if the device is small enough that not much of the panel acts as a piston, then there might be an issue)

(the BBC papers suggest yes due to the resonance of the sealed air cavity in a panel design)

which particular paper are you referring to? There are an awful lot of BBC papers on acoustic treatment, most of which deal with their own standardized modular devices: Those aren't very big, so there might be some size-related factors there.

will a 2x2 ft trap (with the same panel thickness, depth, construction materials etc) target the same frequency as a 8x8 ft trap?

In theory, yes, but a 2x2 panel is not very big, and doesn't give you much surface area. I'll have to go read that chapter again: it's been a while... - Stuart -