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Wall Traps

Wall Traps
 The Newport Acoustics Studio needed as much wall absorption as practical because of the closely-spaced, parallel, sheet-rock walls. Many companies sell such products but the author wanted to a) limit the cost of such simple treatment and to b) have the experience of designing/constructing/measuring acoustic absorbers. If you don't do the work, you'll never know the why or the how of your room's sound.

Construction
The wall traps are simple to construct and there are even accessories, like pre-sewn covers which just slip over one end and finish with a zipper. The photos below are of the author's own build of the Wall Traps's 2-in thick, 24x48 inch wall traps which use MineralWool 1280 at 8pcf. At the time, MineralWool was selected because it was denser (8pcf) and cheaper than OC-705 so it was hoped that with just 2-inches thickness, absorption would not suffer as much with only 2-inches.

A batch of 6 traps were built; that number being selected because that's the number of 2-in MineralWool 1280 panels in one package and because the studio needed at least that many traps. Notable Wall Traps features include:
  • The frame is cut from light-weight pine with two strips per 3/4-in width so the frame thickness is just under 3/8 in x 2-in.
  • Two cross-pieces (wall-side) are only 2-in wide and, with the corner supports, keep the frame stiff while containing the MineralWool panel while adding very little weight.
  • MineralWool bats are a little larger than 24 x 48 x 2 inches so a bit had to be trimmed off the edges to make them fit the frames. The frames needed to fit the micro-suade coevers so they had to be 24 x 48 x 2 as they stretch a little, but only a little. (Hint without the frame, the covers would be even more difficult to put on -- the frames helped.) So, the frames are 24 x 48 x 2 OD; outside measurements. Even then, the corner and back-side braces cause the MineralWool to protrude out the front of the frame about 1/4-inch. Not a problem, just a detail to note.
[ Remember, the frame has no purpose other than to a) give some rigidity to the shape and b) make it easier to mount the panel away from the wall. ]
  • Note that the pre-sewn cover has two velcro strips for hanging a rigid fiberglass panel on the wall without a frame. These were not used because acoustic absorption is significantly enhanced by spacing the panel off the wall.
  • The frame hangers and spacers, made from oak for strength, are mounted to the frame cross supports after the cover installed.  The hanger and spacer are felt-lined to reduce the induction of vibrational noise into the panel from the wall -- as from an adjoining room.
  • The spacing is only about 1-inch instead of the recommended 2-inches for a 2-inch thick panel. The spacing can easily be increased later but present room limitations dictate 1-inch for now. The rationale is that 1-inch is better than nothing.

 
 
 Note the corner and cross-trap stiffners
 6ea MineralWool-1280 stuffed 2x24x48 inch traps
 
 
 Sliding the pre-sewn cover over a trap
 Hanger and spacer mounting on back
 
 
 Oak hangers - one on wall, one on trap
 Oak spacers - keeps panel spaced from wall

Materials
  • MineralWool-1280 (NO -FRK !), 2-in thick, spaced off the wall 1-2 inches is, in theory, the best option to conserve space
  • Weight is also very important to minimize, so the frame is 5/16 x 2 inch white pine cut from 3/4-in clear 'shelf' material (hand select lower density boards to reduce the weight)
  • Slip-on, micro-suade covers reduced the labor at a reasonable cost and look professional in Lt-Grey
  • Mounting brackets of Oak are both strong and, felt-lined, they reduce wall-induced vibration
  • The felt pads were cut from 6-in squares of double-stick pads available at the local hardware store for cheap
  • These traps are <15 lbs each so light enough that one was hung as a 'cloud' -- spaced 2-inches below the ceiling
  • Labor was DIY ("Free") and material cost was about $50-$60 each - about 25% of the cost of commercial traps
Subjective Performance
The RPlusD (EFT) application, with a Behringer ECM-8000 mic, was used to measure bass reverberation times. Subjectively, the room became much less reflective to the point of sounding somewhat "dead". While this is not the goal, there was significant improvement in speaker clarity; a good first step. These wall-mounted traps are not in the corners and so low-bass reverberation improvement was not the goal. These Traps were designed for mid-range frequencies and did appear to help. Was the choice of 8pcf RockWool over OC-703 worth it. There is no way to really know based on measurements. These work but other choices could do as well.

Measurement Tools
To make acoustic measurements of a room, and so attempt to measure the relative changes related to room treatment, a sound source, a microphone detector and some control/processing software is required. Fortunately, these are more easily found associated with a recording studio than just a listening room. However, despite having the tools, it's still not trivial to make reasonable measurements.
  • Speakers, the usual sound source can "drive" the room but is the speaker, it's position and mounting able to produce sounds with a flat spectrum? Does it "drive" the room uniformly at all frequencies?
    • No. However, good monitor speakers can at least be better than the room.
  • Microphones cone in a wide variety to match specific sound sources. Are any of them "good enough"?
    • Yes, though really good ones (with really flat frequency response) are very expensive.
    • Fortunately, ones like the Behringer ECM-8000 are good enough and fairly inexpensive.
  • Software is advanced, very sophisticated and reasonable in price.
    • Tools like RPluD (R+D) (EFT) can measure room response and produce waterfall plots for about $150
    • The hard part is learning to use them and learning to interpret the plots

Measurements
At first, the plots below appear unimpressive. However, they are real and they show the real-world details involved when working on room acoustic treatment. It's not clear-cut, dramatic or "just do steps 1-2-3 and you're done". Significant improvement has been realized but more treatment is still needed. Remember, these are not bass traps, they are "mid-range" traps, on the walls, designed to just dampen the room's largest flat surface areas. Here's some observations about the plots below:
  • The room frequency response improved below 400 Hz but much ringing remains.
  • Reverberation time has been shortened.
  • Sharp room resonances are clearly visible in abundance without the wall traps.
  • Though not dramatically visible in the plots, the room "sounded" less "live" - initially an improvement.

 
 
 Frequency Response (Before)
 Frequency Response (After)
 
 
 Impulse Response (Before)
 Impulse Response (After)
 
 
 Waterfall Plot (Before)
(Bare room w/Gear)
 Waterfall Plot (After)
(5 Wall-Traps + 1 Cloud-Trap)