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Bass Traps

Bass Traps
The next phase of acoustic treatment is adding corner bass traps. The "trick" here is to create the following:
  • There are 8 corners to a rectangular room; 4 at the ceiling, 4 at the floor. Treat as much of them as possible.
  • Surface area matters more than panel depth but use at least 3-inch depth if at all possible.
  • Leaving some air space behind the panel improves absorption and is less weight and less costly.
  • Leave the edges of the insulation exposed where possible.
  • The frame need not weigh much. The frame's ONLY purpose is for mounting and to keep the panel from sagging.
  • OC-705, at 5pcf, is better than OC-703 for bass traps and OC-7XX is much easier to work with.
  • Adding a "room-side" -FRK membrane (OC-705-FRK) does improve bass absorption.
  • Weight IS and issue. Remember, the traps will be mounted over your head.
  • Straddling the corners is easier to construct and has an integral air space behind.
Construction
The frames are again made from clear white pine, splitting 1x6x8-ft lengths into 1.5x5/16-in stock. Carefully sawing the 1x6's 3/4-in thickness in half will yield two 5/16 pieces which are sufficiently rigid at 4-ft length when glue, screws and bracing is added. These frames, when completed, weigh less than 2 lbs each.

Many trap designs use overly heavy construction techniques like 3/4-in shelving, 2x2's or MDS particle board. These are 1) not necessary, 2) are very heavy and 3) excessively costly. The inverse, using no frame at all is also problematic because 1) even OC-705 and OC-703 are relatively fragile, 2) wrapped insulation can loose shape and sag over time and 3) spray-on adhesives are often necessary to hold the fabric in place. Instead, these Bass-Traps are build with very light-weight white-pine frames whose purpose is to 1) give support and structure to the fragile OC-705/703, 2) provide a framework for stretching the fabric and 3) making it easy to support a 4x24x48 panel from only two mounting points. Note: While staples can be used to secure the fabric, these frames are designed with sufficient flat surface area to allow using double-stick tape to secure the muslin in place. Since there is very little force on the fabric, the large tape surface area is more than sufficient and provides a very simple assembly process.


Materials
The following list of material was sufficient to build 8 Bass-Traps with enough insulation, wood and muslin to build a few smaller GOBOs at a later date. Build times (each, w/o coffee breaks) were Lower-Frame (30 min), Upper-Frame (60 min), Assembly w/insulation (45 min) and fabric-wrap (45 min). Assembled Bass-Traps weigh only 11 lbs each fully assembled.
  • 6 ea 2x24x48 OC-705-FRK 5pcf insulation
  • 6 ea 2x24x48 OC-703 3pcf insulation
  • 8 yds, 108-in wide, fire-retardant, heavy-weight, grey muslin
  • 20 ea 1-in x 6-in x 8-ft white pine - selected for clarity, no warp and light weight
  • 1.5 rolls of 2-in wide x 36yds NETTO Permacel P2 double-stick tape
  • 100 ea 2-in sheetrock screws
  • 25 ea 3.25-in course deck screws
  • 400 ea #6 x 3/4-in wood screws
  • 16 ea #8 eye hooks
  • 16 ea #10x3-in hooks
  • 16 ea 5-3/4-in turn-buckle

 
 
Assembling Lower-Frame using a jig
Assembled Lower-Frame corners
 
 
Assembling Upper-Frame using same jig
Assembled Upper and Lower Frames
(2x18x48 OC-705-FRK below)
(2x14x48 OC-703 above)
 
 
Frame showing FRK membrane
2 Frames shown from side
 
 
16 Attachment blocks
Wrapping frame with fabric
(Fire-retardant muslin + 2-in double-stick tape)
 
 
Completed corner-mount Bass-Trap
Installed Bass-Trap

 
Corner Bass-Traps plus Ceiling Cloud-Traps

Installation
As a result of size and quantity of OC-705 and OC-703 material, the best option was constructing 8 corner bass traps. However, this room is very small (8x9x10) and with the door style and position, it was not possible to install all 8 traps in ceiling corners. As a result, 2 of the traps were mounted flat with the ceiling but located as close to the corner as was reasonable. Due to the trap construction, this still provided an air space between the panels and the ceiling.

Because these traps were constructed using OC-705-FRK, it is not desirable to mount them near "first-reflection-points" because of the added mid-range reflections from the FRK membrane. Obviously, in the real world, there must be compromises. By locating the 2 "flat-mounted" bass traps as close to ceiling corners as possible, and separated by a mid-position, ceiling-mounted mid-range trap "cloud", the "first-reflection" issue for FRK material has been largely avoided.

Because they are light-weight, mounting in ceiling corners is made easier using hooks, eye hooks and turn-buckles.

Measurements
The following plots show the room response before and after installing 8 bass traps. Note that the "before" acoustic response does include the 6 mid-range traps. Note the following from the plots. Note that due to differences in measurement processing, the plots may have differences in frequency span or magnitude scale.

These plots show the improvement from adding 8 Bass-Traps over using just the Wall/Cloud-Traps:
  • The frequency response variance is down from 13 dB to a respectable 8 dB
  • The notch at ~230 Hz has been eliminated
  • The upper end, above 3 kHz, is much flatter
  • The first reflection, at about 4 mS, has been significantly reduced
  • Some sharp low-frequency room resonances remain but the waterfall plot shows improvement
  • The reverberation time dropped from about 350 mS to about 200 mS
  • The peaks and notches are damped and much broader
 
 
Frequency Response (Before)
(includes 6 mid-range traps)
Frequency Response (After)
(includes 6 mid-range traps)
 
 
Impulse Response (Before)
Impulse Response (After)
 
 
Waterfall Plot (Before)
Waterfall Plot (After)


These plots show the improvement from the 5 Wall-Traps, 1 Cloud-Trap and the 8 Bass-Traps over just the bare room:
  • The frequency response variance is down from 13 dB to a respectable 8 dB
  • The notch at ~90 and ~230 Hz have been eliminated
  • The upper end, above 3 kHz, is much flatter
  • The first reflection, at about 4 mS, has been significantly reduced
  • Most of the <300 Hz resonances have been smoothed; 9 sharp peaks have been reduced to 5 broad ones
  • The reverberation time dropped from about 500 mS to about 200 mS
 
 
Frequency Response (Bare Room w/Gear)
Frequency Response (w/14 Traps)
 
 
Impulse Response (Bare Room w/Gear)
Impulse Response (w/14 Traps)
 
 
Waterfall Plot (Bare Room w/Gear) Waterfall Plot (w/14 Traps)