Quote:
Originally Posted by anseo 
I did built a number of the absorbers you mentioned, and they are terrrific, but not for bass trapping in my experience. For instance, there's an ugly standing wave at 76 Hz that's obviously not adressed by these units.
Um, aren't bass traps, like panel absorbers, supposed to be air tight? Because if it's just a frame with rockwool covered with chicken wire and burlap, that's an absorber, right? |
Anseo- bass traps do not need to be airtight. Also a bass trap is effectively a bass module absorber- the design will effect the effectiveness of absorbing very low frequencies.
It is recommended that there be a gap behind the bass trap and the wall. Theory being low end goes into the trap (layer or rigid fibreglass) through it, bounces back off wall, and the cavity between the wall and the bass trap fibreglass layer traps the bass. Alton Everest recommends a gap of 6-7 inches between the rear of the fibreglass and the wall.
So although stacking rockwool in the corner is good, even better if you can effect a gap between the "basstrap rear" and the rear of the wall. Alton Everest has written a guide on basstraps- and gives the gap between the wall and rear of the basstrap.
The front of the basstrap can be covered with cloth- which means it is not a basstrap per se- but a broadband absorption trap. To have the trap only absorb low frequencies, you need to place over the front of the trap thick plywood, which then reflects back out the upper and mid frequencies.
You then tune what level frequency your trap absorbs by the size of drill holes in the plywood and the perforation percentage of the plywood surface.
So for instance- no cover or perforation of at least 25% of the front cover results in effective absorption coefficient of 1.0 from about 250 hz all the way flat lining past 10khz. So- this will be a broadband trap, absorbing everything above 250 Hz upwards.
A plywood cover with 0.5% percentage perforation (equates to 26 3/16 holes per square foot drilled into the plywood cover) is a true bass trap- as it starts absorption from 40Hz at some 0.4 absorption coefficient, peaking between 90 and 170hz, and from 200 hz becoming less effective until it does not do much absorption at 900HZ onwards. This is great design for bass trapping but not absorbing the higher frequencies if you are concerned about deadening the room.
After looking at the commercial "basstraps" which I thought was a joke, I bought 7 foot pine corner cupboards, with 4 deep cornershelves. I drilled out the two rear walls of the bass traps facing the rear corner (as well as the top and bottom) with heaps of 3/16 holes to make it effectively "open" at the rear, and on top/bottom. I then stacked up with acoustic rockwool and covered the front with nice black patterned cloth. These I placed 3 inches off the rear corner walls to give a "airpocket" between the room corner and the back of the corner basstraps.
My monitors stand in front of these monsters and I can really say that the results were incredible. I had huge problems before- I use Event ASP8's which really need good bass trapping and had huge issues before hearing the low end- now its great-tight but so deep.
So good that I built a further 2 for the tracking room, for drums, but covered the entire front of the unit with thick plywood. I wanted to absorb low bass at the rear, but reflect back from the front higher frequencies into the tracking room. So I drilled the plywood rear two walls facing the corners with heaps of holes to make the rear "open", and the front had a very thick plywood cover. I drilled that cover with about 0.5% perforation to preserve frequencies above 1000hz in the tracking room as my tracking room is already too dark because of other treatments.
Works great on drums.
Good luck and hope this helps
GJ
Newcastle/OZ