The panel will weigh more like 5 to 7 kg. Play with the depth to get the freq you want. It won’t be deep! Tuned absorbers tend to be used for probs under 100 Hz because porous absorbers work well above that.
Bass traps and diffusers
- Thread starter Spencerleehorton
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Slat absorbers can work well and are simpler than perforated panels to get right.
What are the dimensions of your panels again?
Spencerleehorton
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my panels are 100 x 60 x 20
helmholtz sounds like the answer, just dont really know yet where to drill the holes?
but looking at that video it has about 83 holes and looks like its about 32 holes each side then about 20 holes in the middle, all evenly spaced.
helmholtz sounds like the answer, just dont really know yet where to drill the holes?
but looking at that video it has about 83 holes and looks like its about 32 holes each side then about 20 holes in the middle, all evenly spaced.
Spencerleehorton
Well-known member
i got loads of slats, im all ears for something easy to sort this 120hz problem!!!
True, just a bit more woodworking involved. It just the efficiency drops when you get down to the lower frequencies unless you have a big surface area. The perforated ones seem to go lower with less space.
The holes should be uniformly distributed across the face. Your panel size with a front plate 19mm thick (and a sealed back of course) would need 228 holes, 10mm dia, leaving the Rockwool inside. This would give you a resonant absorption frequency of 115Hz - and up to 80% efficiency an octave either side if you leave in the Rockwool. This gives a 3% perforation percentage where the minimum is 1%. This can be a grid of 12 x 19 holes
If you use the same drilling pattern with 7mm holes you get 83Hz with 1.44% perforation percentage, you don’t need as much absorption at 83 as at 118 (I calculated it to be 114 from exact 3M width but measured you get 118?) from the plots you posted anyway. It may not be necessary to built the 83Hz as the bandwidth of the 115Hz absorber covers that as long as the absorber cavity is filled.
Link to calculator again:
http://www.mh-audio.nl/Acoustics/HelmholzPanelResonator.html
So all you need to test this is two sheets of timber 600 x 1000, one 19mm thick for the front plate and you could use 12mm for the back seal plate, for a couple of your existing panels. Remember the thickness of the front plate affects the tuning depth of all the ports so if using a different thickness front you need to recalculate the number of holes. I’ve built quite a few of these and they’re very effective.
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Spencerleehorton
Well-known member
I was going to build the helmholtz out of these rack panels I have, I’ll get the dims tomorrow
Ok but you already have ready made panels you could tack a front and a back to so as to test. I think you said the rack timber was 18mm - for the front panel that lifts resonance to 118Hz.
Slat absorber calculator here. I usually use 1 x 8 or 2x 8 slats with something like a few mm air gap to start with…
http://www.mh-audio.nl/Acoustics/SlotAbsorber.html
If you make perf panel, you’ll need to grid it out and get a good forstner drill bit.
Also, don’t let the fiber touch the front panel and fabric on the face is detrimental to the effectiveness. You do gain some diffusion/reflection effects for mid and high freq.
Hard to get down to low frequencies without a deep cavity
Spencerleehorton
Well-known member
This is the size of one of my rack units which I could make into box and then drill the holes and put one layer of rockwool glued to back and not touching the front.
I can make several of these but don’t yet know where they will go.
I have seen that material is put on the underside of the holes side but have also read not to do this? This is confusing?
Something like this would work- could be placed in the lower part of each side wall
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The holes need to be free of any obstruction per BBC documents. The box must be airtight and rigid and you’ll need probably 16 or more square feet of surface area.
BTW, you never showed any decay graphs of the room. That measurement is perhaps more important to explain how a room feels to mix in. Freq response is only per of the story.
My recommendation is to define a few different possible solutions, like perf panel, slot panel, tubes, etc. Then imagine some scenarios for each. For instance, the slat absorber could be along the lower wall with its top creating a shelf or bench. The panel absorber could be faced with an absorber to maintain HF control, etc…
There are trade offs that need to assessed, but first take some better measurements and clearly define your listening position and speaker placement. Speaker placement will affect SBIR and stereo imaging and possibly even modal behavior if they are near corners. Change one parameter then measure then one more and measure again.
Spencerleehorton
Well-known member
sorry i thought the waterfall was the decay time, is this ok the RT60?
Spencerleehorton
Well-known member
putting on the 120hz sinewave and moving around the room is interesting!!
literally only the mix position has the null everywhere else is fine.
i then stood on a chair and took my head up to either side of the ceiling panel and the 120hz definetly gets louder so i will try hanging another couple of panels either side of the one ive got there and see if any improvement.
literally only the mix position has the null everywhere else is fine.
i then stood on a chair and took my head up to either side of the ceiling panel and the 120hz definetly gets louder so i will try hanging another couple of panels either side of the one ive got there and see if any improvement.
That would be 3 or 4 slats ending with only 2 or 3 slots at 400mm wide slats, not much slot area for absorption - these also providing a large reflective surface area for reflecting highs. As would panel and other LF absorbers I guess.
I built a studio in Sydney in the ‘80s where I had walls which were completely slatted with diagonal slats in the large vocal/drum room. The air gap behind was 90mm, with absorbent material in the cavity, framework covered with special acoustic fabric, slats over that and the timber slats were of varying lengths with free ends to allow them to vibrate as well. We did impulse and instrument tests in the room - a small starter cannon was used by the guy who did the analysis for the impulse tests. The room had very little in the way of peaks or dips and was brilliant for vocals, acoustic and electric guitar as well as drums. It was a semi live room.
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As this is the focal point for your monitors this is the area to get full wall and ceiling coverage. Your speakers at 145cm apart are each sitting 1/4 of the way across the room, your seating point and the speaker focal point is at 2/4 in the centre - I think this may be an issue here.
Another little test is shifting your monitors down the room (to the other side of your desk) ie. placed at the current listening position - same spacing and orientation and see if there is a hotspot at the new listen position, further down the room - or if reversed in direction to face the front of the room at that spot, a hotspot at the new listen position at your front wall. I think you’ll find you get one no matter where down the room you place them although it may improve bringing them out from the front wall.
Most larger control rooms are wider than they are deep but yours is a tricky room to shoot across instead of down the length of the room as the current back wall has a window and door and symmetry may be hard to achieve. However you can try setting your monitors up to fire across the room to see what results you get from that
Spencerleehorton
Well-known member
this position is speakers 134cm apart and mic is now just before the desk, pretty much where my head would be over the keyboard.
the main change here is i now have 3 panels above my head and ive put a makeshift panel on the floor on the front wall, as below the desk there is a big build up of 120hz, so i could make a few more panels and put them on the front wall, as when i listen this seems to be the culpret.
pretty major change now.
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