100 - 125 Hz Cancellation in Control Room - Ethan?

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DigitalDon

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Ethan or others,
I need some help with a freq cancellation problem in my control room. This control room has 6 corners (not including ceiling and floor of course). In the 2 corners behind me I installed Knauf 2" rigid across the corners from floor to ceiling. There is a couch behind me which I've installed a panel (4" deep framed Knauf 2" rigid) on each side between the couch and corners. I also have panels (same as above) on the wall behind my JBL 4311 full range monitors (a very wide angle corner is here). The cancellation problem I have is from 100 t0 125 Hz using the Minirator (from your website) as a signal source. It's there whether I use the JBL's or my M-Audio nearfields. Mic used is a Behringer ECM8000 into an Aardvark Q10. I tried other mics just to make sure it wasn't a bad mic. The cancellation is very audible when sitting in the mix position also. I'm losing as much as 15dB at these frequencies. As you can imagine, the low end of my mixes will almost blow the woofers out when played outside of the studio because I cranked in so much bass. I've learned to adjust my mixes to compensate but it's really getting old. I plan on building 2 more traps to place in the remaining 2 corners. My question is this: I have a vaulted suspended ceiling (14 feet at peak) with metal roof 18" above the ceiling. Do I need to place traps at any of these "corners" (wall meets suspended ceiling) at the side/back/front walls or do the low freqs pass on through the acoustic tiled ceiling and through the metal roof? Sorry this got so long but I'd like your opinion before buying materials to do all this. Oh yeah, floor is painted concrete and walls are sheetrock if it matters.

Thanks,
DD
 
Don, for anyone to seriously try to answer your question we'd need more info from you - ideal would be a scale drawing - at least we'd need exact dimensions in all three axes of your room, your ears, and the centerpoints of your woofers. Your comment about 6 sides tells me this isn't a straight rectangular room (last I checked, those had 4 sides :=)

So if you could measure across the face of your speakers side to side both from wall to wall (to hard surface, not to face of absorbers) and from speaker to speaker (note if they aren't plane symetrical thru the center of CR) then, side to side measurements at your head, front to back measurements at speaker placement and at your head (if different), and finally height measurements; floor to ceiling at speakers (and head if different) and speaker height from floor. All speaker measurements should be from center of woofer cone.

With those kind of variations, you may still need more trapping; but it sounds like either the speakers or your head are in a serious null, and we can't find that without the above data.

One way to quickly tell if it's your head in the null; listen at the mix position, (preferably with a well-recorded COMMERCIAL release) then slowly stand up without moving in any other axis; note any changes in perceived bass. If they're there, you're in a vertical null. Next, sit back down and slide your chair forward and back by at least a foot, slowly; any change in PBL (Perceived Bass Level; just made that up :=) then you're in a longitudinal null. Finally, move your head side to side. You'll lose stereo pretty quickly, but listen for any PBL changes; if there, it's a left/right null. This is worst case in some ways, since you need to be centered left/right to get good stereo imaging.

If none of these test make a diff (or not much of one) then it's likely your speaker locations will be the problem. Make sure that distances from woofer center to ANY surface are not even fractions of that room dimension at the speaker location - not 1/2, 1/3, 1/4, 1/5 for example. This needs to be checked in all three axes as well.

If you or your speakers are sitting in a null, there is no amount of power you can add (EQ) to compensate; something will HAVE to move. More traps would lessen the problem, once you find out which dimensions are causing it so you can figure out where the traps should be placed; but positioning should be optimised FIRST, especially where speaker juxtaposition is concerned.

So, break out the tape measure, make us a purty pitcher, let's see if we can getcha "back on bass" (sorry, can only go so long without bad puns or good funs)

Let us know... Steve
 
Steve,
Thanks for the advice. Yeah there's definitely some null activity going on here. Unfortunately the null is dead centered at my mixing position. I noticed yesterday when I raised or lowered my head there was a significant difference in sound intensity at the trouble frequency. This also happened when I moved back and forth horizontally. Sounds wierd but the physical sensation is like being in water and diving to a different level where the temperature suddenly changes.

I had a pretty serious problem at 250 Hz but it was reinforcement rather than cancellation. It was pegging the meter. I estimate the traps reduced it by about 10dB. Still not ideal but more manageable.

We have to load up equipment for a gig tomorrow night then play the gig Wednesday night. Thursday night is vocal dubs so it'll be this weekend before I can draw something up. I'm terrible at CAD (don't have the patience) so I'll probably just hand draw it, scan it and post it this weekend. Please check back this weekend.

Thanks,
DD
 
Don,

As Steve said, a simple drawing with dimensions will help a lot. But I can tell you a few things now:

If you have the same null from both sets of speakers, the problem is either modal or position dependant. Using the Virtual Minirator, play 100 Hz and move forward and back from the mix position to find where the null is the worst. Now increase the frequency and see if the null moves back, closer to the rear wall. Do this again at the next higher frequency, then try lower frequencies. If the null moves closer to the rear wall as you go higher, and farther from the wall as you go lower, then the cause is basic acoustic interference.

As for the ceiling, I suggest you lay 12-inch thick fluffy fiberglass batts on top of all the existing tiles. This is cheaper and easier than climbing all the way up to put rigid fiberglass at the ceiling peak above, yet it's still very effective. It won't do much to reduce nulls in the front-rear direction, but it will help generally.

Also, rigid fiberglass only 2 inches thick is not enough for corner treatment. You'll get much better results if you double it up to be 4 inches thick.

--Ethan
 
Thanks Ethan. I'll do that check as soon as possible

DD
 
Steve/Ethan,
Sorry it took so long but here's the control room drawings. One is from above and one from the side. I'm not a CAD expert so have pity on my attempt at using CAD. I had to really squash it to fit the 64K attachment requirement so I hope it's readable.

Don
 
Don, it looks like both of your speaker pairs are within 3" of the third harmonic of your axial mode related to front/rear dimension; the nearfields are 2" toward the front wall from dead null @ 120 hZ, and the mains are about 4" further away from the wall than that same harmonic would be nulled; so both pairs are too close to that null point. You didn't mention whether there is ANY place in the room where the 100-125 hZ range is strong enough; if not, then this front wall distance is likely the culprit. You could try moving the NFM's 3-4" closer to the front wall, and the mains 3-4" FURTHER from the front wall.

I didn't see any axial mode problems with speakr height; however, now that I see which direction your roof peak runs I'd strongly suggest putting a 3 or 4" fiberglass or rockwool "cloud" that's around 4' x 8', centered over your desk; as it is, you've got some unbalance going there due to ceiling heights, and you're also most likely getting some strong early reflections back from that ceiling - those could even be part of the dip at 100-125, although it looks more like what I mentioned in the first paragraph.

Try sliding your nearfields farther away and your farfields nearer by 3-4", gotta start somewhere... Steve
 
Thanks Steve. I'll give that a try.
Here's some freq measurement info if it helps. Top numbers are frequencies. Don't directly compare dB levels between the mains and nearfield monitors because they use two different amps. I'm sure the output levels aren't the same. Oh yeah, at 250Hz it pegged the meter. The numbers you see here are after installing two 2'x4' traps, one on each side of the couch about 3 feet from the floor and 4 inches from the couch. Measurements made with a Behringer ECM8000 omni mic at the mix position using the Minirator on Ethans site.

Freq 400 315 250 200 160 125 100 80 65 50 40

Mains -20 -13 -8 -16 -15 -26 -26 -11 -12 -15 -24

Nears -20 -13 -13 -19 -12 -29 -27 -15 -19 -25 -25

My speakers probably aren't very efficient below 50Hz so I wouldn't put too much confidence in the 40Hz levels.

Thanks again,
Don
 
Don,

> Measurements made with a Behringer ECM8000 omni mic at the mix position using the Minirator <

The Minirator is okay as a very rough guide, and we put it on our site mainly to give people a way to see they have a problem. A surprising number of people have no idea their LF response is so badly skewed! But to get a real measure of the low end response you need to measure at 1 Hz intervals. Using 1/3 octave sine waves (or pink noise) is simply too coarse to see the true extent of the peaks and nulls or their exact frequencies.

I use the ETF software (PC only), and at $150 it's the best value in a serious room analysis program.

--Ethan
 
Ethan,
Yeah I realize this was a very coarse measurement but it did indicate the same thing I am experiencing with my mixdowns.

Steve,
I tried moving both nearfield and main speakers like you said but it had no effect.
The greatest loss I am hearing at 100Hz is at 7 feet from the front wall. This is pretty much dead center of the room from front to back wall. At 80Hz the greatest loss moves a little over 1 foot toward the back wall. At 50Hz it's back to the middle of the room. Yes there are places in the room where 100Hz is very loud. About 2 feet from the side walls (door on the right side and opposite wall) again centered in the room from front to back. Also about 1 foot to the right of the right main speaker/8 inches from the wall behind this speaker. Same applies to the left main speaker but in the opposite direction.

There is a definite 100Hz "hole" in the center of the room. At it's worst it measures about 2 feet (centered between front/back walls) and about 6 feet wide (centered between side walls).

Just to see, I placed 2 of my 2'x4' traps on the wall above the couch covering a 4'x4' area directly behind me. Of course this had no effect. I'm starting to wonder if I should put a couple of bookcase units on the left side wall to break up that wall. I've also got several pairs of big Peavey PA speakers (SP2's and subs) I could place there in-between gigs. Gotta put them somewhere :D Just grabbing at straws here.

So, does it look llike the problem is due to front/back wall distance or side walls distance??

Thanks once again

Don
 
Don,

> does it look llike the problem is due to front/back wall distance or side walls distance?? <

Both probably. The center of any dimension is the worst place to sit while listening.

--Ethan
 
Sooo, I'm considered moving the desk and all to the left side wall. That will place me facing the wall (door behind me) with mix position about 5 feet from that wall. It would put me about 6 feet from room center. Seems I read somewhere the sound should project the longest distance in the room instead of the shortest like I have it now.

Don
 
Ok, I moved everything to the left side wall. My back is to the door. There is a definite improvement at 125Hz. Not quite as much at 100Hz but very noticeable. I can even hear the low rumble of 40Hz now. I'll be placing traps on the back of the door and just to the right of the door. 250Hz was very intense here before I set a couple of traps there. Still have to beef up the corners with 4" instead of 2" as Ethan said. All in all I'm much more satisfied with my room but still have a little way to go. I'm sure I'll put a "cloud" over the mix position because the ceiling is lower. Might try some thick insulation on top of the tiles here as Ethan suggested first.

DD
 
Don, there are likely a couple of reasons why you're better with the new orientation; one, your ceiling isn't causing non-symmetry left to right, since ceiling height is now the same on either side (even though it's sloped up to the rear of the mix position) - also, as you said it's generally better to have the deeper dimension behind you.

Unless your room is REALLY bright sounding, I wouldn't put a cloud over the mix position where you are now; it's not necessary there, because you have that sloped ceiling acting to make a Reflection Free Zone (only as far as the ceiling is concerned) - What I would be more inclined to do is to build a frame for the peak of your room that is a mirror image of the peak itself, with 4 foot sides; fill with standard UN-faced fiberglass batts, put 703 between the braces so that's the first thing the sound sees, cover with cloth. This will act as a broadband trap that handles down into the low mid range, and also minimise reflections that would come back to the mix position from the rear.

If you don't cover this with cloth til you hear the result, then if the room isn't still bright sounding enough you could staple thin plastic over the surface of this trap in sections til it's slightly too bright, then add the cloth.

Something like this... Steve
 

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