Low end troubles ...

knightfly · Post by **knightfly** » Sun Sep 11, 2005 9:51 am

Still 'way too short on time to catch ALL your q's, but placement isn't going to be intuitive (even with several years study) for a pressure-type trap - it's likely you'll need to do several tests to find the biggest peak (along walls/ceilings where you can mount such a trap) to find the biggest peak pressure at design Fo - that's where a panel resonator will have the greatest effect.

In a rectangular room, it's easy; you figure out which harmonic of which dimension is causing the problem, then place the trap either against one of the offending walls, or if it's an upper harmonic you can fairly easily find ALL the peak pressure points (especially easy with the Harmon mode calc) and place the trap at any one or two of those points along the wall or ceiling, etc -

In your case, you'd need to "fish around" with your mic (omni, I hope) until you find the highest peak pressure at Fo, looking (obviously) at places you could actually PUT the trap.

HTH, I'm off to the last night shift (then I get a glorious 1-1/2 days off, woopee) - I'll check back on this as soon as I can.

Meantime, one more quick answer -

http://www.mbr-design-group.com/tech/no ... gmatic.pdf

r.baby · Post by **r.baby** » Sun Sep 11, 2005 5:39 pm

Hi Steve,

EXCELLENT link!

Just to recap (which reminds me: I gotta recap some Neve preamps tomorrow! hehe) ... you did suggest I fill up the right corner with dense fiberglass, right? That still stands?

Also, a side question for Ethan or whoever is in the know: Does the trap HAS to have the exact width to heigth ratios from your Build A Better Bass Trap-page? The 2' by 8', I mean? Could I make it wider and shorter? Or would that alter its characteristics? I don't have any 8' free space anywhere.

best regards (and sorry again for messing up everything with my limited printout. Sigh)

Fl.

knightfly · Post by **knightfly** » Mon Sep 12, 2005 1:12 am

Yeah, there's probably no place in your room that absorption would NOT help

As to panel trap sizes - as long as the trap is large enough to work at all, the panel size doesn't affect the frequency (in theory anyway) - 2' x 4' is probably the smallest size that wouldn't start to unnecessarily restrict panel movement.

Beyond that, as you can see from the formulae, size doesn't matter; only front panel weight, overall depth inside, and having the requisite rigid fiberglass or rockwool NOT touching the front panel, with typically half the volume filled (6" deep trap, 3" insulation, spaced so there is 1/2" to 1" of free space between the front panel and the absorbent.

Also, the formula assumes that the back of the trap is an infinitely rigid surface; so for best accuracy in calculations, make the back as thick and heavy as possible. 3/4" MDF is kind of a de facto minimum for backs... Steve

Ethan Winer · Post by **Ethan Winer** » Mon Sep 12, 2005 1:51 am

Flemming,

> It doesn't really say anything about the size of the plywood <

It really does! Look for the words '1/4" plywood' and '1/8" plywood' on the right sides of the panel trap portions of the drawing.

--Ethan

r.baby · Post by **r.baby** » Tue Sep 13, 2005 9:18 am

Steve: Thanks again. I'm a bit ill these days so not a lot of construction work right now, but I'll get on to it as soon as I'm up again!

I think I've decided against the EQ option, btw. I tried it a bit, and it's a mess. Makes the sound go all thin and weird at the back of the room (where there's a small lounge area for my clients), plus I'd have to get some really good quality hardware unit to get it to work, given the fact that I use both direct DAW output, mixing desk output (I've got a tla 16 channel desk), and CD output when monitoring. I mean, I COULD look into building a discrete line level 8 dB/45 hz filter with a wide q and plug it on the right amp input, but as you said: it just feels SO wrong! Also, isn't filtering gonna affect the signal higher up at its harmonics?

I think I'll try the trapping and treatment methods first and see how close I get. The good thing is that the problems are so low that they aren't really affecting my stereo image (which is surprisingly good, actually, given the circumstances), and that the boost is so wide that it actually just sounds a bit like an A&R-rep eq setting!

Ethan: Yeah, I know I know! I already posted my apologies for any confusion a few posts back. Sorry, mate! My bad!!

Now, back to bed!

Fl.

drfrankencopter · Post by **drfrankencopter** » Sun Sep 25, 2005 5:35 am

I think I've decided against the EQ option, btw. I tried it a bit, and it's a mess. Makes the sound go all thin and weird at the back of the room (where there's a small lounge area for my clients)

Did you measure the spectrum at the mix position, and at the client couch? You shouldn't apply different criteria for satisfaction on a whim....better to be objective and methodical, then build your opinions on fact. Your first post showed frequency response spectra which suggested too much bass buildup on the side located in a near tri-corner. You EQ it out, and now complain that it's thin....without taking the time to measure if it's flat or not.

It's your control room, and you have to be happy with it...and maybe you don't really want a flat response, but I think you ought to verify with a spectrum analyisis if the bass buildup is indeed a boundary effect, or if it's a mode.

Actually, most people prefer a little boost in the lows, makes the mix feel bigger, and keeps the finished product from being too bass heavy.

I COULD look into building a discrete line level 8 dB/45 hz filter with a wide q and plug it on the right amp input, but as you said: it just feels SO wrong! Also, isn't filtering gonna affect the signal higher up at its harmonics?

You'd want a shelf filter...pretty easy to make with 3 op-amps. Affecting higher harmonics....no, a shelf filter should have no effect on anything in the pass band. You really need to think that (if it is a boundary problem), what's coming out of your speakers is already improperly EQ'd....adding another EQ can actually fix that problem.

Cheers,

Kris