Nasty Room Null: What to do? - Home Theater Forum and Systems -

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post #1 of 6 Old 09-21-07, 11:36 AM
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Nasty Room Null: What to do?

Nasty room calcellation problems...

I'm in the middle of replacing my existing VMPS "larger" subwoofer with a now-completed pair of "typical" 650 liter, ~14 Hz tuned SoundSplinter RL-p 18 monsters. Unfortunately, one of the two woofers arrived with a frozen voice coil, so I was only able to hook one up for its maiden voyage last night. As a side note, Mike at SS was more than helpful in getting the defective woofer returned and replaced...

I always had a nasty room null at ~33-38 Hz with my VMPS larger, no matter what I did. When measured, it's more than 10 dB... I was hoping that by corner loading each of these two new subs in each front corner, things may act a little differently. The VMPS sub, for reference, was on the front wall about 2/5 of the way in from the side wall...

First, let me just say that the amount of air even ONE of these things moves is a little more than ridiculous. The VMPS larger, for those of you that know, is a of a sub in its own right: a 15" and 12" active driver arrangement with a 15" passive radiator. Aside from the nasty null already mentioned, it was fairly flat to ~20 Hz in the room. No comparison with just one of the RL-p 18's going though!

My question is simple: is there anything I can do to tame this null right in the worst of spots? I've got a BFD and can boost the snot out of it, but this is obviously not the most ideal scenario.

Specifics: Amplifier = QSC PLX2402, right now with each channel driving a voice coil of the single woofer. Sub is in the front corner of the room.

Room dimensions: 23' long, 14'9" wide, 8'2" ceiling height, but half of the ceiling is lowered in order to hide ducts and a beam (it's in the basement)... and in these spots, the ceiling is 7'3" high. I suspect my problem lies in that the 1/4 wavelength of a frequency in the mid 30's is about 7.5', and I have two dimensions here that may be an issue with the 7'3" ceiling in parts, and the 14'9" room width... Anything I can do besides make the most of it with the BFD? What's the highest boost anybody has applied via a BFD in order to level out a very deep null like this?
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post #2 of 6 Old 09-21-07, 12:38 PM
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Re: Nasty Room Null: What to do?

I suspect my problem lies in that the 1/4 wavelength of a frequency in the mid 30's is about 7.5', and I have two dimensions here that may be an issue with the 7'3" ceiling in parts, and the 14'9" room width...

Room responce is your biggest problem. I recommend base traps 1st. You will probably need quite a bit (not so bad DIY)--corners (wall/wall, wall/ceiling, even that ceiling drop down will be a great place). After this, you can try adjusting the signal. There are very good articles on DIY traps & placement on this forum.
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post #3 of 6 Old 09-21-07, 12:53 PM
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Re: Nasty Room Null: What to do?

Can you please point me in the right direction in the way of information on bass traps? I don't know a lot about them... can they actually change the acoustical properties of the room just by absorbing certain frequencies? Does this help even in the case of a deep null?

For more info, I've constructed this room in typical AVS-Forum style fashion, with TheaterShield Plus (now InsulShield Plus) on the entire front wall, and the bottom half +/- of the side and rear walls. Top half of side and rear walls are reflective. I don't suppose this 1" thick treatment is anything compared to an actual bass trap though, is it?
post #4 of 6 Old 09-21-07, 03:53 PM
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Re: Nasty Room Null: What to do?

Is it feasible in your setup to move the sub to a different location? If so, place the sub in your listening position and crawl around the room to see where the bass response sounds the best. That will be the best place for your subwoofer. If that is not an option then a bass trap or traps will be your next option. I haven't used them myself but to deal with a room mode in the 35 hz range will require a large trap.
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post #5 of 6 Old 09-21-07, 05:42 PM
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Re: Nasty Room Null: What to do?

Can you point me in the right direction for how to treat a room for such low frequency problems as this one (~35 Hz)? Much of the bass trap info I've read caution that they don't do much below 60-100 Hz... what's my best shot at taking care of this?

post #6 of 6 Old 09-22-07, 08:19 AM
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Re: Nasty Room Null: What to do?

How to treat will really depend on the cause. Are you positive that it's not seating position related? Boosting a null with an EQ is just a bad idea - sorry.

The first thing I'd do is to try to move the sub in the vertical dimension. People forget that there are 3 dimensions in a room and sitting on the floor against the front wall is still in a corner. Try getting it up about 1' off the floor. Oh, and you can try the corner but it'll likely be worse in a ton of other places since you're maximally exciting every mode in every dimension of the room.

If you end up finding that the soffit undersides are causing issues (I'm not personally thinking so but it could be), the only way to effectively absorb that low is either:

- A very thick broadband absorber with a might membrane bonded to it. It may not be perfect at 35Hz but it can certainly help. We're talking probably 7-8" thick here at a minimum.

- A wooden sealed full blown membrane absorber tuned VERY low. Something like this would be a bit thinner than the above but will would likely be somewhere in the neighborhood of 5" thick.

Once figure out the CAUSE of the issue, then we can determine where best to place something like this.

On anothe note, there is more to room tuning than frequency response. Decay times and reflections are just as or more important than frequency response. For those things, more broadband treatments are in order. This is part of why I mentioned the soft/faced type absorber above - it will help with both FR and with decay times over a wide variety of frequencies whereas the membrane (wood) absorber is only good for maybe 1.5-2 octaves.

Let the fun begin....


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