# Bass Traps for In-Room Response?



## aboroth00 (May 15, 2010)

The room is irregular shaped with dimensions about 13'x15' with a 3'x5' nook in the rear right corner. The speakers are situated on the longer wall. I also have a drop ceiling that is 3" from the actual ceiling which makes the height of the room about 7.5'. The ceiling slopes from 94" to 90". I have 1" OC703 2'x4' absorbers at the first reflection points on the side and front walls (between the speakers). The LP is about 3' from the rear wall and 10' from the front wall. Perhaps moving the LP a foot or so closer to the speakers might yield better results to be in the bass sweet spot of 33-38% of the room dimensions?

I am running stand mounted speakers (Revel Gem) which are about 8 feet apart and 3 feet from either side wall and 2-3 feet from the front wall. The subwoofer is situated in the middle between the main speakers at about 3 feet from the front wall. The positioning of the subwoofer has yielded the smoothest response thus far. The speakers are rolled off starting at 80hz and the subwoofer (James Loudspeaker L4000P) rolled in at 50hz. So far I was able to achieve quite a flat FR without the need for any EQ. 

I can probably experiment a little further with the speaker positioning to try to get rid of the nulls in the bass region. I anticipate being able to place corner bass traps in the front corners if that'll help? I was planning to purchase a pair of Tri Traps and experiment with the positioning and fill the entire corners if things look good. 

I'm a little noob with room treatments and REW but I have a couple measurements available and tweaked my current setup to the best of my ability. Any tips or advice would be helpful.


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## bpape (Sep 14, 2006)

A little more low bass absorption to tame ringing would be helpful. As for moving things, theory is great but the results you have below 90 is very good. I would experiment and move the seat about 1 foot forward and see if the 90Hz changes center frequency.


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## Greenster (Mar 2, 2013)

Brian
Looking at the graphs, could it be something picking up those frequencies and ringing in the room?


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## bpape (Sep 14, 2006)

I kinda doubt it. Usually when it's something resonating, it's pretty narrow and pretty intense - sticks out like a sore thumb.


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## aboroth00 (May 15, 2010)

Greenster said:


> Brian
> Looking at the graphs, could it be something picking up those frequencies and ringing in the room?


Hmmm... interesting... I never thought of this possibility. I actually had a lot of junk sitting on top of the subwoofer when I was taking the measurement and I believe I heard a little ringing as the sweeps were playing. I didn't know whether that would be picked up by the microphone and be of consequence. I actually played a 20hz tone and walked around listening for vibrations. Some of the junk sitting around on the subwoofer and adjacent audio stand vibrated a tad, but again nothing I thought of consequence. I cleaned off the junk since and haven't heard that vibration again or ran any sweeps.


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## bpape (Sep 14, 2006)

Not saying it couldn't be something in the room - just didn't look like it.


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## aboroth00 (May 15, 2010)

bpape said:


> Not saying it couldn't be something in the room - just didn't look like it.


I'm not quite convinced yet either. I'll do a little more testing this weekend.


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## Erin H (Aug 26, 2009)

What crossover slopes are you using? If LR4, I wouldn't be concerned about the sub as much as I would be the mains, given how low the sub is crossed relative to the mains. 

I'd do this: measure the response of the mains by itself. Then the sub. This should tell you if the out of phase response at 90hz is caused by the room or the interaction of the mains+sub (ie; crossover phase relationship needs to be evaluated).


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## aboroth00 (May 15, 2010)

Erin H said:


> What crossover slopes are you using? If LR4, I wouldn't be concerned about the sub as much as I would be the mains, given how low the sub is crossed relative to the mains.
> 
> I'd do this: measure the response of the mains by itself. Then the sub. This should tell you if the out of phase response at 90hz is caused by the room or the interaction of the mains+sub (ie; crossover phase relationship needs to be evaluated).


Using a NHT X2 active crossover at 12db/octave. The response with just the speakers has that big dip at 90hz. The response with the sub has a huge peak in the 80-90hz region. I've already evaluated the phase relationship between the speakers. Pretty much tried all the settings on the crossover in regards to phase at each setting to see how they'd react. Pretty much the response above is the best I could get by just adjusting the phase (least dips/cancellation).


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## Erin H (Aug 26, 2009)

aboroth00 said:


> Using a NHT X2 active crossover at 12db/octave. The response with just the speakers has that big dip at 90hz. The response with the sub has a huge peak in the 80-90hz region. I've already evaluated the phase relationship between the speakers. Pretty much tried all the settings on the crossover in regards to phase at each setting to see how they'd react. Pretty much the response above is the best I could get by just adjusting the phase (least dips/cancellation).


Okay. I wasn't sure if you were trying to fix the overall problem or if you managed to find the culprit. 

What happens to the dip as you move the mic a couple feet to either side of your listening position? Does it stay the same or change? 

When you were playing with the crossover values, did you ever try asymmetrical points/slopes? Like 18dB/octave on the right and 12dB/oct on left. Sometimes different values helps this out. It's not the ideal solution but it's an option.


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## aboroth00 (May 15, 2010)

Erin H said:


> Okay. I wasn't sure if you were trying to fix the overall problem or if you managed to find the culprit.
> 
> What happens to the dip as you move the mic a couple feet to either side of your listening position? Does it stay the same or change?
> 
> When you were playing with the crossover values, did you ever try asymmetrical points/slopes? Like 18dB/octave on the right and 12dB/oct on left. Sometimes different values helps this out. It's not the ideal solution but it's an option.


Yup any help is appreciated! I'm guessing it's probably the room. Definitely at ~80hz. Play that test tone and the room obviously rings... especially when the sub is playing it at its position, lesser so than from the speakers. However the dip at 90hz is a mystery to me... Not sure what's going on there.

I have no played around with shifting the mic around. I played around with taht when I had my SMS-1 and moving the mic around didn't really do anything. Unfortunately the NHT X2 does not let you choose the XO slope as it is a fixed 12db/octave. I would imagine differing XO slopes would cause phase problems. Which I assume can be simply fixed with the phase control on the sub?

Not sure what you mean by assymetrical point... currently the speakers are being crossed at 80 and the sub at 50. The NHT allows me to set the high pass at 50, 80 and 110. The HP set at 50 or 80 works fine for the speakers but also yields the 90hz dip. Setting it at 110 causes an even bigger suck out at 90hz and little to no drop of the peak at 80hz. Setting the speakers and sub at 80hz causes reinforcement of the peak at 80hz. I pretty much exhausted all my options in that department. I continually played with the LP knob of the subwoofer with each setting of the HP. Setting the gap between the LP and HP has yielded the smoothest response so far.


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## aboroth00 (May 15, 2010)

After rereading your post, I cannot set my left or right speakers independently crossover-wise.


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## Erin H (Aug 26, 2009)

I meant asymmetric between the left and right mains. My bad. Like my example of a 18dB slope on one and a 12db slope on the other. 

Stupid question, but what do you get when you flip the sub polarity? Does that suck out at 90hz disappear? Wonder if that might fill it in. 

I want to suggest more but you said you tried all sorts of crossover values, so I'm assuming you've tried altering the points and slopes between the front mains and the sub (ie; 50hz/18dB on the sub instead of 50/12dB or setting the fronts at 80/24 and the sub at 50/12). That kind of thing.


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## aboroth00 (May 15, 2010)

Erin H said:


> I meant asymmetric between the left and right mains. My bad. Like my example of a 18dB slope on one and a 12db slope on the other.
> 
> Stupid question, but what do you get when you flip the sub polarity? Does that suck out at 90hz disappear? Wonder if that might fill it in.
> 
> I want to suggest more but you said you tried all sorts of crossover values, so I'm assuming you've tried altering the points and slopes between the front mains and the sub (ie; 50hz/18dB on the sub instead of 50/12dB or setting the fronts at 80/24 and the sub at 50/12). That kind of thing.


Doesn't fill in, makes it worse IIRC. I would imagine an asymmetric slope between the left and right would cause wild cancellation/phase issues. Again, the NHT doesn't let me adjust the slope. One fixed slope, 12db/octave.


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## aboroth00 (May 15, 2010)

Not sure what differing slopes would solve? Sounds like a headache and asking for more problems. Seems to me the issue lies with the room unless you beg to differ? Again I'm not sure what the advice is getting at.


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## Erin H (Aug 26, 2009)

aboroth00 said:


> Not sure what differing slopes would solve? Sounds like a headache and asking for more problems. Seems to me the issue lies with the room unless you beg to differ? Again I'm not sure what the advice is getting at.


Slopes drive relative phase at the crossover. Since the speakers exhibit a null near the crossover, I'd be curious to see if altering the slopes (which would then change the phase relationship) would change that dip. If not, it's a room problem for sure. If so, then it hints that the problem may be in the set up. 

Most likely the problem is the relationship of the speaker location vs the room vs your listening position. BUT, the first thing I personally would test is the phase relationship based on crossover settings just to rule that out. 


Anyway, it's all for nought if you dint have this capability. Was just trying to give you options for troubleshooting.


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## aboroth00 (May 15, 2010)

Erin H said:


> Slopes drive relative phase at the crossover. Since the speakers exhibit a null near the crossover, I'd be curious to see if altering the slopes (which would then change the phase relationship) would change that dip. If not, it's a room problem for sure. If so, then it hints that the problem may be in the set up.
> 
> Most likely the problem is the relationship of the speaker location vs the room vs your listening position. BUT, the first thing I personally would test is the phase relationship based on crossover settings just to rule that out.
> 
> ...


Got it. Cancellation due to phase issues. Yeah... I played around with the XO between my sub and I didn't see any issues when the subs were in phase and when they were out of phase it created problems. Couldn't I just inert the phase on my speakers (red to black, black to red) to see the same problem with different XO slopes? I know I did this accidentally once and the bass was all whacky. That's scientific lingo for you...


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