# Starting down the path...



## Bullitt5094 (Mar 5, 2012)

I would like some advice on how to attack a specific problem in my room. In my favorite seating position there is a significant dip at 65 hz. As you can see in the graph it's about 15dB. It exists throughout the center of the room and actually gets worse in the back row. On the edges of the room it totally disappears. What would be the best solution for eliminating this and getting more consistent low end in all seating positions? My guess is the addition of a second sub. I am going to do acoustic treatment but I don't think bass traps are going to eliminate this. Moving the sub seems to have very little effect, which seems strange to me. The sub is crossed at 80 and this plot was done with the B&W 804s active, so you do see their effects too at the cross and above 80.

Room is 14ft W x 19ft L and the ceiling height varies from 8ft at the front to 10ft in the middle to 8ft in the rear under the projector shelf. Double doors in center rear. Fronts are B&W 804s and I built a DIY sub which is two opposed 18" drivers in a 24" square sealed enclosure powered by a Crown 1000. I have a BFD to equalize the Sub. It was out of the loop on this measurement except for 5dB or boost at 20Hz. I can duplicate the sub and place it if necessary but if there is something I'm missing, I would prefer not to have to do that. Any advice is welcome and appreciated!


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## bpape (Sep 14, 2006)

What you describe acts like a width mode but plotting your dimensions, that's not the case. I see nothing at 65Hz. 

Are you cutting off your mains or are you running them full range and then augmenting with the sub? Where is the xover? 

Bryan


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## Bullitt5094 (Mar 5, 2012)

I have the mains/sub output crossed within my B&K 317. X is 80Hz with an upper slope of 12dB/octave and lower at 24dB/octave. If I put the mike anywhere near the perimeter of the room I get something like this... What dimension should I be looking for to cause a problem at 65? My 2ft riser is 11 ft from the front wall. My ceiling is multi-soffeted and slopes from front to back, so there is no constant dimension between the floor/riser and ceiling. The drop from the ceiling is 16ft back and drops 2ft from the ceiling in the center. Do I need to do a detail of the room?


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## bpape (Sep 14, 2006)

As I said, there aren't any modes specifically at 65Hz - axial or otherwise. If moving front to back doesn't change much in terms of the center frequency, then it's likely not a cancellation off the rear wall. It acts like a width mode but the calculations don't bear it out. Just for giggles, can you take a measurement at your front to back position but with the mic say 1' off center either left or right.

Bryan


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## Bullitt5094 (Mar 5, 2012)

Shirley I can do that.
:bigsmile:


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## Bullitt5094 (Mar 5, 2012)

There was no significant change in the response at one foot to the side. I have to move to the next seat over to get any change and then it changes a lot. You can hear/feel a lot more bass on the two outer seats than you can in the two inner seats. I'm pretty happy with the sound except the fact there is a very noticeable change in the base levels between the middle two seats and the outer two seats in both rows. I need to figure-out a way to fix that.


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## Bullitt5094 (Mar 5, 2012)

Bryan, surely your out there somewhere? Didn't lose you, did I?

What should I try next to figure this out. It drives me crazy that I can move one seat in the same row and seem like the low-end SPL drops by half. This HAS to be a reflection/cancellation problem below 300, right?

I really, REALLY appreciate your help/advice. :T


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## bpape (Sep 14, 2006)

Sorry - been swamped at work and missed this one.

Was there any difference when moving a foot? If so and more when you go off center, likely it's a combination of a closer width mode and a 2nd or 3rd order mode somewhere. Generally, something in that range in that size room would be coming off the rear wall but then the rear row would move in frequency rather than getting worse at the same frequency - which leads back to the room width.

Bryan


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## Bullitt5094 (Mar 5, 2012)

Not doing well understanding your answer. No, moving one foot didn't change anything. 

What should I try? 1)Bass Traps in the corner(s)? 2)Moving the Sub other places? 3)Adding another sub? 4)Build a new house with a room with different dimensions? 4)Sell all my on Craigs list and turn the HT into a storage room. 
Leaning toward option 4 at this point. :crying:


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## bpape (Sep 14, 2006)

A sketch and or some pics would help me visualize better. What I'm saying overall is that it's acting like a reflection issue but those don't normally happen that low. Not saying it can't happen.

Bryan


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