Tame Mid-Room Null? - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 27 Old 11-07-09, 03:16 PM Thread Starter
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Tame Mid-Room Null?

Well, after months of lurking, research and construction, I am finally posting! I have completed our HT (Ha!) and now in the process of tweaking. The never-ending process. I have written a thesis here to enlist the best in the arena of HT set-up...the Shacksters. Please forgive the long post, but I wanted to cover as many HT factors as necessary for you guys to suggest a plausible solution.

Feel free to comment on any aspect of construction taht may need improvement, but my main complaint is the bass null in the front row seating. I have also reached out to GIK and RealTraps for possible solution.

First of all, thanks for everyone's posts in the spirit of DIY and product guidance. I have been a DIY guy to this point, but have reached an impasse and would like to know if anyone can suggest an improved configuration or product that can fix my problem.

The HT is a 2-row seating arrangement, and the front row has a HORRIBLE null at most practical bass frequencies in music and movies. This is due to the mid-placement of the front row, but cannot be changed due to room layout, screen distance, legroom etc. My room is dedicated HT 12’x17’x10’ with a sloped/flat reflective ceiling and absorptive carpet floor.

For EQ, I am using a RS SPL meter and a spreadsheet, 1/6 octaves test tones, 16-160Hz, played on the Okyo DPT-1. The Oppo BD83 had trouble with the track splits on the burned test tone CD.

I began EQ with a parametric sub-amp that allows one cut/boost, and the AVR which allows another at set frequencies (40Hz being the only usable one). I was trying to decide if a BFD would help and discovered that I have much bigger problems.

My brother proclaiming “it needs more bass!” from the front row during a movie, while I was being bass-massaged in the back, did not settle well with me either.

The sub is slightly off the screen wall and 1/3 from the corner. I tried lateral 2/7 with reduced front seating output; 3/7 is not feasible placement. 7” off the wall was better than 3”; 12” was worse. I also tried side-wall, fore of the front row, but that also reduced output. No other sub-placement is really feasible due to the small room layout.
-Sub-amp X-over is ~90Hz, mains are set "small" in the AVR
-AVR X-over is set at 80Hz
-Sub-amp is reduced -2dB at 18Hz, 1/6 octave BW to reduce clipping in bass intensive explosion scenes. Gain is at 0.5.
-AVR amp is set at -2 at 40Hz as a result of measurements in the back row, and +1 at 80Hz to help the X-over dip. Raising the 40Hz range helps mitigate the null attenuation in the front row, but causes overwhelming increases in the back row.

I may remove that 18Hz cut and reduce the amp gain to prevent clipping, but right now the front lacks so much that I was compelled to turn up the gain and cut the sub-sonic for the benefit of the “audible” frequencies.

Walls are ½” + 5/8” sheetrock with 60oz GG per sheet, on 16” centers. Ceiling is the same except on 24” centers. All are covered in very heavy “mud” texture.

Floors are second story wood sub-floor with 1/8” polyethylene underlayment beneath 1/8” MLV with padding and carpet.

There are curtains in the screen-wall corners behind the mains, and soon there will be parted curtains mid side-walls for appearance and some first reflection absorption. Back walls are empty for now, with good mix of reflection/diffraction due to the heavy texture.

I have a 8’x10’x8”riser along the back wall that has the top-back 9” of deck open, and filled with 50% ultra-touch for the purpose of adding some bass trapping. No additional holes or ports. Measurements open and covered show that this does not make a difference, concluding that a small riser-trap is ineffective (except to raise the back row. I did use GG between the deck ply and MDF layers, attached on 16” centers.

My trapping consists of front corner super-chunks of the R21 UltraTouch, 23” across the face, floor to ceiling, covered with 1mil plastic. No trapping exists in the back corners where the tremendous bass lives, and I am not undertaking any more construction in the room.

Bass Measurements in trouble:
20Hz: FrontRow 76dB; BackRow 78dB (OK)
25Hz: FrontRow 90dB; BackRow 95dB
28: 91; 100
31.5: 92; 106
36: 82; 104
40: 86; 102
45: 95; 96
50: 94; 91
56: 93; 100
63: 98; 101
71: 97; 100
80: 90; 88
89: 80;91
100: 88; 87
111: 92; 84
125: 81; 65


So, how do I fix this? Will Monster Bass Traps or Mondo Traps in the one available back corner help the 25 to 56Hz range? Will 2 stacked make an improvement?

There are French entry doors in the other back corner that preclude traps there, so I am left with only one more available wall-wall corner to treat.

I am also considering bass-shakers of the musical variety to bridge the bass gap in front. I do not want anything overwhelming since I will also be listening to music in this room, probably from the front row due to proximity of the AVR and disc player.

AVR: Onkyo DTR6.5
BD: Oppo BD83
Mains: Paradigm Studio 80's
Sub: Velo F-1500, re-edged and re-amped
Note: I tried a spare SVS PB12-plus with no benefit. However, it did exhibit unacceptable port chuffing in the low frequencies that I never noticed before.
Sub-Amp: Dayton HPSA1000
Center: Paradigm CC350
Surrounds: Paradigm ADP190
Projector: 6500UB with latest FW..."low" FI is awesome now
Screen: 92" diag DaLite DaMat...perfect for the room and 12' throw
Dark brown walls, dark wood trim, black curtains, dark brown faux leather media couches (2x3 seats with cup/storage consoles between seats and leg rests), medieval and Gothic art.

Best Regards,
Brad Minter
HT Enthusiast
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post #2 of 27 Old 11-08-09, 03:38 AM
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post #3 of 27 Old 11-08-09, 08:33 AM
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Re: Tame Mid-Room Null?

Hi there Brad. Welcome to the Shack!
For starters, it sounds like you're already aware, the middle of the room is going to be a difficult place to get a lot of bass when you have modes cancelling it out.
To make things easier, I'd suggest you check out Room EQ Wizard, a software that you can use with your RS Meter and a soundcard make the work of taking measurements easier, and automaticaly display in graphical form. We have a whole section of this forum dedicated to using this software and interpretting the results. You can find this here:
And the download, here:
And help files for getting started here:

If you could provide a sketch of your room, that would probably help as well.
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post #4 of 27 Old 11-08-09, 10:06 AM
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Re: Tame Mid-Room Null?

A few things. First, your listed measurements seem pretty good to me. >90db in all critical bands? Pretty good, even with the Rat Shack meter which really gives you more of a ballpark measurement rather than precision. But with all readings taken with the same meter under the same conditions, you can use them as relative representations of SPL differences in different locations. Don't get too hung up on the numbers as that can drive you crazy. Trust me, I speak as one who has been driven crazy by them before. Use the meter to get an idea of your relative levels, but really your ears are just as good a judge of that as that meter in most cases as a 3 or 4 db difference is not really worth worrying about from a room mode perspective.

Second, it looks from your description that you've already made some good steps toward taming the space. While it is always possible to improve things, you want to beware of over-damping and eliminating the sense of reality that a live room creates. Once the room is no longer resonating and your major reflections are minimized, adding more sound treatments often only makes the space sound dead. Some people like this effect. Me, I prefer not to listen to music or movies in an anechoic chamber.

If your goal here is to create a uniform bass field at all seating locations (which is near impossible under the best conditions) your likely solution will involve moving your subwoofer around. Try it in different locations and you may discover a sweet spot where the results are more acceptable. You may have to add a second subwoofer system. But you really are unlikely to ever get a uniform bass field. It is just difficult. Create a situation where you reach a happy medium and the LF is acceptable in as many seating locations as possible and let it go at that.... unless you're looking for room certification or something. Let me know if that's your goal and I can recommend a THX certified tech in your area who you could hire to help you...

I also highly recommend ringing the room with a genuine RTA so you are not relying on SPL readings alone. You can do this yourself with a few tools, but again if you want to get serious (and it sounds like you do) hiring a pro in your area is always a better bet.
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post #5 of 27 Old 11-08-09, 10:24 AM
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Re: Tame Mid-Room Null?

OH and I thought of one point I forgot to make. You may just need to raise the overall LF dbSPL in the entire room rather than try and fight modes that may or may not be there. Have you tried a more powerful subwoofer system? Have you considered installing an architectural IB system?
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post #6 of 27 Old 11-08-09, 02:22 PM Thread Starter
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Re: Tame Mid-Room Null?

Greg, David, thanks for the post.

I checked REW and BFD...looks like awesome tools for taming bass. The goal is get the "wow" bass factor in the front row, since the front is weak across the bass range compared to the back. May not be possible, but I am sure gonna try to get close. Unfortunately, I have tried the two other sub locations, and the problem was worsened. That leaves bass traps and/or EQ. Since I do not have the proper soundcard for REW ( have the SoundBlaster X-Fi Notebook), my thought is that the RS meter and thorough SPL logging would confirm what our ears are hearing, and even pinpoint the deficient frequencies. Therefore, I would like to try and work on the " balance" front-to-rear rather than a nice curve in one seat.

David, thanks for the encouragement on the measurements. It surely does impress even my friends and family that have dedicated entertainment rooms. But alas, it is I who must be satisfied, especially if I have to sit in the front row while entertaining for guests sitting in the back. Sometimes we have to give up the "money seat."

I am stepping back and trying not to let the numbers get too overbearing relative to my ears. I have always been able to achieve excellent results by ear and general knowledge of acoustics, but realize that there is often more efficient and effective means of setup with tools proposed here.

Last night while demo'ing several concert videos, I noticed a significant shortcoming in the HF range. The room IS too dead. The curtains behind the mains are partly to blame, but i think the biggest factor is the very heavy texture on the walls and ceiling. I had the room covered in 6 boxes of heavy mud (approx. 760 SF of surface) that was applied with a "stippling" technique. It is very deep texture and obviously too absorptive. It sucked up six gallons of primer and paint that was applied with a sprayer. Unlike curtains, I cannot pull this down so easily. It looks great and gives the room a "huge" sound, but wow, did it ever kill that live concert effect.

So, now I have two deficiencies to overcome. Since I started this post with the bass mid-room null issue, I will attach a sketch for your comments. As for the HF/dead room issue, I would like to explore wall/ceiling treatments or even re-mudding (aaarrrrg!!!) specific areas to bring back some liveliness to the room. I did not realize how much I would miss it in concert features.

Hmmm...how do I attach a sketch?
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post #7 of 27 Old 11-08-09, 10:42 PM Thread Starter
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Re: Tame Mid-Room Null?

I re-evaluated the first reflection treatment for the mains and tried a test by hanging moving blankets in those areas. Wow- what a difference! The highs are now much better and imaging/soundstage is getting better, although some exists on the ceiling towards the rear, probably due to the reflections from the screen coupled with very absorptive chairs and carpet. I will try additional absorption along the first reflection wall in the next few days, but still awnt to recover more of the HF "sizzle" in the room.
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post #8 of 27 Old 11-09-09, 09:34 AM
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Re: Tame Mid-Room Null?

Hi Brad-
Yes, too much absorption in HF can certainly deaden a room way too much. Many times I see the recommendation to put absorption where it's needed (by finding trouble areas) and that's it.
As for posting pix, you should be able to do that as soon as you reach 5 posts. As it seems you're not a spammer, you can do this in the post-padding thread located here:
Even thought you think you're goal isn't flatter response at one seat, modes and other reflections still seem to be your problem here... the front row can be in a null while your back row in a peak... you mentioned trapping, the mondo traps attentuation drops off significantly below around 100Hz or so, so much of the range you're not happy with may only exhibit minimal gains from these... That being said, and I'm not an acoustics expert here, that the null is created from reflection that aren't necessarily modal (think FRP)... It's also possible your corner traps could do you more good in the back of the room, or on the floor/ceiling corners...
Did your expereiment with sub locations include near-field placement by the couch? j Is your sub currently in a corner? Although that often results in the MOST bass, it also often results in the most DIFFERENT bass experience from seat to seat, or row to row...
What I see people often do is put the sub where it has the least variation from seat to seat, or row to row... this can result in a reduced max SPL at the seating locations without running the sub so hot as to distort. But then you can add extra subs to make up the difference, depending on just how loud you want it to go... Another way to go would be to place the sub where it doesn't excite as many room modes, and add a second one in a different location to excite different modes, letting each one fill in the gaps the other one leaves...
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post #9 of 27 Old 11-09-09, 10:06 AM
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Re: Tame Mid-Room Null?

Welcome Brad. Have fun. Dennis

L Dennis Doan, DC
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post #10 of 27 Old 11-09-09, 10:34 AM
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Re: Tame Mid-Room Null?

Insted of adding additional absorption at your first reflection points try diffusion instead. And don't neglect the ceiling reflection points. Also, back to your LF issue, you may want to read Dr. Floyd Toole's excellent paper on using distributed subwoofers. LINK to Harmon white papers.
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