# Tame Mid-Room Null?



## minterbrad

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

WTH??

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.

Components:
AVR: Onkyo DTR6.5
BD: Oppo BD83
DVD: DPT1
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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## Sonnie

[No message]


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## glaufman

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:
http://www.hometheatershack.com/forums/rew-forum/
And the download, here:
http://www.hometheatershack.com/forums/downloads-area/19-downloads-page.html
And help files for getting started here:
http://www.hometheatershack.com/roomeq/wizardhelp/help_en-GB/html/index.html

If you could provide a sketch of your room, that would probably help as well.


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## dyohn

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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## dyohn

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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## minterbrad

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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## minterbrad

UPDATE:
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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## glaufman

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:
http://www.hometheatershack.com/forums/testing/21659-post-padding-thread.html
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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## drdoan

Welcome Brad. Have fun. Dennis


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## dyohn

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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## minterbrad

Greg,


> the null is created from reflection that aren't necessarily modal (think FRP)


What is FRP? If my thinking is not flawed, modal bass nulls can only be balanced by sub placement and number of subs, which would mean my only resolution would be to add another sub. I have tried near field placement by the front couch and it lowered the levels in that front row. I was only able to move it 2 feet or so due to the small room and big honkin' couches.

I considered the concept of a second sub and trying to balance the new modes, but there are only two available locations:
1. Front wall on the other side of the AV rack, which is where I ran tests to confirm that the same front row null exists with one sub. Could this change with two subs? Imaging front wall layout:
Left-Sub(new)-AV Rack-Sub(Exist)-Right
2. Back Corner

Neither are optimum, and I am afraid of what peaks may develop with corner placement. It is also to my disadvantage that the subs I own are different, and HUGE. One is the Velo F1500 (in service now) and the other is the SVS PB12-plus (awaiting service somewhere). Maybe opposite corners? Or should I consider buying two new smaller subs?


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## minterbrad

Finally a crude sketch of the HT layout. The "superchunk" bass traps are in the front corners, floor to ceiling. I know, heads on the back wall are a no-no, but the bi-pole ADP-190's work quite well from the side walls near the back. The bass is probably exagerated on that back wall as well.


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## minterbrad

David,
Testing absorption points is easy enough by hanging moving blankets etc. How can I test diffusion? I think that the very deep texture is quite diffusive, despite my comment earlier about being absorptive. Since it is a relatively hard, very irregular surface, would you agree?

I read read the article on multiple subs...thanks for the link. The best arrangement appears to be two subs centered on opposite walls. I would need to buy two new small subs to get this done. Can you suggest a good value?


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## minterbrad

Some pics:
Wall Texture
Riser-Trap -not my "whiteys"
Front Superchunk Traps


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## glaufman

minterbrad said:


> What is FRP?


First Reflection Point... more commonly talked about WRT higher frequencies, but I believe it is still a factor in bass response...


> If my thinking is not flawed, modal bass nulls can only be balanced by sub placement and number of subs, which would mean my only resolution would be to add another sub.


Sort of true, yet sort of not true. The problem is the word "only." Modal bass nulls CAN be balanced (somewhat) by sub placement and number of subs, the effect of them can also be minimized by listening position placement. In the higher frequencies of the bass region, traps are effective, though the commercially available ones seem to roll off on their absorption before you get down to the real subwoofer range. In that range EQ can be effective.
Looking at the sketch you provided tells a great story. You're front row could very well be in a good place after all, while your back row is so close to the rear wall that it's the one getting the most effect from the room modes, it's sitting in a peak, so you get LOTS of bass, but as you can see from your numbers, the exact level is all over the place as you vary frequency. So, if it was me, I would consider whether I could move the BACK row FORWARD to where it got a flatter response (like the front row), eve if that sacrificed SPL. I would then consider adding more subs to get more headroom, and therefore, more SPL. But I'm addmittedly not an expert here.

It's fairly difficult to properly calibrate two different subs to each other. It's time-consuming enough to do 2 identical subs, let alone 2 with different capabilities. I think the commonly accpeted approach is to "stagger" the FR they each handle, say use one from 20-40Hz, and the other from 40-80Hz. But for sure, the one less capable of big, undistorted SPL will limit the ability of the other. But If you already have a second, you might as well try it and see if you like it.

When you took your numbers originally, you took one set for front row and one set for back row, have you done any looking at how the 2 rows vary from seat to seat?

It's better to measure how a sub performs in a given location than to speculate about it. It's difficult to move these subs sometimes. Old school no-measuring-equipment people used to say to put the sub in your LP, play it, and crawl around the room. Where the bass sounds the best is where you should place the sub. Take this one step further. Instead of crawling around the room, put your SPL meter in the locations you're consdiering and take your measurements. Whichever one shows the best will be a decent place for the sub. But I think it'll be extremely difficult to balance two subs with just an SPL meter. Better to spring for the soundcard and run the scans in REW.

BTW: when I said you could use EQ, I mean to tame the PEAKS. It is inadvisable to try and boost bass nulls with EQ.


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## minterbrad

In agreement here. We are now getting to the compromise options of my little HT. In the design stage, I was aware of some challenges that would accompany the small room+large furniture and a second row. Obviously, the results have greater deficiencies than expected.

In my comment about modal bass nulls, I was being inflexible about the seating positions since the riser, pathways, legroom are all crammed into such a small space. Now I am struggling with "form vs function" and may try some adjustments in the back row position. It would also liven up the rear surround field to move heads forward off the back wall.

Your split sub idea has merit. I considered this months ago but was discouraged by the same guy that told me to put the old Velo15 out to the curb when I asked about amp repair. I will try both and see what happens time permitting.

I did take measurements in various seats with approximately the same results across a row. The right side seats were a little higher due to the wall and corner boundaries.

Do you know of any traps that can help in the 20-40 range? I am more interested first in full musical sound and "seat of the pants" in the front row rather that close scrutiny of measurements, although they are closely related If the next sensible step would be to graph REW on a soundcard, can you recommend one, or should I go to the guides?


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## SierraMikeBravo

glaufman said:


> First Reflection Point... more commonly talked about WRT higher frequencies, but I believe it is still a factor in bass response...


Yes it does, but more importantly WRT to clarity. First reflection points are by far more important than people realize. It needs to be treated as one of the most critical aspects of the room, and thought of a little bit more than just throwing straight diffusion and absorption at it.


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## glaufman

minterbrad said:


> Do you know of any traps that can help in the 20-40 range? I am more interested first in full musical sound and "seat of the pants" in the front row rather that close scrutiny of measurements, although they are closely related If the next sensible step would be to graph REW on a soundcard, can you recommend one, or should I go to the guides?


I don't know of any that perform very well in that range. I'm not sure if it's a size or material issue, but I suspect the size would be impractical regardless of any material issues. I always like to see nice pretty graphs myself, and they sometimes can tell more of a story. No promises there though. I personally use the soundblaster live external card, really any card with a line in and line out that works with your OS should work fine with REW (except firewire cards on a mac). 

You might take measurments just a few inches back from your front row and see if you get more power and what traedoff you get with the flatness... it might just be an interesting experiment with not much real value, though.


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## Jason_Nolan

Middle of the room, the only real solution is to reposition seating (although I know you don't want to). It may only take one foot forward to get out of it, but if the null is in a low frequency the only cheap way to deal with it is to lose a foot on those front rows.


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## Jason_Nolan

And RPG makes a modex corner that's tuned at 40hz, very narrow band absorption focused at that frequency with some absorption upto plus/minus 10hz. That's about it, and they're expensive. I've been looking into lower absorption for my room as well.


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## glaufman

Looking at the diagram, I'd try a few inches to a foot back before going forward.


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## glaufman

Ah, I was thinking broadband, never thought about tuned... 
That's an area where if it was me, I'd like to see the graph before comitting to a tuned treatment (might need more at a different tune...)


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## Jason_Nolan

Doesn't matter. Displacement is displacement. Foot forward, foot back. It get's you out of the middle. Most importantly I'd set up a mic with rew and measure both places to see direction and seating position. Since you have two rows you'll have to live with some compromise. It's hard enough trying to get one single seat perfect in the middle, but doing two rows will be difficult to impossible.

My best advise it to take measurements and document each step and position so you're not chasing your tail.


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## glaufman

Jason_Nolan said:


> Doesn't matter. Displacement is displacement. Foot forward, foot back. It get's you out of the middle.


Depends: middle of what? He's already a little back from dead center on the room, which should be the predominant mode, so moving a little further back would get him further out. If he's in the middle of another mode, though, that's not centered on the room, then yes.


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## minterbrad

Great feedback guys. I have considered and failed at most of the viable solutions you have offered. This problem is all due to the greedy over-use of a small room, trying to seat too many butts, unkowingly relegated to a thin lifeless place in the room. There is still hope, however...

There is merit to moving the front row forward, but then it puts the front row uncomfortable close to the front channels...I like it at near reference level, which is disturbing to many guest at "point-blank range." Moving back, well now my feet are on the back of their head. No problem for me, but you get the idea Moving the back row forward will help reduce the peaks for that row, but again, toes-in-back-of-head when reclined.

Corner/wall LF absorbers could lessen the problem peaks, but not raise the null considerably since the amp/sub is already pushing max in these testing phases to try and bring some life to that cursed front row. For now it is "seat of the pants" testing since the problem is so obvious. I will stop whining very soon!

I mounted Clark Silvers in the front row to help round out the "feel." That was a bit of an ordeal. First tried them mounted to the metal floor rails with pine. Do not make this mistake if you want any articulation. Second attempt was pine on the seat frame...no-go. The final and quite acceptable installation is with red oak 3/4" plank screwed to the seat-frame. Another notch in the budget. This provides great detail and immersion, especially when your head/neck is on the cusion of the headrest. I know that there are many that dislike this technology due to it's potential "gimmick factor," but if gain and xover is set correctly, it really adds a great missing dimension to the music and movies in the front row. I resist overloading the effect with excessive gain.

Now, moving to the back row and feeling/hearing what it SHOULD be brings me to the eBay auction for a Velo 8" Minivee to be placed beside the front row. This will hopefully allow me to turn down/EQ the main sub a bit to reduce boomyness along the back wall, and help fill the void in the front row. For the sake of clarity, picture the front-right chair having a sub between it and the wall (inches off the wall). I have not made the purchase yet...does this arrangement seem plausible? Should I just auction off the subs I have and go for a set if identical 10" velos? Ouch on the budget...but we are obsessed aren't we???

I am reserving the time and effort of REW for the moment until we get some LFE in the front row. Then we can EQ ...


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## minterbrad

Greg, Jason,

I will be REW'ing the room before the end of the year, after getting close to some balance. These results will tell us where we would need to be with any additional absorbers if that route is chosen...

Looking forward, just how expensive are these RPG corner units? I was previously interested their "diffsorption" technology and it seems that they are fairly cutting edge.


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## glaufman

minterbrad said:


> There is merit to moving the front row forward, but then it puts the front row uncomfortable close to the front channels...I like it at near reference level, which is disturbing to many guest at "point-blank range." Moving back, well now my feet are on the back of their head. No problem for me, but you get the idea Moving the back row forward will help reduce the peaks for that row, but again, toes-in-back-of-head when reclined.
> 
> Corner/wall LF absorbers could lessen the problem peaks, but not raise the null considerably since the amp/sub is already pushing max in these testing phases to try and bring some life to that cursed front row. For now it is "seat of the pants" testing since the problem is so obvious. I will stop whining very soon!


Here's some more suggestions: pull the front row forward to where it sounds best for you. If it'suncomfortably close for guests, it becomes your seat. Then achieve the best possible compromise for the rear row... you can stagger the seating, so the back row feet aren't on your head but between you and the next seat...

when you do a scan, I'd love to see scans with the mic 6.5' and then 10.5' from the front wall...

Bass absorbers should help the dip you're talking about, as they'll absorb the reflections that are cancelling out the sound. So the sub won't work as hard to get the same SPL in the null areas, although you will lose SPL in the peak areas, but it'll be cleaner and flatter all around. Or at least that's how it works in the frequency ranges where we use broadband absorption.


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