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post #1651 of 1676 Old 11-08-2016, 08:48 AM
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Originally Posted by rms8 View Post
OSHA and the USAF E&I Standards would disagree with you.

Installing a 100lbs on a shelf vs. hanging from the front ears ONLY is a much safer option. If he can rig something to support the rear as well, that would work too. Just because something worked for 3 years does not mean it's a safe option.


.
Wait, you're telling me that OSHA would disagree that I had amp hanging in my rack by just the ears? They must have missed in their last inspection. Phew.

I'm not saying that a shelf is safer, I'm not saying it's not. I'm not implying anything about the safety of this option. I'm pointing out that it is an option and readers and make the decision for themselves. After three years of being in a stationary rack both the amplifier and the rack were fine. I did have concerns about the rack, so I contacted MA who gave me the all clear. I know you received other guidance, but we have different racks. As noted above and in the manual for the amp, Sherbourn does not indicate that rear support is required. But each person can make that decision for themselves. For me, it wasn't work the extra cost of another shelf. For you it was. Nothing wrong with either decision. Frankly, the wire idea is great. I wish I would have thought if that.
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post #1652 of 1676 Old 11-08-2016, 10:12 AM
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Quote:
Originally Posted by rms8 View Post
OSHA and the USAF E&I Standards would disagree with you.

Installing a 100lbs on a shelf vs. hanging from the front ears ONLY is a much safer option. If he can rig something to support the rear as well, that would work too. Just because something worked for 3 years does not mean it's a safe option.


.
Fred only has front rack rails, so options are limited and your shelf won't work unless the shelf itself is rigged with support in the back vs. directly on the amplifier.

And there are PLENTY of amplifiers - and even heavier UPS / isolation transformers which weigh WELL over 100 pounds and mount only with front rack ears with zero problems because the chassis are specifically designed to work with the ears to handle the weight, compression and shear. A Richard Gray Powerhouse is 375 pounds with front ear mounting and no problems. Almost all Torus and ExactPower Isolation Transformers are front ear mount only. All Levinson amplifiers, ATI amplifiers, etc. are front ear mount only and nearly all of them weigh about 100 pounds or more.

The Sherbourn will hold as @BllDo implies and as Sherbourn stated, but there's nothing wrong with adding a bit of rear support for peace of mind, especially if you already have rear rack rails and it is easier to do.
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post #1653 of 1676 Old 11-08-2016, 10:34 AM
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My two amps are the lowest things on my rack. I actually cut a couple 2x4 blocks just long enough that they were a friction fit beneath the back of my amp and the floor. We're only talking a couple inches here, and just long enough that I could see the back of the amp move up a hair (that's the precision term for it, though there is one unit of measure finer ). Then another set between the first amp and the second. Again, just long enough to take some of the weight off the back and transfer it down to the floor through the first amp.

Not as elegant as some of the other solutions suggested, but this was also free, quick, and easy
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post #1654 of 1676 Old 11-08-2016, 10:52 AM
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Quote:
Originally Posted by TMcG View Post
Fred only has front rack rails, so options are limited and your shelf won't work unless the shelf itself is rigged with support in the back vs. directly on the amplifier.

And there are PLENTY of amplifiers - and even heavier UPS / isolation transformers which weigh WELL over 100 pounds and mount only with front rack ears with zero problems because the chassis are specifically designed to work with the ears to handle the weight, compression and shear. A Richard Gray Powerhouse is 375 pounds with front ear mounting and no problems. Almost all Torus and ExactPower Isolation Transformers are front ear mount only. All Levinson amplifiers, ATI amplifiers, etc. are front ear mount only and nearly all of them weigh about 100 pounds or more.

The Sherbourn will hold as BllDo implies and as Sherbourn stated, but there's nothing wrong with adding a bit of rear support for peace of mind, especially if you already have rear rack rails and it is easier to do.

We could sit here all day and provide one another anecdotal examples to support each case, but facts are facts and the standards bodies determined that items in this weight class and above required rear support as well. I followed those standards while I worked E&I. No one ever stated that hanging from the ears won't work. Don't shoot the messenger.

I wasn't aware that he was only using front rails, so you are correct, the shelf won't work in his case.


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post #1655 of 1676 Old 11-08-2016, 03:25 PM - Thread Starter
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Quote:
Originally Posted by TMcG View Post
Fred only has front rack rails
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Originally Posted by rms8 View Post
I wasn't aware that he was only using front rails.
Actually, I have all four rails. I am mostly sure I will do something to ease the rear weight, but I am not sure what yet. Picture wire seems smart, but I want to be neat about it. Thanks for the tips guys!
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post #1656 of 1676 Old 11-11-2016, 06:36 PM - Thread Starter
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In celebration of five full years of build thread, I ate too much chili and spent the evening lounging on the sofa instead of building my theater.

That's right folks! We now begin the sixth year of work on this project...

I'll have some photos and such tomorrow to remind us where we are and where we started. Sincerely, thanks for for helping and encouraging me along the way. I'm getting there.
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post #1657 of 1676 Old 11-12-2016, 04:50 AM
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.......I ate too much chili and spent the evening lounging on the sofa instead of building my theater.
........
.....remember, pics, or it didn't happen!
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post #1658 of 1676 Old 11-12-2016, 05:24 AM
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Quote:
Originally Posted by HopefulFred View Post
I ate too much chili and spent the evening lounging on the sofa instead of building my theater.
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.....remember, pics, or it didn't happen!
I think there are certain instances where exceptions are encouraged!!
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post #1659 of 1676 Old 11-12-2016, 05:49 PM - Thread Starter
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Here's another tour. I hope it's not too long to sit through (12 minutes). Please pardon the newb move I made when I didn't put my phone in airplane mode before I started - I received a text message at the 7 minute mark.

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post #1660 of 1676 Old 11-13-2016, 06:17 AM
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Great vid Fred... Room is coming along. Will be interesting how you choose to close in the screen wall. The steampunk lights are a nice touch.
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post #1661 of 1676 Old 11-14-2016, 12:41 PM
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Holy smokes HF! You're so close! I don't know how I missed the memo, but you're on the home stretch. The room looks great. I really like how the wood turned out and the overall aesthetic of the room.

If I kept score correctly I believe you only need to finish out the front (screen and panels/curtains), the bass trap, and carpet? Are you planning to finish the outside of the room before hanging the PJ?

Do me one favor. Screw some scrap ply or drywall to temporarily enclose that return duct, and turn on the fan. I'm still not sure what caused the issue I had in my theater, but I'd hate for you to have the same issue and discover it after you've buttoned up everything.

Keep it up! You're almost there!

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post #1662 of 1676 Old 11-14-2016, 06:13 PM - Thread Starter
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Originally Posted by J_P_A View Post
Holy smokes HF! You're so close! I don't know how I missed the memo, but you're on the home stretch. The room looks great. I really like how the wood turned out and the overall aesthetic of the room.
Thanks! It is starting to seem close. The messes and troubles begin to come to solutions and things come together... I just have to stay at it.

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If I kept score correctly I believe you only need to finish out the front (screen and panels/curtains), the bass trap, and carpet? Are you planning to finish the outside of the room before hanging the PJ?
You've got the score right, mostly. I am too nervous to put anything in the rack (permanently) until I can close the doors to it, so that will need to be finished before the electronics go in. So, I expect to hang the projector as soon as I am done with the things that make the theater work the way I need it to (for now, before the revisions and upgrades start). The lobby work will get contracted out once I have a dust-resistant rack. I am just getting started with some acoustic measurements, and that process will probably take months more for complete treatment. I will start watching movies without a complete treatment scheme, however.

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Do me one favor. Screw some scrap ply or drywall to temporarily enclose that return duct, and turn on the fan. I'm still not sure what caused the issue I had in my theater, but I'd hate for you to have the same issue and discover it after you've buttoned up everything.

Keep it up! You're almost there!
I will get the return duct closed in soon. In fact, I think it might be influencing my bass measurements a little. However, there are a couple hidden things to get finished before that happens. The duct isn't connected to the air handler at the other end! I also need to run an extra wire for power I forgot. I have everything I need to make that happen, but I haven't taken the time yet. I have confirmed that with the door closed the incoming air flows quietly and smoothly. The return duct also has a slight breeze at the face of it as the positive air forces back out, so it's nice to confirm that the laws of physics seem to be holding up. I hope the flow will be adequate to keep the air cool and fresh - we will see.
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post #1663 of 1676 Old 11-14-2016, 06:54 PM - Thread Starter
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I'm going to try to keep the verbiage to a minimum here, but there is a lot that might deserve saying.

I took these measurements as a first step in my acoustic analysis and treatment design. The goal for these measurements isn't to produce an EQ setting or even to determine optimal sub or seating locations. The goal is simply to see how the sub is coupling to the space and how the room is ringing. There is no cal file loaded, and the measurements are not corrected for SPL. The mic is a ECM8000 and the single UM 15-22 (wired to 4 ohms) in a 3 cubic foot sealed enclosure was driven off a single channel of a Behringer EP4000. The sub is on the stage, very close to the front left corner of the room. I have some ideas about what exactly I can learn from these data, but I would really appreciate any thoughts you may have.

All 13 of the measurements here were taken in the same sitting without adjusting anything in the signal chain. No smoothing is applied. The only thing changing among these sweeps is the location of the mic. The first measurement (LWR +0) was taken near the left wall (LW) and was offset from the rear (R) by +0. In fact, it was about four inches from each wall. The mic is on a small stand that you could see in the video I shared yesterday. From one measurement to the next, the mic was moved toward the screen by a distance approximately equal to the size of the base of the mic stand - about 10-12 inches. By the time I got to the last measurements, I was beyond the midpoint of the left wall. Assuming that the response changes are symmetrical fore/aft and left/right, this set of measurements should describe the bass response along both walls along the full length of the room. As the mic moved from the riser to the floor, I extended the mic to maintain the distance from the ceiling - about 34 inches above the riser, or 48 inches above the floor. That happens between the tenth and eleventh measurements (LWR +9 and LWR +10). I don't want to say where the midpoint of the wall was geometrically.



I guess the central questions I want to answer are: What are the center frequencies and bandwidths of the major axial (length only) resonances? Where in the room is the center of the first axial mode?

If you see anything else that deserves attention, or you would like me to explain something, please chime right in!

Edit: I was going to try to attach the .mdat for anyone interested, but it seems the forum doesn't allow non-image attachments. if you want to see it, send me a message and I can email it to you.

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post #1664 of 1676 Old 11-16-2016, 04:01 PM
 
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What is your general idea about your bass treatment strategy ? What were you planning on doing additionally (that is beyond what is done for these measurements)

I am not really sure what to tell you based only on that measurement chart. @Ellebob might know better than me.
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post #1665 of 1676 Old 11-16-2016, 06:57 PM - Thread Starter
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What is your general idea about your bass treatment strategy ? What were you planning on doing additionally (that is beyond what is done for these measurements)
Honestly, I was hoping that measurements like this set would inform what was needed for a treatment system. There is nothing going on here at all - just a sub in a room, with some (small amount) of assorted stuff laying around (tools and a couple boxes alongside the speakers). The biggest change I expect is from firing up all found subs (all four corners). The only other treatment I have actually planned is the helmholtz trap I am building from the "speaker stand" (seen in my post from 11/3 and in the video). I don't have a specific plan for EQ either. My processor (AVR) doesn't have any active EQ for the .1, so I apart from the XO setting in the AVR and the level settings in the amps, I don't have any options before I add equipment. I am happy to do that, but haven't worked out a plan - again, I was hoping to determine my needs first, then pick the equipment that would meet those needs.

My general feeling for strategy is that you start with positioning, then move on to passive treatment (absorption) and finish with EQ assuming adequate headroom. I have committed long ago to no more than four subs in four corners. The small size of the room makes a lot of passive treatment challenging, but that's why I have made arrangements for a helmholtz trap 40 inches tall, 8 feet wide (2/3 of the width of the room), and two feet deep (as deep as will fit behind the screen). I could add some additional absorption on the screen wall (and I will to some extent for sure - but maybe not for bass), but I don't think I can have anything meaningful for bass traps in any other places in the room. And for EQ I am flexible but have no current equipment.

My feeling looking at this set of plots is that the stuff below 50 Hz is going to change once I drive all four subs - but I'm not sure if it will be for the better. I know it's difficult to discern from the plots all overlapping like this, but I figured this was better than 14 individual images. The highest output between 20 and 50 Hz is measured nearest the corner. At 50 Hz, where the level measured was very similar at all positions, the highest levels were still measured in the corner, and the level came down smoothly as I moved toward the middle of the room. So you can look at the pattern of the plots and work out which one was in the corner and next closest and so on. There's something going on length-mode-wise around 30-40 Hz near the middle of the room. What's causing that? How should I expect it to change driving four subs? What if half of them are out of phase (for example)?

Hopefully, my helmholtz trap will help above 70-80 Hz. Obviously, there will be mains integration details to work out once I get there. To what extend should I expect my mains to mitigate those problems (mains are sealed f3 near 80Hz, flexible crossover, but assuming THX)?

I will need more measurements, I am sure. For instance, I feel confident that the spatial uniformity I see in these plots will not continue when I measure away from the side walls. Again, that will change once I start running subs on both sides of the room, but in what ways?

I guess I will work most of this out through trial and error eventually, but I am trying to get my head around what the actual modal resonances are - not what I think they should be based on a calculator. Once I get that straight in my head, I can start to make sense of what changes I can expect and get a better idea of what treatments I will need and what I can hope to gain from various EQ options.
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post #1666 of 1676 Old 11-17-2016, 08:24 AM
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How well do your measurements agree with the room sim in REW?

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post #1667 of 1676 Old 11-17-2016, 08:27 AM
 
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Fred I don't have time to comment right now intelligently, but you are opening up a big can of worms. You should break it down smaller into stages and just take it one step at a time. I would not worry to much about it until you get your actual treatment plan (or closer to it) in place because honestly there is not much worth worrying about now if it might not be such a big problem later. You can do a lot with DSP and electronic means too. You can't really absorb well below 100hz without going thick, and while I don't recall all of your build thread (it has been too long!) I don't recall you taking that approach. What did you do inside your riser?
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post #1668 of 1676 Old 11-17-2016, 10:23 AM - Thread Starter
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J_P_A - not very well. That's at the center of my interest. The sim isn't informative, so I want a functional model.
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post #1669 of 1676 Old 11-17-2016, 12:35 PM
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I dunno, HF. I ran it again based on the measurements in your first post, I get an estimated response that seems to have peaks and dips in roughly the same places.

Did you measure your subs outside to make sure they don't have anything funky going on?

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post #1670 of 1676 Old 11-17-2016, 04:49 PM - Thread Starter
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J_P_A - you're not wrong... I had/have some concerns.

First, and perhaps trivially, my ceiling height is almost 9 feet. (8' 8.5")

What's weird and what I have been trying to get straight has finally become clear to me after a little more (careful) playing with the simulator: the peak around 50 Hz (actually 52) which coincides with the predicted peak on your plot is an axial (length) resonance and should change with position, but mine does not. What I realized is that the peak I measured is actually associated with an axial mode (width) indicated in green on your plot (also on mine, but it's harder to see). In order to get the width mode to center above 50Hz, the room needs to be functioning shorter than 11 feet. 52 Hz resonance comes from a 10.865' wide room, at least in theory.

What's weird to me is that the argument about online modal resonance calculators that I have heard is that most walls are lossy and therefore are slow to respond to incident sound. As a result, the actual resonance tends to be lower frequency - the room functions as though it were larger because the reflected wave needs the extra time of the wall reflex to actually reflect - the same as though the wave had traveled further. In my case, the room is functioning smaller than predicted. I'm not sure why, but I have two guesses. First guess is that the soffits are shifting the average width to a smaller number - soffits are about 11 inches tall and 18 inches wide. That takes 25% off the overall width of the room across more than 10% of the height. Second guess is that the calculators (REW simulator and others) have a correction factor built into them to lengthen rooms assuming some standard of stick framing and drywall construction and that my stiffer walls are not performing to that standard.

Similarly, the peak at 57Hz that changes with position in my room makes sense to be the secondary axial (length) mode. That should be reaching at minimum at the acoustic center of the room. It never really seems to get there, as the peak seems to blend with something else happening slightly higher in frequency. Maybe I can figure that out... 57Hz peak in the corner means the room is behaving the way the sim would predict a 19.7' long room to behave. However, if the secondary axial mode is at 57Hz, the primary should be higher as well. It looks to me like the primary modal resonance would be at 26Hz, based on the peak measured in the corner.

Here's a new screenshot with the modal resonances adjusted to match the 52 and 57Hz peaks, as I have described here. Notice how the 26Hz peak doesn't seem to make any sense?


...and no, I have not measured the subs in any freespace or half-space setting - only in the basement.

Last edited by HopefulFred; 11-17-2016 at 04:53 PM.
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post #1671 of 1676 Old 11-18-2016, 07:00 AM
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Which measurement is the seating location? Can you give dimensions to that location as well the height you measured it at?

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post #1672 of 1676 Old 11-18-2016, 08:58 AM - Thread Starter
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None of those measurements was taken at a seating location. They were all about 4-5 inches from the left wall and about 33 inches above the riser (47 above the floor)
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post #1673 of 1676 Old 11-21-2016, 07:53 PM - Thread Starter
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I took some more measurements. Again, I have more questions than answers.

J_P_A, this first one is MLP only. That's just as before (though level was adjusted between day 1 and day 2): single sub (front left), mic at listening position - front row, center, height approximated (no measurement).



This second plot shows all the measurements I took tonight. I began at the MLP (same plot as above) and moved to the left 5 inches at a time. So there is better spatial resolution than there was last time, but the overall process was the same. In this case the mic did not traverse any changes in height. The numbers in the label for each plot refer to the distance (in inches) I moved the mic from the MLP to the left. MLP-65 got me all the way to the wall for the mic stand; the mic was still a few inches off the wall (for example, MLP-25 was taken 25 inches to the left, or approximately in the front left seat)



I kept the same dimensions (at least I think I did) in the EQ tab of REW so that the predicted modal resonance lines at the bottom are the same as from last time. I find several items of interest here and I am not sure which if any are actually interesting from a practical perspective, but I can't explain them and would like to be able to.
  • First, working from left to right, I can't explain why the output peaks in the bottom octave at 24Hz and then comes down. The peak we saw in the first set was around 26.
  • There is a ripple in the measurements closest to the center of the room (MLP-0, MLP-5, MLP-10, MLP-15) around 29Hz, which does correspond to the first axial length mode I predicted, but why is it only visible in the measurements near the center of the room? And shouldn't it be stronger?
  • There is a second ripple around 39Hz which is present in all the measurements to some degree, but strongest near the wall (MLP-65, MLP-60, MLP-55). I see no reason for this at all - could the theater be coupling with the lobby via the open air conditioning return duct?
  • Why do all the measurements diverge between 27 and 33HZ? That's too low a frequency for any room mode associated with the width.
  • Not visible in this plot, there associated ringing at about 34 Hz. It's mild, but inexplicably visible in the waterfall below.
  • Between 35 and 50Hz, the measurements all diverge again, even though we are still below the primary axial width mode. I can't explain that at all.
  • When we come to what should be the primary axial width mode, around 52Hz, we do see something of a null, but it is not at the MLP in the center of the room as one should expect from a primary axial mode, it is displaced left (and presumably right as well?) by 25-30 inches - putting it at the 1/3 width mark - and not really a null anyway, just a dip.

I'll come back to this in a day or two and see if it looks different to me, like happened last time. In the meantime, your thoughts are appreciated.

Here's the waterfall referenced above - it was calculated from MLP-55. Note ringing at 34Hz.

Last edited by HopefulFred; 11-21-2016 at 07:59 PM.
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post #1674 of 1676 Old Today, 08:42 AM
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A couple quick questions.
  1. What is the x,y,z location of the sub in relation to the front wall, side wall, and room floor?
  2. What is the distance to the MLP from the front wall?

I need have to digest this sort of stuff in small pieces since I'm not as familiar with your room as you are. With the diverging sweeps between 25 Hz - 50 Hz, that would be consistent with what the sim shows if you're measuring closer to the back of the room than the front. I haven't had a chance to think through why that would be the case, though.

Have you posted this in the theory thread?

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Originally Posted by J_P_A View Post
A couple quick questions.
  1. What is the x,y,z location of the sub in relation to the front wall, side wall, and room floor?
In this image, the red line is 25", the green line is also 25", and the blue line is about 11"

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  1. What is the distance to the MLP from the front wall?
That's just a bit over 12 feet - about 148 inches.
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With the diverging sweeps between 25 Hz - 50 Hz, that would be consistent with what the sim shows if you're measuring closer to the back of the room than the front.
^^I'm having trouble getting my head around this. Whatever words you can put to it may help us come to some understanding, because my brain turns to jelly when I think about it.

I imagine this --> Modal Analysis is the thread you are refering to. No, I haven't posted any updates there. As of right now, that thread only has 48 views and no replies... I plan to, but I was hoping to have something useful to say.
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^^I'm having trouble getting my head around this. Whatever words you can put to it may help us come to some understanding, because my brain turns to jelly when I think about it.

I'm struggling with it as well. So far, my only theory is it's a combination of some of the higher order modes. E.g., the 1, 1, 0 mode around 53 Hz has the right shape to cause variation along the width and behind the MLP. Of course, that mode would have to have a pretty wide bandwidth to cause an issue all the way down to 25 Hz. If we are assuming such a wide bandwidth, then the interactions between the modes get hard to understand. At least for my poor brain.

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