I've had a pair of GIK Corner Bass Traps for a long time so I recently contacted them and provided them with the dimensions and REW measurements of my room. They suggested extensive treatments and I decided to start out with some of it. I bought 4 of their 50Hz FlexRange Bass Trap Panels and 2 of their 80Hz FreeStand Bass Trap Panels. The FlexRange panels are all on the front wall and the FreeStand panels are on each side, placed with a mirror. After doing before and after measurements I think at best the room just measures differently or possibly, worse. To begin with, I haven't found a position for the speakers or MLP that improves the deep null in the right channel. Once I've completed doing to the room whatever I can, the next step will be to add a pair of subwoofers to try and address that problem. The picture of the room makes it look larger than it actually is. The doors and drapes are normally closed while listening. In some ways, the After Spl looks worse to me than the Before? I think the After Topt might be slightly better? Does the Waterfall show the After has more Ringing? I use Focus Fidelity to make Convolution filters so I've included those measurements as that's the way I listen to the system. I've tried to follow Dr. Wong's directions on how to present the measurements as closely as possible so if I've done something wrong, please let me know so I can correct the problem. If anyone has some suggestions for what to do next, I'd appreciate it.
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Before and after Room Treatment
- Thread starter Daverich4
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kemmler3D
Master Contributor
I guess my first question is did you place the speakers and mic in the *exact* same spot for the before / after measurements? It looks like there's been some improvement in the 80-200hz range (some of the peaks/dips look smaller) but it's hard to be sure what's going on between 90-100hz, as it looks like the null got worse.
Topt looks better IMO but it's a little debatable as you're also lowering the reverb time of the treble. At >300ms I think that's fine but it does indicate that if you go with further treatments, you might want to use the reflective / diffusive facing on the panels to preserve energy above 1khz.
The convolved response doesn't bad to me, except for those stubborn nulls at 30 and 80-100hz. Subs might be the cheaper / easier way to fix those at this point.
Topt looks better IMO but it's a little debatable as you're also lowering the reverb time of the treble. At >300ms I think that's fine but it does indicate that if you go with further treatments, you might want to use the reflective / diffusive facing on the panels to preserve energy above 1khz.
The convolved response doesn't bad to me, except for those stubborn nulls at 30 and 80-100hz. Subs might be the cheaper / easier way to fix those at this point.
- Thread Starter
- #3
Yes, everything except the additional room treatment stayed exactly in the same place for all the measurements. I'm planning on adding a pair of subs when I've done whatever else is appropriate.
PirateQuinn
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I would remove them one by one and remeasure as you remove each one. You may find one or more that aren't doing much and can potentially help more repurposed in a different location in the room. Its all about experimenting if you want that last 10 percent.
- Thread Starter
- #5
I’m not quite sure what you mean by remove them one by one? The front wall bass panels?
Keith_W
Master Contributor
This is how you can tell that the mic was not moved between measurements: if you look at "Info", you will see that the first measurement was taken at 12:47AM and the last at 12:57AM. So it's the same session, not days or months apart. OK, so this evidence is circumstantial, so we keep looking.
We can see that before and after measurements have the exact same timing delta.
... and the before and after have the exact same pattern on the ETC. The only difference is that the "after" curve has less amplitude on the reflections, but otherwise it's the same.
Anyway, to comment on the measurements: the RT60 target needs to be calculated for room volume. You supplied your room dimensions, but not ceiling height. I assume it's a standard 9 foot ceiling, so room volume = 4365 ft^3 / 123.6 m^3. So once you plug in the room volume and target RT60 (I used 400ms) into REW's EBU limits, you get this:
We should be cautious about reading the RT60 too closely since it's a single measurement from a single mic position (see the book about how to take a RT60 measurement) but we can see that the room treatment has brought it down nicely into target. Please note that we should ignore the RT60 below a certain limit, because those are not reverberant fields, those are room modes.
What's not so great are all these loud and early specular reflections in the Energy-Time Curve (ETC). That reflection at 2.6ms is particularly bad. As it happens, I am due to give a talk on REW measurements at the Melbourne Audio Club in a couple of weeks and I have prepared some slides for the talk. Go to this link, open the first talk ("MAC Talk Treble"), then go to Slide 43 where I explain the ETC in detail. Pay particular attention to the method I describe for working out where those reflection spikes are coming from. Given that your RT60 is now within target, those reflection points should be treated with diffusers. Or you could move your foam there.
Also: in the same folder where you downloaded the eBook, there is a new document called "Acoustic Measurement Standards for Stereo Living Rooms" by Nyal Mellor. Download and read it, it is excellent. Then compare your measurements with the targets published in that document.
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Keith_W
Master Contributor
As for ringing ... use the spectrogram and flip between before and after. I made an animated GIF for you, you can easily see the difference:
Keith_W
Master Contributor
- Thread Starter
- #10
Thank you for your help. I clearly have a lot to learn. I read through MAC Talk Treble and am going to see if I can track down the the problems shown by the ETC. Lots of other good information in the rest of that talk as well.
I could be wrong, but looking at the L/R/L+R graph, there may be some under-summation around ~80–105 Hz. At the same time, the individual L and R responses are already pretty irregular there.
It might be worth testing a few speaker positions, especially forward/back from the front wall, while measuring L, R, and L+R each time. If that dip moves around in frequency, that would point more toward SBIR/placement.
It might be worth testing a few speaker positions, especially forward/back from the front wall, while measuring L, R, and L+R each time. If that dip moves around in frequency, that would point more toward SBIR/placement.
- Thread Starter
- #12
The tweeters are currently 39” from the front wall. I’ve tried them 2 feet closer to the front wall and 1 foot further away in 1 inch increments. I got different measurements at some of the locations but nothing that was better, just different. Same with the MLP, forwards and back. Several people here have suggested multiple subwoofers and that will happen after I get this sorted out a little.
Is this your speaker?
One thing I noticed in the Sophia manual is that Wilson puts a lot of emphasis on WASP / Zone of Neutrality and boundary interaction. They describe speaker placement as a way to reduce interaction with nearby room boundaries, and they mention that sidewall reflections can cause out-of-phase cancellations / comb filtering.
So it may just be that the speaker/room geometry is producing cancellations that are hard to fully solve with panels. Wilson also notes that low-frequency standing waves are difficult to treat and that small placement changes can significantly alter tonal balance.
Since you already tried a wide range of front-wall and MLP positions and got different but not better, it seems like a challenging room/speaker setup problem rather. I do not think this is a simple fix. Subs may be the more effective way to address the stubborn naulls.
I don’t think that is simple either, because if the stubborn issue is around 80–105 Hz, the subs would need to be crossed high enough to contribute in that region. Ideally you’d also have DSP/bass management to test different crossover points, slopes, delay, and EQ. Two subs would probably be ideal to get smoother response across the room and to match the capability of your speakers.
One thing I noticed in the Sophia manual is that Wilson puts a lot of emphasis on WASP / Zone of Neutrality and boundary interaction. They describe speaker placement as a way to reduce interaction with nearby room boundaries, and they mention that sidewall reflections can cause out-of-phase cancellations / comb filtering.
So it may just be that the speaker/room geometry is producing cancellations that are hard to fully solve with panels. Wilson also notes that low-frequency standing waves are difficult to treat and that small placement changes can significantly alter tonal balance.
Since you already tried a wide range of front-wall and MLP positions and got different but not better, it seems like a challenging room/speaker setup problem rather. I do not think this is a simple fix. Subs may be the more effective way to address the stubborn naulls.
I don’t think that is simple either, because if the stubborn issue is around 80–105 Hz, the subs would need to be crossed high enough to contribute in that region. Ideally you’d also have DSP/bass management to test different crossover points, slopes, delay, and EQ. Two subs would probably be ideal to get smoother response across the room and to match the capability of your speakers.
- Thread Starter
- #14
The speakers are Wilson Sasha ll’s. The subs I’m looking at are SVS SB-3000 R’s. For my speakers SVS recommends crossing over at 80 Hz. There is a big null between 80-90 Hz so maybe I would have to cross over a little higher? In any event, I have some other issues to deal with first and then I can look into doing something with a pair of subwoofers.
I took a quick look at the Sasha Series 2 manual. Really interesting speaker. Wilson clearly put a lot of thought into setup and seems very aware that the speaker is sensitive to the room and nearby boundaries.
A few things from the manual seem relevant here.
I would probably recheck WASP and the Zone of Neutrality for each speaker individually, especially because your room is asymmetric.
I would also recheck the listening position. Wilson suggests the listening distance should be around 1.1 to 1.25 times the tweeter spacing. They also mention avoiding sitting too close to a boundary and avoiding the exact center of the room if bass is weak.
Sidewall reflections also seem important. The manual mentions that close sidewall interaction can cause out of phase cancellations and comb filtering, so sidewall distance and toe in seem worth checking along with front wall distance.
Front wall distance also looks like a tradeoff. Closer to the wall can give more low bass and a stronger center image, but it can also hurt upper bass quality, stage size, and bloom.
Wilson also notes that low frequency standing waves are difficult to fix with normal absorption.
Since you already tried a wide range of speaker and listening positions and got different but not clearly better, this may be several overlapping room issues rather than one simple placement fix.
If the stubborn null is around 80 to 90 Hz, subs may help, but they would need to be crossed high enough to contribute in that region and integrated carefully with delay, polarity, slopes, and EQ. I would treat 80 Hz as a starting point, but also test 90 and 100 Hz when you are ready to turn your attention to that.
- Thread Starter
- #16
I'm trying to work out the 2.6ms reflection. Using the 1m= roughly 0.3ms sample in your Mac Talk Treble presentation I get a distance of a little over 8.5 meters? Is that right, it seems really long? However, that is almost exactly the distance from the midpoint of the speakers to the back wall and then to the MLP. There is something on the back wall that could be reflecting sound but before I take it down, it's big and heavy and I'm not sure I'm doing this right. If I put the numbers into the actual formula to get distance I get a distance much different from my first estimate. I'm clearly on the wrong track here.
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PirateQuinn
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All of them you bought six I thought?
- Thread Starter
- #18
I bought 4 bass panels for the front wall and 2 other panels to go on each side. You're suggesting I try removing the front wall panels one at a time?
For the 2.6 ms reflection, I’d keep the test simple: measure L-only ETC and R-only ETC, then temporarily cover one suspected surface at a time with a thick blanket, absorber, or one of the panels. If the 2.6 ms spike drops, that surface is probably the source.
For the panels, I think the idea is to use removal as a diagnostic: measure with everything in place, remove or move one panel, remeasure, then put it back and test the next one. That should show which panels are helping, which are doing little, and whether any might work better somewhere else.
For the panels, I think the idea is to use removal as a diagnostic: measure with everything in place, remove or move one panel, remeasure, then put it back and test the next one. That should show which panels are helping, which are doing little, and whether any might work better somewhere else.
Keith_W
Master Contributor
The 2.6ms spike in your ETC represents the path length difference between the direct sound and the reflection. And I calculate a 2.6ms delay to equal 8.92m. It depends on the speed of sound of course, and the speed of sound varies with ambient temperature, elevation, and so on. So it's roughly 8.92m and not exactly 8.92m. I get that all you have are the slides and my book, but:
Speaker to mic: 0ms (i.e. the main impulse)
Any spike after 0ms has travelled an extra distance to get to the mic.
I then gave a few examples. For a front-to-back reflection, you can work it out with simple addition. But for a side wall reflection or ceiling/floor bounce, you have to use Pythagoras' theorem.
One way to work it out is to sit down with a diagram showing measurements where your loudspeakers and mic are, along with the height of the mic and dimensions of the room. Then calculate one by one when each reflection is expected to arrive. This is a bit onerous.
Another way is to take some thick acoustic foam and plonk it on where you think the reflection is coming from. It might be a bit tricky if it's a ceiling reflection! Do a before-after measurement, and compare the spike on the ETC.
I don't know what those numbers mean, some of them are clearly room dimensions but I have no idea why your left speaker has "54" and "39" on it. And I am assuming that the listening position labelled "9" is where you are sitting. Assuming that "9" is how far the MLP is from the rear wall, that's about an 18 foot extra distance, 5.5m. That's about 1.6ms. You're the one with the tape measure, so you can work it out
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