Another room mode thread - please help me interpret REW results

[klettermann]

As a newcomer to REW I'm rather overwhelmed with how to interpret the results. And some functions I can't figure out at all. To be clear, At this point just I'm trying to optimize speaker and MLP positioning before starting with other room treatments, DSP equelization or whatever. I'm hoping that you kind folks can help get me on track. Here's the story.

My room is L22ft x 12.2ft x H7ft. Speakers are Magneplanar MGIIIa's positioned 4'6" from front wall and 2' from side walls (to center of speaker, speaker edge 1ft from side wall). The room has minimal treatment - it's fully carpeted and there are ASC tube trap towers in the 4 corners. MLP was originally about 13ft from front wall because this created an equilateral triangle with speakers and was a good TV distance. The LF room modes were simply horrendous. After several iterations the MLP ended being 17'3" from the front after successive scans farther and farther back. Doing so seemed to improve matters greatly, leading to the last series of testiing.

It turned out that the best result seemed to be MLP 18ft 3in from the front. Please see scans below. The only difference is moving MLP from 17'3" (green) to 17'9" (red) to 18'3" (blue). Obviously MLP has a substantial impact! Can anybody comment on these scans? My sense is that blue is best, though the low end is still all over the place. Is the only solution a lot of bass traps? There are already 4 in the room. Comments welcome!

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Then there's the question of spectrograms. Shown below is the reference blue trace. I really don't know how to interpret these. Is this OK? Does it indicate obvious problems? Comments?

Again, I'm not sure if these results are pretty good, terrible, or what. How many more bass traps does this translate to? Set up some kind of DSP in pursuit of total flatness? Or call it a day? I've gottas say, the system sounds very VERY good - incredible clarity, imaging/sound stage, no obvious muddiness. Thanks in advance and cheers,

 

[dweeeeb2]

To position things in a room I use pink periodic noise and RTA averaging (All in REW). This provides a real time FR graph which changes as you move things. Position the screen so you can see it and slowly move stuff till you get the best result.

EDIT: Also a single trace at a MLP is pretty deceiving as to FR at MLP. Its best to do an average over the listening position (try 5 measurements within 0.5 meters of each other and you will see what I mean), or use the Moving Mic Method (this uses the RTA function in REW and I use it predominantly for FR but you can't look at Impulse response etc)

 

[Keith_W]

Those crazy fluctuations you observe in the bass are normal. Read amirm's post here.

In summary: if you have issues below the transition zone (Schroder frequency), DSP is better than room treatment. Both have downsides - room treatment becomes impractically large and intrusive, and DSP has a learning curve. I think it is better to deal with the learning curve since so much intrusion into your living room is intolerable for most people. FYI, your options are velocity absorbers or pressure absorbers. Examples of pressure absorbers: Helmholtz resonators or membrane absorbers. These are narrowband and need to be tuned to the frequency you wish to absorb.

Velocity absorber = thick foam. This needs to be 1/8 as thick as the lowest wavelength you want to absorb - in your case, 70Hz. So your absorber needs to be 2ft thick. How effective it would be depends on how much wall coverage you are able to achieve. The problem is, achieving sufficient wall coverage also deadens the upper frequencies too much.

If you have issues above the transition zone, the first thing to realise is that your measurements aren't telling you the whole story. If you have issues here, the solution is better speakers or room treatment. You can use DSP, but it has less of a role. Since you say you are already happy with the sound, I would leave it alone.

To calculate the Schroder frequency, use the formula Fs = 11,885 * sqrt (T30/V). You can find the T30 in REW's "RT60" tab, and V is your room volume in cubic feet. The transition zone is between Fs and 4Fs.

 

[ErVikingo]

As a newcomer to REW I'm rather overwhelmed with how to interpret the results. And some functions I can't figure out at all. To be clear, At this point just I'm trying to optimize speaker and MLP positioning before starting with other room treatments, DSP equelization or whatever. I'm hoping that you kind folks can help get me on track. Here's the story.

My room is L22ft x 12.2ft x H7ft. Speakers are Magneplanar MGIIIa's positioned 4'6" from front wall and 2' from side walls (to center of speaker, speaker edge 1ft from side wall). The room has minimal treatment - it's fully carpeted and there are ASC tube trap towers in the 4 corners. MLP was originally about 13ft from front wall because this created an equilateral triangle with speakers and was a good TV distance. The LF room modes were simply horrendous. After several iterations the MLP ended being 17'3" from the front after successive scans farther and farther back. Doing so seemed to improve matters greatly, leading to the last series of testiing.

It turned out that the best result seemed to be MLP 18ft 3in from the front. Please see scans below. The only difference is moving MLP from 17'3" (green) to 17'9" (red) to 18'3" (blue). Obviously MLP has a substantial impact! Can anybody comment on these scans? My sense is that blue is best, though the low end is still all over the place. Is the only solution a lot of bass traps? There are already 4 in the room. Comments welcome!

Again, I'm not sure if these results are pretty good, terrible, or what. How many more bass traps does this translate to? Set up some kind of DSP in pursuit of total flatness? Or call it a day? I've gottas say, the system sounds very VERY good - incredible clarity, imaging/sound stage, no obvious muddiness. Thanks in advance and cheers,

Could you please try to move your MLP to 14.5' from the front wall and report back?

 

[klettermann]

Could you please try to move your MLP to 14.5' from the front wall and report back?

OK, thanks, but traveling now. Will try when I get home. In any case, I finally decided to bite the DSP bullet and exploring what would work best for me. Cheers,

 

[ErVikingo]

I find DSP useful but dont use it all the time.

 

[basscleaner]

There is one more original method for getting the best frequency response result for LF range. It calls ADT-method (Acoustical Dimensions Theory) in case you have possibility to change your initial room dimensions by a little bit. Briefly speaking, among the many sets of possible room dimensions you will find these, which are capable to ensure the optimal FR in LF range for your front speakers pair. It will require, of course, some repair of masonry construction. In the simplest way, you can compare results for the listening position with minor changes of your dimension set by REW. For example, try to begin with one dimension like longitude or width and have a look at FR. The aim will be smooth FR at the listening point. In general, is not a difficult task to automate such a search.

 

[klettermann]

There is one more original method for getting the best frequency response result for LF range. It calls ADT-method (Acoustical Dimensions Theory) in case you have possibility to change your initial room dimensions by a little bit. Briefly speaking, among the many sets of possible room dimensions you will find these, which are capable to ensure the optimal FR in LF range for your front speakers pair. It will require, of course, some repair of masonry construction. In the simplest way, you can compare results for the listening position with minor changes of your dimension set by REW. For example, try to begin with one dimension like longitude or width and have a look at FR. The aim will be smooth FR at the listening point. In general, is not a difficult task to automate such a search.

Thanks, actually I did such a room dimensioning as best I could. Unfortunately there were many constraints that couldn't be reasonably changed so what I ended up with was the best compromise. Cheers,