RPG Modex Plates (35Hz) placement question

stevekale

New Member
Aug 8, 2012
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I might take you up on that!

In the interim I will see if I can make any progress to make the job simpler. With this room I won't be buying any more subs and I am quite constrained regarding positioning of the existing sub but I'd like to get the crossover, phase and delays sorted as best they can. (When I build or purchase then I will hopefully have more flexibility and I'm sure my modest purchases of acoustic treatment will be useful in that room as well.)

I've read through the process recommended by Barry Ober. Has anyone purchased his CD?
 

Nyal Mellor

Industry Expert
Jul 14, 2010
590
4
330
SF Bay Area, CA, USA
I might take you up on that!

In the interim I will see if I can make any progress to make the job simpler. With this room I won't be buying any more subs and I am quite constrained regarding positioning of the existing sub but I'd like to get the crossover, phase and delays sorted as best they can. (When I build or purchase then I will hopefully have more flexibility and I'm sure my modest purchases of acoustic treatment will be useful in that room as well.)

I've read through the process recommended by Barry Ober. Has anyone purchased his CD?

If you have an acoustic measurement rig, which you do, then you should be using that to integrate your subs!
 

stevekale

New Member
Aug 8, 2012
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Ha! Not surprised about that answer. :) I need to learn more about how to use the tools already at my disposal. I had thought RTA was useful but this phase/delay point was always bugging me. Revisiting my LF decay picture post the BAD Panel installation reminded me of my intuitive discomfort with adding large delay to a sub that wasn't materially further away from the listening position than my main speakers. I "get" the concept of potential "group delay" but how to measure it? Phase charts or other? Or play useful tones and tune by ear? I'll do some digging. Thanks a lot.
 

Nyal Mellor

Industry Expert
Jul 14, 2010
590
4
330
SF Bay Area, CA, USA
Ha! Not surprised about that answer. :) I need to learn more about how to use the tools already at my disposal. I had thought RTA was useful but this phase/delay point was always bugging me. Revisiting my LF decay picture post the BAD Panel installation reminded me of my intuitive discomfort with adding large delay to a sub that wasn't materially further away from the listening position than my main speakers. I "get" the concept of potential "group delay" but how to measure it? Phase charts or other? Or play useful tones and tune by ear? I'll do some digging. Thanks a lot.

The beginner way is by trial and error adjusting the sub distance / delay on the pre-pro 1ft at a time and remeasuring. I'd start with sub distance at it's measured physical distance and then adjust from there. Note the bit in the Soundoctor article I linked to earlier about the backwards way most pre-pros work with respect to distance / delay. It may be that with a significant overlap between mains and subs as you have it then there is no best setting, just compromises as there may be phase cancellation between the subs and mains that can't be avoided.

There are other ways to do it but they require looking at phase traces and impulse responses which if I were you would steer clear of at this time.
 

Jose Almagro

Well-Known Member
Dec 10, 2010
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913
Madrid (Spain)
www.mundohifi.com
Ha! Not surprised about that answer. :) I need to learn more about how to use the tools already at my disposal. I had thought RTA was useful but this phase/delay point was always bugging me. Revisiting my LF decay picture post the BAD Panel installation reminded me of my intuitive discomfort with adding large delay to a sub that wasn't materially further away from the listening position than my main speakers. I "get" the concept of potential "group delay" but how to measure it? Phase charts or other? Or play useful tones and tune by ear? I'll do some digging. Thanks a lot.
I use Smaartlive in Transfer mode, it's nice and quick.
 

stevekale

New Member
Aug 8, 2012
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0
I spent some time on this yesterday. It was interesting to focus on the spectrogram when making adjustments and less so on the SPL response. I started out by delaying the main speakers by 10ms and the sub 0ms. (The distance difference from the main listening point to the sub versus each main speaker is relatively immaterial.) I also looked at a crossover to the sub rather than full range to the main plus LF pass to the sub. I had originally thought that broadening the sources of LF (in effect increasing the number of subs) would lead to the best result and in may in fact still do that if optimising for more than one seating position. (Note the position of the sub in the graphic on page one of this thread.)

In the end I settled on a Butterworth crossover to the sub at 100Hz, slope 24dB (both Hi and Lo). (Previously it was set at 80Hz and the main speakers were running Full with a LP copied to the sub.) I set the phase on the sub to "normal" and began working through delay settings, finally setting on a 6ms delay to the mains. I was using full 5-22kHz sweeps. (For awhile I played an 80Hz tone and walked around the room. This was very eye opening with respect to the impact of room modes. However, it also gave me a headache very rapidly!) The changes in delays manifested mostly in differing decay in the 60-100Hz region.

So to the results. Well I still have a lot of base to absorb at c35-40Hz, it would be nice to soak a little more around 60-100Hz and I have a null at 118Hz. I would be interested in any other thoughts people have.

One thing I found very intriguing was the relationship between SPL response and delays. I was not expecting the two to interact so much. I guess this is combing.

Here are some graphics. All are based on mains and sub running all together (3 speakers) as I was only focused on the low end.













The last graphic shows a comparison of SPL response between the previous settings (sub delayed 8ms and phase reversed, mains running full and LF to sub) and the current ones (mains delayed 6ms, sub 0ms, normal phase, Xover to sub):



I might try several 40Hz Modex Corner Plates to soak up some of the bass there. I'm not sure what I can do about the 120Hz null or other stuff. Roll-on Theta's implementation of Dirac Live...
 

Nyal Mellor

Industry Expert
Jul 14, 2010
590
4
330
SF Bay Area, CA, USA
Some good work there. What's the root cause of that 40Hz issue?
 

stevekale

New Member
Aug 8, 2012
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0
I think it's the room length. If I measure the room length ignoring the recesses on either side of the fireplace (4.4m) 38Hz is, I believe, a primary axial mode.
 

Nyal Mellor

Industry Expert
Jul 14, 2010
590
4
330
SF Bay Area, CA, USA
I think it's the room length. If I measure the room length ignoring the recesses on either side of the fireplace (4.4m) 38Hz is, I believe, a primary axial mode.

If that's true then I would fix the problem via room mode cancellation:
- crossover front speakers to sub at 38Hz
- level set sub so that it is same level as front speakers playing together
- time align fronts and subs at listening position

If you do it right then there will be no room mode as the front speakers and sub will be driving it destructively. Ringing and frequency response peak will disappear.
 

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