A summary of how I optimize subwoofers in a system. Take 1

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
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Chicago, IL
www.seatonsound.net
#1
How I optimize subwoofers in a system. Take 1

While responding to some questions elsewhere over Sunday morning coffee I found myself with a decent summary on subwoofer optimization using Room EQ Wizard (REW) or other measurement systems like the Dayton OmniMic or XTZ Room Analyzer. This is by no means comprehensive, as we could flesh out many bullet points on a few of these items, but I thought it was a worthwhile summary to post up before I go ahead and organize the measurements and data on thebland's system. This is a generalized post that won't apply to all systems depending on number of subwoofers, inclusion of manual subwoofer EQ, and features and functions of each surround processor.


To summarize some of my thoughts and experience, here's my suggestion for most attacking their system with REW and other measurement systems:


  1. Take a subwoofer only measurement in your first subwoofer location while noting signal levels to the subwoofer and volume setting.
  2. Do NOT change any signal levels and start moving the subwoofer to any other possible locations saving a measurement at each location. The response should be louder or softer from different locations and you want to see this.
  3. Choose the location with the balance of fewest dips and most efficiency across the subwoofer range.
  4. If you have more than 1 subwoofer, pick locations that compliment eachother where dips do not overlap.
  5. With multiple subwoofers overlay individual responses and then measure both together driven at the same level to see how they sum. Maximum summation of 2 equal sources is +6dB. Unless there is a big peak, the goal is to get the sum to be +0-6dB and not negative. Moving the sub or adding delay to the closer sub is my preferred way to make this adjustment.
  6. If manually EQ'ing, now apply EQ to the subwoofer alone while also watching for peaks out to 120-200Hz that go above the target or above flat flat if no HP is in the measurement.
  7. Now run any auto-EQ/room correction.
  8. Measure the subwoofer alone and center channel alone, then the combination of the two.
  9. Make adjustments to the subwoofer distance setting, most often adding to the physically measured distance at 0-8ms.
  10. Choose the smoothest interaction and adjust the crossovers up or down if you see room for improvement.
  11. If you have separate EQ on the subwoofer you can sometimes improve a lumpy/peaky summation by adding a notch to the subwoofer around the crossover range. The benefit to dialog and music balance far outweighs a small dip to the LFE channel.
  12. Next compare the interaction with the left & right speakers making crossover setting and delay adjustments to get a best fit for both.
  13. The more you listen to movies & surround concerts, the more you should prioritize the center channel vs. left/right.
  14. Now measure what happens when you increase the subwoofer level by 3,6 and 9dB to understand what happens when raising the sub level.
  15. Do some listening and adjust the subwoofer level and/or shape to what you are after, and then re-measure subwoofer-center interaction to insure any ramping up in response you add is as smooth as practical.
  16. Enjoy!

There have been many times when people have asked "What does Mark do that an auto-EQ system or typical enthusiast can't?" when they hear people raving about the results after I get into setting up or adjusting a system for 3-12 hours. My answer is that anyone COULD do it, very few do. Few will take the time to gather all the measurements and learn how to manipulate what they see. I encourage more to be some of the few, or search them out to set up their system. :)
 
Last edited:

LL21

Well-Known Member
Dec 26, 2010
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#4
great post - a reference thread. thank you.
 
Apr 3, 2010
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Seattle, WA
#6
Wow, best point by point instructions I have seen. And one where the underlying science very well supports it. Definitely a future reference thread.
 

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
355
74
435
44
Chicago, IL
www.seatonsound.net
#8
Wow, best point by point instructions I have seen. And one where the underlying science very well supports it. Definitely a future reference thread.
Thank you Amir,

I've given plenty of suggestions online before, but many always were very system/case specific. In the process of working on a handful of systems recently, both in person and advising remotely, the more important concepts have become more clear. The "tricks and tips" lie in the "how," but understanding the goals and what you are looking for is the far more important part required to make some sense of all the squiggly lines. Once the goals make some sense, it's easy to give tips or hints of how to address a problem that is identified.

For those looking for help from others on subwoofer placement and EQ, the most important details are:

  1. Overlaid (one graph) measurements of available subwoofer locations with no EQ and no changes to the settings between measurements.
  2. Overlaid measurements of combined responses of desired locations AND the individual responses. Again no changes to settings.
  3. Overlaid measurements of any significant adjustments that have been made with delay or other settings to change interaction.

Here is one graph showing front and rear subwoofer locations with the subwoofer producing the same sound output (ie same signal in and same gain). The differences amount to how efficiently the subwoofers deliver frequencies to the listening position. Take note of how the rear corner is almost worthless below ~15Hz, but is as much as 10dB more efficient in the critical 20-35Hz range. Yellow = Front/center sub, Green = Rear right corner sub



Due to available space, the efficiency of the front wall at very low frequencies, and more even distribution laterally in the room, we used 2x as many subwoofers along the front wall vs the rear corner (4 dual 15" SubMersive HP subwoofers across the front, 2 dual 15" SubMersive F2 subwoofers stacked in rear). Driving all subs equally we see the front subs raise in level by ~6dB, but remember we have all subs doing the same workload (signal in and volume setting):Yellow = Front wall subs, Green = Rear right corner subs



Of course things are not always so simple as plug-n-play...

Here we see the same yellow and green curves and the result of both driven at the same time. Red = Identical signal to each, Cyan = 7ms delay on the rear subwoofers. The rear subwoofers are ~9' closer to the microphone, but due to the modal interaction with the room, distance alone is not usually an ideal adjustment. Note this shows ~25dB improvement around 45Hz. This was hugely audible.



Remember the subwoofers are each doing the exact same amount of work in each curve. We are only changing how they sum together at the listening position with a delay adjustment. All of these measurements are from the SubMersive HP & F2 models dropped in with no EQ. I'll use another post to show how I settled on this delay for a best fit with 2 other rows in another post.
 

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
355
74
435
44
Chicago, IL
www.seatonsound.net
#9
Here's a look at what happens to the combined response at the main listening position as measured above when we adjust the delay to the rear stack of subs (a few pics of the room here).
The bottom Red, Magenta and White curves are with 0ms, 3ms and 5ms of delay applied to the rear subs (no change in level or EQ). The tighter grouping of curves are 7ms, 8ms, 10ms and 12ms delay. From this I wanted to next look to the front/rear rows and see what is the best fit for all within the very workable responses of 7-12ms you can see below:



Next I took a look at the rear row as we adjusted the same delay range. Note these are all progressively changing responses. There are no magic surprises to be seen at small increments in between...

Here we have the main row with 7ms delay shown in green for reference. Yellow is the rear row with 7ms delay, Red with 8ms, and Magenta with 10ms.



Now repeat for the front row where we again have the main row with 7ms delay shown in green for reference. Yellow is the front row with 7ms delay, Magenta with 8ms, and Red with 12ms.

 

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
355
74
435
44
Chicago, IL
www.seatonsound.net
#10
Looking at the front/rear rows we could select 7 or 8 ms of delay and get a nice looking starting point. Looking at the 7-8ms difference for the rear row shows a reduction in the 40Hz range for the 8ms response while the main row increases at the same frequency, which would increase the row-row variation. The rear row is not against the back wall, and gets a little more use from guests and kids than even the front row, so the 7ms curves work out as a nice best fit.

While I didn't save all the measurements, I did check what was going on laterally, at each row, and didn't find huge variations, and the 7ms curves looked similar or better at those locations than other delay increments. Overlaying the 3 curves from the middle of each row confirms that a very impressive balance was reached. No EQ applied, just 7ms delay to the rear subs:



For another look here is a show of deviation from the green middle row (subtracting from the middle row response):



While this would be merely "pretty good" for a single row, and possibly 2 rows, across 3 rows this is an exceptionally good consistency, not to mention a very smooth response. Note this was an empirical setup where years back I moved a SubMersive around a few spots in the rear, along the side walls and front wall. Those measurements actually forced me to scrap an original plan for the room to put a big supersub/VLF monster at the rear right corner. While corners are usually good locations for deep bass, the differences in construction from external front vs interior rear walls was enough to be far from expected. As you can see in the first graph I posted, below 15Hz a sub at the rear delivers 1/4 the power to the listener as the same sub placed up front. Of course around 25-30Hz this rear location is as efficient as 3 subs at the front wall... It was clear a combination of the 2 could be used to good effect.
 

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