Interesting AVR/ Pre-Pro Info: Adjusting Subwoofer Phase w/ Distance Control (LONG)

Tour2ma

I spent a pleasant 90 minutes or so a couple Sunday mornings ago talking with my good friend Dr. Spec (AKA Ed Mullen), a long standing Club Polk member. Doc’s very busy with subwoofer reviews and family obligations (including a new addition, Doc, Jr.!!!) and has very little time to post in forums anymore. Nonetheless because of family feel here, CP is still one of his favorite forums, and he looks in on us every once in a while.

Anyway… after catching up on life situations and such we started talking about acoustics and general HT. When we got to subwoofer set-up, Ed mentioned that Mark Seaton (of Seaton Sound) had discovered that most surround sound processors (SSP’s) have a bass management circuit, signal delay on the order of 3-5 milliseconds relative to the other 4 or 6 speakers in the typical HT rig. This delay affects both redirected bass and also the LFE (.1) channel.

While this doesn’t seem like much, if the subwoofer is located in the front stage, the delay is enough to create a phase mismatch between the front stage (mains & center) and the subwoofer. To compensate for this latency, you simply input a subwoofer distance about 1 meter greater, i.e., farther away, than the actual distance. While intuitively this may seem to be “moving” the sub in the wrong direction (after all, you want the delayed sub info to arrive sooner), what you are doing is fooling the SSP into producing the sub signal sooner.

Since Auto Set-Up and Auto EQ routines in the latest SSP’s are measuring the actual arrival time from the speakers and the subwoofer to the mic at/near the XO frequency, they do this automatically. If your SSP has this capability, check the distances that have been auto-set for your speakers vs. their physical distance from the mic position you chose. Don’t be surprised if you find that your sub is the only speaker that your processor has “misplaced”.

For those enthusiasts that lack an Auto Set-up routine, Doc recommends that you begin with the subwoofer distance set about 0.3 meters in front of the front stage (less than the actual distance to the sub) and electronically “walk the sub away from you" in 0.1 m increments, running an FR sweep at every interval.

Make sure the FR sweep is processed through the SSP bass management circuit and includes either one main speaker or the center channel with the subwoofer. Were you to use both main speakers, you might introduce comb filtering which can create data interpretation issues. Ed believes that using a DPLII setting will produce the signal from the center channel, if you want to use it rather than a main speaker.

With the subwoofer distance set to actual, Ed experienced about a 7-8 dB dip around the XO frequency, indicating a phase mismatch. The dip disappeared and the FR was optimal with the subwoofer distance set about 2.8 feet farther than actual in his HT. However, in addition to room variations, different manufacturers’ SSP’s are different and your results will likely be different.

Ed added the following note to an advance copy of this post that I had sent him to be sure I was close to reflecting our conversation. I was pretty close… :rolleyes:
If you are using a high res SACD or DVD-A player and performing bass management at the player and then sending the signal to your SSP via the External Analog Inputs, there may be even more of a latency delay in the subwoofer, particularly if your SSP has some bass management capability at the External Inputs (for example some of the Denon AVR's can adjust the Ext. Input subwoofer level from 0 to +15 dB).

Bottom Line

• A properly phased subwoofer will greatly improve the integration with the mains and upper bass coherence thus improving the frequency response over the XO bandwidth.
• The subwoofer distance control is essentially a digital variable phase control that can be used to compensate for the latency in the bass management circuit and optimize the phase integration between the subwoofer and the front stage.
• Use of the sub distance control is vastly superior to using the 0/180 polarity switch found on most subwoofers.
• Use of the sub distance control gives owners of subs with no phase management circuitry the flexibility they thought they were missing.
• Even if your subwoofer has an analog 0-180 variable phase control, you can set this control to 0 degrees and use the subwoofer distance control instead. This approach is actually more precise than use of the sub’s phase adjustments. Plus, it’s easier on the back instead of leaning behind the subwoofer with a pen flashlight in your mouth tweaking the phase knob – an important consideration for those with bad backs…. like Doc and I….

tonyb

Nice Tour,hopefully some will learn from this.
Thanks for passing it along.:)

Lsi9

good info I will have to check if my outlaw has a specific distance setting for the sub, makes complete sense, even though I haven't noticed any latency I will have to double check that.

dudeinaroom

awesome read. too bad my cheap avr has no sub distance settings. Still one to bookmark for later ref.

Tour2ma

Y'all are welcome...

If your SSP lacks the ability to "move" the sub, I imagine you could play with the distance of the one main (or center) you include in the calibration and then move the rest of the front stage proportionally. I don't think the surrounds are a significant factor.

As for the latency... while it prompted this approach, it's really the added flexibility of the sub distance's use to "tune a room" that's the find. You'll note in Doc's HT that his sub ended up in a place other than where it should be to compensate for the time delay.

aaharvel

awesome readup and I'm going to try that Tour. Thanks alot.

Skynut

Nice read.
I am going to try this out as soon as I have the time.

schwarcw

Tour,

Did the Doc recommend any particuliar software to run the subwoofer sweep? Or just a SPL with the frequency tones?

I've always wanted to run a series of sweeps with the subwoofer phase set at 0, 15, 30, 45, etc. to see the affect of the phase setting on the curve to see if there are any dips vs phase. Anyone run this type of test?

dudeinaroom

lol. After I read the post, I went and checked the setting on my avr to see if it had any distance settings for the sub. When I realized it did not, bumped my center and mains back 2 feet and noticed the bass was not as muddy. My sub lives just to the inside of my left channel, and sticks out about 6 inche past it. The only way I am going to see a big improvement is to upgrade the sub. It's a klh 15" that's rated at 200 watts, and a fr of 20hz - xover (40-180hz) it'ts currently set at 100Hz same as the avr phase is at 180 degres. avrs x-over cna be set at 100, 150, or 200(don't like it) fronts and rears are set to large and center is small. Thinking of takeing the amp and building a box for a 12" bazooka sub I have(way more accurate than the klh). I was thinking of doing a pr design until I realized I would need 3 12" pr's to cope with a single 12" woofer(as I have read in a few dyi articles) depending on the freq. response curve that win isd gives me, I might do a sealed enclosure, but if it does not dig deep enough I will do a ported even though I really like the sound of a sealed box. yeah yeah I know back on topic sorry all, and again good read tour,

Later,
dude

Mike682

Quick question: When calibrating a sub that is located in the front soundstage, one should adjust the sub's distance relative to the front soundstage and not to the typical listening postion?

I have to try this..