Subwoofer Distance: measure, calculate, or use receiver? - AVS Forum
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post #1 of 8 Old 11-08-2012, 06:09 PM - Thread Starter
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I have been searching for definitive answers to this common question, but I haven't been able to find an evidence-based thread to answer, so any help would be greating appreciated.

I'm resetting up my home theater after a flood and need to enter the subwoofer distance into the receiver to allow it to set the appropriate signal delay. I have 2 identical subwoofers that are equidistant from the preferred seating area.

Should I:

1. Enter the exact distance as measured by a tape measure
2. Measure the exact distance with a tape measure and enter it into an equation for a corrected distance (I seem to recall there is a formula you can plug the exact distance into to get a corrected distance)
3. Use my receiver's autosetup (YPAO) program to measure the acoustic distance.

If the answer to #3 is "Yes", should I trust this program to set up the distances to my other speakers as well? (they are off from the measured distance by a few inches)

Thanks in advance for your help!
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post #2 of 8 Old 11-08-2012, 06:16 PM
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There is phase shift in the crossover and various delays that make the physical distance not match the actual measured delay. It is best to think of the "distance" setting as a "delay" setting instead. Usually the AVR should get it right, but of course there are always exceptions to the rule...

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #3 of 8 Old 11-08-2012, 06:33 PM - Thread Starter
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Hi, Don:

Thanks for the reply.

So...how do you know if there is an "exception to the rule"? My subs are 9 feet from the listening position. YPAO puts them at 19.2 feet. The AVR distances match the actual measured distances for all of the other speakers.

Should I just toggle between both sub distances and see what sounds the best?

Finally--I seem to recall there was a sub distance correction equation that some people used. You basically took the actual measured distance, enter it into the equation and then put that value into the AVR. Do you remember such an equation?

Thanks!
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post #4 of 8 Old 11-09-2012, 04:56 AM
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Originally Posted by mvp2005fan View Post

Hi, Don:

So...how do you know if there is an "exception to the rule"?

Don already explained once why physical measurements can be misleading.
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My subs are 9 feet from the listening position. YPAO puts them at 19.2 feet.

Don already explained once why physical measurements can be misleading.

He said:

There is phase shift in the crossover and various delays that make the physical distance not match the actual measured delay.

Is there something in that statement that is unclear and needs to be explained in more detail?
Quote:
The AVR distances match the actual measured distances for all of the other speakers.

Would you believe that not all speakers and their built-in crossovers have the same amount of phase shift near the crossover frequency that is chosen by various means? ;-)
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Should I just toggle between both sub distances and see what sounds the best?

If you change things one little iota, there will be lasting severe physical damage to your subwoofer, your AVR, your house's structural integrity, your dishwasher and your neighbor's swimming pool. ;-)

Of course, if you are skeptical about how YPAO adjusted things, do some experiments and educate yourself!
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post #5 of 8 Old 11-09-2012, 05:49 AM - Thread Starter
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I've previously set up my home theater system old school with a tape measure, SPL meter, a real time display of a Velodyne SMS-1 (for the subs), and my ears. Via the usual time and experimentation process, I eventually got it to where I liked it.

I was planning to do the same this time, but decided to mess around with YPAO just for fun.

My issue is this: while Audyssey does not allow you to manually enter adjustments while engaged, YPAO does.

Therefore, if the sub "distance" is really the "acoustic distance" rather than physical distance, how would the user measure (or calculate) that corrected value to enter in manually?
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post #6 of 8 Old 11-09-2012, 05:53 AM
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Originally Posted by mvp2005fan View Post

I've previously set up my home theater system old school with a tape measure, SPL meter, a real time display of a Velodyne SMS-1 (for the subs), and my ears. Via the usual time and experimentation process, I eventually got it to where I liked it.

I was planning to do the same this time, but decided to mess around with YPAO just for fun.

My issue is this: while Audyssey does not allow you to manually enter adjustments while engaged, YPAO does.

Therefore, if the sub "distance" is really the "acoustic distance" rather than physical distance, how would the user measure (or calculate) that corrected value to enter in manually?

In general the output of the sub at the crossover frequency is maximized when it is in phase with the mains. If you adjust the timing setting on the AVR's LFE crossover for maximum output in the crossover range, then you are going to be very close to optimum.

Any loss is most likely due to cancellation due to out-of-phase operation, which is an undesirable loss.
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post #7 of 8 Old 11-09-2012, 06:12 AM - Thread Starter
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post #8 of 8 Old 11-09-2012, 12:28 PM
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Arny did an excellent job of covering my, err, answers. Thanks Arny! smile.gif

Going for max output at the crossover frequency is the usual, and easiest, way to optimize the delay/phase shift so I agree with Arny. If you want to get fancy, do an impulse or step test and time-align the system so the waves at the listening position so they are the same. I have done it both ways, and the first method is pretty durn close and takes far less equipment and hassle.

The distance measurement for the other speakers tends to be in midband where phase is pretty flat ('ish); it changes rapidly near the crossover point(s). And what Arny said. The sub crossover is a different beast most of the time.

Exceptions I would not worry about, and the only definitive way to tell is to measure the acoustic response at the listening position.

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