# Subwoofer phase question (again)



## THX-UltraII (Nov 19, 2008)

I have found the right phase (delay on AVR) setting for my subwoofer and main (L and R) speakers. I did this by running a 80Hz test tone, run both my subwoofer and mains at approx the same volume and then find/set the subwoofer distance in my AVR than gives me the highest value on my dB meter.

I hope I did this correct.

If so, for a 2.1 system you are good to go but what about a 5.1 system? How can I set the subwoofer distance that s results in a correct phase between my subwoofer and all 5 speakers? Or is that practically not possible?


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## wgmontgomery (Jun 9, 2011)

If your sub is set correctly for 2.1, those settings should carry-over for 5.1. The specific AVR that you are using is not listed, so I'm curious if it has an "auto set-up/room correction" feature? If it does have room correction, it should address the sub's phase, too.

As far as the sub distance is concerned, why not actually measure the distance and use that info? It's not unusual for auto set-up to get the sub distance wrong; I always double check the "auto's" measurement with a measuring tape.


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## rab-byte (Feb 1, 2011)

wgmontgomery said:


> If your sub is set correctly for 2.1, those settings should carry-over for 5.1. The specific AVR that you are using is not listed, so I'm curious if it has an "auto set-up/room correction" feature? If it does have room correction, it should address the sub's phase, too.
> 
> As far as the sub distance is concerned, why not actually measure the distance and use that info? It's not unusual for auto set-up to get the sub distance wrong; I always double check the "auto's" measurement with a measuring tape.


Sub measurements are usually better with the auto EQ rather then a measuring tape as base likes to do odd things in a room. As for your speakers yes it's best to actually measure you speakers distance.


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## wgmontgomery (Jun 9, 2011)

rab-byte said:


> Sub measurements are usually better with the auto EQ rather then a measuring tape as base likes to do odd things in a room. As for your speakers yes it's best to actually measure you speakers distance.


I certainly agree that the auto EQ is a good idea and can help correct/tame room modes.  Perhaps I'm missing something, but how will having the sub's distance set _incorrectly_ fix/correct room modes?

Either way, the point that I was trying to make was that the "sub distance" should be set with either "auto set-up" (which often gets the distance wrong) or a measuring tape instead of just an SPL meter.


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## primetimeguy (Jun 3, 2006)

Actually, the physical measured distance is more wrong than what auto setup chooses. A sub typically has a crossover and other delays so an acoustic distance that is a couple feet farther than the physical measured distance is the correct value.


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## tabalji2 (Jul 27, 2012)

I am a new poster so if I talk of something which has already been dealt with before, please excuse me.

Getting a correct phase is not difficult and can be done empirically without much mus-and-fuss. First of all we have to presume that the main speakers are correctly phased. If we are satisfied with the phase of our main speakers then follow the next few steps.

1) Determine what the crossover frequency of your subwoofer is.
2) Connect the audio output of a computer to the input of your system.
3) Using Audacity (or any software that allows you to generate a sine wave), feed a signal into your system.
4) Move your main speakers (left, right and centre) to a position which is as close to your subwoofer as possible.
5) Play a sine wave which corresponds to your crossover frequency.
6) Now start reversing the polarity of the wires to your subwoofer. Go back and forth a few times. You will find that the sound is much louder in one arrangement than another. The arrangement which is loudest corresponds to the correct phasing of the subwoofer.

This has always worked for me.

peace

David Courtney


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## wgmontgomery (Jun 9, 2011)

tabalji2 said:


> I am a new poster so if I talk of something which has already been dealt with before, please excuse me.
> 
> Getting a correct phase is not difficult and can be done empirically without much mus-and-fuss. First of all we have to presume that the main speakers are correctly phased. If we are satisfied with the phase of our main speakers then follow the next few steps.
> 
> ...


Welcome to HTS, Dave! Most people have their subwoofers connected from a sub/LFE output on their AVR/SSP that is either an RCA or XLR cable, so it's a bit difficult to change the phase from the wires. Your logic is quite sound however (pardon the pun), and the same result can be achieved with the phase control on most subwoofers.


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## rab-byte (Feb 1, 2011)

I was writing a long post about this but thanks for saving me some time. 

You want the sub to crossover at the point the mains can't handle the low frequencies you're demanding of them. 

For instance my towers reach 35Hz but SPL drops off around 50Hz so I want my sub to pick up the slack or "cross over" at around 50Hz down to the 20Hz mark. Since I'm using an AVR with an active crossover I just set my crossover to 50Hz and the AVR will direct audio traffic to where it needs to go. 

As for sub placement google "sub crawl" and you'll find a good bit of reading on the subject. Martin Logan has a good YouTube video on how to setup a subwoofer that is step by step.


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