# Multiple subs, distance to listener?



## aceinc (Oct 24, 2006)

When running multiple subs in various locations in a room, how do you set the distance to the listener?


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## Moonfly (Aug 1, 2008)

Tricky question that one with no real answer. Ideally you want them at the same distance, or you need a setup designed to run 2 subs. At the very least you would prefer to have a system with auto setup to take care of it for you. Failing all that your into guess territory, where you can choose either one of the subs distances, or the middle ground between them, perhaps even test all 3 and see which sounds best to you. It may just be best that you stack the subs if yo have no eq systems to help, but it would be a shame not to take advantage of improved eq from spreading them out if you can.

Sorry not to be of more help, but there really isnt a single right answer for that one other than to let auto setup take care of it. Good question though :T


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## salvasol (Oct 31, 2006)

aceinc said:


> When running multiple subs in various locations in a room, how do you set the distance to the listener?


If your AVR has the autocalibration feature you can use it... the AVR will adjuts the the sub output.

To do it correctly, you need to set the gain of each sub/amp one by one first, then run auto-calibration... if you don't do it that way, probably one sub will sound louder than the other.

Do you have a SPL meter??? if you don't, buy one... then using the AVR test tones calibrate first sub to get 75db, then turn off/disconnect and do the same with the second, then the third, etc. until all are set to 75db... Turn on all subs and run auto calibrate, the AVR will set the distance/delay using the internal sub test tone :T.

(Hope I didn't lost you :innocent

That's what I did when I was using two subs (one in the front and one in the back)....


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## recruit (May 9, 2009)

Yes, if you have a latest AVR then it should take care of the distances for you as it is very accurate if using something like Audyssey, otherwise it gets tricky and its not just distances that come into it but also phase so you don't end up with cancellations in the room which will mean less bass :rolleyesno:


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## aceinc (Oct 24, 2006)

No you didn't lose me. Relative loudness isn't the problem I was trying to address. 

The problem is that when different speakers are at different distances from the listener the sound will arrive from each speaker at a different time unless there is calibration in the time domain. Most sound processors provide for this.

I suspect that different frequencies will manifest the time domain issue in different ways. In the sub frequencies the wave lengths are so long, I would think the main manifestation should be a phase problem (more or less) although there will be the traditional time delay issue as well.

Since most (all?) sound processors have only one sub out they can only synchronize one sub. I am not sure what it does with multiple non equi-distant subs. My guess is that it would set the distance for the closest (perhaps loudest) sub, and "see" the subs farther away as "reverberation/echo."

I never really thought about it until today. When I originally setup my multi sub system, I had limited places to place the individual subs. I tried a variety of places (within those constraints), and the best configuration was to cluster the subs in one spot. 

Because of changes in my system I might have additional flexibility in sub placement, and I got to thinking about the issue. Does anyone know if the Behringer Feedback Destroyer pro allows setting time delays per channel? If so, I might be able to put one pair on one channel, and the third sub on a different channel. This would allow setting up different freq curves as well.

I recently posted a messgae on the Emotiva Forum suggesting that it might be a desireable feature to have additional outputs beyond 7.1, and allow those to be programmable as additional subs, or different zones.

Paul


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## Moonfly (Aug 1, 2008)

OK, the proper way to do it is to get stuck into REW, learn it and use that to set everything up. Most modern subs have variable phase rather than just a reverse phase switch, and its the variable phase you need.

You would then use REW to find the best spots for your subs out of the positions you have available to use. You need to look for 2 positions who's responses best compliment each other, not forgetting to do all your measurements at a fixed reference level (typically 75db, which is what I would recommend). One you have identified the 2 positioned your subs will live, its then time to phase align them. You need to take measurements of the 2 subs together, and one each of them alone. The response graphs will clearly highlight any areas of cancellations within the response, and you efforts would then be targeted at removing these dips.

Any single sub processor will treat the 2 subs as though they are a single one, and set the distance as it measures them. With the phase of the subs set optimally, I would have thought (and I'm not 100% sure on this myself) that them middle distance between the subs would be the figure to use. Its possible Audyssey does something different to this.

Thats a less than all encompassing explanation, not least as you need to phase match the subs with your speakers too. Basically, to do that I would do the phase alignment thing using REW with the speakers and the sub closest to them first, then do the subs phase by adjusting only the second sub to match the first, and thus the speakers too.

Doing it that way is about as good as you will get if you were to do it manually. Distance settings are closely related to time alignment and phase alignment, and doing the phase setup properly will take care of the majority of your distance issues. I couldnt answer your BFD question, even though I own a 2496, because Ive never actually needed to employ it, and try not to if I dont need to.


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## ironglen (Mar 4, 2009)

I don't think the bfd 1124 has any phase/time adjustment, though I wish it did, as I have a similar setup with an older preamp having dual front subs on one channel, and another rear sub on the other channel. I too have got to get going with REW so I know what's going on.


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## aceinc (Oct 24, 2006)

Been using REW for a few years.

REW will help in regards to phase, but not time. I say this because making the combined subs loudest at the listening position is in effect just aligning the phase at the listening postion, I think.

What you are suggesting is an empirical approach, this is how I determined that clustering the subs was the best within the available positions I had at the time. This can be a time consuming back breaking work.

The more I think about multiple sub locations, the more I wonder how well that might work for multiple seating positions. If you tune multiple subs for a specific listening position, the other positions may have cancellation nulls.

Paul


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## Moonfly (Aug 1, 2008)

Making the subs loudest at the listening position isnt a dead cert indication your phase settings are optimal, and wont tell you at all how well they are time aligned.


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