# Time aligning subwoofers



## Amfibius (Aug 13, 2008)

Hi all, long time lurker but very few posts. 

My situation: I own two JL Audio subwoofers and a pair of front speakers. The subs are currently placed on either side of the main speakers, about 1.5m into the room. However, I am thinking of placing the subs in the corners, where they will be approximately 1m behind the main speakers. I also run REQW, external sound card, ECM8000, etc. 

My questions: 

1. How do I measure the time difference between the main speakers and the subwoofer at the listening position? If there is a page on how to do this on REQW I must have missed it. Apologies if this is a common question. 

2. What is a "ballpark" figure where the time difference becomes insignificant or inaudible? 

Thanks all for your help.


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## wgmontgomery (Jun 9, 2011)

What kind of preamp/receiver are you using? If it has a "room eq" or "auto set-up" it should have a section where you enter the speaker distances. That should take into account the differences in distance.

FWIW, the speed of sound is 340.29 m / s; the speakers would have to be a _long_ way apart to hear a difference in sound because of distance (~1,123 feet=1 second delay). This is especially true of low frequencies (<80Hz) which are not usually localized anyway. 

One more note, the "phase" control on the sub is useful for placement issues as well. The 1m difference shouldn't be audible, but you can make small adjustments with the phase control until you get the best bass (all other things being equal).

Just my 2¢; I hope it helps. 

BTW-I have REW on my laptop, but I don't feel knowledgeable enough about it (yet) to address your questions regarding the program. I'd rather be honest about that than give you wrong advice/information about it. I plan to use it on my room/system, but I need a better mic first.


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## wgmontgomery (Jun 9, 2011)

I may have misread your post; when you wrote REQW did you mean the REW program from this site? That's the program that I have on my laptop and referred to in my previous post. If it's a set-up program on your receiver/processor it should have a speaker distance section in the set-up menu.


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## Amfibius (Aug 13, 2008)

Thanks for your response. I am not using a HT receiver, I am using a conventional analog preamp. There is no way for me to digitally adjust the delay between the sub and the main speakers. I do, however, have an analog active crossover between the sub and main speakers (a Marchand XM-44) which can accept a time delay module. Before I get such a module custom built, I have to learn what the time difference is between the sub and the speaker. 

And yes I mean REW on this website.


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## wgmontgomery (Jun 9, 2011)

The time difference would be _extremely_ small. From what I've read, it's about 1ms per foot. A meter is 3.3 feet, so 3.3 ms...an extremely small time difference. For more info, check out:

http://www.recordinginstitute.com/da154/ARP/chap3Sig/0304delcor.html

I hope that this helps, but with a difference of ~1m (and it's a subwoofer reproducing frequencies that are not easily localized anyway) it _really_ shouldn't matter. The subs will be producing sounds with a wavelength longer than the difference. Room interactions (peaks, standing waves etc.) will be a bigger concern. Again, I hope the info helps. If not, please let me know. I'm happy to help in any way that I can.


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## wgmontgomery (Jun 9, 2011)

BTW-the REW program will help with the room interactions that I mentioned. I really need to use it in my room, but I'm moving in a month (or so) and don't want to invest a lot of time "taming" a room that I'll be leaving. I plan on using REW after I move, but it should help tackle any big problems that you have.

Anyway, the 1ms per foot is a good, basic rule. I believe that the true delay is about 10% higher, so if you want to get it as close to exact as possible, add 10% (1m= 3.3 ft= 3.3 ms + 10%= 3.63 ms).


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## Amfibius (Aug 13, 2008)

Yes, I do know that the 3.3ms time delay is very small ... but I want to know how small is acceptable before it is completely imperceptible to human hearing. Does anybody know?


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## Amfibius (Aug 13, 2008)

Actually I may have answered my own question with some googling. 

http://www.ethanwiner.com/phase.html

They were unable to hear a difference with 7ms of delay. I suppose that's good enough for me.


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## wgmontgomery (Jun 9, 2011)

I infer from your last post that you're "good to go?" If not, please feel free to post any other questions. We'll do our best to help!


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## Amfibius (Aug 13, 2008)

Thanks Gary  At the moment it seems as if approx 4ms is going to be the time delay imposed from physical positioning alone. What I didn't mention is that I have a digital EQ doing room correction for the sub, which will have its own overhead. Ultimately I need to measure it with REW to see what the time alignment is. I have spent some time RTFM'ing and trying to understand _how_ exactly to find out the time difference between my various drivers. I guess the only way is to go and play!


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## wgmontgomery (Jun 9, 2011)

I read in Stereophile that the DSP processing used in some subs adds delay just by the fact that there are a couple of ms that pass as it processes the info. I'll try to find the article and see if it's online. From what I remember, when the reviewer did an auto set-up on his processor, it mis-placed the subs by several feet. He concluded that it was due to the time used by the sub's DSP processing. 

Which JL Audio subs are you using?


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## Amfibius (Aug 13, 2008)

Hi Gary, I am using the JL Audio F110's. 

I used the weekend to reposition my sub, do some sweeps, and try to interpret it. I am still very much a n00b when it comes to REW so I will have to redo them. At the moment it looks as if the group delay (relative to the main speakers) is ... 50ms!! That is definitely audible, and I can hear it. 

I also suspect I have sound card issues - REW indicates that my system rolls off at 8kHz (it most definitely does not!); and it also suggests that I need a lot more bass. When I tune the system so that REW indicates the F/R is flat, I have WAYYYY too much bass. After tuning the system with REW, I re-tuned it by ear. REW now thinks I am running my bass 15dB too cold. 

Anyway, I have more investigating to do. I don't trust this set of measurements because some of it does not correlate with what I am hearing. I have RTFM'ed several times over. Next thing to do is calibrate my sound card (which I have not yet done because I don't have the cable).


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## Amfibius (Aug 13, 2008)

Hey! This forum changes "R T F M" to moo baby moo!!!


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## wgmontgomery (Jun 9, 2011)

Calibration of the soundcard AND the mic is very important. Also, I think that you may need to do room sweeps in multiple locations...at least for bass. I hope that someone with more REW experience will comment as my experience (as noted) with this program is limited.

The last time that I looked there were calibration files for specific mics that you can download. Calibration of the soundcard (if there isn't a download) can be done by following instructions on the REW thread. 

The room plays such a huge role in the sound of your system that addressing its "problems" should pay huge dividends. Good luck; I'll help as much as I can.


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## Dual-500 (Aug 1, 2010)

Amfibius said:


> Yes, I do know that the 3.3ms time delay is very small ... but I want to know how small is acceptable before it is completely imperceptible to human hearing. Does anybody know?


In the frequency range where the subs cross over to the mid/high end section, human hearing isn't real sensitive to phase angle differences.

To make it right, time alignment via digital conversion is a real good solution. I don't know the cost of hardware to upgrade your Marchand analog. If in the $250 range or more, have a look at a DBX DriveRack PA as a possible solution.

My setup has correction of 24" between the low mids and subwoofer as they sit out front of the sub. Also corrections of 9" between 8" low mid drivers and high mid horn compression drivers. Also, a 12" correction between 8" low driver and midrange horn on the center channel.

The low crossover point between subwoofer and low mid drivers is 200 hz. When I converted the system from analog crossovers to DSC's (Digital System Controllers) what I noticed more than spacial or time differences was eq settings and overall tonality. Putting the drivers in phase allowed for better acoustic coupling and less overlap correction in the controller and less equalization around the crossover region.

In the upper frequency range the phase alignment or misalignment is much more noticeable and comparing errored against corrected phase relationships is noticeable in terms of time and also toneality. The same applies in terms of correction in the crossover region for controller correction and equalization requirements.

Overall time correction/alignment in my system was a big plus.


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## terry j (Jul 31, 2006)

depends on the crossover point from subs to mains as well.

if you can do a sweep of JUST your bass (like you can? with the subs) then do that, if not then just limit the sweep from rew to the freq range you know/think the bass driver covers.

also do a sweep of the subs, best to do them all singly.

then go to the overlay tab, and pull up the impulse responses. They can be placed on top of each other to check alignment.

We are after the ACTUAL time arrival of the sub/woofer impulse, that may or may not be in accordance with any given physical (tape measure) measurement.

Using normal click and drag tools of rew, you can measure very accurately the arrival times of each impulse (oh, check the manual, but you need to have loopback etc etc operating)

Look at the woofer impulse, note the time of the first positive peak (neat check that the two mains are time aligned too! They need to be excatly the same at the LP) We want the subs first positive peak to arrive at the same time as the woofers.

I can fix that stuff easily with delays, I guess otherwise it is move and measure??


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