Subwoofer Timing without a Subwoofer Channel

[Ice32]

Hi everyone, I've been trying do time alignment in REW. My preamp doesn't have a dedicated sub out so it's fed a full range signal from both left and right channels. I'm using a CSL Umik-2 to measure. The problem I have is that using an acoustic reference still sends a sweep to the reference channel (right speaker in this case) in addition to the chirp. Also, I noticed the timing in my meaurements seem to be off. When using right front as the reference, REW shows an estimated delay from left front as 1.6 feet when in reality it's about 4.5 feet. Measurement position is about 10 feet away from both speakers. I do have a Behringer HD202 and a Dayton EMM-6 but I don't know how to create a loopback (I tried connecting output 2 to input 2 using a guitar patch cable) and the Behringer is unreliable and doesn't work all the time.

Is there any way to get just the chirp to play from my right speaker while still measuring the sub?

Should I be concerned about the possible errors with estimated delay shown in REW?

 

[John Mulcahy]

When using right front as the reference, REW shows an estimated delay from left front as 1.6 feet when in reality it's about 4.5 feet.

Your left front is 4.5 feet further from the mic than the right?

Is there any way to get just the chirp to play from my right speaker while still measuring the sub?

Not clear what you mean by that. The timing signal will come from the channel selected as the ref output, the measurement sweep will go to whichever channel is selected as the measurement output.

 

[JStewart]

To clarify, acoustic timing reference measures difference in time between reference channel to mic and measured channel to mic.

So if the left is both the reference and the measured channel the ”delay” is zero or very close to it.
The assumption for a 2 channel system is the speakers are equidistant from where the listener and microphone would be, therefore when using the same left reference channel and measuring the right, the “delay” should also be zero or very close to it, since the distance from left reference to mic and distance from measured right to mic is the same.

The sub must be off when measuring the left or the right.

To measure the sub with a left timing reference, disconnect the right speaker. Now the right measurement will be the subwoofer only.

If you are aligning sub to mains, any XO related settings/controls must be made before taking measurements.

If you are aligning subs to mains then REW’s trace arithmetic will be used to combine the left/right together and REW can add 6dB to the sub measurement to equal the spl of having simultaneous left/right inputs.

Now REW’s Alignment Tool can be utilized to find the best timing relationship between subs and mains.

If your kit doesn’t allow independent left/right/subwoofer delay/distance settings then a bit of trial and error may become necessary to remeasure with changed settings on the sub and see.

It’s possible to get away with delay only on the sub. For it to work the actual delay of the subwoofer must be equal to or less than the mains. If greater than the mains then the mains will need delay to match. Looking another way, assuming the subwoofer had no additional latency from its electronics, the sub would need to be the same distance as or closer to the listening position, so it could delayed to match the mains.

 

[Ice32]

Your left front is 4.5 feet further from the mic than the right?


Not clear what you mean by that. The timing signal will come from the channel selected as the ref output, the measurement sweep will go to whichever channel is selected as the measurement output.

Thank you SO much for that comment about the distance. Both are almost the same distance from the mic. Now I understand what REW's doing here.

As for the second part, my system doesn't have a dedicated sub out. Whatever goes to left sub in also goes to left main and same for right. So the answer to that question is no. However, I'd like to try what @JStewart is suggesting. If I disconnect the right speaker but keep the sub connected, then do a right channel measurement that should work for timing. Then I add 6dB to the sub measurement as he suggests and I can use the alignment tool. The same should work when I add a second sub. Thanks to both of you.