Trace arithmetic of 2 sub positions

[FargateOne]

Hello,
I have one sub. I took 20 measurements in 20 different places for this sub in my room. Before to buy a second sub, is it a good idea to try A + B the sum of 2 postions to see what improvement could be acheived with the second sub?

 

[jtalden]

Yes, that is absolutely the way to do it. If you are buying an identical sub you can determine the combined response for any 2 selected positions. The REW alignment tool is the easiest way to adjust timing, polarity and gain as desired to optimize the response for each set of optional positions.

 

[FargateOne]

Yes, that is absolutely the way to do it. If you are buying an identical sub you can determine the combined response for any 2 selected positions. The REW alignment tool is the easiest way to adjust timing, polarity and gain as desired to optimize the response for each set of optional positions.

@jtalden OK for the fun of it ( and maybe helping somebody here) I made some measurements with my only sub at different places. In the receiver the sub delay is at 285cm from mlp and I placed the sub elsewhere without changing the delay. Knowing that my receiver has only one setting for sub delay and knowing that the receiver has 2 sub outputs and those get the exact same signal (are like a y splitter on one sub cable), am I wright to conclude that the best place for an hypothetic second sub is :

1- aligned sum 1 because the only thing to do is inverting the polarity of one suf without changing the distance in the AVR;
2- aligned sum 2 is the second best but less good because I changed the delay for the subs (the same amount is applied for the 2 subs)

P.S.: I know that a miniDsp device (DDRC-24 or 2X4HD) could be a solution but out of budget for the moment.

 

[jtalden]

Yes, you have correctly identified 2 good choices.

2- aligned sum 2 is the second best but less good because I changed the delay for the subs (the same amount is applied for the 2 subs)

Changing the distance the same amount for both subs is not a change so it didn't matter that you did that.

When you setup the 2 subs with the mains (the AVR XO should be active) the distance and possibly the polarity setting for the 2 subs will change anyway.

 

[FargateOne]

@jtalden.
Here is two alignment sum of the same 2 measurements. The difference is that I hit "time align" button before doing the sum with the aligment tool. Which one is the best prediction of a second sub effect?
In fact there is my question: when using alignment tool do we need to time align the measurements befor even if all those were taken with an accoustic reference?

 

[jtalden]

The original calculation is the correct one. It reflects the distance difference between the 2 SW.

REW 'time align' shifts the selected measurement impulses to t=0. The original timing relationship created using acoustic timing is lost. To implement that change in relative delay timing between your 2 SWs would require; the MiniDSP you mentioned, a new AVR with that capability, or some other device that can delay one of the SWs. The new SPL trace correctly shows the result of that particular timing change. Often, time aligning 2 SWs to t=0 is a favorable situation as shown here. It is the starting point for setting up multiple SWs, but sometimes a better result can be obtained with some timing offset.

The REW alignment tool provides an easier way to investigate the impact of any change to the relative timing so it is the more versatile tool for this type of job.

 

[FargateOne]

@jtalden Sorry for the dealy to get back. Thank you for this clear explanation!