Originally Posted by AustinJerry
Have you looked at the 2x4 guide linked in my sig? There is a section titled “Checking Sub Alignment using REW Impulse Response Measurement”, starting on page 22, which I think will provide some guidance.
I actually read it yesterday before posting last evening. I realized I probably made a mistake in inverting the rear sub from page 22. And starting on page 23, I wondered if impulse charts were valid (is the USB loopback timing accurate or good to use). Except for the fact that step 3 calls for sending signal thru center channel with crossover redirecting it to the sub, I have made these measurements. I think their validity still depends on whether the USB loopback is and adequate timing reference.
I am unclear in step 6 about the alignment. My read is that the start of measurement should align (in example its at 8ms) and I'm guessing the initial peaks should peak at same time. In my impulse chart, the start is same (lookback delay is 0.0 for both measurements), and initial peaks line up. But if this is good alignment, the FR is not good with a chasm between 20-60 Hz (first two attachments). I realize that I didn't run it thru a crossover. I could disconnect both L&R speakers and play signal through both or one of them. Would that be close enough to how you used the center channel? Am I interpreting the impulse correctly?
On the REW alignment tool is appears that choosing different frequencies to either align phase or phase slope and affect the target curve (except for noted inaccuracy in mid bass). If it works that is improvement over trial and error.
I could try to do this with the Marantz but it is the NAD, I am trying maximize the quality.
The third chart is where I ended up via trial and error with rear closer sub inverted and a 6.3 ms delay. The subs are in diagonal corners but facing the opposing side wall. The reason I did that (speaker placement) is no longer valid, so I intend to rotate them to face front and rear before configuring 2x4 again. That would allow a reasonably accurate distance measurement with a laser tool. I was thinking to start there and make .1 ms adjustments if alignment tool isn't useful.
Thank you for producing the guide and helping folks on this and other forums!