There may be an issue or there may not. The "Time of flight" or delay is a function of frequency. The SW to Main timing is not easily evaluated by looking just at the IR peaks. The conventional wisdom is to adjust the timing such that the SW and Main provide close phase tracking throughout the XO range.
If you post a measurement of the SW, Main, and SW+Main I will review it and comment in more detail.
It's interesting to note that when I did sweep the sub full range, there was some tiny analog leakage in my system into the other channel (barely audible, could have been 60 dB down), and there was enough high frequency leakage into the other channel so that the impulse measurement of the sub showed up as identical timing to the mains! In other words, REW got a "clue" from the slight high frequency leakage that made it think the sub was timed the same as the mains; I saw a narrow initial impulse in the sub display, which I intuitively knew as wrong. But when I muted the other channel, then the sub time delay measurement returned to its extreme that we see in these measurements.Bob,
I see your concern is valid.
The SW is excessively delayed according to the files you provided. This is not good timing for any philosophy regarding proper XO timing alignment. Of course if the timing was somehow corrupted by your setup/calibration then all the following comments are not relevant.
[There was a 2 dB SPL offset between the SW and Mains as indicated by the SW+Mains. I just ignored this for these comments as an SPL offset does not impact any comments regarding the delay/phase.]
[To be safe, I normally sweep full range for SW and Main as this provides REW a nice sharp loopback IR signal to set the timing on. I have not seen any error even when reducing the SW sweep range, but it is safer to use the full range measurement. It is pretty safe to assume this was no issue here.]
Did you make the timing offset using the "estimate IR delay/shift IR" settings in REW? I don't see any other way to shift that. I also don't see any way to reverse that "shift IR" setting once you perform it, and there are times when I would want to see the genuine time of flight... ???Your XO is very steep (high order) such that the XO range is very narrow (about 55-85 Hz) and thus the phase matching is not overly critical as it impacts the SPL in only a very narrow range. The more important issue is that the delay/timing is reasonable at the XO.
View attachment 39832
The current delays provide the following phase tracking through the XO range. It is okay for phase at 60 Hz, but the phase tracking through the XO is not the best and the SW IR delay relative to the Main is excessive.
[Note that I offset both the SW and Main measurement IRs by 8.475 ms (the offset of the peak of the mains). This is necessary to best evaluate the phase timing around the 60 Hz XO.]
John, please help me find where you make that delay change in REW. I can't seem to find it.Option 1:
Reducing the delay of the SW (or increasing the delay of the Main) by 14 ms And inverting the polarity of the SW we get:
This provides better phase agreement through the entire XO range and brings the SW timing more in line with conventional practice.
Thanks, yes, it's a great start. Again, it would be great to find out why the big subwoofer delay. Maybe this weekend I'll bypass both lowpass filters and take another measurement and see if the excessive delay goes away. Well, my plan is to learn how to use AudioLense first on this system, within the limitations of having one DAC. Then to move to multiple DACs, a digital crossover and a dedicated HTPC to drive the whole system. That's going to be a big move, and since this is a mastering studio doing actual production work, I have to do it slowly and carefully!Option 2:
Reducing the delay of the SW (or increasing the delay of the Main) by 22 ms we get:
This provides better phase agreement through the entire XO range and brings the SW timing more in line with the Main. With this alignment the GD will be minimized compared to the other 2 alignments.
Because of the very steep XO the impact on SPL through the XO range is minor with any of these alignments.
I can provide more information regading the process used as needed.
I hope this helps.
My comment was only to point out that for some reason the level of the SW+Main was somehow shifted -2 dB in the measurements you provided as shown below. Something must have changed the level of that measurement. [I was not commenting on the chosen relative level of the SW and Main.] A shift like this will not happen if the only change is to apply appropriate muting. It was just an observation FYI, one that does not affect the analysis.…
The so-called 2 dB SPL offset you notice is in my opinion a misinterpretation on your part. I never use the SPL measurement reported in REW or any analysis tool as the basis for adjusting the sub level; this is just not a useful tool. For one thing, SPL of a narrow bandwidth signal is going to always be different. Anyway, first I crerate a nominal 0 dB line in 1/1 octave smoothing mode setting the midrange to center around that line using REW's vertical offset adjustment. That's partly an art of course because "what is the midrange", but this method gets the bass very accurate and rarely is a further tweak necessary, no more than 1/2 dB of sub level variance. Anyway, that becomes the 0 dB reference. Then using 1/6 octave smoothing I set the subwoofer level so the positive and negative extremes of the subwoofer output are centered on the 0 dB line, and the result is always quite correct. That's my method, at least.
In addition, the splice between the non-filtered satellite speakers and the sub is level-perfect, you can tell that the sub level must not be offset because you can see the accuracy of the splice in the combined measurement.
Higher Order minimum phase XO filters cause more total phase rotation and more GD between high and low freq. With this steep XO I am not surprised by the delay shown here, but I am not able to confirm if it is entirely due to the filters.The main question, however, is how the hell did the subwoofer get so delayed? To repeat, there is only one D/A converter in my current system and all the frequency dividing and summing is performed in the analog domain. Perhaps it's due to the extremely steep slope of the low pass filter, which is done by two 24-dB/octave filters in series? But in my wildest imagination I can't imagine that causing an apparent 10 ms. or more offset between mains and sub... perhaps some technical expert on analog filters can respond here to that conundrum, but let's move on and respond to your wonderful detailed response! Please forgive the excess quoting and try to find my questions below.
See below the “Offset” box. I just measured the 8.475 ms offset of the FL Main IR peak and entered that value into the offset box. I applied that SAME 8.475 ms offset to each of the 3 measurements to keep the same relative timing.…
Did you make the timing offset using the "estimate IR delay/shift IR" settings in REW? I don't see any other way to shift that. I also don't see any way to reverse that "shift IR" setting once you perform it, and there are times when I would want to see the genuine time of flight... ???