# Case Study: Sub Alignment using REW v5



## laser188139

*OVERVIEW*

I was looking forward to using the new features of REW v5 to evaluate the distance calculation of my sub. The distance calculation made by my Audyssey-enabled Denon receiver always left questions as to why its times differed from the physical distance, particularly why the calculated sub distance was less than the physical distance. 

The best article I’ve seen so far on how to align the sub is “Subwoofer Alignment with a Full Range System”, by Charlie Hughes and published by Syn-Aud-Con, kindly cited here on the REW Forum by Dr Who. This article describes the problems with using the Impulse plot to align the sub, because of the low resolution provided by the low frequencies, and recommends using a Group Delay plot. 

REW v5 provides two new features very helpful in aligning the sub distance with the main speakers. There is the Group Delay plot, that shows the relative or absolute arrival times of the impulse at various frequencies. There is the ability to use the Left Channel for timing, which gives an absolute time reference to the Impulse and ETC plots, so that one can compare these across measures of multiple speakers. In my situation, I needed to use both of these, as the alignment exercise was trickier than normal. For my front speakers I have classic Klipsch floorstanding all-horn speakers (LaScalas) so there is a significant delay between the woofers and the tweeters. I chose to align the sub sound with the woofer, as it seemed most important for these two to work together.


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## laser188139

*SETUP*

For the phase and group delay information in the new plots to be accurate, the soundcard calibration files must be rebuilt with REW v5. V5 now includes phase information in the soundcard file, which was not present in earlier versions. 

In the Settings -> Analysis screen, you need to check the Use Left as Timing Reference box to use the Impulse plot to compare multiple speakers. You will also need to loopback the left channel on your soundcard from its output back to its input. This is similar to the loopback needed to build the soundcard calibration file, but now it is on the left channel instead of the right and you need to leave it connected all the time. As described in the REW helpfile, this involves one cable in addition to those shown on the REW Interconnect Diagram, looping left to left. The helpfile shows this optional cable in the pictures illustrating the connection to a BFD Pro equalizer. 

You can examine the Group Delay plot without using the left channel as timing. If you are using the left channel as timing for impulse curves and are using the first beta build of REW v5, you will need to uncheck the Allow 96 PPO Log Spacing option or update to a later build.


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## laser188139

*GROUP DELAY PLOT*

Unlike the nice smooth plots in Charlie Hughes’s article, when I measured mine the data was not nearly as easy to interpret. The following plot shows the Group Delay view of two measurements of the left front speaker plus sub before this exercise. The red curve represents the sub with the Audyssey calculated distance, 12.6’, and the green measures the result after increasing the distance in the AVR to match the physical distance to the sub at that time, 14.8’.









One can see in this curve that the midrange and bass frequencies are delayed about 2 msec relative to the tweeter. The sub might be delayed about 3.8 msec relative to the woofer. More significant for me was the noise in the measurement, making it hard to judge the group delay over the range of each component. 









Changing the smoothing to ½ octave made it easier to see an edge, but it suggests that the sub is too late by 7 msec relative to the woofer, which is probably excessive. It even indicates at one point that the smaller AVR distance value (green) is appearing earlier than the greater AVR distance value (red).

This noise is to be expected in a complex room environment. Essentially peaks and nulls from reflections and modal resonances cause phase changes that appear in the group delay view, making it hard to isolate differences in distance. For comparison, I took a near field measurement of the front speaker, still indoors, and the 1.7 msec delay from tweeter to midrange is obvious; at the low end room resonances still have a significant effect.


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## laser188139

*BAND-LIMITED IMPULSE PLOT*

As described in the article, the problem with using the Impulse plot on the sub is that the limited frequency range of the sub acts as a filter and limits the resolution of the information. To resolve this problem, I came up with the idea to measure the sub and the front speaker over the same frequency range. By using the same frequency range, I would be looking at the data through filters of the same bandwidth, and so the results should look similar if they are aligned in time/distance. 

To compare measurements of multiple speakers taken separately, one must use the Left Channel as Timing described above under Setup. As the upper limit of the sweep spectrum is raised, a finer resolution of the curve is obtained. So the first step is to determine how high a frequency can be used. First, I raised the crossover in my receiver to its highest possible value, 250Hz. When I measured the sub, I observed that its frequency response with Audyssey On was flat to about 200Hz. I then changed the AVR to the lowest possible crossover, 40Hz, physically turned off the sub, and measured the frequency response of the left front over 10Hz-200Hz.









In this graph, the sub response (green) is even up to 200Hz, so this is a good upper bound to the sweep. Any higher, and the impulse curve of the main will show too much resolution and may have a different character than the sub impulse. The main curve, with a 40Hz crossover, is even from about 35Hz on up to 200Hz. To cover the range shared by these speakers, I chose to examine 50Hz-200Hz. 









In this graph, an Impulse plot showing % FS on the vertical axis, the teal is the impulse curve of the left front over its full range, and the green shows what happens when I limited the measurement range to 10Hz-200Hz. The red line shows the impulse response of the sub, located physically very near the left front so it should see roughly the same room response. Its current peak appears 3.06 msec (3.4 ft) after the left front woofer. After increasing the distance value in the AVR by 3.3 ft, the sub’s new impulse curve (red) shows its earlier arrival, now aligned with the left front woofer. 









Note that I was able to examine the effect of changing the AVR’s distance to the sub by re-measuring the sub only because it was not the farthest away speaker. The receiver uses the speaker the farthest away as its point of reference, and delays the others to match. Had my sub been farther away than the front speakers, changes to its distance would have been seen as moving the impulse response curves of the front speakers. 

Looking at the leading edge of the impulse curves instead of the peak, one calculates almost the same difference in distance. From reading Charlie Hughes’s article and the experiments described below on the effect of the crossover’s low-pass filter on the sub, there is a sound basis for using the leading edge as the point defining the location of the sub. The tricky part is deciding which part of the curve to use as the leading edge, when there is a negative pulse in front of the curve. In the following graph, I measured from the 2% point on the sub curve forward to the center of the leading negative peak of the front woofer and found a difference in distance of 3.2’, very close to the 3.4’ found above measuring peak-to-peak. 









To get the curves to line up like those above, depending on the polarity of the sub when you start, you may need to check the Invert Impulse box in the Controls for the sub’s individual impulse plot. A nice feature in REW v5 is that changes you make in the individual plot are immediately reflected in any Overlay plot displaying the same measure. 

The above technique seeks to obtain a reliable measure of the time/distance to the front speaker and the sub by avoiding the effects of the crossover. It does so by using a very low crossover when measuring the front speaker, and a very high crossover when measuring the sub. 

I looked at whether a similar result could be obtained measuring narrowly at a selected crossover frequency. As discussed in the next section on the Crossover Filter, its high-pass and low-pass filters have different effects on the two speakers. These make it a challenge to define the correct measurement points for their impulse responses. 









In this graph of the system with the crossover set to 100Hz, the teal is still the left as full-range, the green is the left woofer measured over 95-105Hz, the sub’s response at the original 12.2' distance over 95-105Hz is in red, and the broad sub response, 50-200Hz is in light red. Comparing the sub’s impulse response measured narrowly to the left front, I can only observe that the distance from the sub’s peak to the large negative peak in the left front, 3.3’, tracks well with the distance determined above. That these two peaks are in opposite directions may explain why I needed to invert the sub’s polarity for the best frequency response. That the sub’s peak is very close to the second positive peak in the left front’s response may explain why the 12.2’ distance calculated by Audyssey gave a frequency response about as good as that seen at an adjusted 15.2’.


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## laser188139

*EFFECT OF CROSSOVER FILTER ON IMPULSE RESPONSES*

To interpret what will be seen in the impulse curves, it is useful to experiment and see the results of running sweeps of different ranges through the various components of the system. 

First let’s look at simple case of the front speaker configured as Large, so we can later isolate the effect of the crossover on the test spectra. 









Measuring the AVR preout as Large, with no crossover, shows that the impulse response is symmetric around the zero point. As the frequency lowers, the resolution becomes less sharp and the curves broader, from full range (red), through 200-400Hz (orange), 150-200Hz (yellow), 95-105Hz (green), 60-80Hz (blue), and 10-40Hz (purple). There is, perhaps, a slight advance in time in the 95-105Hz curve, of 200 μsec. 

When a 100Hz crossover is introduced, the behavior of the impulse response curve of the left preout changes. 









The 100Hz crossover provokes a slight advance in the time of the curves above the crossover frequency, 400-800Hz (orange), 200Hz-400Hz (yellow), 150Hz-200Hz (green), then the curves become more significantly advanced and broader at and below the crossover, 95-105Hz (blue), 60-80Hz (indigo), 10-40Hz (violet). We seem to be seeing two effects here. There is still the broadening of the curves from the lower resolution of the lower frequencies, as seen above when the speaker was Large. There appears a new effect from the crossover, that it is introducing a phase shift and advancing the time, the more the frequency is reduced below the crossover frequency of the high pass filter. 









Looking at similar tests of the LFE output shows different behavior below the crossover. In this test run with Audyssey Off giving smooth curves, sub range sweeps, 10-400Hz, were taken varying the crossover, 250Hz (red), 120Hz (orange), 100Hz (yellow), 90Hz (green), 80Hz (blue), 60Hz (indigo), 40Hz (violet). The gray pulse shows a full range measure of the left preout, to establish a point for the starting time. As described in Charlie Hughes’s paper and already seen above, with the lower resolution of the lower frequencies, the curves become broader. The beginnings of all the curves appear to start at almost the same instant, 1.0-1.5msec before the pulse from the left channel preout. 









This graph shows the impulse curves of the LFE output from the AVR of a sub configured 1.7’ (1.5 msec) closer than the front speaker. If we look at a typical crossover frequency, 100Hz, it is interesting to compare the curves as the sweep range is varied, 10-400Hz (red), 150-200Hz (orange), 95-105Hz (yellow), 50-80Hz (green). Here one sees that the 50-80Hz curve is delayed later, the 95-105Hz yellow curve has a similar peak to the full range, but has a leading negative peak, and the 150-200Hz range (orange) above the crossover looks as if its phase is inverted, with a full negative peak before the positive peak. The center of the negative peak of the narrow range around the crossover, 95Hz-105Hz, matches very closely the initial time of the full range curve. 









Comparing the group delay curves of the left preout to the LFEout with both speakers configured to the same distance in the receiver confirms the impression from the impulse curves above, that the lower frequencies are delayed relative to the higher frequencies. Here the blue curves are the overall LFEOut and the narrow ranges used above, and the red curves are the left preout, full range and narrow sweeps. Well above the 100Hz crossover, the curves show the same 4.7 msec processing delay in the receiver. But as the frequency decreases near, through, and below the crossover, the crossover introduces delay. Because the Denon crossover is 24dB/octave on the low-pass side to the sub, and 12dB/octave on the high-pass side, the LFEout shows twice as much delay as does the left preout. There are two important implications for setting the sub distance: (1) the front speaker itself appears at a different distance at the crossover than it does at the other frequencies, and (2) even if the sub distance is set to match perfectly at the crossover, its distance will only be close at the frequencies above and below the crossover. 









In this graph, I compare the Left preout to the left speaker measured in the room. Particularly interesting is how the narrow measure 95-105Hz at the crossover changes. The leading positive peak does not appear as large, the negative peak is just as large, while the second positive peak now appears larger than the first. This illustrates why narrow measures at the crossover are so hard to interpret.


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## laser188139

*CONCLUSION*

The Group Delay plot after aligning the sub appears to show the sub frequency range more-or-less even with the woofer/midrange. This confirms that the band-limited impulse approach used above had the intended result.









After adjusting the sub time/distance setting in the receiver, one must go back and review the polarity of the sub and the choice of crossover frequency. I looked at the frequency response curve of the (left +sub) and (right + sub) pairs separately, seeing which polarity and crossover frequencies worked with the left, then choosing among these which worked best with the right. Only after checking each front speaker separately with the sub, did I look at the overall response of both driven together.

After swapping the polarity of the sub, one can see from the phase curves why the corrected 15.2’ distance works a little better than the original 12.2’ distance. Looking at the phase curves over the octave either side of the 100Hz crossover, one can see that the new distance (blue) stays between the green and red curves over most of the range. The sub curve at the original distance (teal) gets more out of phase sooner below 100Hz; its phase is changing more rapidly because its audio is delayed and seems farther away. 









I can't say I can hear a difference, but the total adjustment I made relative to the Audyssey calculated distance, +3.1', is below the threshold of what is considered audible. Correcting the sub distance gave me a clearer choice of sub polarity and crossover frequency, and a better overall response curve. That is audible, and I'm happy about that.


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## laser188139

RESERVED


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## JohnM

Bill, the band limited impulse measurements is an interesting idea, looks to work quite well. I'm not clear what you are measuring with the deltas in those group delay plots, though. In-room measurements in a small space (e.g. HT room versus stadium) will result in GD plots with lots of variation, but comparisons between plots need to be made at specific frequencies - for example, in the 12.6' versus 14.8' measurements the GD should show a difference of about 2ms at any frequency where the sub's output dominates, say below 50Hz or so. To use GD to compare the sub's timing with the main you really need separate measurements of sub only and main only and to make comparisons of their GD's around the crossover frequency.


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## laser188139

The deltas in the group delay plots were just arbitrary frequencies compared against what seemed to be the leading edge of the woofer. 

I was hoping to see a broad trend in the group delay plot, but as you say the issues with a small room dashed that hope. Being able to look at a broad trend, if that were smooth without so much noise in the data, would have been a nice technique to recommend for people whose mains don't have a wide range of overlap with the sub. 

I did look at the Group Delay of the sub compared to a measurement of the main just over the range 10Hz-200Hz. Even over that limited range, there was so much variation between different frequencies that there was no obvious point at which to compare them. Depending on which frequency I picked to look at, the sub was significantly early, significantly late, or just on time. 

I didn't think to take a sweep just around the crossover to see if that yielded a more stable measure. I may play with that and see what it says. When I started this exercise, I was thinking more of synchronizing the sub with the woofer for any improvements one might hear in music by eliminating the time difference. You may be right, though, that focusing on the crossover could yield a better result, by ensuring no interference between the sub and the woofer and improving the overall response. 

Another idea for the statistically inclined would be to export the data, calculate a mean group delay over the interesting range, say 60Hz-120Hz, and see if comparing these would yield the same recommended change in time/distance as does aligning the impulses.


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## JimP

JohnM

What would you recommend for time aligning a subwoofer to mains?


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## JohnM

JimP said:


> What would you recommend for time aligning a subwoofer to mains?


I like Bill's idea of band limited measurements of sub and mains over the range both operate. Another option would be to make measurements at the line level outputs to sub and mains to accurately characterise the delays in processors etc, free of any room influences, then add in the delays from the relative distances of the speakers/sub to the listening position. An option for fine tuning is to set the sub to opposite polarity and tweak the delay adjustment to get the largest dip at the crossover frequency, then flip the sub back to correct polarity, though that is subject to room influences on the measurement which may make the evaluation difficult.


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## Moonfly

I am wandering how adjusting the time delay in your AVR, after Audyssey is run, affects what Audyssey is trying to do. I'm all for improving your system, and your results sound positive, but if you manually change settings after Audyssey stores its settings, your then basically altering the response Audyssey is trying to base its calculations on.

This is one of those things that would niggle away at me, even if I cant hear the difference, at least for a while. Ive always been an advocate of helping Audyssey as much as possible to get the very best results, so I really wander how this figures into the equation.


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## JimP

JohnM said:


> I like Bill's idea of band limited measurements of sub and mains over the range both operate. Another option would be to make measurements at the line level outputs to sub and mains to accurately characterise the delays in processors etc, free of any room influences, then add in the delays from the relative distances of the speakers/sub to the listening position. An option for fine tuning is to set the sub to opposite polarity and tweak the delay adjustment to get the largest dip at the crossover frequency, then flip the sub back to correct polarity, though that is subject to room influences on the measurement which may make the evaluation difficult.



I've basically been adjusting distance in my prepro to peak out the crossover region also. Inverting the subwoofer signal gave me the same results. 

One tweak that I'd like to see in REW is the ability to send out a DD 5.1 signal where the user can select which speakers to turn on/off so we can tweak distances in the other channels. For example, the user could turn off all but the left and right main, run the signal generator and adjust distances to peak spl. Then turn off the left channel and turn on the center and repeat the test. So on and so forth with the other speakers.


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## laser188139

Moonfly said:


> I am wandering how adjusting the time delay in your AVR, after Audyssey is run, affects what Audyssey is trying to do. I'm all for improving your system, and your results sound positive, but if you manually change settings after Audyssey stores its settings, your then basically altering the response Audyssey is trying to base its calculations on.
> 
> This is one of those things that would niggle away at me, even if I cant hear the difference, at least for a while. Ive always been an advocate of helping Audyssey as much as possible to get the very best results, so I really wander how this figures into the equation.


Unless you are using an Audyssey Pro system, that actually looks at how the satellites merge with the sub and calculates the results of each choice of crossover, this doesn't impact the Audyssey did it's equalization at all. Audyssey generates filters to equalize each speaker individually to 75dB up to the points where the speaker rolls off. Time/distance alignment of the speakers doesn't affect that, Audyssey still runs, although it might make the little square or green light turn off that indicates you are using the original Audyssey values. Also, especially for the sub, Chris recognizes that Audyssey's distance calculation is often off, hence the advice when it's too low to set it to the physical distance. With REW v5 we now have a measurement tool to make it right whether the Audyssey calculated distance was too low or too high. 

I actually took advantage of this in the band limited impulse plot. Because Audyssey equalized the sub and the mains, the impulse curve, generated just over the range where both can respond, should look similar. With Audyssey equalization, the sub's range is extended as high as possible, to give the best available resolution. I intend to add a plot showing how I picked 200Hz for the upper end, and why I should have used a higher lower end, e.g., 30Hz or 40Hz for the sub impulse to match the main's range. 

It is important to have Audyssey On when taking any of the measures for estimating distance. With the left channel as timing, one can see the delay from enabling the Audyssey processing. In my Denon, I'm seeing about 9 msec (10') difference between the impulse curves of Audyssey Off and On.


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## laser188139

Moonfly said:


> I am wandering how adjusting the time delay in your AVR, after Audyssey is run, affects what Audyssey is trying to do. ...


As promised, I updated the description to include a freq resp curve showing why I stopped the impulse sweep at 200Hz. 

Just for you, Dan, I also included a comparison showing the initial Audyssey chosen distance.


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## Moonfly

Cheers Laser :T

Ive just got in from work an hour ago, and I'm just settling down for the night, I'll sink my teeth into into it in a little while and have a think.


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## laser188139

I've expanded my description to reflect later experiments I made investigating the effect of the crossover in the AVR. 



JimP said:


> I've basically been adjusting distance in my prepro to peak out the crossover region also. Inverting the subwoofer signal gave me the same results.
> ...


Of course you are right, had I started with the physical distance to the sub and stepped its distance upward, I would have eventually found a distance setting that gave the best frequency response. 

I've seen comments elsewhere in the forum, perhaps yours, which described how someone adjusted the sub distance over a wide range looking for the best frequency response. I was always reluctant to do this, both because of the time involved and because frequency response is only one criteria, the absolute time relationship matters too. So I did not follow this route until v5 gave us tools to look at the timing. 

I did some more experiments to see if a narrow sweep range around the crossover could give a starting distance as good or better than I determined with my first technique. This proved not as satisfactory as using a range 50Hz-200Hz while moving the crossover filter out of the way. 



JohnM said:


> ... Another option would be to make measurements at the line level outputs to sub and mains to accurately characterise the delays in processors etc, free of any room influences, then add in the delays from the relative distances of the speakers/sub to the listening position. ...


I have made some more measurements of the line level outputs from the AVR, some of which I've included in the article. For me, the physical distance of the speakers to the listening position is itself problematic when considering an all-horn floor standing speaker, where the woofer faces backward to use the back of the cabinet to extend the horn. Add to that, there is some delay in the custom crossovers in the front speakers. 

Bill


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## laser188139

After looking at impulse responses over narrow ranges to see the effect of the receiver's crossover at different frequencies, it occurred to me that one should see the same thing in the group delay curve. So I've added that graph, confirming what crossover design would lead one to expect. 

Bill


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## glaufman

Somehow I just found the time to read this thread fo rthe first time... thanks for the great work Bill.
But I'm almost thoroughly confused... so please correct me if I'm wrong...
The sweeps you took to generate the two IRs you used to measure your delay were both done from 50Hz-200Hz, right? One was the Left speaker set with a 40Hz Xover in place, and the other was the sub with a 250Hz xover in place.

Right?


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## laser188139

Correct, Greg.

Because of how the range of the sweep affects the shape of the impulse curve, essentially the rise time at the start of the curve, looking at both speakers over the same frequency range lets one try an apples-and-apples comparison of the relative timing of the curves.


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## glaufman

Right. But later on, don't you prove that the exact crossover frequency changes that delay time and therefore phase relationship?


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## laser188139

Yes, it is annoying, isn't it. My inference was how much the crossover affects the shape of the impulse curve. This shape change makes it difficult to compare curves with the crossover at its normal value; narrow sweeps around a chosen crossover frequency did not seem to yield curves that could be used to verify the timing/distance. 

This serves as a justification for why I measured the impulses with the crossover well out of the way. By greatly lowering the crossover on the main to look at its impulse, and greatly raising the crossover to look at the sub's impulse, each measurement is being performed on frequencies in the pass-through range of its filter. 

This is similar to how Audyssey calculates the distance/timing. It does a sweep of the sub and main with the crossover out of the way, calculates its equalization parameters, then determines where to recommend the crossover be set based on where it found the rolloffs to be. 

I've not seen any feedback yet from anyone trying what I did, to see if my experience was accidental, and somehow dependent on the construction of the crossover filters in the receiver.


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## glaufman

True. And since I'm still running without a sub, I won't be any help there unless/until I can get into a friend's setup. 
But my concerns are less around the "shape" of the impulse, which I think you've done very well with by measuring with the xover out of the way, but what is at least an apparent delay caused by the xover, that varies with the xover's setpoint. So it seems you've measured and set the delay with the xover out of the way, but presumably are then running with additional delays once you set the xover appropriately. 
I call it an apparent delay because I can't tell if it's real or if it's due to - in the sub case for example - due to a further lack of high frequencies. I don't remember much about the characteristics of digital filters, but if we were in the analog realm, having a different delay at a given F that's varying with as you vary the Fc of the filter would be absolutely normal.

Hence, I suspect, John's comment above that if I interpret it correctly, uses your method to determine a start point, but then measuring the additional delays by moving the xover to where it's going to be, and adding/subtracting that from your delay as appropriate, to come up with the final delay needed. I'm basically just wondering what your thoughts are on that and if you've tried it?

I have no beef with moving the xover out of the way to see the response of the speaker to determine it's usable range.


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## laser188139

Actually, I read John's comment as an alternative, not as a follow-on tweak. I didn't try the analysis route; I figured measuring the effect in the room had the advantage of taking into account whatever delays are present in the sub's amp or the receiver's amps, after the preouts. It was interesting to compare the timing of the preouts with Audyssey Off and Audyssey On, to see how much delay Audyssey adds to all the speakers. 

Looking back again, I find I did compare the left, right and sub in a narrow range around the 100Hz crossover. (I fixed the last image in the post so this should make more sense.) Looking at a narrow range of sweep around the chosen crossover gave me essentially the same result. The corrected image does show why the Audyssey-determined distance wasn't terrible. 

It makes me wonder if a different procedure might be more generally valid: choose a plausible crossover frequency, measure the impulse with a sweep over a narrow range, ±5Hz around the crossover, and adjust the timing/distance looking at these impulse curves. This would probably be an equally valid procedure, provided the crossover is well above the rolloffs of the main. I don't know how well it would work if the crossover was at the point where the mains rolloff, as happens with THX speakers, but hopefully the range of the sweep would be narrow enough that the effect of the rolloff would be negligible. After the timing/distance value has been set correctly, changing the crossover frequency later should have little effect on the frequency response.

For my verification, I varied the distance around the point determined from the impulse curves, just to make sure that the overall frequency response with each front appeared better than where I started with the Audyssey calculated distance. 

I do appreciate your comments, of course. It's great for someone else to think through the problem, even if you don't have a sub to try it yourself. 

Bill


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## glaufman

laser188139 said:


> I figured measuring the effect in the room had the advantage of taking into account whatever delays are present in the sub's amp or the receiver's amps, after the preouts.
> 
> 
> 
> Yes, my only concern there is if the phase changes when you bring the crossover into range.
> 
> 
> 
> It was interesting to compare the timing of the preouts with Audyssey Off and Audyssey On, to see how much delay Audyssey adds to all the speakers.
> 
> 
> 
> Speaking of which, was it an equal delay across all speakers (with the xover out of range)?
> 
> 
> 
> Looking back again, I find I did compare the left, right and sub in a narrow range around the 100Hz crossover. (I fixed the last image in the post so this should make more sense.)
> 
> Click to expand...
> 
> Hmmm... I think it makes a little more sense (to me at least) now... so that was done with the 100Hz xover in place, and seems to show that that xover's effect on both speakers is very close to being the same. some might say close enough to be trivial. That just might be intuitive to those who remember this stuff better than I do... Very interesting indeed... I expected them to be off here, due to at least to the different frequencies each source is now being fed, but I guess that since you're measuring over such a limited range the difference isn't all that great (about or less than an inch)?
> 
> 
> 
> Looking at a narrow range of sweep around the chosen crossover gave me essentially the same result. The corrected image does show why the Audyssey-determined distance wasn't terrible.
> 
> Click to expand...
> 
> Hmmm...
> 
> 
> 
> It makes me wonder if a different procedure might be more generally valid: choose a plausible crossover frequency, measure the impulse with a sweep over a narrow range, ±5Hz around the crossover, and adjust the timing/distance looking at these impulse curves. This would probably be an equally valid procedure, provided the crossover is well above the rolloffs of the main.
> 
> Click to expand...
> 
> Because of the inherent inaccuracy in locating the impulse being on different sides of the xover, which I suspect could have a worse effect with different AVRs if there's a different characteristic to the xover, I kind of like the idea of using your method, with a check once the xover is in place to make sure it's still close.
> 
> 
> 
> 
> I do appreciate your comments, of course. It's great for someone else to think through the problem, even if you don't have a sub to try it yourself.
> 
> Click to expand...
> 
> And one day I may find myself back in my friend's or cousin's HT where I can try and verify. In between my tweaking, I like thinking about the best way to tweak, so when I get the rare opportunity, I know exactly what I want to look for.
> 
> Click to expand...
Click to expand...


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## laser188139

glaufman said:


> ... Speaking of which, was it an equal delay across all speakers (with the xover out of range)? ...


You would ask; I had to go back and look again. The difference between Audyssey Off and Audyssey On is consistent across speakers, all other things being equal. 

The Denon treats the case of Large fronts with Audyssey Off as a special case. The difference between Audyssey On and Off when the speakers are Large, 9.1 msec, is greater from the difference when the crossover is enabled, 5.1 msec. Apparently in this special case it bypasses bass management and the processing speeds up. 

Another peculiarity in doing measurements like this, is the Denon bases all of its timing on whatever speaker is the farthest away. As my sub distance increased beyond the fronts, changing the sub distance setting causes the absolute timing/delay to the fronts to change. It was the reverse when the sub distance was set less than the fronts. So I've learned to record my sub distance setting with all my front speaker measurements. 

Bill


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## jtalden

Good thread - lots of great info to help describe the impact of a change in freq range or delay to the IR and GD measurements. Thanks for all the work!

I believe I followed most, but maybe not all of the points made. I have previously aligned my system using other methods so I was interested to confirm my current SWs-Main timing. 

I did not clearly see a step by step process, but used my interpretation of the information to form one. I was NOT able to confirm my current setup or to converge on another setting using the GD measurements. There is just too much variability in the curve. I believe this is due to the same multipath issues that render the phase unreadable. I understand why the GD would work just as phase would work if the multipath “noise” could be avoided. It’s very possible that my particular setup is more problematic than other setups so this method may work fine for others.

I was able to confirm my alignment using a band limited IR measurement as was mention here. I find this pretty easy to do using REW and it does not seem to be impacted by the same noise issue as is the GD.

BACKGROUND INVESTIGATION:
1) System was set to a previously determined XO, EQ, level and delay alignment that was identified as good from other work. 
[My system setup includes 2 SWs. A DCX2496 provides delay for the closer SW to match timing at the LP. An Integra 9.8 pre/pro is set to XO at 100 Hz for the front mains. A DCX also provides EQ for the 2 SWs and Mains, a 20 Hz But-12-HP filter to protect the SWs from subsonic frequencies, and additional filters at the XO (But-48-LP at 121 Hz and But-12-HP at 100 Hz. The net result is significantly steeper filter slopes at the XO than a receiver or Pre/Pro provides by itself.] 

2) Mic was set at the main LP and this example is evaluating timing between the 2 SWs together and my FR main speaker.

3) Used “Use Loopback as Timing Reference” in REW 

4) Measured the Noise Floor, SWs and FR over the 15-2000 Hz range to see/confirm the actual acoustic XO characteristic.
In this case the handoff range I highlighted is from about 70-140 Hz to provide about 22 dB of isolation. 

Fig 1 SPL 15–2k








Fig 2 Impulse 15-2k Measurement Sweep 








Fig 3 GD 15-2k Hz Measurement Sweep (focusing on to 70-140 XO area)








The overlay does not look to bad at first glance until the scale is considered. At 100 Hz the offset bulges to over 50 ms or over 5 wave lengths at that freq. It varies across the range so much that it is impossible to make any conclusion as to how to shift the delay to align them within say 1 ms which is 36 deg at 100 Hz. I show that the offset is still about 10 ms at about 115 Hz where the lines appear to be close. This is still a huge error so the variation makes this method unusable at least in this particular case.

5) Measure the SW and FR again using 70-140 freq range as the start and stop measurement limits.

Fig 4 GD of the 70-140 Measurement range Vs the 15-2k Measurement range.








The results overlay closely in the 70-140 Hz range as we should expect them to and thus this is still not helpful.

Fig 5 IR of the 70-140 Measurement range 








The larger peaks seem to be aligned within 0.5 ms so that is encouraging. Also note that some of the other peaks are as large so there could be other alignments as well. This represents the entire XO range however and we know relative phase may be shifting over this range so it is not clear if the peak alignment is really representing the best phase alignment at 100 Hz or if the IR distortion due the freq range is enough to mislead us.

So let’s reduce the freq range further to see if the situation changes. 

6) Measure the SW and FR again using 100-105 freq range as the start and stop measurement limits.

Fig 6 IR - Narrow Range (100-105 Hz):








I picked 100-105 as this is near the middle of my acoustic XO in Fig 1. I could have just as easily picked 95-100 or 97-103 for that matter. It just should be something very near the center of the XO handoff range. 

This IR curve one gives me confidence that the 2 drivers are in phase in this narrow range as the peaks are well aligned. If you look carefully you may decide that there would be even larger peaks in alignment if the FR was inverted and the signal delayed another 5 ms (1/2 wavelength). Let’s see.

7) Invert the FR and increase its delay 5 ms. Measure the SW and FR again still using 100-105 freq range as the start and stop measurement limits.

Fig 7 IR - FR Inverted the and delay increased another 5 ms








This really aligns the largest peaks well. This is actually the alignment I currently use for this XO. I have evaluated both of these 2 settings with very good results. I have also evaluated another 1/2 wavelength alignment as well with good results. What are the tradeoffs? Well the phase rotation is different for each of these so, while they are aligned equally at the XO midpoint, they diverge at different rates so there is a different phase differential at the XO range extremes (70 and 140 Hz). The alignment of Fig 7 provides the most parallel alignment and there is there is little overall phase/timing error even at the 70 and 140 Hz XO extremes. I presume aligning the largest peak will generally be the preferable alignment. Also note the symmetry of the 2 curves between 90 and 120 ms. 

So the process is simple:
1) Measure the SWs and Main separately with all XO, EQ and Level setting in place.
2) Look to identify the XO range and chose your narrow measurement range in the center of that range. It will normally be okay to just use the XO freq. setting plus and minus 2-5 Hz.
3) Measure the SWs and Main separately again using start and stop freq selected.
4) Adjust the delays ( or speaker distance in an AVR or pre/pro) as needed and repeat the measurements until the largest IR peaks are aligned as shown. (Invert either the mains or the SWs if needed.)
[Of course you should keep all the mains in the same polarity.]
5) If the peaks are aligned within about 2 ms the alignment will be pretty good.

Comments:
> My system is sensitive to alignment so, if I move my delay 1 ms either way, I can see a dip start to form in the SPL response. This suggests that the traditional method of just looking the impact on SPL of the XO range as the delay is changed can work pretty well, but since there are alignment points (each 1/2 wavelength) it is not possible to tell if you are on the best one. The narrow IR method described here can give a sense that the best alignment is chosen if the 2 largest peaks are aligned. (There also should be some symmetry in the overlay 2 leading and 2 following peaks. My system always has some additional trailing “noise” in the IR of the Main that is always lower in magnitude. You can see that from 120 ms to 150 ms in Fig 6 and 7 (the blue curve). I don’t understand why this occurs, but I think it is safe to just ignore it.

> Of course this narrow IR method doesn’t show the phase rotation through the XO range and does not give any visual sense of what is happening to phase overall. That’s too bad, but it’s not critical to get your XO handoff aligned properly.

Bill, Are my findings supporting yours or have I missed the boat and confused the issue? 
I hope I have helped and not just corrupted your thread.


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## ccclapp

This is a very interesting and complex thread. Thanks all of your for your detailed walk-through!!

Im wondering why there was no comment on the workflow proposed in the last post above, from the OP or others who previously commented. Is that an indication that you disagree with his summary approach?

I am preparing to integrate my 2 subs with my two mains and am trying to learn of the recommended workflow from you who are far more experienced. 

Thanks!!

--Caleb


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## jtalden

ccclapp said:


> This is a very interesting and complex thread. Thanks all of your for your detailed walk-through!!
> 
> Im wondering why there was no comment on the workflow proposed in the last post above, from the OP or others who previously commented. Is that an indication that you disagree with his summary approach?
> 
> I am preparing to integrate my 2 subs with my two mains and am trying to learn of the recommended workflow from you who are far more experienced.
> 
> Thanks!!
> 
> --Caleb


Hi Calib,
Measuring using very narrow band sweeps at the acoustic XO and evaluating the resulting IRs to align phase as suggested by others in this thread made sense to me and that is why I investigated it and then reported my positive results. I believe the process steps outlined should result in good phase alignment at a reasonable level. Aligning the largest peaks as shown in Figure 7 should result in a "conventional alignment" that is the one with the closest phase alignment throughout the entire XO range. 

While I have experience with evaluating hundreds of XO filter settings and phase alignments in my system, I wanted to point out that this is the only time I actually investigated this particular method of doing it. That means, there is a fair risk I am overlooking something that may skew the results at least in some cases, e.g., possibly a room mode right at the XO freq? 

It would be advisable to also confirm your results using another method. Possibly you can you check the phase result directly as I do, but this can be difficult in some rooms particularly using REW. A simple check of the SPL magnitude of SW-Main combination measurement verses the individual SW and Main SPL magnitudes can also be a very effective indirect method of confirming reasonable phase alignment.

Higher frequency XOs as from MW to TW are easier to do using phase charts and will show the phase alignment over the entire XO range.


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