# Impulse aligned vs phase aligned?



## AngelEyes (Nov 6, 2006)

Hello,

I have been using REW to perfectly align all my speakers and subs using the impulse response graphs. I used the loop back and ir estimate to get me close and then tweaked the delays to get the graphs to line up.

Hopefully I am correct in getting the first major peak of each channel to line up? For some reasons my MK CS22 surrounds have a little wiggle before their major peak, so I am not sure if I should ignore that and go with the large peak immediately after?

Anyhow it does all sound a lot better but I am conscious the phase of each speaker are not particularly well aligned. My 2 subs are ok and the l and r surrounds but the rest are all a bit amiss and also the phase seems to vary with time so even if I could line them up perfectly at one FR they would be out somewhere else.

I would appreciate advice on the pros and cons of favouring impulse response alignment vs phase alignment, what sound qualities each gives over the other? 

FYI my system is in an untreated lounge room with an ADA Suite 7.1 HD Processor (No auto EQ, but PEQ) on a 6.1 setup with AE AV15-X twin DIY subs.

I can provide a link to my latest REW measurements if that would help .

Thanks.

Adam


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## jtalden (Mar 12, 2009)

For time alignment between channels (FR, FL, CC, SL, Etc.) the IR is a very accurate method. 

For alignment between drivers within a channels it is ideal to look at the phase to fully understand the alignment. The SW is a driver within each channel so it can be done this way also. Be advised though that fine tuning the using phase is much more time consuming and is difficult to interpret correctly. 

The much easier way is to use the RTA method. It is normally recommended and will accurately find a good setting for drivers within a channel. Just align the initial IR rise (or the initial IR peaks) and then switch to RTA mode and make minor distance/delay adjustments until the SPL is maximized in the XO area. If the adjustment was more than 1.5 m then you may want to invert the SW polarity and start over. You can evaluate both settings and see which you prefer.

To comment you your particular settings I need to see measurements with REW loopback activated of the SW and each main channel (all separately).

The easier RTA methods work fine and is recommended for those that want to setup the system accurately and enjoy using it. The phase method just assures which possible alignment point was reached. It is more a learning/understanding exercise for a hobbyist like myself who like to fully investigate the impact of various options/tradeoffs. I would not suggest that the resulting sound would be any better using one method or the other. Even an error of 0.5 m is not a major issue at <120 Hz where most of us XO the SW.


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## AngelEyes (Nov 6, 2006)

Thanks very much for the reply.

You can download the REW file here. It has all the channels separate, before EQ, including separate and combined subwoofer sweeps. My understanding was to select a zone where the phase of the subwoofers and the speakers are in phase for the crossover point, which I believe is 70hz and that is what I am now using.

http://www.creativefreedom.co.uk/work/6.1-Impulse-Response-test.zip

There are also a few full range sweeps for good measure as well as individual sweeps after PEQ which I applied to all channels up to about 400Hz.

Sorry that makes it a fairly big file (40mb) but I am very interested in what people might think to how I have things set, I am sure I have a lot to learn 

Thanks,

Adam


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## jtalden (Mar 12, 2009)

AngelEyes said:


> I have been using REW to perfectly align all my speakers and subs using the impulse response graphs. I used the loop back and ir estimate to get me close and then tweaked the delays to get the graphs to line up.


This worked well for the 3 front speakers. All 3 are timed well to provide an in-phase condition at the XO freq. 



> Hopefully I am correct in getting the first major peak of each channel to line up? For some reasons my MK CS22 surrounds have a little wiggle before their major peak, so I am not sure if I should ignore that and go with the large peak immediately after?


The design and placement of the surround speakers has resulted in this messy IR. I also first aligned the initial large IR peaks at the same timing of the L, R and found that the phase was close to 180° out at the XO so you may want align this way and also wire them with opposite polarity to get the best SPL support in the XO range. The IRs also showed the TWs were in phase with this reversed polarity. Because of their role primarily as ambience I don't think it is a big deal either way. 



> Anyhow it does all sound a lot better but I am conscious the phase of each speaker are not particularly well aligned. My 2 subs are ok and the l and r surrounds but the rest are all a bit amiss and also the phase seems to vary with time so even if I could line them up perfectly at one FR they would be out somewhere else.


Yes, the 2 SWs are ideally time aligned! The L, R, C, are good also. You are getting SPL support through the XO range. 

The measurements should not vary with time, but small positional differences in the mic position at LP will make the IRs shift noticeably. This is not a concern however as your head position is not fixed either. If the mic is not moved the IR peak should shift less than 4 mm due to REW/measuring system repeatability (assuming my setup is typical).



> I would appreciate advice on the pros and cons of favouring impulse response alignment vs phase alignment, what sound qualities each gives over the other?


Your method led to an alignment that reduces the total GD and still provides Phase alignment near the XO. It is not the alignment that is normally recommended to maximize the phase matching trough the entire XO range. I call that one the "conventional" alignment. I am currently using an alignment similar to the one you found, but have evaluated many different XO and timing setups. The conventional alignment sounds a little to heavy on male voices on some material, but sounds better on a lot of music. These results may differ greatly with different rooms speaker and LP positions and speakers themselves. So this is just my impression with my setup. It would be easy for you to try both alignments.



> ...
> My understanding was to select a zone where the phase of the subwoofers and the speakers are in phase for the crossover point, which I believe is 70hz and that is what I am now using.


Yes, that has worked for you in this case. I would say to select the XO point desired and then adjust the delays until phase is matched there.

*Other thoughts:*
> The C speaker is designed differently than the L, R pair. The TW polarity is reversed relative to the L, R pair. So there is a choice to be made. Align the XO point as shown here and then have the C TW out of phase with the L, R pair or reverse the speaker polarity and allow the upper frequencies to be phase aligned at the expense of the SW XO. The speaker designer chose the former option (assuming that the C was intended to match to the L, R pair).

Below is the "current target" alignment but with surround and back speakers inverted and delays slightly adjusted.








Below is same target zoomed to show how I aligned all the IR peaks. Minor deviations from this idealized alignment is not an issue. It basically agrees with your front 3 and inverts the surrounds and improves their timing a little. Note that the Surround IRs now peak the same as the L, R pair and the C IR peak appears inverted to all the others because the TW is wired in the opposite polarity from the others. 








To setup to this target we need to know the relative offset of these 6 main speakers relative to the SW. This can be done by measuring the offset of a point on the SW IR to a point on the Mains IR. I chose the zero crossing point on the SW IR and the peak on the mains IRs. Below shows that offset to be about 1.456 ms with the mains trailing the SW crossing point by that much.








Below, zoomed closer, we get 1.455 ms








To get the more "conventional" phase timing we need to delay the SW by 1/2 wavelength (at 70Hz) or 7.143 ms. We also need to invert the polarity of the SW so it will still be in phase with the mains. Now the phase handoff will better track throughout the XO range. Below is the "conventional" target alignment. The offset is now measured at 5.737 ms with the mains leading the SW crossing by that much.








The EQ setting will not be influenced very much as the phase is still similar in the XO range. There will be some small differences in that range.

If you decide to evaluate both setups let us know what you think. Is there a difference? Which do you prefer? 

P.S.
I should have mentioned that to setup to these targets just measure the SWs and then adjust all the main to have the offsets shown in the IR charts. If the SW crossing point is found at 52.000 ms for example then the target for all the main IR peaks is +1.455 or 53.455 ms for your current alignment.

My values are at 0 ms only because I offset all the measurements by about 52.xxx ms so I could investigated the phase.


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## AngelEyes (Nov 6, 2006)

What a terrific reply, now if I had not just consumed the better part of a bottle of wine I might be able to make head or tail of it.  

I will take a considered look tomorrow, thank you so much for putting such time into your reply. I will certainly be doing some further experimentation, my initial concern is the centre speaker, as it is from the same family as the L&R (XTZ 99.26 and 2x 99.36s) whose crossovers I updated myself, so if there is something not correctly wired up I will be a bit surprised as well as embarrassed!

Thank you.

Adam


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## jtalden (Mar 12, 2009)

XO design will impact total phase rotation so if the XO is different (different order filters or 2-way vs 3-way, etc.) that may be the reason as opposed to wiring. Even room location seems to have some influence (although I don't know why in that case).


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