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Old 20th December 2004, 06:06 PM   #1
tiroth is offline tiroth  United States
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Default Time aligning subs, mains, etc.

What is the best/easiest way to digitally time align multiple loudspeakers, for instance, subs with mains? Before you respond that time alignment is unnecessary, especially at low frequencies, let me say that I am specifically dealing with a case that has significant path length and/or phase differences, and that I have the ability to add arbitrary delay to any and all channels.

I can measure step responses, but I'm not sure aligning the step response is really meaningful--would it be better to configure the loudspeakers so they are out of phase at crossover, find the largest null, and then then reverse the phase? The only thing that concerns me is knowing whether I am at 0 degrees or 360 degrees! (I guess this is impossible at subwoofer frequencies, but it could be harder to determine in the midrange and up)

Going farther afield, is there benefit to doing this at higher-than-sub frequencies? My midbass/midrange crossover is not time aligned (perhaps this should be called impulse aligned?), yet the phase and FR sum nicely on and off axis. The mid/high transition is a little more ragged off axis, but given the rapid phase rotation at HF I'm not sure time alignment at a given point could help...

Thanks for any input.
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Old 21st December 2004, 04:16 PM   #2
tiroth is offline tiroth  United States
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No input on this? I thought that time alignment was all the rage.
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Old 21st December 2004, 04:43 PM   #3
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mains delay in seconds = 1/1150/distance in feet
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Old 21st December 2004, 05:15 PM   #4
tiroth is offline tiroth  United States
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That only accounts for path length differences though (estimated in the case of horns), not phase errors.
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Old 21st December 2004, 05:24 PM   #5
tiroth is offline tiroth  United States
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For example, here is the phase response of ESP's 36dB/octave rumble filter. Add this to the changing phase of the driver itself near resonance and I think calculations won't cut it...some kind of iterative or other measurement process is needed.

I know there are other solutions (allpass filters) but if the problem can be directly solved, is there a reason to much around with something like that?
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Old 21st December 2004, 05:35 PM   #6
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Quote:
Originally posted by tiroth
That only accounts for path length differences though (estimated in the case of horns), not phase errors.
That's all I was offering. I haven't bothered with alignment problems myself and given the excellence of my bass the reproduction of impacts, I'm sure I'm not going to.
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Old 21st December 2004, 06:22 PM   #7
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anything in search or google?
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Old 21st December 2004, 06:37 PM   #8
tiroth is offline tiroth  United States
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Well, the problem is that there does not appear to be widespread agreement on what time alignment means. To me, it ideally means two things: that the start of the impulses of two transducers are aligned. And two, that they are in phase at their crossover point.

To me this poses several questions. The first is whether the impulse start times mean anything if they are less than 1 cycle off and the drivers are in phase at crossover. Reviewers often use this as the only criteria for "time alignment."

Now, a delay (positive or negative) can affect both impulse start time and (effective) phase, although not independently. Unless I am mistaken, an all-pass can alter phase but has no effect on impulse start time.

I am not confident in these assertions though...I think there are people here that can respond with more authority. If you'd like you can replace "crossover" above with "frequency of interest", since adding filters to the mix will probably hoplessly complicate this discussion.
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Old 21st December 2004, 06:45 PM   #9
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The crux of the problem is that just about anything affects phase, including all the electronic components and even the wire in the system. That's why the only thing that is generally worried about is pathway differentials, because those can be easily fixed. Getting both that and perfection of phase alignment is next to impossible, especially in the bass. where room interactions also glitch everything up phase wise.
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Old 21st December 2004, 06:53 PM   #10
tiroth is offline tiroth  United States
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That bolsters my suspicion that the empirical technique I offered above might be the easiest solution--do you agree? I think your "fix the easy stuff" suggestion is valid, but even if worst-case phase summation errors only take 6dB off of your low end that is equivilent to losing half your subs!

The reason this piqued my interest was Harman's white paper on multiple subs. They standardized around 2-4 in part because the destructive effects of higher numbers of transducers...but they specifically noted that they were not considering the possibilities of aligning the phase of individual drivers.
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