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Author Topic: Phase align subs to mains  (Read 23615 times)

Mac Kerr

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Re: Phase align subs to mains
« Reply #20 on: March 02, 2014, 11:26:33 am »

So Ivan - a somewhat ignorant question.  If both cars cross the finish line at the same time (amplifying each other) but one is a lap behind, how do you see that on measurement software?  On an RtA it would follow that things 'look right'. Yes?  Or am I missing something critical (I probably am).

On a dual FFT analyzer like Smaart, or SIM, or SysTune, you see a phase trace that is frequency vs phase. To align different subsystems you look at their phase trace and get them to match at the frequencies of interest.

In Ivan's race car analogy the cars represent a sine wave. In real audio it is more complex, but still understandable, measurable, and alignable (over a limited range of frequencies).

A simple RTA cannot show this information.

Mac
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Jerome Malsack

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Re: Phase align subs to mains
« Reply #21 on: March 02, 2014, 11:33:36 am »

My example to your question would be Latency.  Because my signal goes threw a system and becomes late but arrives in polarity with the signal that is on time does not present it self in photo.  Our ears will know that the sound is not on the correct time.   The Kick drum with a significant amount of smack from the beater in the 4 k area is heard and the sub 80 hz segment is not there.  The next beat you get beater and sub together.  Until the song ends and the beater is missing but you get the 80hz kick extra after??
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Ivan Beaver

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Re: Phase align subs to mains
« Reply #22 on: March 02, 2014, 12:19:53 pm »

On a dual FFT analyzer like Smaart, or SIM, or SysTune, you see a phase trace that is frequency vs phase. To align different subsystems you look at their phase trace and get them to match at the frequencies of interest.

In Ivan's race car analogy the cars represent a sine wave. In real audio it is more complex, but still understandable, measurable, and alignable (over a limited range of frequencies).

A simple RTA cannot show this information.

Mac
Exactly-you would see a 360 phase shift.

I would also add that it is kinda easy to get 2 traces to align at one freq.

But one should ALSO look at what is happening on a octave on either side of crossover.

This is something that the simple "RTA and sinewave-adjust till max cancellation-then flip polarity method) does not show.

You could end up with cancellations on either side of crossover if the time is not correct.

So you have to look at the phase-the amplitude (over a range of freq) to see if you have got it correct.

Save trace-adjust time-what happened? adjust again and so forth.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

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Jay Barracato

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Re: Phase align subs to mains
« Reply #23 on: March 02, 2014, 12:23:17 pm »

The critical point is that you are not looking at what the phase is, but how it is changing with frequency. The slope of the phase trace is the critical information.

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Jay Barracato

Thad Kraus

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Re: Phase align subs to mains
« Reply #24 on: March 02, 2014, 04:24:26 pm »

So i did a blog post a few days ago where i explained a little about how you might eq a system using smaart7.
Since i also measured the output of the processor you can see the effect eq filters have on the live ir.
The measurements on the low driver in that particular system and it's processor output show the ir being stretched more in time with every additional eq filter i put in (as expected) and at the last screen shot on the low driver it moves because i implemented a low pass filter as was also expected.

http://timobeckmangeluid.wordpress.com/2014/02/12/eqin-a-system/

Now would i use the ir for the final alignment : no i would have to phase align the whole thing in the end.
This was a low driver and as you can see the lp filter was set at 1200Hz. If you implement a low pass at say a 100Hz on a sub-woofer the same thing will happen (only the values will be a bit more then at 1200Hz:-).

Just read through this article and it was quite helpful. How does one handle a reference signal when not measuring after the DSP? For example if using crowns internal DSP on their amps I am unable to take a measurement after processing.
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Tim McCulloch

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Re: Phase align subs to mains
« Reply #25 on: March 02, 2014, 04:55:16 pm »

Just read through this article and it was quite helpful. How does one handle a reference signal when not measuring after the DSP? For example if using crowns internal DSP on their amps I am unable to take a measurement after processing.

Timo's article was for demonstration purposes, to show the absolute affect of filters on time by comparing the output of the DSP to the acoustic output of the loudspeaker.  For the purposes of aligning subs to tops, you don't need to measure the DSP as all we care about is how the acoustic crossover works out.

Jay B. says "The slope of the phase trace is the critical information."  I agree, and add to it - the other critical info is that the phase traces of the tops and subs are overlapping or at least parallel and not separated by more than 90 (less is better), and the overlap should extend at least 1/2 octave (a full octave is mucho better) on either side of the acoustic crossover.
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Ivan Beaver

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Re: Phase align subs to mains
« Reply #26 on: March 02, 2014, 05:10:30 pm »

Just read through this article and it was quite helpful. How does one handle a reference signal when not measuring after the DSP? For example if using crowns internal DSP on their amps I am unable to take a measurement after processing.
The reference signal is gotten from the output of the interface and looped back into one channel.

When you measure the loudspeaker, you are measuring everything inbetween the noise source and the loudspeaker-processing-amplifier-mixer-room-air etc.

So you adjust as needed/desired to get the response that you want.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
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Cailen Waddell

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Re: Phase align subs to mains
« Reply #27 on: March 02, 2014, 05:26:47 pm »


The reference signal is gotten from the output of the interface and looped back into one channel.

When you measure the loudspeaker, you are measuring everything inbetween the noise source and the loudspeaker-processing-amplifier-mixer-room-air etc.

So you adjust as needed/desired to get the response that you want.

So - I have a lot of questions now...

One - if you are comparing your reference signal to what is coming out of the speakers, there is a time (phase?) difference due to processing and the distance from the mic to the speaker(s).  Do you correct for this?  Does it matter?

Two - when looking in smart at the phase response, do you just slowly nudge your delay on your subs until it looks right?   Surely I am over simplifying...

Three - multiple microphone - because a singal mic represents a single seat, so how many places do you measure from?


Thanks all... This is still a bit of a dark art for me, though I understand the approach is actually quite scientific, logical, and the art perhaps comes in when knowing what compromises to make where....
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Mac Kerr

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Re: Phase align subs to mains
« Reply #28 on: March 02, 2014, 05:36:45 pm »

So - I have a lot of questions now...

One - if you are comparing your reference signal to what is coming out of the speakers, there is a time (phase?) difference due to processing and the distance from the mic to the speaker(s).  Do you correct for this?  Does it matter?
Yes it matters, and the offset is done in the analyzing software so that it is comparing 2 signals in time with each other.

Quote
Two - when looking in smart at the phase response, do you just slowly nudge your delay on your subs until it looks right?   Surely I am over simplifying...
Yes you do, but it may be that you will have to delay your mains to the subs.

Quote
Three - multiple microphone - because a singal mic represents a single seat, so how many places do you measure from?
Unless your mains and subs are very close to each other you will only be able to get it right for a limited number of seats, pick the good ones. If your mains and subs are very close together, the area of good phase match will be much larger, and it won't matter much where you measure, pick good seats.

Mac
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Tim McCulloch

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Re: Phase align subs to mains
« Reply #29 on: March 02, 2014, 05:47:24 pm »

So - I have a lot of questions now...

One - if you are comparing your reference signal to what is coming out of the speakers, there is a time (phase?) difference due to processing and the distance from the mic to the speaker(s).  Do you correct for this?  Does it matter?

Two - when looking in smart at the phase response, do you just slowly nudge your delay on your subs until it looks right?   Surely I am over simplifying...

Three - multiple microphone - because a singal mic represents a single seat, so how many places do you measure from?


Thanks all... This is still a bit of a dark art for me, though I understand the approach is actually quite scientific, logical, and the art perhaps comes in when knowing what compromises to make where....

ONE - Yes, there is a time difference.  You use the Delay Finder in Smaart to apply delay to the reference signal.

TWO - Uh... only if you know for sure that your subs are AHEAD of the mains, time-wise.  My experience is that it ain't necessarily so.  You may end up delaying the tops to the subs and then aligning the rest of the system to the tops.

THREE - It depends on what you're measuring and why.  If you're doing a sub/top alignment Mac's advice is spot on.  If you're doing more, you need to measure where various subsystems (mains v. ff, balcony/under-balcony fills etc) have overlapping coverage within 6dB of each other.

The art of measurement is in several areas:  interpretation of displayed results and developing an intuitive grasp of what you can align and what you can't.  Based on science but executed in a more artisan way.
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

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Re: Phase align subs to mains
« Reply #29 on: March 02, 2014, 05:47:24 pm »


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