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Author Topic: Smaart procedure for aligning subs and tops  (Read 93516 times)

Mark McFarlane

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Smaart procedure for aligning subs and tops
« on: July 30, 2012, 03:08:53 pm »

Below is a draft workflow for aligning subs to tops using Smaart that I have 'adapted' from this discussion: http://forums.prosoundweb.com/index.php/topic,126499.0.html.

I'd appreciate anyone willing to take the time to 'beat up' this workflow so we can refine it, add more comments, fix problems, etc so it can serve as a future reference.  It's likely there are a few errors in this draft version.

This is not intended to be the 'only' workflow for aligning subs and tops, just one method that works.  This workflow assumes the sub(s) are going to be deployed ground stacked with the tops.  The test is performed on 'one side' of the rig, i.e. test R or L, not both together. Ideally, any EQ used to flatten the sub and the tops low-bass response is performed before the alignment process since these EQ filters will affect the phase response. 

Accurate alignment will take an iterative approach to get the best system response, iterating the delay calculation, crossover setup, and corrective EQ since these three adjustments are interdependent.

Here are the basic steps:

  • Ideally perform the test outdoors, in an open space away from any structures, where there are no room modes or reflections to interfere with the measurements
  • Lay the top and sub cabinets down on the ground (if the sub isn't too tall you can stack the top on the sub)
  • Place the measurement microphone at least 10' away from the speaker stack to get a similar path distance from both top and sub.
       ⁃   If your sub stack is tall, move the mic further than 10 feet from the sub so your distances from the mic to the sub driver(s) and the top's low driver are within a foot or two.
  • Have the mic element touching the ground on a hard surface (concrete, plywood) at an ~ 45 degree angle (to avoid reflections from the ground). 
       ⁃   Having the mic touch the ground is more important for higher frequencies, less important for the sub alignment, but you might as well be as accurate as you can.
  • Engage your estimated electronic crossover for both the sub (LPF) and top (HPF) because the LPF on the sub will have a significant impact on the acoustic delay time.
  • Apply any corrective EQ (if known) to flatten the sub and top frequency response in the acoustic crossover band.
       ⁃   Don't try to determine this corrective EQ if you are in a small room because you will likely be measuring and correcting for room effects.  It's best to determine the corrective EQ requirements outdoors, away from structures.
  • Using a pink noise source (either Smaart, your console, or whatever is handy), level-balance the tops and subs (or balance them like you would for a normal gig, perhaps a bit sub heavy)
       ⁃   It doesn't need to be very loud, just enough above the ambient noise to get a good coherency trace.
       ⁃   Changing relative levels between subs and tops changes the acoustic crossover point, which is where we are performing this alignment, so get the relative levels as close to your show levels as possible.
  • Generate some pink noise through just the subs (or you could do the tops first)
  • Set Smaart's phase averaging to 4 seconds
  • Zoom in to the sub crossover frequency range, say 30-250Hz.
  • Allow SMAART to find the delay (auto delay), and tweak the delay until the phase trace at the acoustic crossover frequency is near 0, and the phase response is as linear as possible over the acoustic crossover band.
       ⁃   Ideally the phase response around the acoustic crossover will be a horizontal line (0 group delay), but that is unlikely Which is more important, to try to get the absolute phase at the crossover to 0, or to get the phase trace flat (0 slope) within the acoustic crossover band?
       ⁃   Successive Smaart auto-delay estimations should produce the same result. Try 3 auto calculations and make sure they settle within a few samples (0.02ms/sample)
       ⁃   If Smaart has a hard time finding a consistent delay time, you can initially set an unusually high LPF (crossover) on the sub (like 1KHz) for the delay detection.  But before continuing you should lower the sub's LPF to the value you expect to use for the crossover while simultaneously increasing the delay to flatten the phase response.  This can be done in steps, increasing delay as you lower the LPF to the desired frequency.
       ⁃   Smaart's calculated delay will be greater than what you would expect from the physical distance between the mic and sub alone (~1ms foot), because delay is also introduced by the physical cabinet design, the crossover filers, and any corrective EQ applied.  A reasonable delay time with the mic at 10 feet might be in the 15-40ms range.  If the mic is 10 feet away and Smaart calculates a delay of 100ms, something is wrong.
       ⁃   If Smaart returns an obviously wrong delay time it may be because the tops and subs are out of polarity.
  • Save a snapshot of the sub's phase and magnitude trace
  • Turn off the subs, and turn on the tops, with the electronic crossover (HPF) filter on the tops engaged.
  • Without moving the mic, and without adjusting the internal SMAART delay determined for the subs, manually add delay to the tops within your DSP (or console or delay unit if no DSP is available) until the slope of the top's phase trace matches the saved sub's phase trace over the acoustic crossover band (i.e. at a range of frequencies around the acoustic crossover frequency). 
       ⁃   Optionally you can use Smaart's auto delay feature to align the traces, and then enter the time difference between the auto-detemrined sub and top delay into your DSP.
       ⁃   Identify the acoustic crossover point: The middle of the frequency range where the energy contributed by the sub and top is equal. 
       ⁃   You want the top and sub to align at the acoustic crossover point, not the electrical crossover point. You want the frequencies on both sides of the acoustic crossover to be in phase. If your delay is set correctly, follow the acoustic crossover point up to the phase traces and you should see that the sub and top phase traces lay right on top of one another at that frequency (and hopefully around that frequency).
       ⁃   As you add delay to the tops, the top's phase trace should move toward the saved sub trace in the acoustic crossover region. If the top's trace moves away, start over with the mains as the saved trace and delay the subs.
       ⁃   The sub and top phase trace may not align exactly on top of each other, but they should have nearly identical slopes and be close (+/- 60, degrees, 120 degrees max) within the entire range of the acoustic crossover range.  The phase trace's slope direction will typically be down-to-the-higher frequencies in 99% of all cases.
       ⁃   A large phase discontinuity around the crossover point may indicate a polarity reversal. Polarity should be checked prior to calculating delay.
       ⁃   If inverting the sub or top polarity allows you to get your desired phase alignment with less delay, then do that. (I'm not conceptually comfortable with this claim.  In my mind there should be a 'correct polarity', not a 'pick your polarity and fudge the delay') but I haven't thought through the issues in detail
       ⁃   You can add a LPF to the tops at 500Hz or above if you want (this shouldn't affect the measured results in the sub crossover band, but it will get rid of the 'hiss' that may be annoying to your neighbors :))
       ⁃   Usually the delay is applied to the tops, but not always.  If the calculated top's delay is more than the subs, then apply the difference to your subs.
This gives a starting point for the required delay to align your tops to subs.  Changes to the electrical crossover point, application of a LPF to the subs, or corrective EQ applied to the subs or tops may require refinements to optimize the delay.



Questions: 

What is the simplest way to determine the relative polarity of tops and subs?  It seems you may be able to phase align at the wrong polarity, but then perhaps transients won't be aligned if you are out of polarity.

If I am going to apply corrective EQ to the sub to get a white (flat) response, which is better, subtractive or additive?  Does the best choice depend on the DSP implementation, or is there  a safe rule of thumb?


Sorry for the long post, but I spent two nights putting this together and figured it was worth sharing and refining.  Unfortunately I haven't actually tried applying the workflow yet, which undoubtedly will uncover some issues and questions.  That's for tomorrow night.
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Mark McFarlane
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Jamie Chappa

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Re: Smaart procedure for aligning subs and tops
« Reply #1 on: July 31, 2012, 05:34:41 am »

This is very helpful im just starting to toy with smaart.  Is it possible to put screen shots of some of the things your talking about. I was thinking the shots of the traces would be very helpful. thanks
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Timo Beckman

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Re: Smaart procedure for aligning subs and tops
« Reply #2 on: July 31, 2012, 10:53:58 am »

@ my blog there are some simulations about this .
There are a few power points . Check out the stuff posted in januari
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Mark McFarlane

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Re: Smaart procedure for aligning subs and tops
« Reply #3 on: July 31, 2012, 12:52:16 pm »

This is very helpful im just starting to toy with smaart.  Is it possible to put screen shots of some of the things your talking about. I was thinking the shots of the traces would be very helpful. thanks

Jamie, I dont want to pollute this discussion with sample screens because people will start commenting on the screens and not the workflow.

If someone has a few great 'reference quality' screenshots feel free to post them.  All I have are a few tests that I ran indoors tonight and there is enough garbage in the plots, probably some due to room reflections, that they aren't worthy of publishing in this thread.  Hopefully over the weekend I'll move outdoors and if I'm extremely lucky I'll end up with some good results, but I'm not holding my breath since I haven't used Smaart before and it appears it takes quite a bit of experience to get good results.
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Mark McFarlane
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Paul Tucci

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Re: Smaart procedure for aligning subs and tops
« Reply #4 on: August 01, 2012, 10:38:43 pm »

Mark,

That's an ambitious undertaking for sure. Big can of worms. I'm just back to work so time is short but I'll throw some points to your effort. I think I have some good screenshots that illustrate key points in the quest to understand all this.
I would avoid Auto-Delay and force the issue to understand what its measuring. Make an Impulse measurement and learn how to make a better one than the Auto-Delay does. By better I mean tweaking the parameters to your advantage. Take more information in...make a better decision. People have a hard time measuring subs early in the learning curve. Auto-Delay doesn't cut it when you need to maximize the S/N ratio of your measurement. I think the screenshots will help get this initial idea across.
I'm working in the same arena for 3 days so maybe I can make some in situ measurements.
PT
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Mark McFarlane

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Re: Smaart procedure for aligning subs and tops
« Reply #5 on: August 10, 2012, 01:44:32 am »

...
I would avoid Auto-Delay and force the issue to understand what its measuring. Make an Impulse measurement and learn how to make a better one than the Auto-Delay does. By better I mean tweaking the parameters to your advantage. Take more information in...make a better decision. People have a hard time measuring subs early in the learning curve. Auto-Delay doesn't cut it when you need to maximize the S/N ratio of your measurement. I think the screenshots will help get this initial idea across.
...
PT

Paul, when you get a chance can you add a few more sentences to describe how Smaart's Impulse Response measurement would be used to calculate the delay?
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Mark McFarlane
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Adam Black

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Re: Smaart procedure for aligning subs and tops
« Reply #6 on: August 10, 2012, 08:37:08 am »

Paul, when you get a chance can you add a few more sentences to describe how Smaart's Impulse Response measurement would be used to calculate the delay?

Mark,

Auto Delay Finding is nothing more than taking an impulse response and reporting the arrival time of the peak. As the peak is usually the initial arrival, this is often sufficient. However, it is possible to have a late arriving reflection with greater amplitude; which would give an erroneous delay time if using the delay finder. Generally speaking, auto delay finding is fine in a pinch. But as it removes the experience and analytical skills of the user out of the process, it's not infallible. The inverse is true of manually taking an impulse response. There are more options available to the user for taking the impulse response, which may yield a better measurement. But most importantly it requires the user to analyze the data and make an informed decision. And that is the true power of analysis software. It measures what is and presents this data in a meaningful way to the user, who then makes an informed decision.

-A
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Peter Vetters

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Re: Smaart procedure for aligning subs and tops
« Reply #7 on: January 07, 2015, 03:20:32 pm »

  • Zoom in to the sub crossover frequency range, say 30-250Hz.

Hello Mark,

Thank you very much for your explaining procedure about using Smaart to align subs to tops!
I'm new to Smaart and this topoc already helped me a lot.
I have a question at the moment, I'm using Smaart 7.2.1.2, how can I zoom in to a specific frequency range like you describe above?

Thanks!

Peter
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Ivan Beaver

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Re: Smaart procedure for aligning subs and tops
« Reply #8 on: January 07, 2015, 09:20:28 pm »

Hello Mark,

Thank you very much for your explaining procedure about using Smaart to align subs to tops!
I'm new to Smaart and this topoc already helped me a lot.
I have a question at the moment, I'm using Smaart 7.2.1.2, how can I zoom in to a specific frequency range like you describe above?

Thanks!

Peter
Right click and "draw" a box (mouse) over the area you want to zoom into.
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Keith Broughton

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Re: Smaart procedure for aligning subs and tops
« Reply #9 on: January 08, 2015, 07:15:15 am »

Thanks Mark!
I have been using SMARRT for a while now but the sub alignment has always been a bit tricky for me.
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Merlijn van Veen

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Re: Smaart procedure for aligning subs and tops
« Reply #10 on: January 08, 2015, 04:21:37 pm »

Hi Everybody,


First I'd like to compliment Mark for his good idea and I'm sure that with combined effort a solid protocol can be devised.

Simultaneously I feel that such an elusive subject could benefit from some clear graphics so I gave it a try. The images attached where created with the greatest effort within the confinement of my Macbook OS using plug-ins. They're free from noise and exhibit realistic phase behavior.

I'd like to start by saying that I'm not a great fan of using the auto delay locator for finding the subwoofer IR. Under controlled circumstances it might work but in practice there's often to much background noise, poor SNR and insufficient data for the analyzer to work with. When using band limited products like subwoofers, less than 1% of the data sent is returned. Only after maximum zoom the IR becomes visible and is clearly stretched over time by acoustical-mechanical design and the phase shift introduced by electronic filters. The main speaker on the other hand has less than 1% of its bandwidth cut and 99% or more of the data sent, is returned for the analyzer to work with resulting in a solid IR.

Check the MS webinar on IR for a better understanding.

http://meyersound.adobeconnect.com/p9o4ms85n0y/

The IR can be used very much to get you into the ballpark but so does a laser distance finder or tape measure. I use the IR but set it manually to where the waveform starts to deviate (take shape) from the zero line. And yes in order to be able to see it I'll have to zoom in maximally.

Like mentioned before, what matters is that the phase slopes of both speakers match and I strongly recommend with as little phase shift as possible. Definitively less than 120 cause this will gain you nothing and neither will you lose anything. IMHO 90 is also not an option cause you'll only gain +3 dB and waste half of your amp's electrical power. 60 is better but try and go for 30 or less and achieve maximum efficiency.

The speaker with the steepest slope at the XO frequency is late to the party. Apply delay to the speaker with the gentle slope. Once you have matched slopes with equal tangents but not necessarily overlapping traces, a 180 phase shift might be visible for all frequencies of interest throughout the XO region which can be solved by reversing the polarity of one of both speakers.

If you don't like the diagonal phase slopes to work with, you can manipulate the phase response plot by playing incrementally with the delay locator setting to "flatten" the portion you're interested in as illustrated in figure 6. Just make sure once you settle on a certain delay locator time to leave it alone. This will be your frame of reference to find the relative amount of time difference.


Regards,


Merlijn

Peter Vetters

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Re: Smaart procedure for aligning subs and tops
« Reply #11 on: January 08, 2015, 04:30:43 pm »

Right click and "draw" a box (mouse) over the area you want to zoom into.

Thanks Ivan, this helps me further! For me, it is not exactly as you describe, I'm a Mac user you know, and that's probably the reason I didn't find it myself. For Mac you must push "Ctrl" during drawing "the zoom box".  ;)
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PiotrSkotnicki

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Re: Smaart procedure for aligning subs and tops
« Reply #12 on: February 03, 2015, 09:34:51 am »

Hi to all,

Here is Bob McCarthy's take on the question: https://bobmccarthy.wordpress.com/2010/02/08/phase-alignment-of-subs-why-i-dont-use-the-impulse-response/

Regards, Piotr.
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ProSoundWeb Community

Re: Smaart procedure for aligning subs and tops
« Reply #12 on: February 03, 2015, 09:34:51 am »


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