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Author Topic: Trace Averaging Separation  (Read 838 times)

Russell Ault

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Trace Averaging Separation
« on: March 05, 2019, 09:49:53 pm »

I can't seem to remember the rule of thumb for how far apart microphones should be when doing TF trace averaging to maximize data quality while minmizing the number of measurements to be taken. I feel like it was something like 1/3 wavelength at 100 Hz (i.e. roughly a metre), but I'm not sure now.

Can anyone enlighten me?

Thanks!

-Russ
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Michael Lawrence

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Re: Trace Averaging Separation
« Reply #1 on: March 06, 2019, 06:43:22 am »

I can't seem to remember the rule of thumb for how far apart microphones should be when doing TF trace averaging to maximize data quality while minmizing the number of measurements to be taken. I feel like it was something like 1/3 wavelength at 100 Hz (i.e. roughly a metre), but I'm not sure now.

Can anyone enlighten me?

Thanks!

-Russ

As part of the Smaart training class, Jamie takes three traces within a couple of inches of each other and shows how that creates a sufficiently reliable average. I think Roger Gans's star pattern used a foot separation.

For what it's worth there is a bit of a divide in the system optimization community about the validity of trace averaging that reaches back to the Don Davis days and it is fairly heated, so you will get different answers depending on who you ask.
Personally I tend to use optical averaging for most applications but if I am going to do a trace average I'll move the mic by a few feet. Moving vertically a bit is also usually just as effective.

M
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Russell Ault

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Re: Trace Averaging Separation
« Reply #2 on: March 06, 2019, 01:24:06 pm »

As part of the Smaart training class, Jamie takes three traces within a couple of inches of each other and shows how that creates a sufficiently reliable average. I think Roger Gans's star pattern used a foot separation.

That's what I was trying to remember. That class was a couple years ago now, and the only thing I could find about the star pattern was that the size of the pattern needed to fit within a speaker's isolation zone (which makes sense, but doesn't tell me anything about minimums).

Personally I tend to use optical averaging for most applications but if I am going to do a trace average I'll move the mic by a few feet. Moving vertically a bit is also usually just as effective.

A few feet is what I've been doing and it seemed to have been doing the job. What I've been debating is whether putting three measurement mics on a single stand using a wide bar might be an effective approach (regardless of how the traces are averaged).

For what it's worth there is a bit of a divide in the system optimization community about the validity of trace averaging that reaches back to the Don Davis days and it is fairly heated, so you will get different answers depending on who you ask.

I always forget about this. I know Jamie Anderson is strongly in favour, and I seem to recall that 6o6 is strongly against, which I feel like is enough of a divide for anyone. (For what it's worth, if I'm going to use trace averaging I'll always compare it to the optical average to make sure I'm getting usable data. Please don't flame me!)  :)

-Russ
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Michael Lawrence

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Re: Trace Averaging Separation
« Reply #3 on: March 06, 2019, 05:29:10 pm »

What I've been debating is whether putting three measurement mics on a single stand using a wide bar might be an effective approach (regardless of how the traces are averaged).

Maybe two on a bar, and one a foot or so lower just to make it less ridiculous to move around and to add another plane of randomization to it.

I know Jamie Anderson is strongly in favour, and I seem to recall that 6o6 is strongly against, which I feel like is enough of a divide for anyone. (For what it's worth, if I'm going to use trace averaging I'll always compare it to the optical average to make sure I'm getting usable data. Please don't flame me!)

I recently discussed this with Jamie, and he said although he was trained to be distrustful of averaging (I'm paraphrasing), he realized through experiment that you had to do some truly contrived situations in order to create an example where the averaging wasn't representative.
I also understand Bob's reservations based on the mathematical difficulties with averaging, and so I see legitimacy in both sides. I have the highest respect for all involved parties and so my strategy is to take it all into consideration and make my own judgement call.
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Riley Casey

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Re: Trace Averaging Separation
« Reply #4 on: March 06, 2019, 08:56:18 pm »

Mild topic swerve. Anyone have an idea for what would constitute best practice for mic placement for getting nearfield curves to average for setting up DSP settings for stage monitors?  Anything not obvious to consider?

Russell Ault

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Re: Trace Averaging Separation
« Reply #5 on: March 06, 2019, 09:12:28 pm »

Maybe two on a bar, and one a foot or so lower just to make it less ridiculous to move around and to add another plane of randomization to it.

Talked me into it, although I'll probably go above instead of below (I spend most of my time in soft-seated venues). Now to find a wide enough bar.

I recently discussed this with Jamie, and he said although he was trained to be distrustful of averaging (I'm paraphrasing), he realized through experiment that you had to do some truly contrived situations in order to create an example where the averaging wasn't representative.
I also understand Bob's reservations based on the mathematical difficulties with averaging, and so I see legitimacy in both sides. I have the highest respect for all involved parties and so my strategy is to take it all into consideration and make my own judgement call.

I'm jealous you had the opportunity to speak to Jamie recently. I think he's been up in my part of the world exactly once (and I count myself lucky to have been there). Still haven't met Bob. I too have the highest respect for both (which is why I consider it to be a serious controversy whenever they disagree). I'm sure both would agree with making your own judgement call (insert your own quote about the real analyser being the one between your ears).

-Russ
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Michael Lawrence

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Re: Trace Averaging Separation
« Reply #6 on: March 06, 2019, 10:22:15 pm »

Mild topic swerve. Anyone have an idea for what would constitute best practice for mic placement for getting nearfield curves to average for setting up DSP settings for stage monitors?  Anything not obvious to consider?

Riley -
I have two comments for you. First of all, I submit for your consideration that the only mic placement that matters for stage monitors is artist's listening position, which is often not on axis. Hopefully it's close, but artists have a nasty and unreasonable tendency not to remain stationary on stage. So I'd start by tuning for the place where their head is actually likely to be, or maybe some representative positions through that area, averaged together or not.

The other one is more of a minority issue concerning phase alignment between drivers in a two-way monitor, which luckily is not something we deal with all that often these days but I find it interesting enough to mention: most people might think that onax to the HF driver is the best location to do the alignment. If you do that, the relative arrival times between the woofer and the tweeter don't hold over distance (unless it's a coaxial setup). If you use the point equidistant between the two, the phase alignment will hold as you move back because the relative time offsets don't change. Bob points it out in his 2nd edition. Probably not relevant but I certainly find it neat.

Oh, also - usually we'd not want boundary effects in a measurement of this type, but since the wedge will be used on the floor, I would say it makes sense to measure it as such.

I'm jealous you had the opportunity to speak to Jamie recently. I think he's been up in my part of the world exactly once (and I count myself lucky to have been there). Still haven't met Bob. I too have the highest respect for both (which is why I consider it to be a serious controversy whenever they disagree). I'm sure both would agree with making your own judgement call (insert your own quote about the real analyser being the one between your ears).

-Russ

Bob recently reviewed a draft of an upcoming article I'm working on and I didn't realize until after I sent it to him that the process I was explaining involved averaged data. He didn't mention anything to me about it but I'm sure he was silently judging :D
The system optimization community has many such 'controversies' because it's still in its infancy. There are things Bob and Jamie don't agree on, things Bob and Pat don't agree on (actually I just published an article that could be interpreted as a direct contradiction of some of Bob's work, although I don't view it as such), things Meyer and L-Acoustics don't agree on, even down to the fundamental mathematics used to describe the same reality - and though of course I have my own personal conclusions about who's 'closer to the truth' in each case, I think the larger lesson for me is that it's wonderful to be in the field during a time where I can learn directly from the pioneers themselves. All of this stuff is just a foundation for human judgement and intuition, so my approach is to learn as much as I can from as many people as possible, do my best to sort it out, and go from there.
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Frank Koenig

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Re: Trace Averaging Separation
« Reply #7 on: March 07, 2019, 01:21:38 pm »

This is an interesting and important topic and I'll throw in a couple of observations.

First, this is a sampling problem and how we think about the spatial sampling rate depends on what we're trying to achieve. If we're combining far-field measurements in a (real or pseudo) anechoic environment we should be thinking in terms of angle, not distance, as we would expect each response to be substantially the same (except for level) if measured at a different distance from the source but at the same angle. If, on the other hand, we're trying to wash out the effects of room modes at low frequencies, as I suspect we're talking about here, then distance is appropriate. I don't know much about the latter and am interested in the guidance given here.

In any case, it comes down to Nyquist, except that it's kind of tricky in that we have to look at the "bandwidth" of the variation in the response at each of a number of frequencies as we vary the distance or angle between the spatial samples. Given this complexity, I think a more practical approach is a statistical one where we try to find the spatial sampling rate that causes the combined measurement to converge sufficiently as the number of samples increases. In other words, if sampling closer together does not make a change we care about in the result then we're close enough. I'm assuming here that the statistic we're using (such as averaging) is a consistent estimator in that the result converges as the number of samples increases.

Second, while "optical averaging", in other words eyeballing a bunch of overlapping magnitude curves, may work better than expected it's not of much use for any sort of automated alignment. We need an algorithmic way of combining measurements. Intuitively we know we'd like to throw out certain outliers and perhaps put more weight on peaks than dips as generally they are more audible. There is also the question of how much smoothing to apply at each frequency and whether some form of non-linear smoothing could be used to achieve the peak/dip bias. Further, we may want to weight measurements differently -- screw the folks at 30 deg a little to get a flatter response onax, for example.

It's pretty hard to argue with magnitude averaging IF a large enough number of samples is available. (Phase averaging is more subtle but similar.) But measurement is expensive and we often would like to get a good enough result with just a handful of measurements. In any case, as above, when more fooling around doesn't result in a material difference in how we set the EQ or what our FIR filter response looks like, we're done.

Personally, I've been using 5 deg in most of my speaker measurements but admit that it's pretty much a wild-ass guess. I've also fooled with the trimmed mean as a way of automatically throwing out outliers. Often I don't find it necessary. Deriving minimum phase from averaged magnitude and averaging excess group delay gets around some phase averaging pitfalls. Too much to write about here but if anyone wants to visit and chat on these subjects I'd be very happy.

--Frank
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Michael Lawrence

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Re: Trace Averaging Separation
« Reply #8 on: March 07, 2019, 04:58:20 pm »

Too much to write about here but if anyone wants to visit and chat on these subjects I'd be very happy.

--Frank

Count me in! Sounds fun... email address is on my profile.

M
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Mark Wilkinson

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Re: Trace Averaging Separation
« Reply #9 on: March 07, 2019, 05:39:53 pm »

Interesting thread...with issues that often drive me crazy !

In my self-taught, measure-in-my-driveway/backyard type experience, I've come to a couple of simple preferences regarding averaging.

I try to average magnitude.
But only when there is high coherence, and response variations tend to be generally the same on and off axis.
So to keep it automated, I smooth measurements to taste before averaging, and the correct the average.
I do this driver by driver, assuming needed corrections are all minimum phase.
If corrections don't respond rather fully, I ditch them as non-minimum phase.
Hopefully I get smoother response optimized for a combination of on and off axis.. 

For phase ...no averaging...it seems to me phase is "inevitably to a spot"
So I use the previously corrected drivers' magnitude response, and phase align those responses to one point, at some distance that is close to equidistant to acoustic centers.

It's never measures as perfect as doing a mag and phase alignment to one spot,
but in today's world of easy to make damn near perfect response at a spot,
it seems like doing so may be its own new can of worms.  (at the expense of off axis)

I hope that made sense...please pass on any critique/observations/reservations on this methodology.


Thanks guys,  Mark
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Michael Lawrence

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Re: Trace Averaging Separation
« Reply #10 on: March 07, 2019, 08:35:59 pm »

Interesting thread...with issues that often drive me crazy !

In my self-taught, measure-in-my-driveway/backyard type experience, I've come to a couple of simple preferences regarding averaging.

I try to average magnitude.
But only when there is high coherence, and response variations tend to be generally the same on and off axis.
So to keep it automated, I smooth measurements to taste before averaging, and the correct the average.
I do this driver by driver, assuming needed corrections are all minimum phase.
If corrections don't respond rather fully, I ditch them as non-minimum phase.
Hopefully I get smoother response optimized for a combination of on and off axis.. 

For phase ...no averaging...it seems to me phase is "inevitably to a spot"
So I use the previously corrected drivers' magnitude response, and phase align those responses to one point, at some distance that is close to equidistant to acoustic centers.

It's never measures as perfect as doing a mag and phase alignment to one spot,
but in today's world of easy to make damn near perfect response at a spot,
it seems like doing so may be its own new can of worms.  (at the expense of off axis)

I hope that made sense...please pass on any critique/observations/reservations on this methodology.


Thanks guys,  Mark

Mark, if you're using Smaart, the smoothing only affects how the trace is displayed, and so won't affect the outcome of the averaging, which is performed on the raw data. As a proof, take a couple of traces, smooth them down to 1 octave, average them, and then reset the smoothing to NONE on the averaged trace. All the data comes back.

On other platforms, I can't say for sure. I only use Smaart.
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Frank Koenig

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Re: Trace Averaging Separation
« Reply #11 on: March 07, 2019, 10:35:13 pm »

Mark, if you're using Smaart, the smoothing only affects how the trace is displayed, and so won't affect the outcome of the averaging, which is performed on the raw data. As a proof, take a couple of traces, smooth them down to 1 octave, average them, and then reset the smoothing to NONE on the averaged trace. All the data comes back.

On other platforms, I can't say for sure. I only use Smaart.

If the smoothing is done by a linear filter, which typically is the case, and the averaging is the point-by-point mean, then the order in which they're applied doesn't matter, as they are both linear processes. Averaging the smoothed responses will give the same result as smoothing the averaged responses.

I'll reply to Mark's post in more detail tomorrow.

--Frank
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Michael Lawrence

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Re: Trace Averaging Separation
« Reply #12 on: March 07, 2019, 11:16:39 pm »

If the smoothing is done by a linear filter, which typically is the case, and the averaging is the point-by-point mean, then the order in which they're applied doesn't matter, as they are both linear processes. Averaging the smoothed responses will give the same result as smoothing the averaged responses.

I'll reply to Mark's post in more detail tomorrow.

--Frank

What I mean is that smoothing in Smaart specifically is just a graphical thing. The trace averaging operation always uses the original raw measurement data, not the smoothed trace data.
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Mark Wilkinson

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Re: Trace Averaging Separation
« Reply #13 on: March 08, 2019, 08:43:52 am »

Mark, if you're using Smaart, the smoothing only affects how the trace is displayed, and so won't affect the outcome of the averaging, which is performed on the raw data. As a proof, take a couple of traces, smooth them down to 1 octave, average them, and then reset the smoothing to NONE on the averaged trace. All the data comes back.

On other platforms, I can't say for sure. I only use Smaart.

Thank you Michael, Yes, I see! Not sure how I've been missing that....

Maybe because I use Smaart for measurements,  then do averaging in FirDesigner.  Still should have seen it though...

In looking at the Smaart user guide this morning, I guess maybe the only way to "fudge" smooth a trace, might be to Export to it  to Ascii and then Import it back in at a lower FFT rate.  But is that even smoothing?  I sure dunno...not going down that path...
Besides, FirD has a number of averaging options I don't even begin to understand, need to learn those first I think...lot's to learn.
As timely luck would have it, I've got class with Jamie next week in Tn.

edit: a little grammar
« Last Edit: March 08, 2019, 11:24:39 am by Mark Wilkinson »
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Mark Wilkinson

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Re: Trace Averaging Separation
« Reply #14 on: March 08, 2019, 08:46:18 am »

If the smoothing is done by a linear filter, which typically is the case, and the averaging is the point-by-point mean, then the order in which they're applied doesn't matter, as they are both linear processes. Averaging the smoothed responses will give the same result as smoothing the averaged responses.

I'll reply to Mark's post in more detail tomorrow.

--Frank

Frank, that's great news...sounds like I might be OK despite not knowing what i was doing LOL
thx dude!
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Michael Lawrence

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Re: Trace Averaging Separation
« Reply #15 on: March 08, 2019, 10:43:07 am »

Thank you Michael, Yes, I see! Not sure how I've been missing that....

Maybe because I use Smaart for measurements,  then do averaging in FirDesigner.  Still should have seen it though...

In looking at the Smaart user guide this morning, I guess maybe the only way to "fudge" smooth a trace, might be to Export to it  to Ascii and then Import it back in at a lower FFT rate.  But it that even smoothing?  I sure dunno...not going down that path...
Besides, FirD has a number of averaging options I don't even begin to understand first...lot's to learn.
As timely luck would have it, I've got class with Jamie next week in Tn.

I asked Johnny at Rational and here's what he said (emphasis added):
Quote
Correct, Smoothing in Smaart is just a visual function and does not change the underlying data used for other calculations (like spatial averaging). Captured traces are the raw data, and the smoothing is re-applied upon loading them (hence why you’re not “stuck” with whatever smoothing was used when the trace was captured). Smoothing IS used when you Export/Copy to ASCII, cuz people wanted that.

So your export / import trick would work if that's something you're looking to do.
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Frank Koenig

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Re: Trace Averaging Separation
« Reply #16 on: March 08, 2019, 01:11:52 pm »

Interesting thread...with issues that often drive me crazy !

In my self-taught, measure-in-my-driveway/backyard type experience, I've come to a couple of simple preferences regarding averaging.

I try to average magnitude.
But only when there is high coherence, and response variations tend to be generally the same on and off axis.
So to keep it automated, I smooth measurements to taste before averaging, and the correct the average.
I do this driver by driver, assuming needed corrections are all minimum phase.
If corrections don't respond rather fully, I ditch them as non-minimum phase.
Hopefully I get smoother response optimized for a combination of on and off axis.. 

For phase ...no averaging...it seems to me phase is "inevitably to a spot"
So I use the previously corrected drivers' magnitude response, and phase align those responses to one point, at some distance that is close to equidistant to acoustic centers.

It's never measures as perfect as doing a mag and phase alignment to one spot,
but in today's world of easy to make damn near perfect response at a spot,
it seems like doing so may be its own new can of worms.  (at the expense of off axis)

I hope that made sense...please pass on any critique/observations/reservations on this methodology.


Thanks guys,  Mark

Mark,

This all makes sense. I, too, am a driveway/backyard measurer, although I've recently taken gear out to the Stanford stadium's overflow parking to get more reflection-free space.*

As you point out, aligning phase over multiple measurements is tricky, but only for the excess phase. The minimum phase can be obtained from the log-magnitude and is not subject to variation in the measurement distance. To get the total (and excess) phase across multiple measurements I align the impulse response (IR) peaks. I know this is not theoretically pure but after much experimentation appears to give the most consistent results.
 
When measuring multiple pass-bands I always measure both without moving the mic so that I have accurate phase difference for crossover work. To average I align the HF IR peaks and apply the same time shift to each corresponding LF IR, preserving the phase difference. I set a fixed time window before the HF peak that is large enough to swallow the leading part of the LF IR as well.

To the extent possible, I work with group delay rather than phase to avoid the problems of phase unwrapping and phase origin. Further, the group delays from multiple measurements are straight-forward to average, just like log-magnitudes. The absolute delay of each measurement is irrelevant to the average just as the absolute level of each log-magnitude is irrelevant. The group delay can be obtained directly from the IR by taking the real part of the DFT of the IR multiplied by time and divided by the spectrum. (Nice trick, huh? Plagiarized from the ARTA users' manual.)

To view the phase you integrate the group delay and get to choose the constant of integration which determines the phase origin. I've taken to using the phase directly from the spectrum at some low, fixed frequency, where the phase is well behaved, as the phase origin. This is an "active research area", in other words I don't know what I'm doing and may decide it's all BS.

I use constant proportional-bandwidth Gaussian smoothing, although any reasonable smoothing kernel will give more-or-less the same results. If all you're doing is averaging then you only have to smooth once and it might as well be after averaging. But since I've been playing with non-linear statistical combinations, such as the trimmed mean, I pre-smooth the log-magnitudes to, say 1/24 octave, before combining them. If nothing else, it make it easier to see what 's going on. After combining I smooth to the desired filter resolution, usually 1/6 octave. This also removes any kinks resulting from the median filtering of the trimmed mean. Excess group delays are pretty smooth to begin with so smoothing is less critical, if necessary at all.

--Frank

*The great thing about research universities like Stanford is that people are totally used to odd stuff going on. The place I do my measurements is near a busy bicycle/pedestrian path through the eucalyptus woods. So here's this weird old guy with a van (this alone should raise suspicions ::) ) hoisting big speakers into the air and blasting the heavens with pink noise. And nobody gives it a second look.
« Last Edit: March 08, 2019, 01:16:19 pm by Frank Koenig »
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Frank Koenig

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Re: Trace Averaging Separation
« Reply #17 on: March 08, 2019, 04:19:01 pm »

Another thought on the original topic of spatial sampling: It might be that the required spatial sampling rate depends on the frequency resolution desired in the final result. The more smoothing we're going to apply in the frequency domain the more disparity we can tolerate between spatial samples. This is a problem for folks a lot better at statistics/stochastic signal processing than I am. But I'd like to crack that nut. Thoughts? --Frank
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Mark Wilkinson

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Re: Trace Averaging Separation
« Reply #18 on: March 08, 2019, 05:01:26 pm »

Mark,

This all makes sense. I, too, am a driveway/backyard measurer, although I've recently taken gear out to the Stanford stadium's overflow parking to get more reflection-free space.*

As you point out, aligning phase over multiple measurements is tricky, but only for the excess phase. The minimum phase can be obtained from the log-magnitude and is not subject to variation in the measurement distance. To get the total (and excess) phase across multiple measurements I align the impulse response (IR) peaks. I know this is not theoretically pure but after much experimentation appears to give the most consistent results.
 
When measuring multiple pass-bands I always measure both without moving the mic so that I have accurate phase difference for crossover work. To average I align the HF IR peaks and apply the same time shift to each corresponding LF IR, preserving the phase difference. I set a fixed time window before the HF peak that is large enough to swallow the leading part of the LF IR as well.

To the extent possible, I work with group delay rather than phase to avoid the problems of phase unwrapping and phase origin. Further, the group delays from multiple measurements are straight-forward to average, just like log-magnitudes. The absolute delay of each measurement is irrelevant to the average just as the absolute level of each log-magnitude is irrelevant. The group delay can be obtained directly from the IR by taking the real part of the DFT of the IR multiplied by time and divided by the spectrum. (Nice trick, huh? Plagiarized from the ARTA users' manual.)

To view the phase you integrate the group delay and get to choose the constant of integration which determines the phase origin. I've taken to using the phase directly from the spectrum at some low, fixed frequency, where the phase is well behaved, as the phase origin. This is an "active research area", in other words I don't know what I'm doing and may decide it's all BS.

I use constant proportional-bandwidth Gaussian smoothing, although any reasonable smoothing kernel will give more-or-less the same results. If all you're doing is averaging then you only have to smooth once and it might as well be after averaging. But since I've been playing with non-linear statistical combinations, such as the trimmed mean, I pre-smooth the log-magnitudes to, say 1/24 octave, before combining them. If nothing else, it make it easier to see what 's going on. After combining I smooth to the desired filter resolution, usually 1/6 octave. This also removes any kinks resulting from the median filtering of the trimmed mean. Excess group delays are pretty smooth to begin with so smoothing is less critical, if necessary at all.

--Frank

*The great thing about research universities like Stanford is that people are totally used to odd stuff going on. The place I do my measurements is near a busy bicycle/pedestrian path through the eucalyptus woods. So here's this weird old guy with a van (this alone should raise suspicions ::) ) hoisting big speakers into the air and blasting the heavens with pink noise. And nobody gives it a second look.

Too funny Frank......'and nobody gives a second look'  ;D

Your procedure makes sense to me also. I think I understand what you're saying about how you handle minimum and excess phase group delay.
I'll try to give a little depth to my procedure, which greatly mirrors yours.

I see you use ARTA. Does it have an auto-find IR peak, or do you manually set it? 
I ask because I just use Smaart's delay finder to set to peak....when there's no LPF in place.  Even for a sub driver, this has been giving consistent location, with excess phase properly removed.  Then (i think) it's only a matter of how many traces do i want to average.

After I have an average trace I want to work with, I use a FIR file to make all the minimum phase corrections, linear phase crossovers, and phase-only driver corrections if helpful. 
This makes phase alignment of drivers together nearly brain dead easy, because there is no phase slope to deal with as part of total group delay.  It also allows me to I tinker with moving xover frequency up or down, without changing timing. (but must build new FIR files)
And it allows me to nail sub timing almost 'to the sample', even with it's low pass filter in place.....
.....Because I know the post xover group delay has to be the sum of the raw group delay with no LPF in place, and the delay added by the number of samples to impulse centering in the FIR filter. Hope that made sense.
After doing that with lin phase crossovers, I can substitute complementary  IIR crossovers, to knock latency down for live sound, .......and know alignment is spot on despite the usual difficulty of locating a driver with insufficient HF energy.

I'm not as fluent when it comes to windowing or the statistical stuff you speak of...
Sorry i can't add anyhing here...

Good stuff, thx again

edit: fixed some wrong terminology, and backwards HPF and LPF

« Last Edit: March 09, 2019, 08:39:25 am by Mark Wilkinson »
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Russell Ault

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Re: Trace Averaging Separation
« Reply #19 on: March 09, 2019, 08:27:25 am »

Another thought on the original topic of spatial sampling: It might be that the required spatial sampling rate depends on the frequency resolution desired in the final result. The more smoothing we're going to apply in the frequency domain the more disparity we can tolerate between spatial samples. This is a problem for folks a lot better at statistics/stochastic signal processing than I am. But I'd like to crack that nut. Thoughts? --Frank

I'm fascinated by all of this; I'm pretty sure your use of spatial averaging and mine are for slightly different purposes.

I'm measuring how speakers operate in a room, and I use spatial averaging to improve the signal-to-noise ratio of what I would like to be a single measurement. I pick a position and measure it, and then take a couple more measurements in close proximity in the hope that those secondary measurements will exhibit different room-bounce characteristics, then take a coherence-weighted average of the lot of them. Comb filtering from room bounce exhibits very low coherence, so that data goes away in the average, and what I'm left with it a trace that looks a heck of a lot like the original measurement, just with much higher coherence and a lot less point-specific room bounce. In this application I still use very little smoothing, because the average is still trying to determine what the sound system is doing at a single point, just with much higher coherence than if I were to only take a single measurement.

-Russ
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Frank Koenig

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Re: Trace Averaging Separation
« Reply #20 on: March 09, 2019, 12:54:11 pm »

I'm fascinated by all of this; I'm pretty sure your use of spatial averaging and mine are for slightly different purposes.

Russ, yes I'm pretty much thinking in terms of coming up with permanent settings to flatten the anechoic response of the speaker, which I understand is not what you're doing. I have no experience getting meaningful measurements in large rooms. I consider meaningful measurements in small rooms impossible, except maybe for near-field (mic 5mm off the dust cap) at low frequencies, which might be helpful for sub alignment when anechoic data aren't available. I'll have more to say eventually but I've got to think about it and have to run now. --Frank
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Yes, it is a giant stereo system!
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