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Automatic Extraction of Minimum-Phase Response
Automatic Extraction of Minimum-Phase Response
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Old 25th July 2021, 10:38 PM   #21
krivium is online now krivium  France
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No this is the same artifact for me.
But intuitively i would consider them from different origins one being 'inherent' to the driver, the other a question related to cabinet.

I wonder about suspension diffraction since i listen the most to coax for 2 years now. I'm sure it is part of the 'ragged' high freq they have but in practice i can't identify it by ear or i'm not sensitive to it?
Maybe it is because i can't compare to a coax driver which doesn't have this issue (or very low like Genelec's or Cabasse's or Kef's) but even then directivity behavior would be different to the Tannoy and could hide the point.
I remember Mark100 described the high of his coax experience as 'diffuse' iirc and wonder if this is not related.



So i take it as part of the sound of the driver... a compromise for the others quality i like.
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Old 25th July 2021, 10:53 PM   #22
AllenB is offline AllenB  Australia
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Automatic Extraction of Minimum-Phase Response
I'd say it both is and isn't depending on the topic.

Have you tried measuring the surround diffraction?
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Old 25th July 2021, 11:02 PM   #23
krivium is online now krivium  France
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No.
It crossed my mind to investigate it but i didn't had time allowed to this between kids and my business to run in this weird time, and had not find a way ( a protocol) to identify the different source which exist on the loudspeaker i have ( Tannoy System800).
How would you proceed Allen?
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Old 25th July 2021, 11:24 PM   #24
AllenB is offline AllenB  Australia
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Automatic Extraction of Minimum-Phase Response
I haven't tried it either.. I guess I'd want to get back a few times the surround dimension, mount the driver flush in a baffle larger than the measuring distance and do a fine set of polars. The information would be in amongst breakup and lobing. If it were hard to spot I could experiment by adding a larger ring around the driver and indentifying that in the differences.
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Old 26th July 2021, 12:21 AM   #25
mark100 is offline mark100  United States
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Automatic Extraction of Minimum-Phase Response
Quote:
Originally Posted by 1audiohack View Post
http://www.excelsior-audio.com/Publi...eive_Delay.pdf

This is a great article by Charlie Hughes and I believe the answer to your question Mark is found in pages four and five.

I guess I have taken having TEF for granted all these years for its ease of use in this respect.

Barry.
Thanks Barry. Yes, Charlie does a great job of explaining where to reference phase. Been a fan of his clarity for some time...

I kinda knew the "best science" answer is the Heyser approach.

Other than maybe TEF (which i've only read about), i don't know how we can realistically make measurements for frequencies well outside the passband of the DUT.

Because it think alot of the time, any measurements that are well past the intended passband are pretty much garbage. And get make sense of the high end of the phase curve. (for those of us relying on Hilbert Transform...Smaart, REW, Arta, etc)

I'm hoping the "measure with a well-higher-low-pass than than the intended-passband", is a pragmatic and at least a more accurate step towards mathematically precise. (using existing tools/software)


Thanks again...much to learn
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Old 26th July 2021, 03:06 AM   #26
DcibeL is offline DcibeL  Canada
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Quote:
Originally Posted by hifijim View Post
Bohdan - it is a very cool paper, and a useful tool.. Thanks!

Your paper answered a small mystery I had with some of my measurements:

When I used a 2-channel audio interface + ARTA + Audix TM1 mic, I was able to measure (directly measure) a TOF difference between the mid driver and the tweeter of 84 u-sec, which is 28.8 mm difference in the acoustic centers.

When I used an OmniMic USB mic, I of course do not capture a valid "time of flight" measurement, so I use the Jeff Bagby method: Box
This method uses inference to iteratively march toward the offset values which most closely reproduce the responses of the tweeter and mid driver.

Sometimes these two methods agreed to within 1 mm. But recently the best agreement I could get was 7.5 mm.

Now I know why thanks to the Bohdan paper. The sampling rate is 48k. This means I get a sample every 20.8 usec, which is equivalent to 7.1 mm this is the resolution limit of my measurement system, both for the OmniMic and the 2-channel audio interface + ARTA + Audix TM1.

Nice to have that mystery cleared up

j.
If your method for determining offset using 2-channel system was to simply measure sample time difference between the peak of the impulse, that will create an error +/- 1 sample time which can be significant as you've found out. However, when measuring using a 2 channel system this process is completely unnecessary, just lock the FFT window start, the TOF differences will be captured in the measured phase. Even though 1 sample may be 7.1mm, you may find that a movement of the mic by 1mm is still captured in the measured phase, it is accurate even at 48kHz sample rate but I recommend 96kHz rate for measuring regardless, most any modern equipment can do it.

In ARTA, instead of simply observing the impulse peak, run your measurements, then convert to FR and observe the excess phase. Adjust the delay for phase estimation until the excess phase looks the same in both measurements, and then the delay value here becomes the difference in acoustic distance and should be more agreeable than simple observing the impulse peaks.

The real question when it comes to minimum phase and 2 channel measurement systems is why do I need it? HBT, IHBT, guiding filters, it all seems rather elaborate for a process that isn't needed at all for loudspeaker design using a 2 channel system. For any measurement of decent SNR, just use measured phase as-is and keep the FFT window start locked and a constant distance fro baffle surface to microphone. Minimum phase is not important or necessary, only relative phase between drivers.
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Old 26th July 2021, 03:28 AM   #27
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by bohdan1232000 View Post
I have produced two papers...
Bohdan, nice work.
A while back I tried to identify excess phase in LTspice.
The obvious way is to calculate the minimum phase from the amplitude response and then compare with the actual LTspice determined phase.
I believe you have Spice expertise, not sure if this is LTspice.
Any advice you can provide?
In any case, thanks for the papers, very educational.

Best wishes
David
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Old 26th July 2021, 11:57 AM   #28
bohdan1232000 is offline bohdan1232000  Australia
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Hi David,

Appreciate your comments. Thank you.

Sorry to disappoint you, but I really do not have that much Spice experience.

Best Regards,
Bohdan
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