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Old 2nd February 2009, 03:32 PM   #541
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Quote:
Originally posted by john k...
Since a few, many or all of you may doubt the results I posted for impulse response based on a simulated in room woofer response I though I would make a measurement of a woofer in my room and use that response as the basis.
...
...
Question: how did you do the min phase eq? It looks VERY precise. Did you use an iterative algorithm (Levinson-Durbin or similar?)

I see your measurement, and presumable EQ goes down to 5 HZ. What is your noise floor?

Your plot also goes up to 1 kHZ. Are eq'ing all of that. If so, there would be no argument that that is motly non minimum phase up there. Also the same question about noise at 1 kHz.
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Old 2nd February 2009, 03:49 PM   #542
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Quote:
Originally posted by cap'n todd

.... (at least in the minimum phase sense, which I contend is more true than not).
Have you looked at the data I posted above? I am sure I can find a position where the response may be more MP than not, just by moving the mic close enough to the source. But I think the converse is true. The response in a room is more likely to be non-MP than MP. At least that is what both my measurement and simulations show.

I don't know if this is perhaps wht you did, but if more excess phase is removed it is possible to get something that starts to look like MP at low frequency. However, when I do this I end up removing more delay that the time of flight from source to mic. So that can not represent a physical system. It might look like MP at low frequency but it is non-causal, therefore non-MP. It implies that the sound arrived at the mic before the source produced it.
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Old 2nd February 2009, 04:20 PM   #543
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Quote:
Originally posted by cap'n todd


Question: how did you do the min phase eq? It looks VERY precise. Did you use an iterative algorithm (Levinson-Durbin or similar?)

I see your measurement, and presumable EQ goes down to 5 HZ. What is your noise floor?

Your plot also goes up to 1 kHZ. Are eq'ing all of that. If so, there would be no argument that that is motly non minimum phase up there. Also the same question about noise at 1 kHz.
Noise is about 3 dividions up frm the bottom of the plot over most of thr range. Down at 5 Hz I'm having a good day and I'm up about another 5 dB (one division).

The MP cals are done using SoundEasy. I am not sure of the exact implimentation other than it follows the integral approach you can find in Bodes original text. I have written my on MP routines and find SE to be highly refined in this regard.

I'm open to suggestions, but I don't see that anything would change the phase response in the pass band where it shows several wraps, and seems consistent with my room simulations.


[edit] actually here is a plot showing the noise level. Better than I thought.

Click the image to open in full size.
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Old 2nd February 2009, 05:21 PM   #544
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Quote:
Originally posted by john k...


Noise is about 3 dividions up frm the bottom of the plot over most of thr range. Down at 5 Hz I'm having a good day and I'm up about another 5 dB (one division).

The MP cals are done using SoundEasy. I am not sure of the exact implimentation other than it follows the integral approach you can find in Bodes original text. I have written my on MP routines and find SE to be highly refined in this regard.

I'm open to suggestions, but I don't see that anything would change the phase response in the pass band where it shows several wraps, and seems consistent with my room simulations.


[edit] actually here is a plot showing the noise level. Better than I thought.

Click the image to open in full size.
OK, your s/n looks good. Just thought I'd check since bad s/n can do wierd things to phase.

The method you use for inversion may have an impact here as well (pole/zero approximation in system modeling). Further there are a number of reasons why dereverberation in a real room does not alway work as expected. John Mourjopoulos has done some research on this ("Errors in Real-Time Room Acoustics Dereverberation"J. Audio Eng. Soc., Vol. 52, No. 9, 2004 September), and concluded:


Specifically it was found that under practical real-time
conditions such “high-Q” ringing poles are extremely sensitive
to small variations in the system (room) properties,
which can occur between response measurements and dereverberation tests even if the source/receiver positions and
equipment settings remain identical, so that any such test
will suffer from significant mismatch error. Hence from
this point of view the room may be considered a “weakly
nonstationary system.”


I'm sure we could argue this one for years to come, but my main point is...


Try this on for size: Look at your impulse response, and tell me at what frequency it is strongly ringing after MP EQ?? Looks like around 40 Hz. now look at your mag and phase plots. Where is the biggest cancellation dip (and phase wrap)? 40 Hz. My theory is that when you do this type of eq, namely trying to boost dips as well as redcing peaks, your end up boosting the response at frequencies where it IS non-min phase. Yes, you are also reducing the level at frequencies where (I contend) it tends to be minimum phase, i.e. the peaks. So, you could argue that the initial response was more non MP than MP, since when you flattened it, boosting the dips exacerbated the ringing more than flattening the peaks reduced it. Maybe - more analaysis of different locations and rooms would be needed perhaps to answer that. BUT, my main point is that if you EQ in a sensible way, i.e. just bring the largest peaks down to a reasonable level, you should reduce the overall ringing.

This is why I content that the responses are mostly non MP. Technically they are not MP, but effectively are with respect to (sensible) MP equalization.

I should like to see the result of a more "gentle" type of EQ in either your simulation or measurements.

Respectfully,
Todd Welti
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Old 2nd February 2009, 06:01 PM   #545
gedlee is offline gedlee  United States
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Personally, I don't see any of this really being significant to the issue of using multiple subs and how to set those up. It never comes up any time I do multiple subs and my results always come out fine. In fact most people suggest the bass in my room is the best that they have heard - MP or not. I should mention that I DON'T use any EQ at all.

John - a boundary condition is only inhomogeneous if the pressure or the presure gradient is zero or the ratio of the two is specified. If the velocity is specified to be non-zero (independent of the pressure) on any portion of the surface then its not the an inhomogenious boundary condition. Since g(r) does not have zero velocity on the boundary no solution of the inhomogeneous boundary can cancel it there.
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Old 2nd February 2009, 06:06 PM   #546
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Quote:
Originally posted by cap'n todd
High resolution in time or frequency? No, I just meant visually, because of hte projection angle. Like if you look at someones nose from 45 degrees its harder to see how long it is compared to from the side.
Do you know FuzzMeasure ? There you can rotate a waterfall diagram freely in 3D space with your mouse.

Quote:
Originally posted by cap'n todd if you EQ in a sensible way, i.e. just bring the largest peaks down to a reasonable level, you should reduce the overall ringing.
Have you found the waterfall data you were talking about that supports this?

Best, Markus
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Old 2nd February 2009, 06:37 PM   #547
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Quote:
Originally posted by gedlee
Personally, I don't see any of this really being significant to the issue of using multiple subs and how to set those up. It never comes up any time I do multiple subs and my results always come out fine. In fact most people suggest the bass in my room is the best that they have heard - MP or not. I should mention that I DON'T use any EQ at all.

John - a boundary condition is only inhomogeneous if the pressure or the presure gradient is zero or the ratio of the two is specified. If the velocity is specified to be non-zero (independent of the pressure) on any portion of the surface then its not the an inhomogenious boundary condition. Since g(r) does not have zero velocity on the boundary no solution of the inhomogeneous boundary can cancel it there.

I guess we are just arguing words again Earl. Seems we always end up there. It would seem we agree that, in simple terms, X must satisfy the wave equation with no sources in the interior and boundary conditions as required so that when added to g, it forms G, the solution for the bounded region. See, I said that without anything about homogeneity.
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Old 2nd February 2009, 07:01 PM   #548
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Quote:
Originally posted by markus76


Do you know FuzzMeasure ? There you can rotate a waterfall diagram freely in 3D space with your mouse.



Have you found the waterfall data you were talking about that supports this?

Best, Markus
The problem is that if you rotate it to the side so you can clearly see the decay, it may be blocked by other resonances in the waterfall. If you view from the side and take a "slice" you could easily see it, but then its not really a waterfall. Of course, with a 3-d cursor you can get the data easily.

I have the data, from one of Floyd's powerpoints. Not sure how to get the image inline in this message. Do i have to host the file (http://) somewhere?

It does show the mode decaying quite fast, and then at a lower rate (harder to tell at that point since it is close in level to the rest of the decay). The initial level is close to the unequalized, with a very quick early decay. Of course, the eq filter is kprobably not perfectly matched to the resonance (it is real world). Personally, I have my reservations about waterfall plots.
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Old 2nd February 2009, 07:07 PM   #549
gedlee is offline gedlee  United States
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Quote:
Originally posted by john k...



I guess we are just arguing words again Earl. Seems we always end up there. It would seem we agree that, in simple terms, X must satisfy the wave equation with no sources in the interior and boundary conditions as required so that when added to g, it forms G, the solution for the bounded region. See, I said that without anything about homogeneity.

John

But X(r) must have sources (or sinks) on the boundary otherwise when it is added to g(r) it will not yield the boundary condition since g(r) is not zero on the boundary. See I can also refute you without using that term either. We agree that there can be such an X(r), but I don't think that we see it as the same function and I don't think that it would be easy to derive. I know that I 've never seen it.
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Old 2nd February 2009, 07:29 PM   #550
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Quote:
Originally posted by cap'n todd
Not sure how to get the image inline in this message. Do i have to host the file (http://) somewhere?
You can attach a file with every message you compose or upload the image to a provider like http://imageshack.us/ and paste the code you get after uploading the image in your message. Alternatively you can link a file that is already online by pasting its URL in your message.

Best, Markus
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