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Old 11th May 2009, 05:45 PM   #5511
gedlee is offline gedlee  United States
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Quote:
Originally posted by xpert

Should we discuss the effect of mechanical mass as a first order filter?
I wanted to see the nature of the response that you would give. It was about what I expected. Apparantly nothing of substance can be obtained from them so I will simply go back to ignoring them.
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Old 11th May 2009, 06:00 PM   #5512
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Hi,

Quote:
Originally posted by soongsc

So what difference will it make when you stop the input at 0 of a sine wave vs peak of a sine wave?
I'm not quite sure what you mean.

Both signals - switching a sine of at maximum or at zero transit - have different spectra, so the result is different.

Concerning resonances in CSD:
you have really to be careful. There are two ways of generating a CSD. The most common one is, where the only starting point of the FFT is shifted to the end of the impulse response. The other one (not so common) is where _both_ - starting and ending point are shifted towards the end.
This is dangerous with reflections because such a reflection may look like a resonance. You have to distinguish between three cases:

1.) the first form of the CSD is used. As the reflection (e. g. a second peak a few milliseconds later) is in the FFT window as long as the starting point is not behind it, it adds to every spectrum until then. This may look like a resonance, because it seems that the decay is slow (which is not)

2.) the second form, when the starting point is before the main impulse and the ending point behind the (2nd, 3rd,...). In this case the second form behaves similar case 1.)

3.) the second form, when the starting point is before the main impulse and the ending point before the first reflection. Then the reflection will show up as a rising edge in the CSD, so you're able to distinguish between resonances and reflections.

You may think that the second form is better than the first. You have to remind, that reflections are often within only a few milliseconds, so a FFT window to get case 3.) will be very short, which degrades the effective frequency resolution.

The solution is to use a burst decay, either with a shaped tone burst or an optimized wavelet (like Morlet). From a burst decay you can clearly distinguish between a resonance and a reflection (above a frequency where the originate burst is down to a much lower level than the reflection). Also, the burst decay does not lie (so much) as the conventional CSD.

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Old 11th May 2009, 06:18 PM   #5513
soongsc is offline soongsc  Taiwan
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Well, I don't think anyone measuring driver performance whould put the window end anywhere else than before first reflection.

I don't even know any software that would move both the beginning of the window AND the end of the window when generating a CSD. Do you know which software does this?
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Old 11th May 2009, 06:40 PM   #5514
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"I don't even know any software that would move both the beginning of the window AND the end of the window when generating a CSD. Do you know which software does this?"

CLIO for one

You can sample anywhere you want. The origin is always "0" for the starting point and the end is however long the sample is

Rob
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Old 11th May 2009, 06:42 PM   #5515
gedlee is offline gedlee  United States
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Soongsc - Agreed

Any decent software and/or user is going to exclude the reflections for all windows. That's clearly what was done in Michaels data. What appears as a resonance IS a resonance. I should know I took the data and I know the system that this data is from. There is a resonance at about 1 kHz, plain and simple.
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Old 11th May 2009, 07:12 PM   #5516
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Why would you put the start time before the main impulse? Seems option three is the only one anybody uses. I'm a little rusty on CSD calculation, but isn't there a resonant "looking" ridge at the lowest frequency that is caused by the limits of the window width? It's not real though?
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Old 11th May 2009, 07:54 PM   #5517
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Hi,

Quote:
Originally posted by soongsc
Well, I don't think anyone measuring driver performance whould put the window end anywhere else than before first reflection.
fine, and then you got a real low frequency resolution. Typical measurement setup, the floor reflection is around 5 ms. That gives you a frequency resolution of 200 Hz, nothing more, and the lowest frequency which is _reliable_ is 400 Hz. That may be enough for tweeters, but how can you evaluate a bass driver?

Quote:
I don't even know any software that would move both the beginning of the window AND the end of the window when generating a CSD. Do you know which software does this?
As I said, this form is not so common (at least in my experience). I saw ARTA doing that once, but I don't know if it is possible.

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Old 11th May 2009, 10:33 PM   #5518
ScottG is offline ScottG  United States
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Quote:
Originally posted by xpert


You persumably referred to this:

http://members.aon.at/kinotechnik/di...20CSD_corr.gif


Completely wrong.



1st - When I stated that I was having a problem with your own statement is was specifically regarding this:

"With this You emphazise that You have concerns. But we don't know where they come from. If I don't bring those effects to my perception, what is the reason? Bad system, non critical listening?"

I'm finding it difficult to understand *what* you specifically are talking about. Is this an attempt to request this?:

2nd - What I was referring to (and have all along), is Michael's post #5398 here:

Beyond the Ariel



Also, I don't think that this was terribly difficult to follow either. My post #5486 here:

Beyond the Ariel

PRECEEDED Michael's post #5488 (same thread page). Additionally, Michael's post actually included as a reference my post from #5486.

My latter reply - post #5496 (same thread page). Specifically referenced *at the beginning* my post from #5486 - i.e. "My recent analysis". Moreover that portion of Michael's post #5488 that I did reply to was referenced - with his other plots/graphs and statements specifically left out.

I guess basically what I'm saying here is that if you are going to drop an "expert" *opinion* into the discussion, that you should probably read that discussion a little more carefully.
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Old 11th May 2009, 11:09 PM   #5519
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I don't intend to get involved in this discussion because it, once again, is arguing over things well understood 3 decades ago. However, one simple thing I will point out is that you should not confuse the length of a window with the FFt length. The window length will set the lowest frequency for which the FR may be considered reasonably accurate. The FFt length, which is completely unrelated, sets the frequency resolution. Windowing (or 1/2 windowing as used in most audio software) is used to smoothly bring the impulse to zero at the end of the window. After that the impulse can be zero padded to what Levey length is desired to set the FFt length and the frequency resolution.
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Old 11th May 2009, 11:58 PM   #5520
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by Robh3606
"I don't even know any software that would move both the beginning of the window AND the end of the window when generating a CSD. Do you know which software does this?"

CLIO for one

You can sample anywhere you want. The origin is always "0" for the starting point and the end is however long the sample is

Rob
But does it move the end in the process of calculating the CSD? What I've seen is "no". As a matter of fact, CLIO could never show the resolution required from one of our suppliers. A few years ago, an agent here revealed that there was somthing wrong in the sample rate of data, details of which I cannot remember now. But if we talk about CSD from a CLIO system a few years back, I agree the data was useless. CLIO did have some other nice features though.
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