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Old 27th July 2007, 10:41 PM   #11
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Mathematically speaking, there is nothing that isn't in an FR curve, that isn't in an impulse. And the CSD is derived purely from an imulse. So, from an information theory standpoint,
This part is true. Both are derived from the information found in the impulse response.

But if the impulse response contains all that information why look beyond the impulse response?

Obviously the impulse response does not display amplitude in any meaningful way. The frequency response does and displays amplitude in relation to frequency.

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the CSD offers no additional information that isn't already in the impulse, and thus, the FR.
Not true. You can see a response pulse in the impulse response in relation to time, but not in relation to frequency and time.

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Again, there is no new information in a CSD that is not present in the impulse, and, hence, in the FR.
Not true. While the single measurement captured the information to display the data for both measurements, you clearly cannot see a decay rate from the frequency response. You can only see amplitude.
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Old 28th July 2007, 01:27 AM   #12
Jay_WJ is offline Jay_WJ  United States
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I think Mark's description is more accurate. Since decay over time is perfectly predicted with FR data, there is no additional information in CSD data. If we did not know the exact model that relates FR with CSD, then we would need additional measurement. Otherwise, we don't.

Danny, you understand Zaph and Mark's point. Stop pretending not to.
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Old 28th July 2007, 09:08 AM   #13
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Hmmm, I think Zaph's response may have been a little misleading, though I wouldn't discount it entirely.

Re: CSD plots, I quite like them if they are done well. What's the point of fussing over accurate data collection when half the time people can't even make the axes legible?

Ucla88, I'm not too worried about artifacts from CSD plots. Windowing functions are usually designed to get rid of artifacts from the classic 'start' and 'stop' clicks, albeit at the cost of losing low frequency information whose wavelength approaches the length of the window.

(edit: on second thoughts, that last bit ^^^ is not quite true, but it'll do )

IMHO, it's usually enough to have a close and careful look at the speaker's impulse or step response (assuming that the manufacturer/tester is kind enough to oblige us with that information). And if there's substantial ringing after the initial "bang" then it's often possible to just measure the dominant resonant frequency/ies manually. I guess CSD plots add an element of mathematical rigour to the method and it may be possible to tune them to provide a visual analogy for the way that humans hear things. I.e.: there's necessarily a compromise between frequency accuracy and timing accuracy, and ringing may be masked below a certain point because it is of such a short duration.

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Old 28th July 2007, 03:59 PM   #14
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Default Csd

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Since decay over time is perfectly predicted with FR data, there is no additional information in CSD data.
Not true at all, and I can illustrate it with Zaphs own measurements. Let's look at a few 6.5" woofers.

Here are two drivers from Seas. One is $63 and one is $70.

Click the image to open in full size.

Click the image to open in full size.

Lets look at the top one. First there is some obvious break up going on in the top end. The frequency response does not hide that. How bad is it? We can't really tell. Zaph claimed though that with a properly designed crossover that it won't be an issue. That's not true either. It will still be an issue. Aside from an infinite slope design that break-up will still be there just down in amplitude.

Here is the real problem though. How do they look in the range you want to use them? How do they look from 4kHz and down? Are they both the same? They both look pretty fair from 4kHz and down don't they? Does the frequency response tell you all that you need to know? Let's see.

Here are the spectral decays.

Click the image to open in full size.

Click the image to open in full size.

Besides the massive break-up in the top end. Look at all the additional stored energy in the range below 4kHz. One rings like a bell and continues to do so through the intended range of use. The other looks really clean.

Which do you think will have the cleaner vocal range? Could you see it in the frequency response? Certainly not !

Lets look at a couple more. Both of these are poly cone woofers. They must be the same right?

Click the image to open in full size.

Click the image to open in full size.

What do you think?

The top one looks like it has a pretty good break up peak at 6kHz, and it has a little roughness in the 1.5kHz to 3kHz range that could be something, but we really don't know.

The bottom one could have some break up going on at 4kHz, 6khz and 9kHz, but how bad is it.

Both look pretty good below 2kHz though right? Let's see.

Click the image to open in full size.

Click the image to open in full size.

Well on the top one, that peak at 6kHz wasn't really much of a resonance at all. It's there all right, but it dies fairly quickly. The smaller ones below it are more of a problem.

However, the biggest problem is in the range below 2kHz. This woofer stored a lot of energy in that range (its useable range).

For what its worth, my experience with this woofer is that it has a horribly smeared mid-range and is not useable beyond 200Hz.

Now the lower woofer is not just clean but real clean. The peaks in the frequency response that looked like might be a problem are no problem at all.

This is information that you CAN NOT get from the frequency response.

Hey let's look at one more. Here is one that is $46 and one that is $45.

Click the image to open in full size.

Click the image to open in full size.

Okay the top one has some small peaks in the top end. How bad is it? We don't know. It looks flat in the lower range though and all the way up to 2kHz so it must be great right?

The lower one looks pretty flat in the lower ranges too but has an elevated area from 3kHz to 5kHz that could be something. There is some small peaks above that. Maybe that is something too.

Lets see what the real tattle tale says (the CSD).

Click the image to open in full size.

Click the image to open in full size.

The top one looks pretty bad all over. Those peaks in the top end are bad. But look at all the stored energy in the range below 2kHz. It is all lumpy. There are areas that are popping up (ringing) where there wasn't even a peak. This thing is ringing all over the place. The mid-range and upper mid-range will be very colored. I have heard this woofer and it is very colored. You couldn't see it in the frequency response either.

Lets look at the lower one. Hey that amplitude peak in the 3kHz to 5kHz area died immediately. In fact the entire response across the board died immediately. You can't ask for it to be much cleaner.

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Danny, you understand Zaph and Mark's point. Stop pretending not to.
Give me a break. I don't think you guys understand MY point. Read the data ! It speaks for itself.
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Old 28th July 2007, 07:33 PM   #15
dlr is offline dlr  United States
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Default Re: Csd

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Originally posted by Danny
Give me a break. I don't think you guys understand MY point. Read the data ! It speaks for itself.
We do understand your point, but that point is just plain wrong. Zaph is without question absolutely correct. There is absolutely nothing in the CSD that is not in the FR. What is evident is that you have a vested interest in maintaining your position. It's wrong, period. All of your comparison examples simply illustrate the fact that the CSD can either misinterpreted or misunderstood as to the data that is there.

The CSD is close to useless. Resonance ridges are obvious in the FR. All one need do is understand the relationship between the Q and magnitude in the FR to not need the CSD. It should be obvious to anyone with experience.

Trying to read too much from the low frequency end also shows a misunderstanding of the limits of the measurement system. The accuracy of the curve and any "resonances" are problematic due to the cutoff frequency. Those CDS examples stop at 300Hz. That also means that there is question about what's happening between successive sample resolution points. A change from a square window to a half Blackman-Harris will likely show two different stories. Even moving the stop time marker a point or two may alter the low end response more than one might think.

No, the CSD is almost useless if you understand the FR curve.

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Old 28th July 2007, 07:57 PM   #16
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We do understand your point, but that point is just plain wrong. Zaph is without question absolutely correct.
Not at all.

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There is absolutely nothing in the CSD that is not in the FR.
Again not true at all. Re-look at the examples I posted.

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The CSD is close to useless.
Nope. It is the most telling of any measurement you can take of a driver.

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All one need do is understand the relationship between the Q and magnitude in the FR to not need the CSD. It should be obvious to anyone with experience.
You are incorrect. And just what is your experience level?

As for me I design drivers and work with manufacturers developing new drivers for various clients. This year alone approximately 20,000 to 30,000 drivers will be built for my clients that are of my design and about 8,000 drivers will be built for my own company that are of my design.

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Trying to read too much from the low frequency end also shows a misunderstanding of the limits of the measurement system.
Decay rates below 200Hz are not accurate and are omitted from the measurements. No misunderstanding here.

Quote:
No, the CSD is almost useless if you understand the FR curve.
You have it backwards as well. Stored energy and inertia information is not found on a frequency response graph. However frequency response is seen in the CSD.

My illustration clearly indicates the value of the information found in the CSD that is not clear from a frequency response curve.

Feel free to fool yourself if you like. Zaph and or any others can do the same, but there is no sense in trying to mislead others.
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Old 28th July 2007, 08:13 PM   #17
dlr is offline dlr  United States
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Default You're just plain wrong

People can be involved in design and still not fully understand some aspects of measurement. This is the case here. The more you protest, the more obvious it is.

I also was not speaking of the cut off area below 300, (not 200 as you mentioned). I'm speaking of the low end of those measurements. The resolution does not allow for accurately describing the true response much below about 1K down to the 300 cutoff. If you think otherwise, then that's a clear demonstration of misunderstanding the limits of the measurement.

Do you truly understand the indisputable relationship between the impulse response, frequency response and the CSD? At this point I'm questioning that even more.

Dave
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Old 28th July 2007, 08:51 PM   #18
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People can be involved in design and still not fully understand some aspects of measurement. This is the case here. The more you protest, the more obvious it is.
Or so you think (in this case).

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The resolution does not allow for accurately describing the true response much below about 1K down to the 300 cutoff. If you think otherwise, then that's a clear demonstration of misunderstanding the limits of the measurement.
In a CSD it is not that there is a point at which everything below point A is not accurate and everything above point B is. It is a graduated curve. Obviously accuracy gets less and less as frequency decreases. Depending on the measurement set up and gated time window accuracy below a 1kHz range becomes less accurate. Below 200Hz (or even 300Hz) it cannot be observed to have any real value as accuracy below that point is not possible.

If YOU think otherwise, then that's a clear demonstration of misunderstanding the limits of the measurement.

Quote:
Do you truly understand the indisputable relationship between the impulse response, frequency response and the CSD?
Clearly you and I know that both are derived from the impulse response measurement. That does not diminish the value of looking to a spectral decay to see stored energy problems. To dismiss a CSD as having no value is absurd.

Maybe you would like to rub your crystal ball and tell us what the spectral decay will look like from looking at a frequency response curve? I could post a few for you if you'd like to decipher them for us.

Clearly from the examples that I posted one can NOT in any way accurately predict the stored energy of a driver only from looking at a frequency response measurement. The suggestion of such is ridiculous. Furthermore to suggest that knowing the information shown on a CSD is of no value is even more ridiculous.
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Old 28th July 2007, 09:04 PM   #19
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Man you need to go to a Buddhist temple. This dustup you had a while back with Zaph has taken a pretty tight hold on your head. Let it go Danny.
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Old 28th July 2007, 09:19 PM   #20
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Man you need to go to a Buddhist temple. This dustup you had a while back with Zaph has taken a pretty tight hold on your head. Let it go Danny.
Hardly, I just hate seeing people mislead and given false information. I know many people in the industry just look at a thread like this and laugh. I guess it is funny to some, but I see it as very sad.
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