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Old 22nd September 2013, 05:23 PM   #11
Juhazi is offline Juhazi  Finland
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Dear mabat,
before you make a complete fool of yourself, please read this about Dr. Geddes or perhaps read his book

Many of us know form own experience how difficult it is to make good indoor or even outdoor measurements. This discussion was however on the FFT method.
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Old 22nd September 2013, 05:43 PM   #12
ra7 is offline ra7  United States
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Quote:
Originally Posted by Juhazi View Post
Dear mabat,
before you make a complete fool of yourself, please read this about Dr. Geddes or perhaps read his book

Many of us know form own experience how difficult it is to make good indoor or even outdoor measurements. This discussion was however on the FFT method.
This does not mean we should blindly accept everything that someone says.

Marcel is right. The resolution of a 5ms gate IS 200 Hz. We can debate whether resonances can exist that are narrow enough to not be detected by that resolution. Toole shows EXACTLY what happens with gating and a high Q narrow resonance and warns against exactly what is being said in this thread.

For your convenience, Markus even put the figure from Toole's book up on the other thread.
Uniform Directivity - How important is it?

The FFT process is clearly explained in ARTA and other software. It is quite well understood. The only argument here is whether 200 Hz resolution is enough from 200 Hz up.

Last edited by ra7; 22nd September 2013 at 05:45 PM.
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Old 22nd September 2013, 05:55 PM   #13
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Charlie,
Thanks for the great examples, they do show well what is happening. I'm not sure what mabat thinks you would see that you don't see in a windowed in room impulse response that would show in an anechoic chamber except at very low frequencies which are generally of little consequence in a small room anyway. The room will skew everything anyway at low frequencies whether the speaker has perfect impulse response or not.

Mabat,
Depending on your analysis setup with or without windowing if the length of the window is long enough what is it that you think is being hidden or missed? You can use a very long window or no windowing at all and the real result is that at some point the room reflections will become a part of the signal you are measuring. So if you want to see all of the hash from room addition just increase the windowing length or look at impulse response and try and determine what is what after a long enough interval. If you are using close miking techniques it is easy to separate the two, it becomes obvious when the room addition becomes part of the signal. Charlie's last chart in post #2 clearly shows this, the initial impulse response has approached zero and the room response becomes obvious after a set time. You could measure the response and for all practical purposes tell the distance to the wall for the first reflection knowing the impulse response that was originally generated.

I would say that Earl has nothing to hide and there is not much to prove by showing the difference between anechoic measurements and room measurements above a minimum frequency. If that be the case I think that all the computer based analysis equipment and software then is a misrepresentation, and that I do not believe. Show me one major audio company that does not rely on this type of measurement system, I know of non that would make that claim.
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Old 22nd September 2013, 06:38 PM   #14
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Quote:
Originally Posted by ra7 View Post
This does not mean we should blindly accept everything that someone says.

Marcel is right. The resolution of a 5ms gate IS 200 Hz. We can debate whether resonances can exist that are narrow enough to not be detected by that resolution. Toole shows EXACTLY what happens with gating and a high Q narrow resonance and warns against exactly what is being said in this thread.

For your convenience, Markus even put the figure from Toole's book up on the other thread.
Uniform Directivity - How important is it?

The FFT process is clearly explained in ARTA and other software. It is quite well understood. The only argument here is whether 200 Hz resolution is enough from 200 Hz up.
You and other are, I believe, still too swayed by the general case and not restricting yourself to thinking about loudspeaker/driver impulse responses - that is afterall what we are all trying to measure, not some arbitrary signal. I did mention in the other thread that some loose restrictions apply. These are that the impulse has sufficiently died out such that most or all of it is captured before the reflections begin. Earl has mentioned that in the case of a smooth frequency response (e.g. in the absense of a high Q resonance) this will be the case. I showed with my example in post #2 of this thread that a "typical" woofer response can be processed without a problem using the windowing technique. And yes one should look at the impulse to see if indeed enough of it exists before the reflections occur, but this is easy enough to see if you increase the gain to see the details around zero.

But perhaps this is a kind of chicken-versus-egg argument. I say that the impulse has to die out sufficiently in order for my argument to hold water. This implies that there are not high Q resonances, at least at a "low" frequency. Another way to say this is that there is a requirement for the response to be "sufficiently smooth", which you have mentioned is like saying that there are no features narrower than the minimum resolved frequency, e.g. 200Hz is typical. All of these imply the same thing in one domain or the other. The only caveat is that I showed that a 400Hz window could partially resolve a 200 Hz wide feature if it is a high (enough) frequency. Since the FFT operates on a linear (not log) frequency scale, 200Hz is 200Hz no matter where in the frequency spectrum the feature is found. So it would seem that this needs a little more study.

But back to the "loose restrictions" thing. I don't think that it is unreasonable to assume that no high Q resonances will be found in the "lower" frequency of the response, for loudspeakers anyway. In this case the criteria for a "smooth" response seems to be fulfilled.

Here is a proposal: if you or others think of a type of frequency response that you feel would cause the "smooth" assumption to be violated, if you post a description of what that frequency response would look like (please constrain it to be something that a loudspeaker could possibly generate) I will try to reproduce it using my method from post #2, generate the impulse and then try the various windowing to see what happens to the resulting frequency response. I think that we could all learn from this kind of exercise, since it will provide some more concrete data to talk about.

-Charlie
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Old 23rd September 2013, 02:00 PM   #15
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The "how much is enough" question is why I posted a nearfield measurement in the other thread.
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Old 23rd September 2013, 02:51 PM   #16
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Dumptruck,
The simple answer to your question is really what type of detail you are looking for. If you are only looking for a general trend you can make the window rather short, but if you are designing a raw frame speaker you want to see everything, you want to see those high Q resonances that would be hidden in a short smoothed response curve. In room response of a finished speaker system would just look messy if you had every perturbation in a response curve, what are you going to do with that much detail if you are just trying to get the position of the cabinet in an optimum place? So I would say there is no one size fits all.
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Old 23rd September 2013, 03:05 PM   #17
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Hey, I didn't ask the question.
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Old 23rd September 2013, 03:26 PM   #18
gedlee is offline gedlee  United States
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Charlie has the right idea. It is the high Q resonances that can cause problems, but I need to point out once again that it is only high Q peaks that are problematic, not high Q dips. A dip would have to be very very sharp to have a tail of many ms.

Let me give an example of when this issue can be a problem but also how easy it is to resolve.

In one of my designs the woofer had an exceptionally high resonance well above where I would be using it. this resonance DID have a tail that exceeded my window length and as such it was not captured correctly. When I designed the crossover for this unit (using this measured data, of course) and then measured the result, it was not correct - owing to the incorrect nature of the windowed response.

However, by simply putting a 6 dB/ octave LP filter in series with the woofer, the tail was decayed sufficiently and the data was corrected. Using this new data in my crossover software, after accounting for the 6 dB/oct filter, I was able to get a crossover design that worked perfectly.

It is interesting to note that the final system easily decayed within the time window once the crossover was implemented, because, of course, the crossover did exactly what was required to eliminate the woofer resonance from the response.

So while "errors" can sometimes occur, they can always be detected and always compensated for. If windowing were indeed a resolution limiting effect then this would not be possible. It all comes down to knowing what you are doing versus not knowing what you are doing.

And as I said to Markus before; If I wanted to trick the measurements I could do so in ways that you could never detect. Anyone CAN do this. (Directly manipulate the impulse response in Cool Edit!! No one could ever detect that.) You either trust that the data is done correctly and is being shown in a fair manner or you don't. Maybe this is why I virtually never trust anyone's data but my own. But then I guess no one else should trust mine either. It's a real dilemma - leaves audio in a state of complete mistrust, total lack of any objective evidence and feeds the marketing peoples claims that "It's what you hear - measurements don't tell the story!" Measurements can tell the story, but just as with economics it all falls apart once trust is removed.

Last edited by gedlee; 23rd September 2013 at 03:30 PM.
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Old 23rd September 2013, 04:04 PM   #19
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Sorry Dumptruck, just went off that last posting as if you asked the question.
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Old 23rd September 2013, 04:23 PM   #20
ScottG is offline ScottG  United States
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Quote:
Originally Posted by CharlieLaub View Post

..Well, for one, we are lucky that we don't really ever encounter a high Q peak at low frequencies!

In the midrange (not really low freq.s)..

How do you know?

Not a break-up resonance, but perhaps something relating to the suspension (particularly in-box operation).

(..of course I'd always look to the impedance trace first before questioning something like this.)



BTW, great post!
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