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Old 12th November 2007, 10:23 PM   #1
Norcad is offline Norcad  Norway
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Default Measurements questions Le & Phase

Hello
I'm using Speaker Workshop to measure my drivers.
The results are good, both frequency and impedance.
But there is two things I dont really understand or getting right.

First; after measurements are done, I estimate S/T parameters, this works fine, and I get good results for FS, Vas, Qes, Qms, Qts and Re.
But Le is always very low, like 0,2mH, L1 says 0,3mH and Series Res is 13,7 ohm ? (this is measurments of a Seas CA18RNX)

I do the impedance measurements through a amp, because my soundcard has not enough output power.

Second; when measuring frequency, I get some strange (?) results of phase. Look at the attached image, this is my measurement of the CA18RNX, distance 1m, and 3mS window.
My measurements are in black, and measurements from Seas are in red. As you see the freq is good, but the phase is very different
Why?
Attached Images
File Type: jpg ca18_vs_seas.jpg (65.0 KB, 233 views)
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Old 13th November 2007, 10:25 AM   #2
kyselym is offline kyselym  Czech Republic
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second:
red is electrical phase
black is acustic phase

but not sure, just a thought of beginner

I measure impedance and phase(electric) together through arta jigbox

when I measure fr response in LSP lab I get similar results of phase like you.
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Old 13th November 2007, 11:04 AM   #3
soongsc is offline soongsc  Taiwan
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Regarding the acoustic measurements in your graph. The two measurements were probably measured at different distances. If you want to show the phase correctly, it is necessary to position the beginning of the window at the root of the beginning of the impulse. SW also has a feature to remove time of flight automatically. I am assuming you have gone through all the calibration procedures mentioned in the SW help file.
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Old 13th November 2007, 11:27 AM   #4
Norcad is offline Norcad  Norway
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These are both acoustic phase, measured at 1m.
Volume and channel difference are calibrated.
It doesnt matter where the starting point of the gating window are, the results are similar.
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Old 13th November 2007, 01:08 PM   #5
soongsc is offline soongsc  Taiwan
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If you could post the .pulse chart with the .pulse.frequency chart, we can know what the problem is. There should be two window markers. If you can tell us what the setting is, it could also give a clue. In the onaxis chart as you have it, you can also use the "remove excess delay" function.
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Old 14th November 2007, 05:22 AM   #6
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I agree with soongc. Both may be measured at 1m, but Seas has removed the "time of flight" from the measurement, you have not. Which is why Seas' response is (closely) minimum phase, and yours is not.
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Old 14th November 2007, 11:47 PM   #7
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soongsc and Brandon speak the truth. The additional phase in your data is due to additional time delay.
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Old 15th November 2007, 07:02 AM   #8
Norcad is offline Norcad  Norway
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OK , that make sense, but I dont understand what "time of flight" means
And how can I get SW to remove this automatically?
I also wandering what "minimum phase" exactly means?

I can post some chart later, all my measurements are on my labtop.

Thanks!
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Old 15th November 2007, 07:35 AM   #9
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by Norcad
OK , that make sense, but I dont understand what "time of flight" means
And how can I get SW to remove this automatically?
I also wandering what "minimum phase" exactly means?

I can post some chart later, all my measurements are on my labtop.

Thanks!
"Time of flight" is the time it takes for the wave to get from the source to the measuring mic.

To get SW to remove the time of flight:
With the frequency response plot highlighted, go to the menu Transform -> Delay... -> check the "remove access delay" with the delay time set to "0", and click okay. The result is not perfect all the time, and sometimes I go though this procedure two or 3 times. Regarding SW, you should be able to get more responses in the Audua forum.
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Old 15th November 2007, 07:49 AM   #10
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Time of flight is simply the time it takes the sound to travel from the speaker to the microphone as it "flies" through the air. This is the additional delay everyone speaks of.

Can't speak for your software but the principle is to re-define t=0 or zero time somehow. I would guess you either enter a distance somewhere or enter a marker on an impulse response plot. Search your menus for some promising options.

Minimum phase - ahh. A tricky one. I think I might get flamed for going out on a limb here but in my mind.... minimum phase just means that the phase response is a function of the magnitude response (and vice versa) via the hilbert transform. It also has the lowest group delay for that particular magnitude response. I think people use minimum phase for everything that isn't linear phase (i.e. pure delay: phase is linear on a linear frequency axis)

Did you sort out your Le/Re problem? If you're not taking the amplifier into consideration in the calculations then that could introduce errors. Can you post an impedance plot?
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