# Qtc measurement/calculation



## jeff8houses (Dec 7, 2010)

First post here. I've been an ARTA user for a while now but was recently turned on to REW and have been very, very impressed so far. Well done! But I do have a question or two.

I'm looking to do some impedance measurements of a dual mf-driver (MTM), closed box system and specifically want to be able to tune to a specific Qtc. The Thiele-Small measurement tool works great for doing single free air drivers, which of course needs either the added mass or closed box system. But to simply get Qtc/Qts one only needs a single impedance measurement. I can do the calculations by hand/spreadsheet from the impedance data, but it's manual, slow, and prone to error from guestimation of points.

I've been messing around with it, but can't seem to find a way to get REW to calculate Q from a single measurement. Any tricks or workarounds I'm not seeing? Many, many thanks!

Jeff


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## JohnM (Apr 11, 2006)

If you just leave the secondary measurement selection blank and click "Calculate Parameters" you will get the basic set, if the Calculate Parameters button is greyed out make sure you have entered a value for the DC resistance.


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## jeff8houses (Dec 7, 2010)

Hey John. Thanks for the quick reply. I must be doing something wrong because when I hit the calculate parameters button, which isn't greyed out, the window blinks, but doesn't display/post anything. The impedance/phase curves look great and the DC resistance is 2.9 ohms. Any other tricks I could try? Thanks again!

Jeff


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## jeff8houses (Dec 7, 2010)

Ahh. Got it. I was only measuring up to 10kHz before. I switched to measure 10Hz to 20 kHz and it totally worked. Sweet!


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## JohnM (Apr 11, 2006)

Good to hear it is working. Could you post or email an impedance measurement or two for me to play with (including one that didn't work)? Looks like that driver has a much flatter impedance than the other measurements I had to work with so I'd like to check the behaviour.


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## jeff8houses (Dec 7, 2010)

Sure thing. Attached is the data set I just took. Let me know if you need more info/data about anything. Cheers!


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## JohnM (Apr 11, 2006)

Thanks Jeff, I've fixed the problem that stopped the first measurement working, the fix will be in the next REW release (though I'm not sure when that will be )


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## Tforanaga (Feb 24, 2011)

Same problem here .I'm measuring on 20Hz-20kHz range but when I hit the calculate parameters button, the window blinks, but doesn't do anything .The same happen for 20-5kHz range . Hope it will be sorted out soon .


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## JohnM (Apr 11, 2006)

Thanks - same problem, though surprising it doesn't work with a wider measurement range - could you attach a 10Hz to 20k measurement? 

Next release is probably a few weeks off. Here are the TS params from that measurement, though I don't know whether the Sd figure is right for your driver.


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## Tforanaga (Feb 24, 2011)

Thank you for your quick replay.
You are right , the SD should be 213.8cm^ .
For now I have only one 20-20kHz witch seems to work , but tomorrow I will make an 10-20kHz . measurement .


The old and broken 20-20kHz measurement seem to have bad/weird phase response , maybe bad cables connections (no file ,sorry) ?


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## JohnM (Apr 11, 2006)

Is that latest measurement a different driver?


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## Tforanaga (Feb 24, 2011)

It is the same driver but before some burn-in. At this moment I am unable to measure Re with an 1% accuracy => some errors . For these measurements I used an 24.5ohm resistor so an low excitation current was used , maybe too low . 



It seems that *the problem lies in selecting the beginning of the measurement interval* : if I select 10-5kHz or 10-20kHz everything works fine but if I try to use *20*-5K or *20*-20KHz nothing happens .
This is an working measurement for 10 years old Celestion AP8 8inch midwoofer 

xxx.mediafire.com/?1rb82ac4mnnec73

The measured parameters came close enough with factory specified parameters .Vas and Fs are a little off but after some burn-in of the driver I will repeat the measurements . This driver was not used for 5-6 years . This measurements were taken by using an 5.6ohm resistor so the driver gets more then 100mA thru it . And yes I am using an amplifier and I'v calibrated REW thru it . This driver is very stiff and correct measurements can be obtained only by using a 50-100mA excitation current . 

Celestion AP8

FS 49
RE 2.9
Sd 33.14 sq.ins
Vas 27.73 litres
Qts 0.478
Qms 2.900
Qes 0.573


Thank you !


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