# Measuring driver T/S parameters with REW



## Horacio (Jul 17, 2010)

Hello.

I've been using REW for room measurement for some time, and now want to use it to measure some drivers towards designing speakers. I've gone through the Help section "Impedance Measurement" and still have some questions.

Probably the first one to get going is the 100 ohm resistor to be used as Rsense. I have several 75 ohm metal-film 1% resistors on hand. If I solder three of these in parallel I would get an equivalent 25 ohm resistor, which in series with a 75ohm equals 100 ohm. Would this work just fine, or are there any inductance-related issues with parallel resistors? Other issues? Metal film ok for this?

Thank you!


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## jtalden (Mar 12, 2009)

That will work just fine.
Metal film is ideal.
No other issues.


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## Horacio (Jul 17, 2010)

Thank you.

So I built it and measures 96.5 ohm. While each resistor is nominally 75 ohm +/- 1% they were all below 75. Oh well.

Next up is the actual setup. I have a tascam US-122MkII that I use with REW for room measurements.

Looking at this: http://www.roomeqwizard.com/help/help_en-GB/html/impedancemeasurement.html#top

The Tascam would be the soundcard mentioned there, and the L & R inputs would be the Line-In L & R. The Help link also shows a headphone output. I could use an iFi Nano or the Tascam "phone output". Any advantage/disadvantage in using one option vs the other?


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## jtalden (Mar 12, 2009)

You can use either.
The Tascam can be calibrated flat in REW. 
The Nano is no doubt flat enough so as to have no impact to the result.


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## Horacio (Jul 17, 2010)

Thank you.

What is not clear to me is what the headphone amp is connected to at the source. I mean, the link above (REW Help) shows the headphone output feeding the soundcard input and also connected to Rsense and the speaker driver. I'm assuming REW has to generate some sort of signal to play through the headphone amp. Should I connect that amp to the computer running REW?

So connect the Tascam to the PC as input soundcard and connect the iFi to the PC as output soundcard?

sorry for the basic questions!


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## jtalden (Mar 12, 2009)

You wouldn't use them both for any reason.
Typical Process:
> Connect the Tascam to the PC via USB
> Setup wiring per the REW help for Impedance measuring (it includes the sense resistor).
> Then short out the sense resistor with a short jumper and do the 'Calibrating the Impedance rig' process per REW help.
> Then remove the jumper so the sense resistor is active and you are ready to make impedance measurements.

The REW sweep signal is sent via USB to the Tascam headphone output. The input from the right and left Tascam inputs is sent back to REW via USB for analysis by REW.

For driver T/S measurements there is no need for additional boxes to be connected in the measurement process. It is a bench test separate from any normal audio system setup.

Does this address your questions?


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## Horacio (Jul 17, 2010)

jtalden said:


> Does this address your questions?


Thank you again!
Not sure if it addresses all, but I believe next step I should try to do it and it will either clarify the process or the questions I have :surprise:

Might be coming back tomorrow.

Cheers!


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