How to measure impedance and T/S parameters using Room EQ Wizard

If you want to measure Thiele Small parameters you need specialized equipment. The most easily accessible one is the DATS v3 ( [Sound Imports] [Amazon] [Parts Express] affiliate paid links) It’s easy to use and affordable(-ish). However, what if you are really stingy and you don’t want to fork out the cash for the Dayton Audio device. Or maybe you just want to do a speaker impedance measurement once in a while, so it doesn’t make sense to buy something like this. In that case, I might have a DIY solution for you. In this article you will find all the parts needed to build the measurement jigs. However, all of the instruction for measurements and stuff are found on the YouTube video.

Parts list

Here are all the parts you will need to make the 2 jigs (Amazon affiliate paid links):ย 

The simple jig for the headphone out:

The advanced jig with an amplifier:

I understand that some parts sell in packs of 10, 25, 50 and you only need 1 or 2. Maybe you already have some at home, though I doubt you have 47 kOhm resistors lying around. Anyway, maybe you have some local electronics store where you can buy small amounts.

Sound Card

Besides these parts, you will also need a soldering iron, and of course a computer or laptop with a working sound card. Since we mentioned a laptop, there might be an issue here, because most laptops have combo jacks for their on-board sound card. We need individual connections for the headphone output and the microphone input. Combo jack has a guaranteed mono microphone and we need a sound card which has a stereo mic input. As a result, getting an adapter from one socket to two sockets will not fix things. Furthermore, getting an external USB sound card won’t guarantee that you have a stereo mic. In addition, most manufacturers don’t specify that the mic is mono or stereo, so it’s going to be a challenge to find a good sound card.

  • External sound card. You don’t need something fancy. Just something which you can connect to USB, and it will give you 2 connections : for the headphone and for the microphone. Also the microphone needs to be stereo. The Asus Xonar U3 found in this link works well. Hopefully it will still be available for purchase at the time you want to build the jig. (Amazon affiliate paid link)

If you are running a desktop computer, you can even use the connections on the front panel if you want (unless it’s a combo jack). I tried two desktop computers and both had stereo microphone inputs and those sound card worked fine for this application.

Measurement cable

The schematic for the simple jig:

ts parameters measurement using sound card

The schematic for the jig with the amplifier”

ts parameters measurement

One quick mention. In the YouTube tutorial I used a calibration resistor of 27 Ohm. However, in the parts list there is 33 Ohm resistor. The value doesn’t matter. What matters is that you measure it precisely. The calibration resistor is not a part of the measurement jig, it’s just used for calibration in Room EQ Wizard. You can download REW here.

Conclusion for T/S parameters measurement

I wrote down the parameters from the 2 measurements and put them side by side with the parameters from a DATS v3 measurement:

measure thiele small parameters results

The results are quite similar. It’s nothing to be amazed at because in essence the DATS v3 is basically a sound card underneath. However, it’s presented in a nice package with dedicated software. In addition, you don’t have to build anything. Not to mention any errors that might pop up in your DIY building process. In conclusion, you have to judge for yourself if you want to pay for convenience or get your soldering iron ready and build your own jig to measure thiele small parameters.