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. It’s easy to use and affordable(-ish). But 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. If you don’t have the patience to read this article, or you want the nitty-gritty details, just watch this youtube video :

PART 1 :

PART 2 :

There are 2 parts on youtube because REW has updated and the procedure changed.We no longer need to make 2 cables, only one which will be used to measure Thiele Small parameters or to make a simple speaker impedance measurement. Basically you need some jack plugs and some resistors. Anyway, I’ll list the full parts list in a sec. The software we will use is Room EQ Wizard. The software is free and easy to use.

Parts list

Here are all the parts you will need to make the 2 cables :

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. Please note that the combo jack option I mentioned in the youtube video won’t work. The microphone section will be mono, when using a combo jack adapter, and we need stereo.

  • Combo jack adapter. This won’t work. If you have a desktop computer, you can use the individual connections on your on board sound card, as most likely the microphone input will be stereo.
  • 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.

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).

Measurement cable

Please ignore the combo jack adapter in the picture. Consider it the output jacks on your sound card (regardless if it’s external or internal).

speaker impedance measurement cable

The connection to the speaker terminals is done using alligator clips. Otherwise, the schematic is self explanatory. However, I want to mention something. It’s not mandatory to do so, but if you want to follow my settings when we head to Room EQ Wizard, you will have to do the connections this way.

The stereo jack has 2 positive poles, one short and one a bit longer. Looking at the schematic, connect the left side of the resistor to the short pole of the stereo jack. The right side of the resistor (the one that connects to the positive terminal of the speaker as well) connect it to the long positive pole of the stereo jack. You can do it the other way around, but you will have to have different settings in REW. For ease of use, just do it like this. Furthermore, make sure you precisely measure the resistor with a multimeter, and note down the value (before you actually make the cable).

Measurement cable impedance

Here is how the cable should look like.

Room EQ Wizard settings

Presuming that you already installed REW, head over to preferences.

REW settings

Before we measure some T/S parameters, we have to make some adjustments first. You have to select your output and input device. I’m using a laptop and use an ASUS Xonar external sound card. If you have some on-board sound card, most likely you will see something like Realtek etc. Make sure you select the L-L / R-L like you see in the image. These are the settings you can copy if you made the cable like I told you.

Furthermore, you would want to check in control panel that you don’t have any sound effects enabled for your playback and recording device. Also, setting the volume to max is a good idea. Finally, you can click measure, and ignore any message about SPL. Here I like to set the input volume at 0.9 (which means 90% volume on the microphone). When we do the first calibration, we might get a clipping error because of this. If we do, we progressively lower the volume until the clipping error doesn’t show up anymore.

calibration REW

Here you have to do the following :

  • Click Impedance button
  • Frequency range 20 Hz – 20 000 Hz
  • Increase the level at 0. Probably at -20  dB.
  • Rsense to 98.4 Ohms. This is the measured value of the 100 Ohm resistor. Input your exact value, not mine.
  • Click “Open circuit cal”

This is the first calibration we need to make. And the alligator clips needs to not touch one another. I got the clipping error message and it went away when I reduced the input volume to 0.75.

measure T/S parameters

Then we need to do the 2nd calibration. Now we have to connect the 2 alligator clips (short them out) between each other and click the “Short circuit cal”.

Measure Thiele small parameters

After that, we need to make the last calibration. Connect the alligator clips to the 33 Ohms resistor. Click the reference cal button and enter the value of the resistor. Mine measured at 32.9 Ohms. Always enter the measured value, not the rated value. When that is done, the calibration is complete and we no longer need to do it again. Even if we close the calibration windows or even REW. We need to redo this process, if we change the sound card, or we use another cable.

Speaker impedance measurement

Now we have everything setup for a speaker impedance measurement and, finally, T/S parameters. If you haven’t done a measurement like this make sure you check out this article. However, that is just a simple impedance measurement. I’ll explain the T/S parameters measurement with a bit more detail.

Click the measure button again. But before we do anything, we need to know the value of the resistor and the resistance of the test leads. We measured the resistor before we made the cable, which was 98.4 Ohm in my case. To measure the resistance of the test leads, short out the alligator clips (connect the red to the black alligator clip). Next, use your multi-meter to measure from the positive of the mono jack (the tip) to the negative of the stereo jack (the large side at the bottom of the plug). From that value subtract the value of the resistor, and the value of the resistance of the test leads of the multi-meter. In my case : 98.65 – 98.4 – 0.1 = 0.15 Ohms.

measure thiele small parameters

Here you have to do the following :

  • Click Impedance button
  • Frequency range 20 Hz – 20 000 Hz
  • Increase the level at 0. Probably at -20  dB.
  • Click “Start”

speaker impedance measurement

The result should look like this. This is the impedance chart. To measure the T/S parameters we need to do another measurement with some modifications to the speaker.

Measure Thiele Small parameters

After we done a free air measurement of the speaker. We need to do another measurement with some modification to the speaker. We need to modify the moving mass to the speaker. To do so, we will place some clay to the speaker cone.

measure T/S parameters

The clay needs to be measured precisely with a scale that shows the weight with at least 1 decimal point.

added mass method

In our case the added mass is 31.2 grams. Make sure you write this number down. Now with the added mass on the speaker, do another measurement just like the one before.

measure thiele small parameters vas

We are going to name the 1st and 2nd measurements accordingly. We are also going to get our multi-meter out to measure the resistance of the speaker. Furthermore, we will measure the effective diameter of the speaker, using a ruler. Simply measure from the apex of the surround on one end to the apex of the surround on the other end.

measure speaker diameter

Having all this data, head on to Tools -> Thiele Small Parameters.

measure thiele small parameters

Here you simply enter the values which we calculated just now. The DC resistance of the speaker. For the speaker area, you can click the little calculator thingy and enter the diameter, and it will calculate the area automatically. Leave the air properties values unchanged and enter the value of the added mass. Simply click calculate parameters and it should display all of the T/S parameters. If you get an error about DC resistance being too high, that means the you didn’t made the measurement from 20 Hz to 20 Khz. You probably made them from 1 Hz. Make your measurements again.

Conclusion for T/S parameters measurement

I wrote down the parameters from this measurement and put them side by side with the parameters from a DATS v3 measurement and the parameters found in the specification sheet of the driver.

Measure T/S parameters

First of all, I want to let you know that the driver wasn’t broken-in. It wasn’t used in any speaker box. As a result, the parameters in the spec sheet are a bit off. This is because they are measured with the driver broken in. You can clearly see this. As the speaker is breaking-in, the resonant frequency goes down. You can see this behavior in our results. But we are interested in the numbers between the DATS v3 and our Thiele Small parameters measurement using the sound card. You can clearly see that the numbers almost match.

This is no surprise, as in essence the DATS v3 is basically a sound card underneath. But it’s presented in a nice package with dedicated software. Plus you don’t have to build anything. Not to mention any errors that might pop up in your DIY building process. So you have to judge for yourself if you want to pay for convenience or get your soldering iron ready.