How to measure speaker impedance?

Making a loudspeaker impedance measurement is not a as easy as you might think. If you compare to resistance, impedance changes value with frequency. Therefore, an impedance measurement is actually a graph, not a number. Normally, there is specialized hardware that does this stuff in a second, automatically plotting the graph. Anyway, let’s be resourceful and make a loudspeaker impedance measurement with what we have.

Equipment needed

The title states that only a multi-meter and 2 resistors are needed, but the measurement requires additional items, which most likely you already have. The resistors can be found at your local electronics store for basically no money.

The list :

Here is how the excel spreadsheet looks like. By the end of the article, it will be all filled out, and the impedance graph will show on the right.

Room EQ Wizard will be used as a signal generator software, in conjunction with the amplifier. Normally, you would have a stand-alone signal generator, but a good one is pretty expensive and doesn’t have other uses. I’m guessing you already have a computer. Since you are making a loudspeaker impedance measurement, I’m guessing you already have an amplifier too. The software is free.

Loudspeaker impedance measurement method

The technique we are using in this tutorial is the voltage divider method, with some additional tweaks. Normally you would have a standalone signal generator which can maintain a constant voltage. In that case we would measure the resistor just once. However, in our tweaked scenario, we will have to measure the resistor just as many times as we measure the speaker. This will make more sense later on.

Advantages :

Equipment readily available or cheap/easy to come by.

It doesn’t take as much time and less tedious than other methods (current source method).

Speaker is connected to an amplifier. This will emulate a real-life scenario, since the amplifier’s damping will be taken into account.

Disadvantages :

High impedance drivers may affect the measurement accuracy.

Now, to give a general picture on how the loudspeaker impedance measurement works, here is a diagram :

The basic principle is that you set a level for your amplifier. With the speaker in place, set the generator to a certain frequency you are interested in. Take a reading on the volt-meter. Swap the speaker with the resistor and take another voltage reading. Since the resistance of the resistor in known, you can calculate the resistance of the speaker at that particular frequency. Write it down, and repeat this step for different frequencies, until you have sufficient data to plot the graph in the excel spreadsheet.

Bear in mind that you will not make this swap every step of the way. First measure the speaker for all the frequencies you are interested in, and then switch to the resistor. Shouldn’t take that much time.

Step 1 : Room EQ Wizard (REW) setup

First of all, download the software here. Normally you would have to make a lot of adjustments for the microphone and stuff, if you plan to use this software for what it was intended to, but we are going to use only the signal generator.

Go to Preferences -> Preferences, and make sure that your output device is set correctly (your sound card, most likely). Then, go ahead and click the signal generator icon. Once there, make sure that the output is set to both channels, and that the RMS level is set to its highest point (-3.0). As for the signal type, select sine wave.

The multi-meter I’m using is the Fluke 177, which has a frequency counter. As a result, I can check if REW is working correctly.

As you can see, REW works great as a frequency generator, and the numbers are spot-on.

Step 2 : Set your amplifier level

The amplifier needs to have a fixed volume level for the whole process. First of all, hook up your speaker in the rig and set the multi-meter to AC volts. Next, set the signal generator to pink noise and press the play icon. Now fiddle with the amplifier volume knob until you reach a reading of around 150 mV on the multi-meter.

When playing pink noise, the voltage will not be stable. In conclusion, if you have an average function on your multi-meter, use it. Try to reach close to the 150 mV mark.

Step 3 : Establish the points of interest

To complete the excel graph, we have to make some measurements at certain frequency points. We decide these values, but there are certain frequencies which must be on the graph. First of all, you have to decide the resolution of the graph. If you look closely, you will see that the file contains 3 spreadsheets : Impedance x35, x50 and x75. This indicates how many measurement points the plot will contain. As a result, a plot containing more measurement points will be more resolute/accurate, but it will take more time. Regarding this loudspeaker impedance measurement tutorial, the Impedance x35 is the pick, since it is less time consuming.

For a driver in free-air or inside a closed box

For this type of setup, there will be a couple of points of interest that must appear on the graph. This is how the graph should look like :

You must find those 2 frequencies :

The frequency where the impedance spikes. This corresponds to the resonant frequency of the driver / closed box. After the resonant frequency, the impedance starts to decrease, reaches a minimum, and then rises again due to voice coil inductance. That minimum point is important.

For a driver in a bass-reflex box

There are 4 points of interest when we are talking about bass-reflex. Here is how the graph should look like :

Find these 4 frequencies:

Bass reflex has 2 impedance spikes. Sweep the frequency generator until you find the highest value of the first spike. Then the impedance will decrease, and start to rise again. Find the minimum point. This corresponds to the resonant frequency (tuning) of the box. Then, the impedance will rise again until it reaches a maximum. Note the frequency of when that happens. Finally, just like with the sealed box, the impedance will drop off, reach a minimum, and then rise again due to voice coil inductance. Note the frequency when the minimum occurs.

Other points of interest

When making a loudspeaker impedance measurement, to have a complete graph, you must have these frequencies in your chart :

The 2/4 frequency points mentioned above for sealed / bass reflex.

2 additional points (or more) close to the previous mentioned values. For example : If the resonant frequency is at 57 Hz, make sure you plot 50 Hz and 65 Hz as well.

The starting points of the graph : 10 Hz and 20.000 Hz.

Every decade : 100 Hz, 1.000 Hz, 10.000 Hz.

Every half of decade : 50 Hz, 500 Hz, 5.000 Hz.

All octaves : 20 Hz, 40 Hz, 80 Hz, 160 Hz, 320 Hz. 640 Hz. 1.250 Hz, 2.500 Hz (5 kHz and 10 kHz already been mentioned).

Half of octaves : 30 Hz, 60 Hz, 120 Hz, 250 Hz, 2.000 Hz, 4.000 Hz, 7.500 Hz, 15.000 Hz.

Add frequency points to complete the table and fill obvious gaps : 12.500 Hz, 17.500 Hz etc.

Step 4 : Measure the resistor

Unless you have an expensive resistor, most likely it’s not exactly 10 Ohm. Therefore, we have to measure it.

Measure the resistance of the test leads by shorting them. After that, measure the resistor and subtract the value of the test leads. In our case 10 – 0.1 = 9.9 Ohms. Insert that value into the spreadsheet.

Step 5 : Complete the frequency column in the spreadsheet

We are going to measure the impedance of a driver in free air : (Seas CA 18 RNX). The driver has a pole piece in its construction. As a result, I placed the driver on 2 books so the vent is not obstructed. As mentioned earlier, we have to find the highest impedance point (at resonance) and the minimum point, before the impedance start to rise.

Place the speaker in the rig, and set the multi-meter to AC volts. Now sweep the frequency, until you see the highest value. Be smart about this. Since it is a mid-bass driver, the resonance will be somewhere between 40 – 80 Hz. If it’s a tweeter aim for 500 – 2000 Hz. After you found it, note down the frequency and the voltage. Then, increase the frequency. You will see the voltage dropping as frequency increases, but at a certain point it will start to rise. Note this minimum point (frequency and voltage), just before it starts to rise.

After we found these important points in the graph, we can begin populating the frequency column, and then we can move on with the loudspeaker impedance measurement. Complete the frequency column similar to this fashion. It is important that the frequency column is in ascending order, otherwise the graph won’t show correctly.

Step 6 : Complete the voltage column (measuring the speaker)

Proceed to place the speaker in the rig. Input each frequency (from the previous step) in the generator, and take a voltage measurement. Each measurement you write down in the corresponding cell. After you’re done, it should look something like this :

When changing the frequency to take another measurement, please do this extra step. Don’t just change the frequency, but also stop and restart the generator. Sometimes I saw obvious erroneous readings. Restarting the generator (depressing and pressing the “play” button) would fix this inconvenience. In conclusion, restart the generator every time you change the frequency.

Step 6 : Complete the voltage column (measuring the resistor)

Go ahead and swap the speaker with the resistor. Do the same thing as you did previously with the speaker. Note down in the correct column all the voltages shown on the multi-meter for each of the corresponding frequencies. After you completed the column, it should look something like this :

As you can see, the impedance column is completed automatically. Please write only in the yellow cells, otherwise you could overwrite the formulas in the impedance column.

Loudspeaker impedance measurement complete

If you have done all the steps, you should see a finished graph on the right.

I know that this looks like a lot of work, but it will take 20-30 minutes. There are other methods out there that are far more tedious. Of course this all makes sense in the absence of specialized measuring equipment. Now you can measure impedance with items that you probably already have.

References