An illustrated guide to measuring a speaker's impedance

SeleniumFalcon

Here is an example of how to measure the impedance of any speaker. Here are the items you'll need:

1) a Volt/Ohm meter
2). an audio amplifier
3). an audio signal generator
4). a medium value resistor of around 1/2 Watt
5). a calculator

Here is a photo of my test set up, an NAD 3020 integrated amplifier, a Polk TL1 Blackstone satellite speaker, a 1.5k resistor, a Fluke 77 multimeter and a Tenma audio generator.

SeleniumFalcon

The first step is to measure the actual value of the resistor using the Fluke. I measure 1495 ohms.

heiney9

Great thread!

H9

SeleniumFalcon

Next, connect the audio output of the signal generator to the auxiliary input on the NAD. Then connect the positive speaker output of the amplifier to one end of the resistor. Connect the other end of the resistor to the positive terminal of the speaker. Connect the negative terminal of the amplifier to the negative terminal of the speaker. Lastly, connect one lead of the Volt/Ohm meter on one end of the resistor and the second lead to the other end of the resistor. Set the Volt/Ohm meter to read AC Voltage.

SeleniumFalcon

Select the frequency you want to use to begin your measurements, I chose 1,000Hz as a beginning point. Turn the amplifier on and adjust the playing volume and the level of the signal generator until you get around 10 Volts AC displayed on the meter. I measured 10.12 Volts AC on my set up.

SeleniumFalcon

With the calculator divide the measured Voltage (10.12 Volts) by the resistance just measured (1495 Ohms) and get 0.0067692 Ampere. We round this off to 0.0068 Ampere or 6.8 mAmpere. Now with everything staying the same, untouched, remove the Volt/Ohm meter's leads from either side of the resistor and place them on the positive and negative terminals of the speaker and read the new Voltage shown. I read 0.033 Volts AC. Now take this new voltage measurement (0.033 Volts AC) and divide it by the current measurement we made (0.0068 Ampere) and read 4.85 Ohms. This is the speaker's impedance when measured at 1,000Hz.

SeleniumFalcon

Now all you need to do is sweep the audio generator up and down in frequency, not changing any of the volume levels and write down the AC Voltages seen on the meter and do the division and have the speaker's impedance at this frequency. The particular speaker I'm using has a self contained high pass filter and as I reduce the signal generator's frequency the impedance goes up because of the internal filter.
I hope this is helpful information.

jbreezy5

SeleniumFalcon wrote: »

With the calculator divide the measured Voltage (10.12 Volts) by the resistance just measured (1495 Ohms) and get 0.0067692 Ampere. We round this off to 0.0068 Ampere or 6.8mAmpere. Now with everything staying the same, untouched, remove the Volt/Ohm meter's leads from either side of the resistor and place them on the positive and negative terminals of the speaker and read the new Voltage shown. I read 0.033 Volts AC. Now take this new voltage measurement (0.033 Volts AC) and divide it by the current measurement we made (0.0068 Ampere) and read 4.85 Ohms. This is the speaker's impedance when measured at 1,000Hz.

Now we just gotta get consenus on how to do the math, lol…

https://forum.polkaudio.com/discussion/198050/

plastic_avatar

FAN-FRAKKING-TASTIC

[two giant thumbs up for the instructions w/ pictures]

SeleniumFalcon

Before I took everything apart I thought I would make a few more measurements since it was set up:

an impedance of 4.15 Ohms @ 640Hz
4.59 Ohms @ 320Hz
13.35 Ohms @ 180Hz (rising impedance due to high pass filter)
8.9 Ohms @ 100Hz
6.98 Ohms @ 50Hz
3.85 Ohms @ 2kHz
2.97 Ohms @ 4.2kHz
2.82 Ohms @ 8.0kHz

mantis

Pretty fun thread, thanks for posting.

skrol

I assume that you need to verify the signal generator output remains 10V over the range of frequencies so that its variations do not influence the results.

SeleniumFalcon

I suppose I should have mentioned that any tone controls, filters, loudness compensations and EQ adjustments on the audio amplifier should be switched off or set to "0". If this is not feasible then as long as after a change in frequency on the signal generator is done you return the Volt/Ohm meter's probes to either side of the resistor and adjust the volume level so the same Voltage reading is displayed the measurement at the speaker's terminals will accurately reflect the speaker's impedance. Any audio amplifier should be able to deliver 10 Volts AC across a 1.5k resistor with uniform response. Quite a few preamps can do that especially if they are vacuum tube based.

Gardenstater

This is a bit nerdy but interesting (what I found online here I mean)



https://www.wavecor.com/Transducer_equivalent_circuit.pdf

Gardenstater

I'm wondering how you would measure impedance of an SDA speaker with the Interconnect Cable in place that is. I know that with mono signals it should be no different from measuring with cable disconnected but what about with different material? Would you need a signal generator that has 2 channels where the 2nd channel can give a user variable phase offset? So at 0deg phase offset it would be like mono and like there was no IC cable in place, but at 0deg to 180deg it would be interesting to see how the impedance of the SDA speaker varies.

Is this doable?

SeleniumFalcon

I wondered if anyone would do an impedance sweep of several frequencies and repeat the process after doing the recommended modifications. In theory even air leaks can effect the system's impedance, so changing the mounting arrangement could alter measurements. Or, do an impedance measurement on a cold speaker, then play it for say an hour and remeasure.

Gardenstater

And if you measure DC Resistance and do an Impedance Curve ( or the needed data points), you can calculate Qms, Qes, and Qts as shown: