How to measure impedance of driver?

lgreen · 05:08 PM

How do I get a rough measure the impedance and Freq response of a driver?

I was thinking of putting a 0.1 ohm resistor in series, applying a sine wave (set at 2V pk-pk or something like that) at a known frequency and measure the voltage drop vdrop (pk-pk) across the resistor using a scope. Do this for multiple frequencies.

Current i would = vdrop/.1 ohm
In this case the impedance will = 2(vpk-pk) /i .... right?

I could also measure the magnitude of the output as detected by a microphone to get a rough idea of how the driver behaved across a frequency range. (I know you should account for FR of mic but lets ignore this for now)

Do this for multiple frequencies and plot?

Is this roughly ok or is there a better way?

Iain McNeill · 30th January 2010, 05:34 PM

You need to use RMS so Vrms = Vpk / 1.414

Current in 0.1ohm resistor will be a very small voltage leading to errors in the impedance. I use a 1ohm resistor myself and this makes it very easy to compute impedance as Z = Vrms(across speaker) / Vrms (across resistor) (which is what your AC voltmeter should be reading)

A good way to proceed is to scan frequencies for the greatest voltage across the speaker first - this is resonance, and then map impedance carefully about this point, expecially to find the frequencies above & below resonance where Impedance = Rmax (at resonance) / Re (DC resistance). Then you can calculate Qts as well.

30th January 2010, 05:06 PM	#1
lgreen diyAudio Member Join Date: May 2003 Location: San Diego, USA	How to measure impedance of driver? How do I get a rough measure the impedance and Freq response of a driver? I was thinking of putting a 0.1 ohm resistor in series, applying a sine wave (set at 2V pk-pk or something like that) at a known frequency and measure the voltage drop vdrop (pk-pk) across the resistor using a scope. Do this for multiple frequencies. Current i would = vdrop/.1 ohm In this case the impedance will = 2(vpk-pk) /i .... right? I could also measure the magnitude of the output as detected by a microphone to get a rough idea of how the driver behaved across a frequency range. (I know you should account for FR of mic but lets ignore this for now) Do this for multiple frequencies and plot? Is this roughly ok or is there a better way? __________________ My DIY audio projects- PartTimeProjects.com. Last edited by lgreen; 30th January 2010 at 05:08 PM.

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30th January 2010, 05:34 PM	#2
Iain McNeill diyAudio Moderator Join Date: Oct 2007 Location: Santa Cruz, California	You need to use RMS so Vrms = Vpk / 1.414 Current in 0.1ohm resistor will be a very small voltage leading to errors in the impedance. I use a 1ohm resistor myself and this makes it very easy to compute impedance as Z = Vrms(across speaker) / Vrms (across resistor) (which is what your AC voltmeter should be reading) A good way to proceed is to scan frequencies for the greatest voltage across the speaker first - this is resonance, and then map impedance carefully about this point, expecially to find the frequencies above & below resonance where Impedance = Rmax (at resonance) / Re (DC resistance). Then you can calculate Qts as well.

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