How can I test a speaker circuit's impedance?

I am working on a circuit that has 2 ceiling speakers and 1 book shelf type speaker. I only have a single pair coming back to the amplifier. I am assuming that all speakers are 8 ohm and wired in parallel. That would give me an impedance of 2.6 ohms.

Can I use multimeter to verify my assumption?

migrated from avp.stackexchange.com Jan 27 '14 at 15:03

This question came from our site for engineers, producers, editors, and enthusiasts spanning the fields of video, and media creation.

  • save yourself the hassle and buy yourself an impedance meter – user16333 Sep 11 '15 at 15:37

You can't just measure the DC resistance, that's not relevant at all.

The easiest way to measure some impedance is to send a standard 50 / 60 Hz sine signal to the speaker (e.g. produced by dangling an unshielded wire from the amplifier input1 and turning up the volume), and then measure both AC voltage parallel to the speaker and AC current in series, which at mains frequency any multimeter should be able to do. The impedance (more precisely, its magnitude) at that frequency is the ratio U / I.

However, using only a single frequency isn't really reliable. You might well obtain a much higher result than you should, since the nominal impendace is basically just the minimum of the frequency-dependant impedance. To do it properly, you therefore need a function generator (any PC with suitable software will do) and a multimeter that allows for AC measurements over the entire audible range. The measurement itself remains the same: supply a signal (varying frequency sine) from the amp, and measure voltage in parallel and current in series.

See the Wikipedia article for more information.

1You might need a preamp for this. Needless to say, an improvised mains antenna offers horrible signal quality, you might actually get more of some higher harmonic than the fundamental frequency.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.