Testing speaker impedance/low ohms


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Old 05-29-15, 04:31 PM
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Testing speaker impedance/low ohms

I have a subwoofer in my car stereo system that may be bad or going bad. To find out, the support tech told me I could check impedence with a multimeter, and gave me the spec of .35 ohms as the proper spec for this subwoofer. My multimeter ohms settings ranges are 200, 2000, 20K, and 200K, and 2000K. So it seems with my multimeter I probably cannot measure an accurate readout because the ohm setting doesn't really go down into that low of range?
 
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Old 05-29-15, 06:00 PM
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First, you can't check impedance with a typical multimeter. What you will be testing is the DC resistance. But the number he gave you is probably a DC resistance reading.

If you have a typical 3 1/2 digit digital multimeter, you can read to .1 ohm on the 200 ohm range. Things like speakers don't usually just shift DC resistance a little, they shift a lot or go completely open (infinite resistance). So if you measure on the 200 ohm range and it measures .2 to .4 that is most likely close enough. You won't be getting a really accurate reading, but should be close enough.

If you have an analog meter, you're probably out of luck in terms of actually reading a value with any accuracy at all. But even with that, you can learn something. If the voicecoil isn't totally open, you will hear a click in the speaker when you attempt to measure the resistance. If you don't hear a click, it's probably an open winding.

All of this assumes you are measuring the speaker directly, with it disconnected from any crossover.

One other point....if you have dual subs...you can compare one to the other.

And one final point....there's a lot that can go wrong with a sub that won't show up in a DC resistance reading so just because the reading is good doesn't give it a clean bill of health.
 
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Old 05-29-15, 07:26 PM
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I have a subwoofer in my car stereo system that may be bad or going bad.
I'm confused..... bad or going bad ??

Is it intermittent ? I would suspect the amplifier more than the speaker.
A resistance check won't tell you much about the speaker status..... just open coil or normal coil.
 
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Old 05-29-15, 08:30 PM
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Yes I was mistaken about what I reported the tech told me. He did indeed say the spec of .35 should be the DC resistance reading (not impedence). I tried checking with my multimeter, which is digital, and checked on the 200 ohm range setting but seemed to get a fluctuating but rather high reading like 1. something so not sure what I was doing wrong. Also, with those multimeter probes touching the terminals, I heard no click. I was measuring the speaker directly, disconnected from any power/crossover. It's a single sub.
The reason I said maybe "going" bad is because yes it was cutting out intermittently. It's an MTX "thunderform" subwoofer which consists of the form, the speaker, a pre-amp, and the amp. It's got two little indicator lights, one red and one green. Green means all is well and it's working, and apparently red means protect mode and it doesn't work then. The red light was coming on and the speaker cutting out intermittently especially it seems at higher volumes.
I checked all connections and all seemed fine, but upon double-checking I decided to ahead and reconnect/re-tighten the ground to the sub, to the body of the vehicle. since doing that, it seems the problem has disappeared. Green light all the time now, doesn't cut out. Hope it's fixed. Had been working fine for the last six years or so since I installed it, but just had this recent issue.
 
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Old 05-29-15, 09:46 PM
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If the amp was going into protect mode due to a speaker problem.... it would not be from an open voice coil. It could happen if the voice coil was rubbing on the magnet. You'd hear a scratching/rumble sound from the speaker. Hard to describe.

Your problem is more likely caused by a lack of power to the amp. You checked the grounds. Make sure all the power wires are solid.
 
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Old 05-29-15, 10:32 PM
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I'll keep in mind too then the possibility of power wires not being solid, if the issue persists. I think I already checked them as good as I can check them (just general visual and checking for looseness at connections) though, and seemed fine. Thanks for the replies!
 
 

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