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Thread: How do you figure out a speakers impedance?

  1. #1
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    How do you figure out a speakers impedance?

    Hi All!
    I have a couple of speakers and need to find out what their impedances are.
    I tried to test them with an ohm meter but get no reading. I do know the
    speakers work.

    Anyone know?

    Thanks much!

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    you will be reading the dc resistance of the coil, the impedance (AC) will be slightly higher. An 8 ohm speaker (in my experience) reads around 6 to 7 ohms DC. If you know the speaker works (tested) and are reading zero (open) you might want to try a different meter.
    Good Luck

  3. #3
    Supporting Member Jazz P Bass's Avatar
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    Speaker Impedance

    For the curious.
    Plane Jane explanation of speaker impedance.
    Link:A New Way to Plot Speaker Impedance: the Smith Chart Secrets of Home Theater and High Fidelity

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    Quote Originally Posted by rudutch View Post
    you will be reading the dc resistance of the coil, the impedance (AC) will be slightly higher. An 8 ohm speaker (in my experience) reads around 6 to 7 ohms DC. If you know the speaker works (tested) and are reading zero (open) you might want to try a different meter.
    Good Luck
    Thanks Rudutch!

    I tried measuring using the DCV setting on my multimeter
    and I didn't get a reading. However I do get a reading on the ohm setting on 200. Is this the meter setting that I should be using?

    On a known 15ohm speaker (Vox Blue) I get a 11.4 reading. On the two speaker with unknown ohms I get a 14.4 reading and 08.0. By the way, the two speakers with unknown impedance are British made Elac speakers from the eary '60s...one 10" (14.4) and an 8" (08.0). BTW, I'm using a new digital Radio Shack multimeter.

  5. #5
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    Hi mrbassman.
    The DCV measurement will get you nowhere, in this case.
    The resistance reading, in this case your 200 ohm scale, is the one.
    There is something else: your cables and test prods have their own resistance, which has to be substracted from what you read,.
    1)Touch the Black and Red proves together, you shall get a low but not 0 reading, say, 0.5 ohms. Remember it.
    2)measure your speaker, say, 14.4.
    Substract those 0.5=13.9 .
    The impedance is conventionally 20% more, so 13.9x1.2= 16.68 or 16 ohms.
    your "8" reading becomes 7.5x1.2=9 ohms. I should call it an 8 ohm unit.
    That substracting the 0.5 "hidden" ohms doesn't look much on high impedance speakers, but becomes quite important on 4 or 2 ohm ones.

  6. #6
    Noodle of Reality Steve Conner's Avatar
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    Quote Originally Posted by mrbassman View Post
    On the two speaker with unknown ohms I get a 14.4 reading and 08.0.
    Then they're 16 and 8 ohms nominally.

    I think using Smith charts for speaker impedance just confuses things even further. Smith charts are for RF, and the point of them is that they let you quickly calculate how a length of transmission line will transform a mismatched load.

    There are no transmission line effects at audio frequencies, so you never need to do this.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  7. #7
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    Quote Originally Posted by J M Fahey View Post
    Hi mrbassman.
    The DCV measurement will get you nowhere, in this case.
    The resistance reading, in this case your 200 ohm scale, is the one.
    There is something else: your cables and test prods have their own resistance, which has to be substracted from what you read,.
    1)Touch the Black and Red proves together, you shall get a low but not 0 reading, say, 0.5 ohms. Remember it.
    2)measure your speaker, say, 14.4.
    Substract those 0.5=13.9 .
    The impedance is conventionally 20% more, so 13.9x1.2= 16.68 or 16 ohms.
    your "8" reading becomes 7.5x1.2=9 ohms. I should call it an 8 ohm unit.
    That substracting the 0.5 "hidden" ohms doesn't look much on high impedance speakers, but becomes quite important on 4 or 2 ohm ones.
    Thanks much JM! I really appreciate the input and the schooling!

  8. #8
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    Hi Steve, I agree about Smith charts and add: most speakers publish frequency response curves, a lot do impedance curves and almost nobody phase curves.
    These are *very* important, even more than impedance ones, to be able to design proper protection circuits.
    The only ones that care about that seem to be the big PA amp and box makers, for obvious reasons.
    Also those nerdy British Hi Fi designers seem to care. , which is clearly seen by their overly complicated, phase-correcting crossover networks.
    American Hi Fi designers seem to be fully unaware of that, considering their simple crossovers, going all the way down to a lonely 2u2 x 250V capacitor hot glued to a piece of Masonite.

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