Measuring Speaker Ohms

Discussion in 'Archived Threads 2001-2004' started by Richie-C, Jun 26, 2002.

  1. Richie-C

    Richie-C Agent

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    Can someone tell me how to measure the speaker ohms to tell whether or not I have a short in one of my speakers?

    Do I just measure across the +/- posts of the speaker with an ohm meter while driving the speakers to see what ohms I am getting.

    What should the reading look like for a 8 ohm compatible speaker? What would a shorted speaker or wire look like on the ohm meter?

    Thanks

    Rich
     
  2. Chu Gai

    Chu Gai Lead Actor

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    what's happening? you could simply run test tones specific to the driver in question and listen, look for movement. ex a 100 hZ sine wave for the woofer...10 kHz sine wave for the tweeter. the resistance of a speaker depends upon the frequency. an 8 ohm speaker simply means that the nominal rating is 8 ohms over a particular frequency range, say 40-18kHz. there's more to it, but let's deal with your problem.
     
  3. Richie-C

    Richie-C Agent

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    My problem is that my receiver is shutting down (49TX) and the technical service guy seemed to think it might be due to a short or that my speakers are getting down to less than 3 ohms.

    I am not a techie so I am looking for help.
     
  4. John Royster

    John Royster Screenwriter

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    Are you driving the receiver pretty hard in 5 channel or 2 channel? What kind of speakers do you have? Many do go down to 3 ohms or below.
     
  5. Chu Gai

    Chu Gai Lead Actor

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    ok, so what you're saying, is that with your speakers connected, the receiver shuts down, yes? if so, there may well be a short in the cables/speakers. how did you connect your speaker cables, bare wire? examine the connections and the wire itself carefully. Disconnect the cables from the receiver end and make sure there's no stray wire bridging the connections. Do the same for the speaker end. If all is ok, leave the wires disconnected. Power up and try listening with a set of headphones. Still OK? add the speakers one at a time. Keep them in phase now! Let us know!
     
  6. Saurav

    Saurav Cinematographer

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    Do they shut down the moment you hook up your speakers, or only when you're playing them loud?

    I would not put a multimeter across a speaker's binding posts. That would send DC voltage into the speaker, and depending on the crossover inside, could cause some damage to the drivers. Even if it doesn't work, speakers aren't meant to take DC, so it wouldn't be a good thing. Plus, like Chu said, an 8 ohm speaker doesn't necessarily have 8 ohms resistance at DC, the impedance varies depending on the frequency of the signal you feed to it.
     
  7. Chu Gai

    Chu Gai Lead Actor

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    well maybe a D cell battery would be ok, no?
     
  8. Saurav

    Saurav Cinematographer

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    Depends. What's the DC resistance of a voice coil? AFAIK, the impedance of the driver comes mostly from the inductance of the coil. If the speaker has a simple crossover with no series resistors..... It'll probably be fine though... a speaker should be able to handle several amps of current, right?
     
  9. Chu Gai

    Chu Gai Lead Actor

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    So what happened? Inquiring minds want to know.
     
  10. Michael R Price

    Michael R Price Screenwriter

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    A multimeter would put such a small amount of voltage through your speaker that it won't hurt. It will give you the DC resistance of your speaker, though, which is probably close to the minimum point of the impedance curve. For example, the DC resistance of a Kit281 (4 ohm nominal) is 2.8 ohms, which is the minimum impedance at 25 Hz. The impedance of a speaker is the DC resistance plus reactance (combination of inductance, capacitance, and frequency). The woofer usually has a peak at the resonance of the sealed box, or twin peaks surrounding a dip at the resonance of the vented box, with a rising curve due to inductance thereafter. The tweeter has an impedance peak at its Fs, then a rising curve due to inductance as well.

    What speakers do you have? The 49tx might just not be up to the task if you're playing loudly into inefficient 4 ohm loads... especially if they're set to large.
     
  11. Chu Gai

    Chu Gai Lead Actor

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    I think the 49tx is Pioneer's top of the line thx certified receiver putting out somewhere in exess of 100 watts/channel.
     

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