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Speaker impedence verses amp impedence

Discussion in 'Amps and Cabs [BG]' started by Tedward, Oct 22, 2013.

  1. Knowing that if a speaker is rated at 8 ohms it is actually less. Say for example a twelve inch rated at 8 ohms when measured with a meter is 5.04 ohms. Now two in parallel is 2.52 ohms. Is that too low for an 4 ohm load from the amp?

  2. I believe you are misinterpreting your meter. From what I remember, meters don't read exact impedance, but some spec which must be translated into impedance.

    Hopefully an EE will pop into the thread to provide the details, but that reading, from my memory of other threads, is typical of an 8ohm nominal impedance cab.
  3. bongomania

    bongomania Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Right, an ohm meter does NOT read the impedance of the speaker/cab. A nominally 8 ohm cab will read around 5 or 6 ohms, and a nominally 4 ohm cab will read around 3 ohms, BUT those readings are false, they do not represent the load the amp will see! So do not do any math based on those ohm meter readings.
  4. Thanks! I knew I recognized the reading as 'typical' for an actual 8ohm nominal impedance cab/driver.
  5. Okay, here comes the nerdy explanation:

    Your speaker impedance consists of 2 components. The first is the DC resistance (resistance to direct current), which is a fixed value, measured in ohms, and is typically slightly less than the speakers nominal impedance. This is what your meter is reading.

    The second component is the speakers additional resistance to alternating current (AC). For a speaker, this is almost entirely due to the speaker's inductance. It is dependent upon frequency!

    Add these two components together and you get the speaker's impedance. It's the fixed DC resistance plus the inductor's component.

    The speaker's impedance therefore is frequency related. It can dip lower than the nominal impedance at a couple of frequencies, but often is many times higher. Go to a speaker site such as Eminence and look at their detailed data sheet and you will see curves showing the impedance versus frequency.
  6. Impedance is resistance with inductance and a touch of capacitance in an AC circuit like a speaker is driven with. You're measuring static DC resistance with your meter. The nominal impedance of a speaker is generally 1.3 times the DC resistance and represents an average impedance over a range of frequencies.
  7. Exactly you have inductive, reactive and capacitive components in an AC signal, your not dealing with DC voltage as input.
  8. Wighat's is the somewhat less nerdy explanation. Here's the least nerdy explanation: The speaker impedance is higher in actual use than the meter measures when it's sitting still, so don't worry about the measured impedance.
  9. jnewmark

    jnewmark Just wanna play the groove. Supporting Member

    Aug 31, 2006
    Stax 1966
    Third St. Cigar Records staff musician.
    I had a no name, plain old speaker in an old Silvertone guitar amp. I took it out and meaured it with my multi-meter. It came out dead on 4 ohms. I borrowed another multi meter, with the same results. So, was this a 4 ohm speaker ?
  10. Correction done as you can not measure "impedance" with the typical hand held meter. :)
  11. No
  12. Nope. You could make two "guesses" from that. 1) It is a 6 ohm speaker. 2) It is an 8 ohm speaker with a damaged voice coil.
    Or buy one of these http://www.parts-express.com/pe/showdetl.cfm?partnumber=390-827 and get a more acurate answer. 390-827_m.

    Also amps do not have output "impedance" of their own, what impedance they can drive is done in the design engineering.
  13. will33


    May 22, 2006
    If you look at the impedance curve on a speakers datasheet, you will see that the DC resistance reading you're getting will be somewhat close to the lowest points along the curve. That means the amp won't see less than that at any frequency, but will see higher than that at many frequencies.
  14. Wow, thanks guys. So' I'm safe with the two 12" in parallel.

  15. If those 12" are rated at 8 ohms and your amp will go down to 4 ohms (two 8 ohm speakers make a 4 ohm load in parallel) you are safe.

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