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Speaker impedance figures don't make sense to me.

Discussion in 'Amps and Cabs [BG]' started by OzzyGreg, Dec 26, 2011.

  1. .
    I'm a little confused here guys....

    Referring to this : Eminence Kappa Lite 3015LF Speakers - Eminence Kappa Lite 3015LF lightweight neodymium woofer. The Eminence Kappa Lite 3015LF is a 15 inch 8 ohm lightweight speaker with extended Xmax. The Eminence Kappa Lite 3015LF 15inch speakers are rated at 450 w

    Eminence states that their Kappalite 3015LF speaker is 8 ohm impedence.

    Yet further down the list it says impedance is 5.31 ohms. I realise that this is the RE resistance, not "actual" resistance (whatever that is!), which is apparently the electrical resistance of voice coil which can be measured across the two terminals with a multimeter I guess.

    I have a new 3015LF here and I've just measured the resistance across the terminals and I get a figure of between 6 and 6.8 ohms, depending on how I wriggle the probes on the terminals.

    Where does Eminence's 8 ohm figure come from?

  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Impedance and DC resistance are not the same thing. Impedance is DCR plus inductive reactance plus capacitive reactance.
  3. f.clef

    f.clef Supporting Member

    Dec 4, 2007
    Yes, OP, it is complicated, with many intersecting curves, like one of those Newtonian 3+ body problems -- calculable, but by whom, and under what caveats? And what does the number mean in the end, except, of course, that you don't want to fry your business?

    There's so much on TB about this. (My personal thanks to Andy from Genz-Benz for his guidance some months ago.) Good lucK!
  4. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    From doing an impedance sweep measurement. Look at the graph on the spec sheet. ;)
  5. Jazz Ad

    Jazz Ad Mi la ré sol

    It's not really complicated. All it involves is basic trigonometry.
    Impedance is the square root of the sum of the squares of impedance and inductive reactance.

  6. Pilgrim

    Pilgrim Supporting Member

    It is Ohm's law. You do not have to understand it, but you must OBEY it. :smug:
  7. skychief


    Apr 27, 2011
    South Bay
    and to make things even more confusing, any given speaker's impedance will vary depending on the frequency of the signal.

    Thats why speakers are generally rated for a "nominal" imppedance-- usually derived from 1KHz reference signal.
  8. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    A sweep will show impedance varying from roughly DCR to ten times that and more, depending on frequency. Rated impedance is a nominal figure, more or less arbitrarily pegged at approximately DCR x 1.4.
  9. Pilgrim

    Pilgrim Supporting Member

    And to help in understanding the comments above, please review the definition of "Nominal"...it has numerous meanings, but these are most appropriate:

    a : existing or being something in name or form only
    b : of, being, or relating to a designated or theoretical size that may vary from the actual : approximate

    In other words, you gotta hang a number on it but that's not the end of the story.
  10. skychief


    Apr 27, 2011
    South Bay
    I think in relation to speakers, 'b' would be the best definition.

    A speaker driver may present 4.1 ohms Z @ 1Khz. That same driver may be 3.6 ohms @ 40Hz and may go as high as 16 ohms @ 8KHz!

    But its rated as a 4-ohm (nominal).
  11. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    A 4 ohm speaker may range from 2.8 ohms to 100 ohms or more. What really matters is minimum Z, as that's what amp designers have to consider when calculating the current capacity of the amp.
  12. Growly Lytes

    Growly Lytes

    Dec 4, 2009
    Downunder Oz
    Bass player
    Aged Horse is the man when it comes to knowing his stuff.
    He also helped me when i really needed it & lucky me that he did,i could have damaged my speakers if he didnt contact me & help out.
    Legend !
    Tip The Hat To Andy.
  13. yes ... I feel like the answer to life itself could be found on this thread, if only we had the capacity to understand. :ninja::p:bassist:
  14. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Rating speaker impedance is difficult because it's not just the driver but the actions (or reactions) of the box and air mass on the moving speaker that defines the impedance curve. Impedance varies with frequency because of the (primarily) inductive and resistive nature of a loudspeaker motor. The nominal and minimum impedance really only matter when considering the entire speaker as a system. (including x-over when present)

    The resistance portion of the impedance curve is essentially the DC resistance of the ~hundreds of feet of wire that makes up the voice coil. The inductive portion comes from the fact that it's a coil of wire wrapped around a steel core (inductor), plus the "back EMF" (or generative effect as the core is magnetic too) that the acoustic loading presents. In general, the minimum impedance and DC resistance are very close BUT under some conditions the minimum impedance can fall below the DC resistance when the reactive components (which represent stored energy phase shifted from the initial signal) are aligned right. This is also responsable for the curious condition where there can be 0 volts but current flowing... which appears to break Ohm's Law. In actuality, it doesn't when the equation is solved using both the real (resistance) and imaginary (reactance) terms.

    The definition of nominal impedance is not at all standardized, and there are several viable ways to calculate it depending on what you are trying to do with the numbers. There is the average impedance over the design or operating bandwidth, which for bass is ~35Hz to 5kHz, there is also the weighted average (which I typically use) which assigns a higher weighting to the areas of the bandwidth with the highest statistical energy. For eample, it's relatively unimportant whatthe impedance is from 3kHz-5kHz because there is so little energy present there but in the 50-500Hz area, this is responsable for probably ~80% of the energy in a typical bass signal. So the first decade is weighted much heavier when coming up with the average. There are different weighting algorithems but the idea is the same.

    We do exactly the same thing when designing amplifiers, understanding the energy-bandwidth profile of the signal helps understand the stresses that an amplifier may encounter in the real world... relative to typical impedance curves versus frequency.

    Hope this helps.
  15. More like LaPlace than Ohm.
  16. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    the caveman answer is that the DC reading off your meter will always be a bit below the nominal rating.

    an "8Ω" speaker might read 6 or 7 ohms, a "16Ω" might read 13Ω.
  17. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    well, i've often hear the expression that 'there's no LaPlace like Ohm'. ;)
  18. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Ouch, that hurt ;)
  19. teemuk


    Mar 1, 2011
    If I remember right, there was a standard defining that nominal impedance may differ from the lowest measured impedance about 20%, but newer to lower side. So, the manufacturers basically just pick the next roundest number (4, 8, 16 ohms etc) above the DC resistance and call it a day. There's a reason why the rating is called "nominal impedance".
  20. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    I don't remember any standard nominal impedance definition. In practice, it generally does end up that we use the rounded up from the lowest impedance and it works out the same.

    Note that not all manufacturers are reasonable with their nominal values, though most are. I had a pretty serious situation occur with a manufacturer (of PA speakers) where the nomimal impedance was almost 50% greater than the minimum impedance AND the minimum impedance was a significant percentage of the tranducer's weighted bandwidth. Cost my customer some blown speakers and amps (speakers because he was inadvertantly overpowering, and amps because the "nominal" 2 ohm load had a wide minimum (with reactance) of about 1.2 ohms). The manufacturer claimed I had no idea what I was talking about, the amp manufacturer got just about the same treatment, the customer got screwed, and the speaker manufacturer is no longer an accepted product on any PA installation project that I design. After they discovered that they were no longer a qualified vendor, they approached me to try and make up. They refused to revise their specs. (probably because they were sitting on a huge inventory of components) and I refused to revise my qualification. Some of these projects are fairly substantial, but they wouldn't budge, they just bitched a lot and called me names ;)

    So, buyer beware. This one really surprised me and caught me off guard, cost me a lot of troubleshooting time and the amp manufacturer ended up eating some too since initially we were focusing on an amp problem.

    Names are not being disclosed for professional non-disclosure reasons.

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