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Why Do 8 Ohm Speakers Measure 6 Ohms?

Discussion in 'Amp Tech Center' started by DennisM, Nov 11, 2020.

  1. DennisM

    DennisM Tele-Meister

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    I've measured quite a few 8 ohm speakers. The highest was 6.8 and the lowest was 6 ohms. 4 ohm speakers usually measure average 3.6 ohms. Never measured a 16 ohm. Is there a reason for 8 ohm ones not to measure closer to 8?
     
  2. Peegoo

    Peegoo Poster Extraordinaire

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    The Ohms rating of a speaker is a measure of DC resistance. In practice, however, the speaker uses AC (not DC) and the resistance to AC is characterized as impedance...which is similar to DC resistance but is not easy to read because impedance varies with frequency. And as you know, signals handled by a guitar speaker can swing anywhere from around 50Hz to around 7kHz. The Ohms rating is an average impedance based on the physical makeup of the speaker's voice coil.
     
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  3. Jon Snell

    Jon Snell Tele-Holic

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    An 8 Ohms speaker is the average AC impedance at any given frequency.
    Impedance is the AC resistance.
    You are measuring with a DC meter and measure the DC resistance of the speech coil, not the impedance of the speech coil.
    Hope that helps.

    Have a google about loudspeaker impedance and coils in general. Very interesting but totally confusing at first.
     
  4. elpico

    elpico Tele-Afflicted

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    It's not the average impedance that would be quite a bit higher than 8ohms.

    It's the "nominal" impedance, which in this context roughly means "the imaginary impedance".

    Nominal
    adjective

    - in name or thought but not in fact or not as things really are
    - used to describe something that is said to be a particular thing, but is not actually that thing

    The impedance doesn't have one value, it's always changing, but you gotta write something on the label. Labeling is what this is all about. Both mentally and literally you need something to tag the thing with to make sure you're picking up the one you want. So for whatever historical reason they decided to take the lowest AC impedance the speaker presents, round it off to an easy single digit number, and slapped that on the label. It doesn't tell you much about the actual impedance of the speaker, but it does tell you that you've picked two of the same kind out of the cupboard, or that one is in a general ballpark
     
    Last edited: Nov 12, 2020
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  5. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Above the resonant peak and below the rise.
     
  6. DougM

    DougM Poster Extraordinaire

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    The rated impedance is NOT the lowest measured at any frequency. It is the average at all frequencies over the range of the speaker. If you look at measurements of stereo speakers at places like Stereophile and Soundstage, you'll often see that a speaker rated at 8 ohm may measure as low as 4 ohms at some frequencies, particularly in the bass region. I have no reason to believe that isn't also the case for guitar speakers.
     
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  7. schmee

    schmee Doctor of Teleocity

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    Some speakers actually measure pretty much 8 ohms, especially old Jensens. Some not though too.
     
  8. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    [​IMG]
     
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  9. DennisM

    DennisM Tele-Meister

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    Holy Smokes!! Way deeper than I thought....confusing too. Way above my pay grade. Youz guys are smart, and thanks for the answers!
     
  10. elpico

    elpico Tele-Afflicted

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    I'm talking about guitar speakers, not the combined impedance of a crossover network and multiple drivers that you'd find in a stereophile plot.

    In the context of an individual guitar speaker, nominal impedance isn't either of the things you mentioned. It's not the impedance at all frequencies averaged together and it's not the lowest impedance either. (that's sometimes listed as well but it's called "minimum" impedance)

    As I said before, the nominal impedance is the labelled impedance. That's it. It's a number, but it's really a name. There's no law that says every manufacturer has to do things the same way so about the most you can say is that usually the manufacturer will look at the lowest impedance above resonance and label the speaker whichever one of the standard names comes closest.

    Impedance after resonance is 6.6ohms? Let call this thing an 8.

    13.2ohms? Well close enough, we'll call that one a 16.

    I'm attaching page 200 from Stark's "Live Sound Reinforcement":

    [​IMG]
     
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  11. Richie Cunningham

    Richie Cunningham Tele-Afflicted

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    They pick a simple number to make the math easy, otherwise we’d be dealing with complex numbers and phasors all the time.
     
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  12. CV Jee Beez

    CV Jee Beez Tele-Holic

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    It looks like the industry just decided in terms of creating standard for what they would produce and then used a tolerance type of approach +/- 2.

    So 8 ohm could exist in a range of 6-10K, 16 ohm in a range of 14-18K, etc.

    I don't think I've ever run into an 8 ohm speaker that read 8.00K on my multimeter.
     
  13. vanr

    vanr Tele-Afflicted

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    In the old days it wasn't 4, 8, and 16 it was 3, 7 and 15
     
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  14. Bill Moore

    Bill Moore Tele-Afflicted

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    If you think it's confusing, I had a D120 labeled 8-16 ohm! I bought the speaker cheap, as the fellow said it was going open, and measured 22 ohms DCR. When I got it measured 19 ohms DCR, and I hooked it to my stereo for a day, retested to 12 ohms DCR. I put it in my 5E3, and it sounded glorious, but really made the little amp too heavy. I installed a Weber 12A125A that's way easier to carry.
    I later sold that speaker to a fellow looking for D120s for a 4-12 cab, it measured 10 ohms DCR the day I sold it!
     
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  15. printer2

    printer2 Poster Extraordinaire

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    There is the resistance of the wire 6 ohms, and the wire is wrapped up into a coil. The coil acts as a transformer of sorts. But instead of having two winding and the magnetic flux produced in the driven coil transferred to the secondary coil (by magnetism) the speaker coil induces a opposite current into itself. Rather than having two curents flowing in opposite directions in the same wire the induced current cancels out some of the current we are driving into the coil (lost you yet?)

    So rather than seeming like 6 ohms, which is what the resistance of the wire is, the voltage from the amp only pushes as much current through as if it was an 8 ohm piece of straight wire. As you go up in frequency the coupling is better, more induced current canceling out the current our amp is trying to put into the wire. So we have less current flowing as we go up in frequency, the coil acts like a higher resistance piece of wire as we keep going up (up to a point but that is way higher than our audio frequency spectrum).

    Which is fine when we start in frequency where the speaker is about 8 ohms in the 200 Hz area, why does it rise in impedance up to the resonant frequency? Rather than the coil self inducing an opposite current in itself you have the speaker cone attached to it. The coil moves the speaker diaphragm and we have the mechanical resonance of the cone and suspension mucking things up. At the resonant frequency of an object it is relatively easy to get it to vibrate. Which means we require less current to flow to get it to flop around. Now the fun part. Because we have the coil in a magnetic field, when we move the coil in it, the magnetism causes an opposite voltage to be developed against the current the amp wants to put into it. But because the speaker is floppier around resonance the smaller current we have flowing in the coil still produces a fair amount of movement.

    That is the two bit, taking shortcut explanation why the impedance goes from 6 ohms (dc through the speaker) rises at the resonant frequency, drops down in the 200-400 Hz range, rises again. Why 8 ohms? They design the speaker to more or less fall there in the 'midrange' of the speaker usable range. Below this the impedance goes up but the speaker resonance helps out with the sound output, where the impedance rises the speaker starts becoming more directional and the effective sound output remains sort of average. That is untill the speaker cone mass makes it too slow to keep up. Much more than I wanted to type at this hour.
     
  16. Jay Jernigan

    Jay Jernigan Tele-Afflicted

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    I have a D-130 in a Vibroverb that is labeled 8-16 ohms. Been working for years. Never measured it.
     
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