Impedance Questions

Discussion in 'Speakers' started by JeremyFr, May 14, 2003.

  1. JeremyFr

    JeremyFr Supporting Actor

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    Ok I was browsing through Parts Express today and happened to run accross the same exact drivers that are used in my Energy Take 5+1's the thing that gets me is the Energy Satellites are rated at 8 ohms, yet the driver used in them is either a 6 ohm or 12 ohm driver so how can the make them an 8 ohm speaker I understand seried and parallel with the tweeter and stuff but still mathmatically doesnt make sense to me.
     
  2. Scott Sabin

    Scott Sabin Stunt Coordinator

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    I'm not familiar with the designs of satellites, but I suspect there is a high pass filter in place of what would be a crossover. Those electronics, if they exist, are what actually loads your amplifier. Or, I should more accurately say, they become part of the overall impedance, together with the driver.
     
  3. Chu Gai

    Chu Gai Lead Actor

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    The satellites are a 2 way design. When one looks at a manufacturer's statement that their speakers are 8 ohm or 6 ohm nominal, they're not talking about an absolute number. For example if you were to take an 8 ohm woofer and measure the resistance of the voice coil, you'd get somewhere in the vicinity of 7 ohms resistance.
    Now when a loudspeaker is created, a woofer and tweeter and maybe a midrange are tied together using some sort of crossover. If you were to measure the impedance of a speaker at various frequencies, you'd see that it's not a straight line but can look something like this...
    [c][​IMG][/c]

    The above impedance curve is for the Total Eclipse Speaker from Coincident Technologies (a nice speaker...pricey bastard though!). The manufacturer states that this loudspeaker has a nominal impedance of 14 ohms.

    Manufacturers generally consider the range of 20 Hz to 20 kHz. Within this range, the impedance goes through minimum and maximum values. Usually, the maximum impedance is located is found somewhere in the lower frequency portion, the bass, region. It is directly related to the bass driver resonating. In the curve above, you see two maximums in the bass region. That's because this particular speaker happens to be vented.

    You'll also see a local minimum value of impedance somewhere in the 200-400 Hz area. This is what's called the nominal impedance of the bass driver in your speaker. Moving to the right, the impedance rises for a variety of reasons and then you'll see another local maximum that occurs around the crossover point. It's more complicated with 3 or 4 way speakers that have intricate crossovers and likely you'll find that there are minimum impedancies that are below any of the drivers.

    You'd think that when a manufacturer is specifying his loudspeaker as being 8 ohms that he's conforming to some sort of standard (like rms continuous power) that all manufacturers conform to. Well that's not the case as far as I know. What many speaker manufacturers do is they'll call the nominal impedance of the speaker the mimimum impedance in a given frequency range that they consider matters. However, each manufacturer has their own idea of what matters. If we look at what the energy content that's present in most music, we'll find more energy in 100-500 Hz as opposed to 5-15 kHz. So most manufacturers will specify the nominal impedance as either the minimum or the average impedance in the lower frequency ranges mentioned.

    Nonetheless, the area of 5-15 kHz is an important one. Not so much because the impedance is generally low. It's because there are some receivers and amps for that matter, that have issues with being able to cleanly reproduce energy content in that area. They may be prone to distortions or instabilities such as oscilations. This is one of the reasons that I favor, as a matter of preference, amps that are both powerful and have specified performance ratings into the lower ohms. A brief perusal of a speaker's impedance curve can shed some light into what sort of amplifiers they should be considering for their short list.

    So there is no standard impedance, only nominal. Hopefully this helps a little.
     
  4. JeremyFr

    JeremyFr Supporting Actor

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    Yeah that makes complete sense it just makes me wonder sometimes how manufactures come up with some of there ratings. Oh well life goes on, the other thing I'm trying to figure out is since P.E. has them so cheap I've though about buying say 2 of em just in case I ever need to replace one but I dont know whether I should get the 6 Ohm units or 12 ohm units.
     
  5. dean_g

    dean_g Agent

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    Good post up there.

    Something else to consider is that you are also looking at the driver impedance spec isolated from the rest of the system. Woofers and tweeters are run together, along with a crossover that is tweaked to iron out any irregularities in the total circuit.
     
  6. Chu Gai

    Chu Gai Lead Actor

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    One would hope [​IMG]
     
  7. Jason GT

    Jason GT Second Unit

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    Chu, that is a fantastic post, and if I daresay to the mods, something worth archiving.

    Thanks.
     

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