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View Full Version : hmmm - wrong impedance?



Etac
12-15-2007, 10:25 AM
ok my speakers have been like this for awhile now.. i didnt measure them the day i got em so i dont know this answer for sure..

but they're rated a t 4ohms.. i didnt test em till i ran them for a couple weeks.. but when i measure through the crossover i get 3.5.. and its either.. i maybe burnt them a little? but both speakers are the same.. both woofers measure 3.6 and tweets 3.4 (or might be other way around forget off hand)... and if i did burn them a little what are the odds i burnt them to the same exact thing?

and i have a really really cheap volt meter and maybe that might have something to do with it...

or maybe thats just the way they are?

just say i did burn them a little... they still sound fine.. and as long as i set the gains correctly... should this affect sq that much?

J31Rob
12-15-2007, 10:29 AM
no coil is dead on it's stated impedance (such as 4 ohm, 2 ohm, etc..)

lilmaniac2
12-15-2007, 10:30 AM
Wrong

Some coils are dead on :)

althought most have a different DCR than the rated impedence :p

Etac
12-15-2007, 10:30 AM
so they arent broke your saying right? cause i've run these speakers pretty hard since i first measured em and they havent changed..
and my memphis were dead on but now they're also completely dead =P

jkulysses
12-15-2007, 12:15 PM
Could be your multimeter isn't exactly accurate too though. Some of the cheaper ones can be +/- 1.5 or more.

squeak9798
12-15-2007, 12:51 PM
You are measuring something other than the nominal impedance, as lilmaniac pointed out.

What you are measuring with your DMM is called the Re (or DCR, the terms are essentially interchangeable). The Re (DCR) is the "Direct Current Resistance" of the voice coil. The DCR is relatively static and is, simply put, the resistance of the coil at rest.

The rated nominal impedance is a different beast. Impedance is the opposition to the flow of alternating current and will vary based on things such as the frequency of the signal the speaker is playing. The "rated" nominal impedance of a speaker is basically an average of what load the speaker will present to the amplifier over it's intended operating range. So while it may be "rated" for an nominal impedance of 4ohm, depending on frequency the actual load presented to the amplifier may be higher (or slightly lower) than 4ohm.....and at the speaker's resonant frequency (Fs) the load could be significantly higher than the rated nominal impedance......30ohm or more depending on the speaker. Impedance also increases as frequency increases due to factors such as inductance.

Most 4ohm speakers will have a DCR in the range of 3.0 - 3.8ohm

helotaxi
12-15-2007, 12:52 PM
They're fine. DCR which you're measuring is different (lower) than rated impedance.

Etac
12-15-2007, 01:02 PM
so when i set the gains should i use 3.5 or 4?

squeak9798
12-15-2007, 01:03 PM
Nominal impedance, if you are using the DMM method w/ accompanying formula.

BrianChia
12-15-2007, 04:56 PM
DCR is different from AC impedance. Impedance varies by frequency and the "nominal" impedance is usually rated near the lowest value of the impedance curve and usually given in increments of 2 ohms. DC resistance is typically somewhere close to but less than the rated nominal impedance. Your measurements are typical. Treat the speakers as 4 ohms nominal.

Etac
12-15-2007, 07:03 PM
just for future reference...
if i go active - treat the tweeter and mid both at 4 as well correct? cause if i measure em as i said im gonan get the 3.4 and 3.6 thing

helotaxi
12-16-2007, 12:29 PM
Yes, treat them as 4 ohm. You'll probably find yourself reducing the gain on the tweet amp to balance them out.