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Topic: JBL 8 ohm speaker actually measures 5.7 ohms. |
Don Griffiths
From:
Steelville, MO
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Posted 11 Nov 2012 5:02 pm |
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I've got an old JBL K145 15" 8ohm speaker here. I was going to list it for sale, but got out the meter and checked the resistance and it actually read 5.7 ohms
on the meter. What are the ramifications of this. Does it mean it's worthless? Thanks for any explanation or input.
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Shobud Pro1,BMI U12, Santa Cruz F, PRS Standard, Fender Twin Reverb, ‘53 000-28 |
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Jerry Overstreet
From:
Louisville Ky
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Posted 11 Nov 2012 5:28 pm
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| There is a wide variance on the DC resistance sometimes. Your reading is not unusual for an 8 ohm speaker. |
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Don Griffiths
From:
Steelville, MO
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Posted 11 Nov 2012 7:20 pm
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Thanks Jerry,
I just spent an hour reading up on some of the old threads on speaker impedance. From what I've read about Twins and JBL's I think I'll try to keep this one to use someday.
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Shobud Pro1,BMI U12, Santa Cruz F, PRS Standard, Fender Twin Reverb, ‘53 000-28 |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 11 Nov 2012 9:52 pm
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5.7 Ohms is not unusual for the DC resistance of a nominal 8 Ohm speaker. In fact, DC resistance of a speaker is virtually always a bit lower than its rated complex impedance.
Complex impedance is the sum of both a load's DC resistance and its AC capacitive and inductive reactance. If a load has both capacitance and inductance, reactance varies with frequency and is non-trivial at some frequcies. Generally, the actual complex impedance of a speaker will only be 8 Ohms at a particular frequency, or perhaps a few frequencies.
Someone posted a graph of typical AC impedance vs. frequency on a thread a few years ago. If you've never seen it before, I think it's worth dredging up. |
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b0b
From:
Cloverdale, CA, USA
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Posted 12 Nov 2012 12:42 am
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Resistance is the opposition to the flow of direct current (DC). Impedance is the opposition to the flow of alternating current AC. Both are measured in ohms. Audio is a form of AC. Your meter measures DC.
The impedance of a speaker is not constant. It changes with frequency and can vary greatly. Manufacturers give you a nominal impedance, which is sort-of the average impedance that the speaker will present to the amplifier when driven within the part of the audio spectrum that its designed to reproduce.
Here's an example that plots the impedance of an "8 ohm" speaker at frequencies from 20 to 10000 Hz. As you can see, the speaker is 8 ohms at just 3 points in the entire audio spectrum.
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 12 Nov 2012 2:44 pm
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Right you are, b0b!
(Of course, this sorta points out that exact impedance matches are important, yet impossible.  ) |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 14 Nov 2012 8:12 pm
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Very nice, b0b  |
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