Measuring speaker impedance

Rotten

Gold Supporting Member
Messages
6,753
I'm a player, not a technical person, but just bought a multimeter and started testing out some speakers. My 8 ohm Jensen C10n from the '60s came in at 6.6. My 8 ohm Eminence GB128 from last month came in at 7.4. My 16 ohm JBL from the early '70s came in at 5.3! Am I doing something wrong?
 

Tomm Williams

Member
Messages
964
No you're fine. I don't think I ever tested a speaker and had it come out exactly at its rating, it's always been lower in every case I'm aware of.
In regards to the JBL, they produced a number of D series (maybe others?) that were all marked 16 ohms but we're actually 8. Or as you found, even less on a meter.
 
Messages
826
The DC resistance of a speaker is always lower than the nominal AC impedance. For an AC signal the speaker has inductance (and believe it or not can even look capacitive at some frequencies due to mechanical effects).
 

swiveltung

Member
Messages
14,483
5.5 to 8 ohm is about where most everything comes in on an 8 ohm speaker. Your JBL is an 8 ohm though not 16 ohm. JBL did some funny labeling for a while, some speakers actually said 8/16 ohm on the label. Some said 16 ohm and were 8. Go figure. A 16 ohm speaker will measure at least 12 ohms.
 

PushedGlass

Member
Messages
888
The various responders are dead on. Generally it's not useful to refer to speakers' DC resistance because they ordinarily won't be passing DC in a circuit. But referring to an AC impedance implies that you know what frequency AC you're using to measure, which doesn't make any sense unless you arbitrarily pick one - and if you do (e.g. 1KHz) hardly going to be representative of what you'd be running through the speaker. And even if you did that I doubt the measured impedance would stay the same at all amplitudes. So it's a wishy-washy thing no matter what.

But that wishy-washyness and the nature of this electronic power circuit that ends in something mechanical is part of what makes it all so interesting and cool. I'm intrigued at the notion of having speaker power reflected back to the output tubes even by things like the edges of the enclosure or the air in the cab.
 

VaughnC

Silver Supporting Member
Messages
19,041
Yup, AC "sees" the speaker coil different than a DC reading of a meter so impedance can't be read directly. So the DCR usually ends up being about 80% of the actual impedance.

Makes me wish that the pioneers of electronics had only called resistance "ohms"...and called impedance something different like "homs" to keep the confusion down ;).
 

Kyle B

Member
Messages
5,290
Yup, AC "sees" the speaker coil different than a DC reading of a meter so impedance can't be read directly. So the DCR usually ends up being about 80% of the actual impedance.

Makes me wish that the pioneers of electronics had only called resistance "ohms"...and called impedance something different like "homs" to keep the confusion down ;).
Well, they did differentiate 'em with "R" and "Z"... that's not enough for ya, eh? ;)
 




Trending Topics

Top Bottom