Stupid DC resistance question

[Eric Lloyd]

While chasing a problem with a receiver, I decided to check resistance of my speakers and speaker wires. What I got was this:
Realistic T-120: a) 4.2 ohms b) 4.3 ohms
Infinity SM120: a) 3.5 ohms b) 3.6 ohms

These are all three way speakers rated at 8 ohms, and I just put the meter across the input terminals. I understand the reading won't be 8 ohms since I'm only reading DC resistance and not AC. Still, it seems like they all read pretty low compared to what I've seen referenced in other threads for 8 ohm speakers. I'm not too worried about their health as the readings are all pretty close, and other receivers run them all just fine. I'm just puzzled.

The question is this - do these resistance readings seem normal for 8 ohm speaker systems?
Also, if not, what could account for them?

 

[Michael Powers]

Electronics technician here. The test you are performing is meaningless, unless you are seeing an open circuit, a dead short, or readings that vary significantly. Then the reading becomes meaningful. The phrase 'resistance is futile' applies to your situation, although not in the manner it is usually understood.

 

[Eric Lloyd]

My initial thought was simply to verify I didn't have a short so I just unplugged the speaker wires from the back of the receiver and checked resistance, then disconnected the wires from the speakers and checked for a short from one wire to another. Everything was good that way.

The reason for the question is that I usually see people say that their 8 ohm speakers, when probed for DC resistance, come out around 5 - 6.5 ohms. I wondered if my readings indicate anything at all. Otherwise, I'm fairly confident my problem isn't on the speaker end.

 

[woodj]

There's a crossover in the mix.

 

[Eric Lloyd]

Oh, yea. Doh! I wasn't thinking about that at all. Thanks.

 

[nj pheonix]

Resistance and impedance are both measured in ohms. Resistance which is only DC wouldn't come into play here. Impedance is combination of resistance, inductive reactance, and capacitive reactance. The last 2 change with frequency.
So a given impedance will change with the test frequency. While i don't know, i'd imagine there is either a standard test frequency or possibly a composite (possibly weighted) frequency (frequencies?) when speakers get their ratings.
It sounds like your speakers are testing normal.

 

[Eric Lloyd]

Yes, once I was reminded there is a crossover in the way it began to make sense.

 

[whoaru99]

Resistance and impedance are both measured in ohms. Resistance which is only DC wouldn't come into play here. Impedance is combination of resistance, inductive reactance, and capacitive reactance. The last 2 change with frequency.
So a given impedance will change with the test frequency. While i don't know, i'd imagine there is either a standard test frequency or possibly a composite (possibly weighted) frequency (frequencies?) when speakers get their ratings.
It sounds like your speakers are testing normal.

Nominal impedance vs actual lowest impedance is a bit of a crap shoot unless the speaker impedance is rated per IEC method. IEC method requires the minimum impedance be not less than 80% of the nominal/rated impedance anywhere in the response curve. In other words, per IEC method, if the speaker has a nominal rating of 8 ohms then the minimum impedance could not be less than 6.4 ohms. For 4 ohms nominal then not less than 3.2 ohms.

Good example is the Infinity Kappa 9. I think Infinity rates the speaker 4-6 ohms, yet it has two low values of like 0.9 ohms in the impedance curve. 0.9 min is quite different than 4-6 ohms nominal. Under the IEC rating method this speaker would've had to list at nominal impedance not more than 1.125 ohms (probably rounded to 1 ohm nominal impedance).

Unfortunately, I think not that many home audio speakers are rated by IEC method.