8 ohm speakers tested lower...

[Addam]

I figured I would ask the people here before any where because, in all honest, I'd take the word over most of you to the people at the guitar store trying to make a quick buck.  Anyways...

I thought my cabinet was sounding a little wierd the other day at band practice so I decided to stop using it for that day and take a look at the insides.  I tested each speaker with a meter.  Each 12" speaker is rated at 8 ohms, however they all, when tested, came in anywhere from 6 ohms flat to about 6.7 ohms at the highest.  Is this normal or have I done something bad?

Also, when I tested 3 of my Sunn 10" transducers (again 8 ohms each) they all tested at about 7 to 7.6 ohms.  This lead me to believe that I may have done some damage to my 12" speakers since the 10" speakers rated higher, even though both are technically 8 ohm speakers.

After a bit of research I've come to the conclusion that speaker companies often don't put the exact ohmage(?) label on the speaker, but are the test ratings I got normal?

I hope this made sense and was clear enough to get some answers.  Thanks for your help!

[mckinnon audio]

  Hi there,this is a common misunderstanding about impeadance/resistance.When you measure the spkr.with a meter you're using D.C. volts because of the battery in the meter.When the spkr.is in use,your amp is putting A.C. volts to the spkr.Resistance in an A.C. circuit is called impeadance.The resistance value is typically 2/3'rds of the impeadance value.There are lots of threads on this topic on this forum.Hope this is of some help,Mel.

[pickinatit]

My understanding is that the standard Impedance designations of
2, 4, 8 & 16 ohms  are actually  NOMINAL quantifiers (in other words there is a +/- factor between the nominal and actual ).  From what I can gather from the many, many threads I've read on this subject, very few 8 ohm speakers will actually generate an 8 ohm reading and the "actual" readings are typically a little lower the 8.
Do I have that right guys ?

[Isaac]

Take this example. It's the frequency response and impedance curve of the Eminence Delta-12B:

The black line is frequency response, the gray is impedance.

Note how it starts out low, rises quickly to a peak around 57-58Hz, falls rapidly to a low around 250Hz, then rises again through the rest of the audible range. This is very typical shape. The actual values and frequencies will change, but the shape is usually a lot like this.

That first peak is the resonant frequency, or Fs. The low is what is often called minimum impedance, in this case around 14 ohms. From this graph, I'd say without hesitation that this is a 16 ohm driver, even though it's at 16 ohms in only two places. The point here is that the DC resistance, the nominal impedance, and the actual impedance can be very different things, and that yes, your DC measurements are normal for 8 ohm drivers.

[Soundmasterg]

The rating that speaker companies rate their speakers with is AC impedance as the others have mentioned. This changes with frequency, so when you are measuring with your meter you are checking the DC resistance, which will give you a reading a bit lower than the number on the speaker. So an 8 ohm speaker will measure out around 6 ohms when checked with DC resistance.

Greg

[Addam]

Many thanks everyone.