Question. Cab reading lower than its rated.

[BlacknWhite]

I'm re tubing my amp today and while I was checking some stuff out I noticed my 4x10 cab which is rated at 8 ohms is reading about 6.2. I know some cabs are a little higher or lower than the rating but I've never had one with a 2 ohm difference. Should I be running my head in the 4 ohm or 8 ohm tap?

 

[ctmullins]

Impedance is an AC measurement, resistance is a DC measurement. It’s normal for the DC resistance to be a couple of ohms lower than the AC impedance.

Additionally, remember that impedance, being an AC measurement, is frequency dependent, so it will be higher or lower at different frequencies. The manufacturer’s specification is nominal.

 

[abarson]

Speakers aren't pure resistance loads but rather reactive, meaning you need to measure resistance (R, ohms units), inductance (L, Henri units) and capacitance (C, Farad units). When you use an ohmmeter to measure a speaker you aren't accounting for inductance, so an 8 ohms speaker will show up as 6.x ohms. Use the 8 ohms tap.

 

[Downunderwonder]

nominal

aka average, aka close enough for jazz, aka one size fits all, number.

 

[Redbrangus]

aka average, aka close enough for jazz, aka one size fits all, number.

Not to quibble, but nominal doesn't mean any of those things. Nominal in this case refers to something being "named"...as in the manufacturer has designated it to be 8 ohms or whatever. I think it's the manufacturer specifying the minimum impedance of the device operated within it's specified frequency range, rounded up to a standard value...I don't believe it's an average of anything.

 

[Warpeg]

Maybe an EE can chime in, but why is there not a standard for this? For example, why not measure a driver’s impedance at 1kHz / 1w or some other standardized methodology?

@agedhorse Do you know how manufacturers go about designating named impedance to drivers?

 

[Redbrangus]

There are standards and industry accepted practice. There is also some room for 'specsmanship' there. I'm sure @agedhorse will contribute as the sun moves westward. But the impedance of a bass driver @ 1kHz wouldn't be a particularly useful yardstick, I don't think.

There's not much reason to sweat it...it's called a nominal number for a reason. It's continuously varying with frequency.

 

[Mr. Foxen]

Driver spec sheets give the DC resistance if you really want to check it is in spec.

 

[micguy]

6.2 Ohms DC resistance sounds very normal for an 8 ohm rated cabinet. It'll be higher at the frequencies you put through it - a lot higher at some low frequencies, but in the upper bass and lower mids, it'll be 8ish. Run it with the 8 ohm tap - you'll be fine.

 

[Rick James]

why not measure a driver’s impedance at 1kHz / 1w or some other standardized methodology?

Look at an impedance chart and you'll see why. speaker impedance curve - Google Search:

That's why the defacto standard for impedance is DC resistance x 1.3, though there's a lot of leeway. You might see the DCR of a driver listed as 8 ohms impedance anywhere between 5 and 7 ohms.

 

[Slevage]

Speakers aren't pure resistance loads but rather reactive, meaning you need to measure resistance (R, ohms units), inductance (L, Henri units) and capacitance (C, Farad units). When you use an ohmmeter to measure a speaker you aren't accounting for inductance, so an 8 ohms speaker will show up as 6.x ohms. Use the 8 ohms tap.

Once you get the L and C, you need to calculate the inductive and capacitive reactance at some frequency, since their values are expressed in ohms.

 

[abarson]

True.
We walk the line between being informative and useful versus TMI.

 

[Redbrangus]

Or is it the fine line between TMI and just plain ol' BS?

 

[agedhorse]

There are standards and industry accepted practice. There is also some room for 'specsmanship' there. I'm sure @agedhorse will contribute as the sun moves westward. But the impedance of a bass driver @ 1kHz wouldn't be a particularly useful yardstick, I don't think.

There's not much reason to sweat it...it's called a nominal number for a reason. It's continuously varying with frequency.

Agreed, and most quality designers insure that there are not significant regions of sub-nominal impedance that could cause issues with amplifiers too. Other designers... not so diligent.

 

[JimmyM]

Did I miss yesterday's "Grossly overcomplicate simple questions" day?



6.2 ohms means 8 ohms. Done.