Dumb question

[drbassman]

How do test a speaker cabinet determine its ohm rating?

[the mojo hobo]

Use a multimeter on the ohms setting and round up. An 8 ohm speaker will generally read 6 some ohms.

[drbassman]

Use a multimeter on the ohms setting and round up. An 8 ohm speaker will generally read 6 some ohms.

On the 4x10cab do I need to pop it open and test each speaker or can I test the entire cab off a 1/4" cord?

Found the answer on Google.  I can test the entire cab.  I am getting some high quality 1/4" amp-speaker cables later this week and will give it a spin.  Got a couple meters, so I'm set.

[Granny Gremlin]

If the goal is to get the load of the entire cab, then just use the multimeter off the main output jack (or a patch cable plugged into same), yeah.  If you want the rating of each individual driver (and they're not labelled) then you gotta open her up or pop the drivers out to get to the terminals (you will also need to disconnect them from the wiring in order to ensure that you are measuring just that driver not it's neighbors as well).

Basically the actual rated (nominal aka weighted average) Impedance (varies with Frequency) of the cab is the next 'standard' rating up (2, 4, 8, 16.... a few hifi speakers, mostly tweeters, but sometimes whole cabs, are 6, but most people treat these as 8... also a couple Traynor cabs - the 6x10  IIRC) because the meter measures only DC R(esistance) which is just the largest component of Impedance, but not all of it (voice coils are coils and therefore have inductance which is reactive; see above re varying with frequency).

As an example, an 8 Ohm cab may measure at anywhere from 4.x to 7.x Ohms DCR.  Modern drivers tend to be in the middle of this range (6+/-); vintage ones are all over the place.

[drbassman]

So logically, a 4 ohm cab will probably measure somewhere south of 4 ohms.  Thanks guys.

[Granny Gremlin]

Yep.

[jumbodbassman]

and that is safe with old tube amps?

and what about new SS amps that are no less than 4 ohm rated.  If a cab rates out at 3.2 is it safe???

[Granny Gremlin]

See above regarding speaker load ratings vs reality.  No speaker is actually 8 ohms (all the time); it is an agreed upon convention in that the load rating is a weighted average of the speaker's impedance over the designated useful frequency range of that speaker.

see this example of a (so called) 8 Ohm woofer's Impedance chart:



Note how the speaker is only actually 8 ohms (or close to it) for a very small part of it's useful range (only actually = 8 Ohms at 2 specific frequencies: 90-something and about 3500).  The peak in the bass region corresponds to the speaker's resonant frequency which, in turn represents (roughly) the lower limit of that speaker's ability to reproduce sound at useful levels.  The slow rise in the top end similarly shows where this woofer stops being efficient and should be crossed over with a midrange or tweeter to maintain an even frequency response throughout the entire audible spectrum (and now you know why modern hifi speakers tend to be 2 ways with a crossover point between 4-6K).

This is the way it's always been, tube era or solid state.  All amps can handle transient spikes (or dips, but the spikes are larger) in impedance caused by the reactance (inductance) of the speaker's voice coil to the frequency of the input signal.  Close enough for rock n roll sort of thing - the speaker is about 8 ohms on average.  You get into trouble when the speaker's impedance is significantly higher/lower (as the case may be - tube vs ss) on average, not in a specific nanosecond when there's a hihat hit or a kick beat.  ... and even then, a decent tube head rated for 8 ohms will take 32 for a while before it dies (been there; done that.... always check you buddy's DIY wiring jobs before turning stuff on.... especially if he's an engineer).