One of the questions that I often get asked about is the proper use of the extension cabinet speaker output, so this seems like a good time to explain it and I’ll start with the speaker impedance.
First you need to verify the impedance of your speakers, both the internal speaker if you have a combo as well as the extension cabinet that you plan to use. If it’s not marked on the speaker or you’re not sure that the listed impedance is correct, all you need is a multi-meter to measure it. You will need to keep in mind that a static speaker, or one that’s not being powered by an amp, would only show about 75% of the speaker’s actual impedance. For example, an 8 ohm speaker may show something like 6.2 ohms while a 16 ohm speaker may only measure at 12.6 ohms.
Figuring out your total speaker load is fairly easy to do and here’s a couple of examples which will help explain the math to you. For example, when you wire speakers in parallel you’ll find that two 8 ohm speakers in parallel would be a 4 ohm total load (8 ohms + 8 ohms = 4 ohms in parallel). In the same manner two 16 ohm speakers in parallel would be a 8 ohm total load (16 ohms + 16 ohms = 8 ohms in parallel).
When you use series wiring on the speakers it would look like this. When you wire speakers in series you’ll find that two 4 ohm speakers in series would be a 8 ohm total load (4 ohms + 4 ohms = 8 ohms in series). In the same manner two 8 ohm speakers in series would be a 16 ohm total load (8 ohms + 8 ohms = 16 ohms in series).
Then you’ll often find series/parallel wiring in multi-speaker cabinets like a 4×12, and in that configuration each pair may be wired in parallel, and then each pair wired in series to arrive at the necessary speaker load.