It is crucial to match your speaker impedance (ohms) with your amplifier’s output impedance. Speaker impedance varies with frequency, so it is possible to approach dangerous conditions for your amplifier with an incorrect impedance load. A lot of amplifiers have multiple taps to accept various cabinet/speaker impedances. On a solid-state amplifier, you gain power by lower speaker impedance. On a tube amplifier, you can safely connect a variety of different cabinets or speaker configurations. Plus, the various taps on a tube amplifier may give you a subtle difference in sonic quality. We recommend that you never connect a different speaker or cabinet impedance than what is listed on your amplifier without checking with the manufacturer to determine if it is safe.
When you see a certain “ohm” specified on a speaker, it is actually a nominal reading. An ohm/multi- meter measures the resistance to direct current (DC), but music is alternating current (AC). A speaker’s impedance to AC depends on frequency and the nominal rated impedance is sort of an average impedance over the useable frequency range of the speaker. You will find that an 8 ohm speaker will measure between 5.1 and 8 ohms, a 16 ohm speaker will measure between 11 and 16 ohms, and a 4 ohm speaker will measure between 2.1 and 4 ohms. You can assume it is safe, if you match the nominal impedance to your amplifier.
Parallel and series wiring are the two methods used to connect multiple speakers or cabinets together. Parallel wiring halves the impedance and series doubles. You can also use a combinations of parallel and series wiring, such as series/parallel, when using 4 or more speakers together.
In multi-speaker or multi-cab configurations, you most likely want the power distributed equally. The speakers should all have the same nominal impedance to achieve this. There are situations, however, where you might not want the power distributed equally. One common example is using a 1×15 and 2×10 together for a bass guitar application. Most people will want half the amp power driving the 1×15 and the other half for the 2×10. In this case, the 15” should be half the nominal impedance of each 10”. The two 10” should have the same nominal impedance. When the two 10” are wired in parallel in the 2×10, they will equal the load of the 1×15. Then you can wire the two cabinets together, either in series or parallel, to achieve the desired impedance load for the amplifier’s outputs.