The following are formulas for wiring speakers for a particular impedance:

Series

In series wiring, the positive from the amp will connect to the positive of the first speaker, then a separate wire will run from the negative of the first speaker to the positive of the second speaker, and lastly a separate wire will then run from the negative of the second speaker to the negative of the amp. You can picture it as: The signal leaves the positive of the amp and goes into the positive terminal of the speaker, then travels through the voice coil, out from that speaker into the next. Then, through that second speaker's voice coil and out of that speaker back into the amp, basically in a straight line.

Total Resistance = Resistance 1 + Resistance 2. Just add the impedance of the speakers together, no matter how many loads are in the circuit.

Example: Two 4 Ohm Subs Wired in Series

Parallel

In parallel wiring, all speakers share the exact same beginning and ending point in the circuit. In the case of two speakers wired in parallel to an amp, both speaker positive terminals will connect to the same positive amp terminal, and both speaker negative terminals will connect together to the same negative amp terminal. Both speakers receive the same signal at the same time.

Multiply the two loads, and divide that figure by the total of those same loads added together. The total resistance in a parallel circuit will always be less than the resistance of the component with the smallest value.

Example: One 6 Ohm Speaker and One 10 Ohm Speaker Wired in Parallel

An easy way to remember the total impedance for two speakers that are of the same impedance, is to just to halve one of the speaker's impedance.

Examples:

For Multiple Parallel (more than two speakers wired together in Parallel) loads, the formula changes a little bit :

Example: Three 12 Ohm Subs Wired in Parallel

Example Two: One 6 Ohm Speaker, One 4 Ohm Speaker, One 8 Ohm Speaker, One 12 Ohm Speaker, and One 3 Ohm Speaker Wired in Parallel

Series/Parallel

Series/Parallel wiring is the use of both wiring methods, combined, to solve impedance problems. For example, let's say that your amp needs to see a 4 Ohm load, but you want to run four speakers. You can do this using four 4 Ohm speakers.

Each pair of speakers will be wired in series to each other, then the resulting single positive and negative coming from each pair will be wired in parallel to the amp. This is exactly the same thing that can be done with Dual Voice Coil (DVC) subs that have 4 Ohm voice coils; you would just be working with two individual voice coils on one speaker instead of two individual speakers with one voice coil each. Word up!

Other Points To Note:

An important factor involving the wiring of speakers that use passive crossovers is that impedance is affected only within the frequencies that the speakers share. In other words, if you have a midrange that plays from 150Hz up to 2Khz, and want to add a tweeter that plays from 2.5Khz up to 30Khz, and both are 4 ohm and have crossovers on them that only allow them to play within their ranges, then they will only present a 4 ohm load when wired in parallel, since they do not play within each other's ranges. (Give or take a little for the resistance presented by the crossovers themselves and the speaker wire.) If you were to then add a second identical midrange with an identical crossover in parallel, the impedance would be 2 Ohms from 150Hz to 2Khz and 4 Ohms from 2.5Khz to 30Khz.

Nominal Impedance is the manufacuter's specification given for the speaker. It is meant to represent the average impedance a speaker presents to an amp. Resistance fluctuates constantly on the speaker while it is playing, so if you try to measure DC resistance on a speaker with a multimeter while it is just sitting on a bench, you will most likely not get a reading consistent with the ohm value stamped on the back of the speaker's magnet.

Be sure to take note of the requirements of the amp as far as how it is affected by the load placed upon it. For example, a mono amp cannot be bridged, so when you hook it up to a 2 Ohm load, that is exactly the load that it "sees". However, if you were to hook up a 2 Ohm load to a 2-channel amp that is in the bridged configuration, you must remember that bridging a 2-channel amp halves what the amp sees, because you are tying two channels of the amp together. Therefore, the 2-channel amp would see a 1 Ohm load when a 2 Ohm load is hooked up to it in the bridged mode.