There are basically 2 methods of wiring more than one speaker to a single amplifier (or head-unit) output. These are known as series wiring and parallel wiring.

The main thing to consider when wiring speakers is the effective impedance of the combination of speaker drivers. Amplifiers are designed to drive a certain minimum impedance. Attach a lower impedance and you risk overheating, distortion and possible amplifier failure.

You may be wondering why a lower impedance is good. This has to do with the way amplifiers work. The Bottom line is this....lower impedance means higher current flow, which means louder sound. Just try to keep the impedance within the manufacturer's specifications.

Parallel wiring:

In a parallel arrangement, more than one speaker is attached to a single amplifier output channel. This is usually the preferred arrangement of connecting multiple speakers to one output but beware, the more speakers you connect, the lower the effective impedance. Go too low and you reach the danger zone.

The total impedance of the arrangement is given by:

1/Z_total = 1/Z₁ + 1/Z₂ + 1/Z₃ +....

For 2 speakers, this formula gives:

Z_total = ( Z₁ * Z₂ )/( Z₁ + Z₂ )

Series wiring:

In a series arrangement, all the drivers are connected together in a chain, positive terminal of one driver connected to negative terminal of another. In this arrangement, the effective impedance is simply the sum of all the individual drivers' impedances. Thus, you do not risk creating a too low impedance. However, presenting a high impedance to an amplifier is wasteful.

Z_total = Z₁ + Z₂ + Z₃ +......

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When wiring speakers, it's usually best to provide your amp with the minimum impedance it can handle (usually 4 Ohms or 2 Ohms). If your amp can go lower, you should be safe once you stay within its limit.

NOTE: When you bridge an amplifier, the resultant channel is only stable to twice what the amplifier is usually stable to. For example, a 2 Ohm stable amp is only stable to 4 Ohms when bridged.

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