Quite simply, The speakers impedance is a measure of how much it will resist the flow of power from the amplifier. Most speakers are rated at 8 ohms. The important thing to remember is that the more speakers you run off a single channel, the less the total resistance is.
If you run two 8 ohm speakers off one amp channel, the combined resistance reduces to 4 ohms. Add a third, and it reduces to 2.6 ohms. A fourth, and it goes to 2 ohms. So, you must be careful when running several speakers that the amplifier will run power into 2 ohms. If it won’t, you may damage the amplifier.
When matching amplifiers to loudspeakers, the output impedance of the amplifier should match that of the loudspeakers as accurately as possible, to ensure that the amplifier is able to deliver its maximum rated power. If the speaker impedance is lower than that of the amplifier’s output impedance, the amplifier will be forced to work too hard, which will cause overheating and possibly failure. If the amplifier has an overload protection circuit built-in, this may operate and shut down the amplifier, either partially or completely.
If, on the other hand, the speaker impedance is significantly higher than the amplifier’s output impedance, the amplifier will be unable to deliver its full power rating, but should in all other respects work normally. Taking an example, if an amplifier with an output impedance of 4(omega) is connected to an 8(omega) loudspeaker, the maximum power available will be half the rated power of the amplifier. Never run valve amplifiers without a speaker connected, as damage may result. This does not apply when using solid state amps, however.
When matching audio signals (eg, mic or line level signals), unlike with the amplifier and speaker example above, it is not power transfer we require, but simply the most efficient transmission of the signal voltage. As a result, the ideal setup here is when the source impedance is significantly lower than the load impedance, ideally by a factor of 10 or greater. If the source impedance is the same as the load impedance, the circuit will work as it should, but the maximum available signal level will be halved. Finally, if the load impedance is lower than the source impedance, the available level will drop, and there may also be audible distortion.