Forget the ratings for a second. An amplifier presents a voltage to the speaker. A speaker presents a resistance (ohms) to the amplifier. This pairing causes a current to flow from the amplifier to the speaker. The current is directly dependent on both the voltage from the amp and the speaker load. This is what defines the power - how much current flows into the speaker load. Or, correctly, how much current the speaker load draws from the available voltage. But defining the power presents a small problem.
One problem is the speaker does not present a steady load resistance because the speaker is not like a light bulb or a space heater, which is pure resistance. A speaker has, in addition to resistors, capacitors and inductors (coils of wire). The speaker driver itself is an inductor. So the speaker, as a whole, is an "inductive load".
Another problem is that the music signal is not a steady voltage. It's ac, which means the voltage's frequency varies. As the frequency varies, the load varies (because of the inductive nature of the load) and the power varies as well. So it is very difficult to define the amplifier power. However, if the speaker were a pure resistance of 8 ohms, the power would be constant over the entire 20hz to 20 khz range.
That's why amplifiers are specified for WPC not only for a given load, but a given frequency as well. For example "200 WPC at 1khz into an 8-ohm load, 400 WPC at 4-ohms".
The way to look at it is that the signal from the source is the "switch" that determines power.
One problem is the speaker does not present a steady load resistance because the speaker is not like a light bulb or a space heater, which is pure resistance. A speaker has, in addition to resistors, capacitors and inductors (coils of wire). The speaker driver itself is an inductor. So the speaker, as a whole, is an "inductive load".
Another problem is that the music signal is not a steady voltage. It's ac, which means the voltage's frequency varies. As the frequency varies, the load varies (because of the inductive nature of the load) and the power varies as well. So it is very difficult to define the amplifier power. However, if the speaker were a pure resistance of 8 ohms, the power would be constant over the entire 20hz to 20 khz range.
That's why amplifiers are specified for WPC not only for a given load, but a given frequency as well. For example "200 WPC at 1khz into an 8-ohm load, 400 WPC at 4-ohms".
The way to look at it is that the signal from the source is the "switch" that determines power.