One of the most confusing subjects in audio? Loudspeaker power ratings.
It’s generally accepted that a large loudspeaker power rating is a sign of quality and something to be desired.
And it’s the performance metric that probably has the greatest influence on the consumer’s buying decision.
But a closer look reveals that power rating is far less significant than other metrics regarding the performance of the loudspeaker.
The term “power rating” requires further explanation to avoid misunderstanding. It’s tempting to associate it with the acoustic output of the transducer, or even the recommended amplifier size.
But it has little to do with either.
First, let’s expand the term to make it more meaningful.
How about “maximum input power dissipation?” The term “input power” is appropriate because the loudspeaker presents a load to an amplifier.
Assuming negligible effects from the cable (a safe assumption if the correct wire selection criteria are used), the output power of the amplifier becomes the input power to the loudspeaker.
And because bigger amplifier power ratings are accepted as better (i.e., a sport utility vehicle versus an economy car), it’s assumed that larger loudspeaker power ratings indicate a better product.
Figure 1: How the power “thing” (amplifier to loudspeaker) works. (click to enlarge)
Amplifiers that connect directly to loudspeakers are called power amplifiers, because their output is a higher voltage and current facsimile of the input voltage to the amplifier. (Figure 1)
Power amplifiers are rated for power generation. A bigger number is generally better as it indicates the potential for the amplifier to do more work.
Loudspeakers are rated for power dissipation. Their power rating describes the amount of continuous power that can be dissipated in the form of heat without damage to the loudspeaker.
While at first glance it may appear that more power dissipation is better, this is only true if the method used to achieve it does not compromise the efficiency of the loudspeaker.
Modern power amplifiers act as constant voltage sources to the loudspeaker. This means that the output voltage of the amplifier is essentially independent of the load placed on it by the loudspeaker.
If you drive an amplifier with a signal and measure its output voltage with no load connected to the output terminals, and then connect a loudspeaker to the terminals, there is no significant change in the reading on the voltmeter.
The difference between the no load and loaded case is that with the load present current will flow from the amplifier terminals through the loudspeaker.
Lower load impedances (more loudspeakers in parallel) draw more current from the amplifier, increasing the total power transfer from source to load. (Figure 2)
Figure 2: Lower impedances (loudspeakers in parallel) draw more current. (click to enlarge)
This is why the total output power of the amplifier generally increases when driving more loudspeakers. Note that the output power of the amplifier increases, but the power is distributed among the connected loudspeakers.
So, if one loudspeaker is connected in parallel with another, the total power output of the amplifier increases but the power per loudspeaker does not.
In fact, it probably drops a little. It is best to keep amplifier loads above 4 ohms to minimize cable effects and avoid excess current demands on the amplifier.
The Arms Race
The power drawn by the loudspeaker from the amplifier is found by multiplying the voltage times the current.