Multiple loudspeakers can be connected in series or parallel to the output of the amplifier.
In either case, the current drawn from the amplifier is determined by the total impedance of the load as presented to the loudspeaker terminals.
Impedance is the opposition to the flow of current.
As the load impedance is decreased, the load on the amplifier is increased, because it must work harder to supply the demand for current. In similar fashion, an automobile trying to maintain its speed uphill is under a greater load than on flat ground.
A “no load “condition means that nothing is hooked to the amplifier, so no current flows and no power is transferred.
The opposite condition, a dead short between the amplifier “+” and “-” terminals, represents the maximum load possible, and current flow is limited only by the resistance of the wire making the connection.
So, the lower the impedance the greater the load - a bit counter intuitive but nonetheless true.
HOLE IN THE BUCKET
An example will clarify this. Imagine a bucket full of water.
Two loudspeakers in parallel require twice the current of a single one, just like two holes in a bucket offer one-half the opposition to water leaving the bucket as a single one. (click to enlarge)
Assuming watertight construction (a good thing for a bucket), there will be no water leaving the bucket (analogous to current flow), and the pressure against the sides of the bucket (analogous to electrical voltage) will be constant.
Now, let’s put a hole in the bucket. Water will now leave the bucket at a rate proportional to the size of the hole.
The hole represents the connection of a loudspeaker - current now flows from the bucket (amplifier) through the hole (load).
If we keep the hole relatively small, the pressure will be similar to the water-tight condition. If we replenish the bucket continually, the flow can continue indefinitely.