I thought Myhrrhleine did a pretty good job of explaining it. I'll just add a few side notes.

First, if an amp puts out 100w at 8 ohms, it is not going to put that same power out at 4 ohms or 16 ohms. At 8 ohms, 1 watt means a signal voltage of 2.83 volts. To keep things fair, other impedances also use that same signal voltage.

But, for this example, let's use 8 volts because it's easier to calculate.

8 volts applied to 8 ohms is P = E^2/R = 8^2/8 = 64/8 = 8watts.

8 volts applied to 4 ohms is 64/4 = 16 watts.

8 volts applied to 16 ohms is 64/16 = 4 watts.

Think of the applied signal as the position of the Volume Control. When you turn the Volume Control up, you aren't increasing the power, you are increasing the signal voltage and more power is consumed as a result.

Now keeping this voltage, current, and power relationship in context, consider that when two equal speakers are in parallel, they both get the same signal voltage but the resulting total current is divided between them. If the total voltage is 8 volts, then the total current is 2 amps, with 1 amp each going to each speaker.

P = I^2 x R = 1^1 x 8 = 1 x 8 = 8 watts to each speaker.

Or

P = E^2/R = 8^2/8 = 64/8 = 8 watt to each speaker.

But note, 8 watts to each speaker means 16 watts total power consumed.

Now in series both speaker have the same Current but the Voltage is divided between them. Since they are in series, whatever current flows through one MUST flow through the other. However, they make a voltage divider, and if they are equal then voltage is divided equally between them.

To keep the numbers simple, let's put two 4 ohms speakers in series for a total of 8 ohms. Now we apply 8 volts which equals a total current of 1 amp. Both speakers get 1 full amp of current, but each individual speaker only gets 4 volts. Consequently, the power to each individual speaker is P = E^2/R = 4^2/4 = 16/4 = 4 watts to each speaker. 8 watts total to the combined speakers.

In the above Series example, if we use 8 ohms speakers, then the power delivered to each speaker is - keeping in mind that each speaker is getting half the total voltage.

P = E^2/R = 4^2/8 = 2 watts per speaker.

Since there are two speakers, the total power seen by the amp is 4 watts.

In the 8 ohm parallel example, the speakers consume a total of 16 watts with a 8 volt input signal.

In the 8 ohms series example, the speakers consume a total of 4 watts with an 8 volt input signal.

This is just a complex way of saying what Myhrrhleine said, but it explains why the amp power is lower when the speakers are in series.

But again, this is all assuming every combination gets the same input voltage. There is a Volume Control knob on the front of your amp for a reason. If it is not loud enough, you just turn it up.

I think more important than sensitivity or efficiency (100db at 1 watt at 1 meter) is getting the individual component of a given speaker system balanced. Making sure the woofer, midrange, and tweeter are all putting out close to the same perceived output level. Once you have a balanced speaker system, you can just turn the volume control to where ever you need it to be to get the sound intensity you want.

That's probably a lot more than you wanted to know, but to understand what is going on, I think you needed to know it.

If it really is way more than you wanted to know, then just listen to Myhrrhleine and RoddyYama, and ignore me.

Steve/bluewizard