I know how you feel! I had this same question when I first came on! Here's another way to view it.
I=V/R where v=voltage, I=current, R=resistance/impedence
To maintain the same V over a speaker, if you decrease the resistance(from 8ohms to 4), you'll need to push more current throught the circuit.
Since
Power = I^2R ,
if you increase the current u'll increase the power required too.
Thus, lower the resistance, higher the power required!
QUOTE]
We still seem to have some confusion. When you say the higher the power required: For what? Not the same output; that will increase too! So one is simply only saying that the louder you want it, the more power you require - which is basic logic.
When one starts by saying "to maintain the same V over a speaker", that means that the output will get louder and louder as the impedance decreases. That does not necessarily make a 2 ohm speaker "harder to drive". Any impedance is harder to drive if one wants more volume. But if you turn it down to give the same volume as before, it is not really harder to drive. It requires more current, but at a lower voltage - one must now define the concept "harder to drive". No more power is required!
I think one must start at the energy required from the system, say 100W. Speaking ideally, for a 16 ohm load you will require 40V at 2,5A (all rms). For 8 ohm you will require 28V at 3,5A, for 4 ohm 20V at 5A, and so on.
For
the same amplifier, the quoted statement becomes a little more true, because usually amplifiers are constant voltage devices. But the "harder to drive" concept is a little confusing as said because you also get extra watts, which one can turn down to compare "apples with apples". To go to practical figures again, if you have a 100W amplifier for an 8 ohm system, you are forcing it to give 200W into 4 ohm maximum if you turn it full blast. That is harder. But you apparently only wanted 100W in the first place - that is not harder.
This is a point often not taken into account in amplifier promotion. Is a 100W amplifier that can do 200W worse than a 100W unit that can do 400W? Sure - but what did one want in the first place - 200W or 400W? Your loudspeaker impedance must come into the equation, and an amplifier designed for that is what you are looking for. (One gets loudspeakers that are "harder to drive" than others, but that is another subject. Just to state here that I do not have too much time for those - it is the year 2007 and designers should be able to maintain a fairly constant impedance. The technology is available.)
I hope this helps despite being lengthy.
