Why are low impedance speakers harder to drive than high impedance speakers

How about this simple example. A regulator on an alternator of a car stops the alternator from putting out more power than it is designed to do. If not regulated the alternator will push as much power as you demand of it until it melts.

A speaker load that has less resistance or push against it or back pressure, will allow an amp to keep putting out more than it can handle. (over heat or melt outputs.) If an amp can do 100 watts pushing against 8 ohms it will want to do 200 into 4 and 400 into 2. If the heat sinks or power supply wasn’t built to handle that it melts down.

The reason someone else in this discussion said it is not necessarily true is because other factors include efficiency of the speaker, but this is only a factor if your taking into account that your trying to fill a room with a certain SPL level, and woofer size because you need more amperage to move a larger motor.

For the same reason you lose water pressure in the shower when the toilet is flushed. More water is drawn, lowering the pressure in the tank. When the speaker has a high impedance, it’s like a faucet that is slightly open. The pressure (voltage) in the tank (amplifier) is maintained as there is little flow (amperage) through the piping (speaker cables). When the impedance drops, the faucet opens which draws more water (amps) causing the pressure (voltage) to drop. If the amplifier does not have enough ’pressure’ it cannot supply the current for the speaker driver to move.

That is why amplifiers that maintain voltage throughout the impedance swings of speakers are heavy and expensive. They have to act as a voltage source regardless of the current demands of the speaker. This requires a bigger transformer, bigger power supply capacitors and high current output devices to put it all togther.

gs5556,
That is a very good analogy!
I’ve often wondered why given the reality of Ohm’s Law, why do many speaker designers revert to 4 ohms rather than 8 ohms (or higher) speaker impedance. As Al noted in his reply, Ralph (Atmasphere) has pointed out that the higher the impedance load ,the lower the amplifier  distortion produced (which is a desirable result you'd think).

It would seem that a concerted effort to design easier to drive high impedance speakers would yield obvious benefits. The most logical explanation for the plethora of lower impedance speakers is that the higher impedance speaker is probably more difficult to implement to get things right and presents more design challenges.
Charles

In a related matter, Roger Modjeski of Music Reference recommends hooking up your speakers to a tube amp on the lowest impedance tap that provides the power you need. So if an amp puts out 45 watts at 8 ohms and 30 at 4, and 30 watts is enough for your needs (with the combination of speaker sensitivity, room size, listening level, etc.) with an 8 ohm speaker, use the 4 ohm tap for lowest power amp distortion and best sound.