Originally Posted by Shaun B
Can anyone explain why resistive loads on amplifier bench test results are more stressful on amplifiers than real loudspeakers?
The resistive load has the same resistance at any audio frequency, and probably well beyon.
The speaker load's impedance is somewhat reactive which means that only part of the load it presents dissipates power, and it is generally higher than its specified impedance over most of the audio range.
Here is the impedance curve of one of the superficially scary speakers on the market:
People report being able to drive it with a good AVR, but given the < 2 ohm impedance above 10 KHz at first thought that almost seems impossible.
However, the speaker's impedance is > 4 ohms below 3,500 Hz, > 6 ohms below 2,500 Hz, and rising above 12 ohms below 200 Hz where most of the energy in the music is. If the speaker is used with a subwoofer then it presents no load at all below 60-80 Hz or whatever the subwoofer's crossover frequency is.
A 4 ohm resistor used in bench test would show up o this chart as a line all the way across the diagram from 10 Hz to 50 KHz (and outside that range as well) @ 4 ohms, which is well below that of the speaker below 3.5 Khz.
Most of the energy in music is at lower frequencies, So from the standpoint of playing music the 4 ohm load is a far tougher load than even this very tough-seeming speaker over most the range of frequencies where most of the energy in music actually is.
Now for a more typical loudspeaker:
This speaker barely goes below 8 ohms at any frequency, and so it is an easier load over the audio band on the average than even an 8 ohm resistor.