Originally Posted by KidHorn
The ohm rating for a speaker is completely meaningless.
Impedance varies not just with frequency, but with the exact waveform passed into the speaker. It's impossible to give it a value like you could for say DC resistance.
There's a reason the impedance value is nominal and it's always in very rounded units. The reason is the number is basically guessed or just plain made up by the manufacturer. Some manufacturers say their speaker is 8 ohms simply because of people scared of 4 ohm speakers. Others put the impedance at 4 ohms so they can protect themselves in case the speakers fail. They can just say the receiver or amp wasn't rated for 4 ohms, so no warranty coverage. OR they can try to get purchasers to get a better qaulity amp or receiver.
Nominal speaker impedance is an important specification..
But like other electronic components it is just 1 spec of many that should be reviewed when deciding loudspeaker brand/model..
The actual nominal speaker impedance is not
guesswork but rather a range
For example, in a 3-way loudspeaker each transducer may have different impedance and when designed into a total system its impedance is determined by the total system design. Frequently conjugate circuits, are used to balance the total impedance load and keep it above certain minimum impedances..
As far as power goes...
It's true that, all things being equal, a speakers power output under 4 ohms of impedance will require more current than the same under 8 ohms, but it's also true that the receiver will have less difficulty delivering said current. By the way, it's not twice the current. Power consumption is a function of the current squared, so it will actually require about 1.4x as much current (p=r*i^2 or p=iv, where v=ir,r=resistance or impedance).
Saying beware of 4 ohm speakers is analogous to someone saying you need to be careful driving an SUV too fast. The extra weight puts a lot of strain on the engine. Without taking into account the SUV having a beefier engine.
Sensitivity or efficiency of the speakers will have more impact on whether or not the receiver overheats at a given volume level than the nominal impedance. So if you have an inefficent 8 ohm speaker, you're more likely to blow an amp than if you have an efficient 4 ohm speaker.
As mentioned previously, the challenge today is that the amplifier sections & power supplies within the price point AVRs (SRP <$999) have been cut back
very significantly to save $.. And since 87% of the home theater market segment are using subwoofer/satellite loudspeaker systems, most users won't notice the cutback....
However, as the listener becomes more experienced and informed, very frequently they will upgrade their loudspeaker system to a higher performance, full-range
system.. As the bandwidth extremes are pushed especially the low end, then the shortcomings
of reduced amplifier sections & power supplies for the price point AVRs becomes more apparent & audible...
Just my $0.02..