Impedance Query ?? Nominal vs Compatible ?

[Gazzza]

I have a Marantz SR5400 receiver. The speaker connections on the back indicate you can use speakers with 6-8 ohms impedence.

I have therefore just purchased Missions M3S cinema pack, plus an additional sat, making it a complete 6.1 set.
The mission spec states these speakers are 8 ohms compatible.

I've received the speakers but when i came to install I noted on the back of each speaker it said "nominal impedence 4 ohms".

Can someone explain the meaning and significance of this ?

I'm assuming the fact that they state the speakers are 8ohms compatible means that i can happily drive them from my recieiver since the receiver states it will drive speakers with impedence from 6-8 ohms ? But it just worries me that the speakers state 4 ohms nominal imedance.

I guess there is a difference between nominal impedence and compatible impedence ?

Thanks for any help/info/advice.

Gareth

 

[Reiner]

No speaker has an impedance of 4, 6, 8 or 6-8 Ohm etc. Impedence greatly varies over the frequency spectrum, it's not unusual that a speakers goes from e.g. 3.5 to 20.3 Ohms.
As such the labels of speakers are just an indicator of compability with amps. Amps do not have impedence as such, the label 6-8 Ohms again is just an indication for compability.
I assume, and I will measure that next time I get a chance to, that a value like 4 or 6 Ohm refers to the DC resistance, which they may call "nominal impedance".

Anyhow, as long as you don't stress the amp too much (setting speakers to small helps) I don't see a problem using 4 Ohm speakers with a 6-8 Ohm amp.

The lower the impedance, the higher the load to the amp (but also sensitivity (measured in dB) effects the load to the amp).