power inversely proportional to impedance?

[apodos]

I'm looking to buy a powered mixer with passive speakers.

Most mixer specs on internet based music equipment sales sites list total power and power per channel but for the most part, they only list the power rating at a specific impedance(usually 4 ohms).

It seems that most pa speakers are 8 ohms and I'd like to know how much power I will get per speaker if hooking one speaker to left output and the other to the right.

So, I'm curious what the correlation is between power and impedance; I have been told by a sales rep(who seemed rather unknowledgeable) that power and impedance are inversely proportional but I have read otherwise: one mixer spec lists 300w @ 4 ohms and 200w @ 8 ohms....the rep told me that it would be 150w @ 8 ohms?

 

[Minion]

Theoreticaly 200w into 8ohms would be 400w into 4ohms , basic ohms law stuff...
But there are other factors which cause Losses , Like the transistors become less effecient when running high output currents which causes losses in power, There could be current limiting on the output so the Transistors stay in there SOA which would limit the output power when running lower impedance loads...

Pluss most equipment manufacturers are just really wishy washy about the specs of there equipment so they have to be taken with a Grain of salt....

Also realize that the sencitivity of your speakers will have a huge impact on how loud the PA is ...a Low power PA with Highly sencitive speakers can be louder than a High powered PA with Low sencitivity speakers....


Cheers

 

[ecc83]

And remember, the quoted impedance for a speaker is only true at one frequency, used to be 400Hz. At very low frequencies, below system resonance it will equate to DC resistance, about 5.5Ohms for an 8Ohms nominal.

Big rise at resonance, down, then a steady rise with increasing F.

All this means is that the quoted Z is there to guide you as to what and how many speakers you can safely hang on a particular amp. As has been said, how LOUD it goes depends on the speakers' sensitivity, expressed as dBSPL at 1mtr for 1watt in (which for the reasons just given is a bit of a guess!).

Actual SPL can be approximated from: SPL=10logW+S, Where W is the power input and S is the rated sensitivity.

This can only be approximate however because as you approach say 30-50% of a speakers rating a thing called "thermal compression" begins to rob volume.

Cut and try son, cut and b*&^%y try!

Dave.