If I have a set of speakers, lets say 2 pair of 16 Ohm speakers, and I test the impedance rating. I'll will get around 4 ohms correct? (Assuming that are all hooked in Parallel) Now if I hook the 4, 16 ohm speakers wired in parallel to an amp and measure the impedance, will the measurement vary? And if so, by how much approximately?

That's what I was thinking. I have a 'sound man' telling me that impedance changes when an amp is applied.

Nice link, 3506string. Simply put, the speakers impedance changes with frequency. It's a MINIMUM of 4 ohms, usually.

His hypothesis was that impedance changes when and amp is connected and just turned on. I assume it might change, but that change is small.. In my assumption.

Two 16 ohm speakers in parallel will give you 8 ohms. Your speaker impedance is independent of your amp. Are you saying you are measuring your speakers while they are still connected to the amp? That will give you an indeterminate and meaningless reading. There is no telling how it will affect the measurement. Speaker impedance is a function of frequency. The impedance given for a speaker is generally a nominal impedance -- a ballpark (but not exact) figure of what you can expect.

When an amp is connected, the total impedance changes so far as the outside world is concerned, as the amp's output impedance is in parallel with the speaker impedance. But the loudspeaker impedance as seen by the amp does not change compared to what the measurement device 'saw'. The impedance does change with frequency. It also changes as more power is fed to the speaker (the coil gets hot and the impedance goes up - look up 'power compression').

It is not a hypothesis - it is the laws of physics and electrodynamics. Resistance is NOT the same as impedance. Impedance is the combined effects of resistance, the inductance of the coil and the voltage created by the coil moving in the magnetic gap. Since speakers and amps are for AUDIO, they are spec'd at audio frequencies, not while sitting still. Using an impedance meter, you will find the speaker has say, 4 Ohms impedance at 1kHz, while the static resistance measures about 5-6 Ohms. To answer your question- 4 16 Ohm speakers all in parallel will be 4 Ohms. You do NOT measure the resistance while they are connected to an amp. The output impedance of an amp is much lower than the speakers and will give you an inaccurate reading.

If you measure the DC resistance across, say, two 16-ohm speakers in parallel, you'll get something around 6 ohms. The AC impedance will typically be greater when signal is applied, but not necessarily exactly 8 ohms. It varies with frequency. The DC resistance will be unchanged if the amp is connected, but no signal is passed.

Replace "typically" with "always". Impedance is resistance plus the AC effects of reactance (inductance and capacitance), and reactance cannot be subtractive.

And for the users benefit, there is a complete discussion of Ohms Law, resistance and related topics in the FAQ thread in the main sticky at the top of the Amps forum. It is well worth your while to peruse it, it will answer many of your basic questions on this set of topics. http://www.talkbass.com/forum/f15/ohms-faq-144244/ This thread should probably be linked in this forum as well, I am sure these questions come up often.