impedance

I was looking through some specs on a few speakers. In the specs they show impendance with a value usually 4 or 8 ohms. After that they show Re: xx ohms. For example I was looking at a Morel tweeter, it shows impendance as 8 ohms then Re 5.2 ohms. What is the Re ohms and when building passive crossovers or figuring out ohm loads in multiple speaker connections do you need to use the Re ohm spec or the normal 4 or 8 ohm they show for the impendance??

Re is the DC resistance of the speaker. If you put a multimeter on it you would read 5.2 ohms as long as the voice coil isn't moving. The impedance is a combination of the resistance, inductance, and capacitance of a speaker in an AC circuit and varies with frequency. The 4 or 8 ohm rating is a nominal rating and is not a measurement. It represents the lowest impedance within the pass band and is usually rounded off at 1,2,4,8, or 16 ohms. Since the nominal impedance is made up, you will find that speakers with the same nominal rating will have differing Re values. IIRC the most accurate nominal impedance to use would be 1.707 of the Re. When comparing amplifier ratings at a given impedance you can use the nominal impedance as most of the information given is generalized anyway. With crossovers however accuracy is important so you want to know the impedance curve of the speakers being used and design accordingly.