John,
If you really, really want to calculate parallel impedances, look
HERE!
O.k., now that you've had your brain bleached, washed, dried, folded and put away.... You can, for all practical purposes, ignore the inductance of the speakers and working only with the resistance, hook up the speakers in series or parallel (or a combination of both) - providing that the speakers are of reasonably the same inductance. By that I mean that a 4 ohm speaker with a power handling capacity of 100 watts will have an inductance much different than one that's rated at only 10 watts. The difference is in the voicecoil wire diameter and number of turns. Simply stated, don't try to mix and match low power speakers with high power ones. You won't like the results. If you want to hook up 4 ohm speakers to an 8 ohm output, put two of them in series. If you want to hook up 8 ohm speakers to a 4 ohm output, put them in parallel. When doing so, series or parallel, make sure you observe the proper speaker polarity. If they aren't marked, just put a "AA" battery across the terminals of the speaker. If the speaker cone moves inward, mark the terminal connected to the
positive terminal of the battery with a "+" sign. If it moves outward, mark the terminal connected to the
negative battery terminal with a "+" sign (a dot of red paint works well too.). Do this for all speakers. If you hook the speakers in series, connect one speakers' "+" terminal to the other speakers' unmarked terminal. For parallel hookups, connect the "+" terminals to each other and the unmarked ones to each other. Then, when connecting them to the output of the receiver, make sure the "+" speaker wires go to the positive terminal on the receiver, and the unmarked speaker wires go to the negative terminal of the receiver. If you don't follow these procedures, some of the speakers may be out of phase with each other and cancel a lot of the sound - especially the lower tones.