I would like to know the difference between a 2 ohm and a 4 ohm subwoofer. I'm looking into buying a pair of Alpine SWR-1242D (4ohm) or SWR-1222D (2ohm). They both are at the same price, just the ohms are different. What difference would the ohms make in performance and sound quality, if any? Any input is greatly appreciated. Thanks.

Your sub-woofer amplifier specs will tell you what is the smallest load (R in ohms) that it can drive. Smaller loads need higher current (I) capability. Larger loads need higher voltage (V) capability. Power = (I^2)*R and also (V^2)/R and V*I so each provides a limiting factor to loudness.

4 ohms is normal for cars. The 2 ohm drivers allow one to put two speakers in series to make their effective resistance = 4 ohms. Something else that you may find is a dual voice coil speaker. Example: dual 4 ohm voice coils. This allows a stereo amp to drive a single sub (4+4), otherwise you will need 2 subs (2 ohm) and a bridged or mono amp or 1 bridged or mono amp and 1 sub (4 ohm).

Clear as mud?

More like murky water! Lol. Had to read it a couple of times for it to make sense. Thanks for the input.

The ohm rating is the resistance (impedence)of the speaker,the sound system output is balance to a certain load,its safe to put say an 8 ohm on a system intended for 4 ohm you lose volume but the system is not harmed but if you put less say 2ohm the amp circ can overheat,but as mentioned by others if you run two speakers in series i.e. a single link wire between speakers and the spare terminal on each speaker receiving the system speaker drive wire the impedance will be correct,its hard to explain but I hope I made sense.