frequency is the pitch of the music i think. Im not good at explaining things but 100 hz is alot higher pitched than 20 hz someone else could explain it alot better than me. It depends on the kind of subs and the enclosure as far as if 2 12"s are louder than 1 15". if you had the same brand of subs and the same kind of box the 2 12" would be louder becaus they would move more air but the 15" would give a lower deeper sound and have a slower response.