Kahn Academy is allegedly very good, although I haven't used it. I usually learn math stuff from old engineering textbooks as I need it, which is very frustrating and hurts my brain, but it's the fastest in the short term. I don't know enough to begin solving problems I don't know how to Google though. Like how do you calculate how low frequency sounds will get through openings of a known diameter? If the wave height is bigger than the diameter of the hole, the sound can get through by clipping the peaks, but how do you calculate that?

Stuff like that I can't figure out, and I don't even know where to start.