Re: Why does low-frequency sound go through walls?

It's because walls are thick and heavy. High frequencies are fast vibrations,
and in order to transmit them, whatever is doing the transmitting needs to
be able to vibrate quickly. The walls can't do that. On the other hand,
the walls have no problem vibrating slowly, so they transmit the lower
frequencies quite well. You can get an idea of the natural frequency of your
walls by thumping on them (perhaps while your neighbor is out so that she
doesn't think you're objecting to the music); it is quite low.

For the same reasons, any good stereo system will have more than one size of
speaker, small "tweeters" for the high frequencies and large "woofers" for
the low end.

You may notice that if you and your neighbor both open your doors at the same
time, you hear her music much louder. This will happen even if you only crack
the door slightly; the sound will diffract in just as well as if the door were
open wide.