I really like this Musical Spectrum Analyzer plugin for my favorite—and free—music player, Foobar 2000.

The spectrum you see here is an ostinato bass-line from a Boards of Canada track called Happy Cycling. (The really low stuff starts around the 5:00 mark.)

The lowest note in the bass line is centered around 25Hz, is pretty loud, and it is absolutely musical in nature. Lacking a subwoofer, or suitably large main speakers, these musical notes are never heard. Many systems struggle to cope with such energy, and transform these sounds into higher-frequency harmonic distortions.

Just an interesting illustration…