40 Hz isn't a note, it's a beat speed. Two tones 40 Hz apart create a fast pulse in the gamma range, the band linked with alert attention. Researchers are studying 40 Hz light and sound; we simply use it for focus playlists.
traditionally linked with
how listeners use it
Frequency music is for relaxing, focusing and enjoying sound. The meanings above come from tradition and from how listeners describe it. They are not medical claims, and the music does not replace care from a doctor or therapist.