@slaw Happy Birthday to your mom Steve. My Dad came home from the hospital Christmas Eve and is back to causing trouble for everyone... he’ll turn 91 in a couple of weeks.
There is quite a bit of information in the literature about the health benefits of the 40 hz tone. Folks set up tone generators in their basements, some say at just below audible level. I wonder about timbre since a plain sign wave doesn’t sound particularly salubrious to me. Further evidence to me, and the real bass players around here can probably say better, is that after a longer session of playing around on my electric bass (I play along to Foghat albums) I can really feel the effect (I think it’s the rich harmonics) in my head as well as the rest of my body. More study needed.