An Hour of Light and Sound a Day Might Keep Alzheimer’s at Bay

[RedFox]

to find out if you can hear 40 hz at all ...

You need good-quality headphones or large speakers to reproduce frequencies that low.

How could a 50Hz device accurately display 40Hz without generating an interference pattern of some sort? Even if that's suppressed the result wouldn't be a 40Hz flicker but some hybrid.

Worse, your display needs to run at at least 80Hz to reproduce a 40Hz flicker. It takes 1/40 s to display both white and black, so it's white at one point and black 1/80 seconds later.

Also, videos and displays don't follow the requirements of signal processing theory for accurate reproduction of signals. Digital audio is filtered before recording to prevent this aliasing you're talking about, and when it's played back, it's again filtered so the output signals moves smoothly from point to point without distortion. Video isn't.