Tapping my monitor screen loudly, not to the verge of breaking of course.
My work computer still uses CRT screen, still no budget to upgrade to LCD screen it seems.
Here’s the interesting part. Playfully knocked the screen with my finger (like knocking door) with my face close. The sound from behind the screen is… interesting. It sounds like an echo room probably as wide as a hangar or warehouse.
There’s echoes from inside, with higher tone than the knocking. Vacuum tube ?