content: Interesting.  I never heard of that.  The ones I’ve seen from the 1960s were all hypersonic battery driven designs.    When I was a kid I could hear ~26K Hz. Although back even into the 70s it was cycles not Hertz.    At any rate I could hear some of the controls. It was Ear Splittingly agonizing!  Like someone taking an ice pick in both ears to meet in the middle.