No idea if its true or how it works but ive heard that said about string instruments like violin/cello type instruments.
I imagine the theremin sort of requires this in its way as well.
As opposed to something with buttons or physical quanta that predetermine the sound that is possible from engaging them as known without need to finetune or ise ones ear to get it right
It’s true for any string instrument without frets or wind instrument without valves.
Of course, even with keys on the trumpet, you need to sound the correct harmonic interval using breath. Otherwise how could you get so many notes from three buttons!
I’m not sure what your question is about exactly. A slide whistle is the simplest version, I suppose?
Yup, thats all. No hidden agenda lol. Aint tryin’ to take over the world just yet