ORIGINAL: AA5BY
Large propellers on gas planes simply offer such a wide variety of noise from when they are loaded, unloaded, and braking... how do you weed out those sounds? The bark of the exhaust stack opposing or facing is different. Even the Doppler shift coming toward or going away plays its factor.
Far more notable to me is time, rather than sound.... whether the engine is failing to spool up quickly as I remember normal for it?
Frankly, I wonder if they are not giving credit to some of the other senses that make it possible.
I think you have the right idea.
Fly a lot - listen to other guys planes, and you will pick up on all this and you will eventually be a expert just like learning the code. I think it is easier to learn than the code actually.