I think he means that he wants to hear what % of throttle planes around him are opperating at
In reality it is very rare to hear someone else's engine over yours (unless your engine is off). The sound also gets distorted by the wind, distance, and direction of the plane. What you can do however is take a plane who's engine sounds on a different frequency than most others (such as 262) and you should be able to hear other engines. I think I have to say -1.
Like, ya know, when that thing that makes you move, it has pistons and things, When your thingamajigy is providing power, you do not hear other peoples thingamajig when they are providing power. HiTech