What is the noise level in decibel on ground of stealth helicopter when its flying on maximum hight?

-Even loud helis cant be easily heard on the ground when they are flying at their maximum height.

Stealth helis are made quiet so they can operate close to the ground without being heard/seen/detected.most noise is created by the rotors of the helicopter,now with the new design in fibreglass and kevlar blades also the wider the blade the more air it can move to create life and forward movement also allows for faster flips or rollovers without the blades taking at lot of strain and they keep the helicopter quieter during flight so there shoud be no sound at all to hear for the human ear but only on a audible sound spectrum will it show up
that's classified