>Breaks up the low-frequency sounds ("exhaust rumble") and turns them
>into high-frequency sounds ("chirp!"), which are then better filtered out
>by the car's insulation. Chirping cars are quieter inside!

It would follow that cars that do not go "chirp," but instead go "blap,"
like mine does, are quieter *outside*. (!) This is not the case. I know it.
My neighbors know it. Idle passers-by know it.

Maybe we all have inadequate insulation. ;)