A320: rrrrrrrrr-sound (tech.)

[Malte]

Hi,

I guess everyone knows the typical "rrrrrrr"-sound of certain engine types when operating with maximum power (for example the Airbus series (A319, A320, A310, A300, A321 but also some 757 and the new 737-800).

Does anyone know by which technical factors this sound is caused?

I was just wondering cause I've noticed it so often (from the in- and outside of airplanes). The strange thing is that you don't hear it when you are "behind" the engines (when you sit in the last row, for example).

thanks

Malte, EDDH