I've read a lot of dicussion about the buzzsaw noise that large, high-bypass turbofans create when they reaxh full power, particularly with the widebodies and of course the C-5. One thing that has puzzled me is the audio frequency of the buzzsaw, which I recall was about 20-30 Hz when I loaded it into a sound editing program. Having had some experience with audio I know that frequency is pretty accurate.
I know the buzzsaw is coming from each individual blade exceeding M1.0, but at 100% N1 there are probably 2000 blades whizzing by each second, not 20 or 30. I suppose the fan itself could be spinning at about 30 revolutions per second, but I'm not clear on why we'd get a sound pulse with one full revolution since there isn't really anything in the engine that should pulse with one revolution. Any ideas on this? Might it be a wave inferference effect from the 2000 Hz shock wave coming from each blade?