rolypolyman From United States of America, joined Mar 2009, 160 posts, RR: 0 Posted (3 years 3 months 2 weeks 1 day ago) and read 6429 times:
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?
tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 80
Reply 1, posted (3 years 3 months 2 weeks 15 hours ago) and read 6261 times:
Each fan blade is shedding a shock wave that's hitting the stationary outlet guide vanes immediately behind the fan. For noise reduction reasons, there usually are a different number of fan blades and guide vanes. I think what you hear as buzzsaw is actually the beat frequency from interference of all these much higher frequency noise sources.
N243NW From United States of America, joined Jul 2003, 1658 posts, RR: 18
Reply 2, posted (3 years 3 months 2 weeks 3 hours ago) and read 6054 times:
That's an interesting observation and a good point.
As an example, the maximum N1 on a RB211-535 is 4500rpm at 100%. Dividing this by 60, we should hear about 75Hz from the fan if it truly makes only 1Hz per revolution.
I played this 757 takeoff video on YouTube and a 69hz triangle wave from a tone generator simultaneously and the two sounds are almost identical. Dividing 69/75 would yield 92% N1, which is well within reason for a normal takeoff. So it appears that the primary buzz sound from these engines is produced more with relation to the shaft rotation and not the individual blades.
There are also overtones of different frequencies based on the geometry of the engine. Multiply this 69Hz tone by 22 (the number of fan blades on the -535E4B), and you get 1518Hz - much higher in pitch. I also played a 1518Hz tone to compare it with the engine sound during takeoff, and that frequency is clearly audible as well.
Hate to plug my own video, but the primary buzzsaw sound you hear during takeoff and initial climb is the 69Hz tone (corresponding to one Hz per LP shaft revolution at 92% N1). When the engine thrust is pulled back a couple minutes later, the high-pitched whine you hear around 5:30 is indeed produced by each fan blade (22 times higher in pitch than the buzz).