Glidepath73 From Germany, joined Mar 2005, 1020 posts, RR: 45 Posted (9 years 4 months 3 days 5 hours ago) and read 4410 times:
Today a Lockheed C130 passed where I work at cruise altitude. On the ground, the engine noise was very loud comparing to passenger jets. Even the Transall seems to be more loud then other jets. It looks for me that turboprops in general seem to be much more louder on the ground when they pass by on cruise altitude.
Did you guys notice something like that as well?
Does it come from the propeller- tips, which reach supersonic speed?
Jetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 1, posted (9 years 4 months 2 days 21 hours ago) and read 4302 times:
It's because lower frequency sound travels further. Propellers produce quite low frequencies so the sound of a big turboprop engine can carry a considerable distance. Also propeller powered aircraft cruising altitudes are usually lower than jets, so you're more likely to hear them anyway.
In the good old days of turbojet airliners you would hear a distant roar caused by the exhaust noise of a cruising jet, but not the whine of the turbo-machinery. The development of turbofans meant that exhaust noise no longer carried to the ground from such a distance, because the noise is masked by the bypass air.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
Jeb94 From United States of America, joined Oct 2004, 604 posts, RR: 5
Reply 2, posted (9 years 4 months 1 day 9 hours ago) and read 4170 times:
The sound produced by props is a big part of the reason unducted fans haven't been employed by airliners. The undected fan jet engine was highly efficient but produced such loud noise that those on the ground actually became physically ill when the aircraft passed by. The ducts of a standard high or medium bypass turbofan hide much of this noise. Imagine the typical buzz saw sound of many high bypass turbofans but completely unmuffled by the bypass duct.