I've often as a passenger noticed the difference of the sound coming from different airliners. I believe the two manufacturers I have compared are Rolls-Royce and GE, where the latter's engines sound more "electric" (funny enough) and the former's more distorted and noisy. Do these observations make sense? And what could possibly create such a difference i sound? I was in the belief that engine design of turbofan engines is somewhat established and detailed.
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2 Answers
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The GE90 engines have a higher bypass ratio than the RR Trent.
The majority of engine noise, especially the "ripping, howling, typical jet noise" comes from the engine core.
The higher the bypass ratio, the less air passing through the core for any given thrust and the more cold air meets the core exhaust at the back reducing the shear
Jet engines with no bypass ratio and deep intakes produce a wonderful sound
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5$\begingroup$ Don't forget the difference in the number of shafts. The Trent should emit at more frequencies, because its parts rotate at three different speeds. $\endgroup$ Mar 21, 2015 at 10:14
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$\begingroup$ @PeterKämpf True, and the Trent has a contra-rotating spool, but I don't know what effect that has on noise. $\endgroup$– SimonMar 21, 2015 at 10:16
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Intake resonance - particularly evident on the Avro Vulcan. It is a sound that speakers can do no justice to.
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2$\begingroup$ I had the pleasure of watching a Wing Commander get grounded (all pre-planned of course) on his retirement flight. Took off, kept the howl going all the way round the circuit, then stood her on the tailpipes directly above the squadron office. Broke a few coffee cups and some windows. Never to be forgotten :) $\endgroup$– SimonMar 21, 2015 at 17:32