Faro From Egypt, joined Aug 2007, 1456 posts, RR: 0 Posted (5 years 9 months 2 weeks 3 days 3 hours ago) and read 2524 times:
One often notices regular, weak "highs" in airliner engine noise which recur at intervals of say 2-20 seconds. Sometimes they coincide between different engines and sometimes not, leading to a recurrent overlap effect. My questions are:
i) what are these local "peaks" in engine noise due to; and
ii) do flight crew actively strive to synchronise them between engines and if so why?
Rwessel From United States of America, joined Jan 2007, 2005 posts, RR: 2 Reply 5, posted (5 years 9 months 2 weeks 2 days 21 hours ago) and read 2360 times:
Quoting Faro (Reply 2): So, in fact the noise is produced by the airframe itself "beating", ie humming at slightly different frequencies between one engine pylon and another? It's not actually produced by the engines?
No, it' just the "beat" from the two engine hums going in and out of phase with one another. You can get the same effect with two musical instruments (pianos or guitars are easy cases) that are slightly out of tune with respect to one another. Hit the same note on both, and you'll get than same "WWWWAAAAHHaaaahhWWWWAAAAHHaaaahhWWWWAAAAHHaaaahh" effect.
It's more noticeable on propeller aircraft since the "hums" are lower frequency and louder.