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A few years back, while trying to snooze on a Jet2 flight up at cruising altitude, my semi-attention wandered to the droning of the engine. I realised there was a very subtle effect of its intensity rising and falling ever so slightly in a repeating fashion. From memory, I'd say it was doing each rise and fall cycle over around 3-4 secs. It was consistently regular in its pattern. I've never heard that before but it was so subtle it could be it always does it and I've just never noticed. I was intrigued as to what the mechanism might be. Any thoughts? I was in the window seat a row or two in front of the wing at the time if that's relevant to what's audible. Thanks!

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    $\begingroup$ I suspect that what you are hearing is the interference pattern between the two engines. The vibration combing from the rotation of one engine is some variable X hertz and the other engine is “X+/- a few” hertz. This creates the interference pattern like you sometimes get with stereo speakers. On many prop and turbo-prop engines you have a prop-synchronizer that lets you make tiny RPM adjustments to find a “sweet spot” where the interference pattern isn’t noticeable. $\endgroup$
    – Max R
    Jun 11, 2023 at 18:42
  • $\begingroup$ Also known as a beat frequency. en.wikipedia.org/wiki/Beat_(acoustics) $\endgroup$
    – Steve
    Jun 12, 2023 at 20:57
  • $\begingroup$ Many thanks for the answers!! $\endgroup$
    – Cantoris
    Jun 13, 2023 at 18:39

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That is almost certainly a sound interference pattern due to a slight difference between the spool speeds of the two engines. It’s a much more common problem on reciprocating or turbopropeller powered airplanes, where it can cause considerable discomfort for the passengers, but can also happen aboard turbofan powered aircraft, especially those with high bypass engines.

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  • $\begingroup$ You said core speed, not fan speed. $\endgroup$
    – John K
    Jun 12, 2023 at 1:54
  • $\begingroup$ I suppose better said as “spool speed”. $\endgroup$ Jun 12, 2023 at 2:58
  • $\begingroup$ Well almost all the lower frequency non-white noise, which is what we are concerned with, is coming from the fan, so I would use that if I was writing it. $\endgroup$
    – John K
    Jun 12, 2023 at 4:48
  • $\begingroup$ Many thanks for the answers!! $\endgroup$
    – Cantoris
    Jun 13, 2023 at 18:39

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