|Quoting Soxfan (Thread starter):|
It's like a middle "G" on the piano scale. Does anyone know what I'm talking about, and what causes that sound?
|Quoting tdscanuck (Reply 2):|
A on most pianos is 440 Hz. Aircraft AC power is 400 Hz. What you're most likely hearing is electro-magnetic interference from the aircraft power system being picked up somewhere in the VHF system.
|Quoting Go3Team (Reply 5):|
If I remember right (from what I read here) it has to do with weight. 400 Hz Generators, Transformers weigh less.
|Quoting flybaurlax (Reply 7):|
Would the fact that I always hear it on UA equipment is because they're older? Although this wouldn't necessarily make sense for the Airbus they fly.
|Quoting KELPkid (Reply 6):|
Used to be a familiar sound in aviation, but you don't hear the 400 Hz hum much anymore (better avionics, noise filtration, etc.).
|Quoting Mir (Reply 8):|
I've heard older Learjets that sound like they've got sirens going off in the background
|Quoting Soxfan (Reply 9):|
I'm assuming, though, that it's still present on newer aircraft because of the power systems, but just not as noticeable?
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