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Comment by ballooney | Hacker News Reader

For a given amount of lift, all things being equal, a single larger diameter propellor will be quieter than several smaller ones at higher speeds, as most of the noise is from turbulence and a function of the difference in velocity between the air accelerated through the rotors and the ‘static’ air surrounding it.

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In practice, the perceived noise is much (much) lower, basically inaudible: https://youtu.be/IX7rdJm5v6s?si=-eGOJKYidDWTRmHw&t=91

I should have said explicitly, rather the subsuming it in all else being equal, that it’s _very_ different between ‘multicopter’ mode and flying under a wing. The later is, of course, much mor efficient, and the propulsion is correspondingly much quieter.

How much of the noise of a helicopter is from the sonic booms of the rotor tips? I read somewhere it’s a substantial portion but I may be misremembering.

Very little, and rarely, if I understand correctly. The characteristic thwop-thwop noise of many helicopters is from the blade passing through the vortex created by the previous blade. In some circumstances, this might lead to local and intermittent supersonic flow, but it's not typical. I think it also only happens when the vehicle is moving in a particular way, but I don't know how vortices propagate from a rotor blade, so I couldn't say how or when the supersonic flow is most likely to occur.

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