For extra clarification, this implies that the helicopter is far enough that you can’t see it and not close enough for the sound to be obvious from where it’s coming from.
In: Physics
It’s not omnidirectional. The sound produced by a helicopter is certainly directional. It might be harder to pinpoint the direction than someone shouting, but that’s generally because it’s out in the open and reflection of sound from nearby surfaces is a lot less impactful than the hit it takes from the distance from the object itself.
I am not 100% certain if this has anything to do with what you hear. I used to be a helicopter mechanic in the Army. I am going completely on how I remember the rotor system working. A hub will have a link to each blade raising and lowering it for the desired pitch causing the lift. There is a round plate that will push up on one side and on the opposite side, pulls down. This is called the swash plate. In order to have each blade at a constant angle, you would have to have an independent system that would rotate with the blades. This cannot be done. On a 2 blade hub, as the blades rotate, say at the 0/180 degree point in the rotation, the blades will be at a neutral point (horizontal). As it rotates to the 90/270 degree position, the blades will be pushed up to the desired pitch. If the constant pitching of the blades is what causes the sound, it would not necessarily be omnidirectional but, will be coming from 2 sources going in opposite directions.
I could be wrong but, this is how I remember the power-train system being constructed.
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