Yeah, the more helicopters in the air, in the same area, the more impressive, and noticable, they are. I was working at a place that was en route to the fort and one day I heard this strange rumble. It wasn't clearly distinguishable what was making the rumble, but I could hear it, inside a building.
I wandered around the building, attempting to determine the source of the rumble, and realized it was coming from outside. I went out, just in enough time to see a flight of 14 Blackhawk helicopters, all in a fairly tight, cluster formation, going overhead.
Now, I heard these helicopters coming, even though I didn't know what they were, for about a minute and a half to two minutes before I saw them. They weren't moving at a leisurely pace, so I have to assume that they were moving at normal cruising speed. That means that a cluster of 14 of them could probably be heard at 10-14 miles distance, possibly more.
So if you get enough of them, helicopters can be pretty impressive.
Oh yeah, I forgot to mention the A-Star helicopters and the "Jolly Green Giant". I worked as an aircraft refueler for a while, so I get very familiar with their sounds. A-Stars are pretty quiet, at least the type I refueled. They flew over the city on tourist flights, so they may have been modified to be quieter. A-Stars can be heard at probably 2-3 miles during the day. A mile or so more at night. The lone Jolly Green that I refueled was plenty loud up close, but I'd estimate that the sound pretty much disappeared after about 4-5 miles. I only saw it during the day, never at night, so I don't know how it sounded at night.
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