Blackhawk v Huey

A flight of three UH-60 Blackhawks performed a flyover this morning
during our local Independence Day parade.
It struck me that they were able to get within one-half mile
(thereabouts) before I noticed the "helicopter" sound.
I had noticed this previously, but had filed it away.
I then got to thinking about the old UH-1 Huey's and how they could be
heard miles before you could see them.
Can someone definitively tell me what improvements were made to the
UH-60's to make them quieter than the UH-1's?
What is it about the main rotor blades and/or tail rotor blades that has
been changed?