State of the art
NOTAR
The MD NOTAR helicopters (e.g. MD 902) are hugely quieter than any other helicopter that I have observed.
I understand that they are slightly more costly to operate and so no one in the UK other than a few air ambulances countenance their use. I suspect that the reason that the air ambulances use them is nothing to do with their low noise footprint but is due to the reduced risk of striking the tail rotor on an obstacle during off-airport landings. There is no tail rotor and so it cannot strike anything:-)
My life would certainly be a lot quieter if the police also selected these aircraft, however I suppose that they perceive that increased noise is a benefit to their mission.