Keith Oakley, from the UK Hoverclub , has done extensive work on the noise small hovercraft generate. He designed a semi circle frame with microphones attached, which was then mounted over a tethered integrated craft , this was then run up to full power and readings taken. The result was then a "picture" of where the most noise comes from. Typically the splitter plate, flow straighteners , drive in front of fan, blade tip speed , silencers etc. When all of these "hot" spots are dwelt with , you end up with a quiet cruiser but its not a racer ! I have found that a twin engine craft can be a quiet cruiser , the lift can be tucked away and using a large prop for thrust is so much better. Regards, Francois Malan