If you really want to get scientifig about it, see Martin Hepperle's home page. He has a java application to design silencers. It really should be easy to quiet a heli, as the rev range is limited. If you use a governor it should even be possible to fine-tune the rpm to a very sharply tuned silencer.
http://www.mh-aerotools.de/airfoils/software.htm