If your field is noise sensitive, you would need to consider a number of things that generate noise:
1) Exhaust system: The MVVS 3204 is the lowest noise system As far as I know
2) propeller: Use 3 bladed props, and keep rpm low (below 6500) Some props are noisier than others, so it pays to ask around
3) airframe resonance. this often generates more noise than you may be aware of. It can be influences by prop balance, engine mounting etc etc.

When you approach 80 dB(a) at 7 meters, the above factors are equally important.