I know they have better fuel economy because they are absorbing more of the ignited charges heat energy through that extra piston the essentially acts as an exhaust recovery turbine in the form of a non-powered piston. I figure there's be less energy available to make loud acoustic pops and also because the gases are allowed to expand down closer to atmospheric pressure that the lower frequency and pressure would also contribute to a far lower exhaust sound signature. Would I be correct in assuming this? What are your thoughts? I ask because backup generators (recently just had the power go out after some particularly powerful storms) are rediculously loud. I wonder if it's make sense to make 5 stroke generators to help with noise and with the same stone help with thermal efficiency.

Here is a link to what (I think is the only firm currently actively developing engines like this):

http://www.ilmor.co.uk/capabilities/5-stroke-engine

Also, yes, I'm aware that it's a novel design and that the economies of scale just wouldn't be there to make a economically viable product yet, I am just asking in general if they'd be quieter. This is more of a thought experiment than anything else. Also, I am well aware that in small engines also produce noise due to the air cooling and cheap construction allowing vibrations from the cams and crank, etc. to permeate outside the engine. Still, I think the pressure characteristics of such am engine design would do wonders for small engine noise, at least at the generator scale. Probably not for higher mobility requirements like lawnmowers and such. More complexity is only useful if it doesn't hurt performance elsewhere.