Smitty Smith, the force behind the Van’s Aircraft Builders website and YouTube channel, recently posted a video showing how they built a soundproofing box for the shop air compressor.
He notes at the beginning of the build for their aircraft, the noise from the compressor was so loud, the neighbors started to complain. That led to the soundproof box, which “really makes for happy neighbors and spouses,” Smitty says.
Soundproofing the compressor seems a fairly good way to reduce the noise. A much better way is to not buy a direct drive compressor. A belt driven compressor is way better, noise wise and otherwise in my opinion.