Silencio Original Earmuff Hearing Protector RBW-71
Because of all the noisemakers I've assembled, I saw fit to invest in a pair of earmuffs for my friend who sometimes likes to see (hear) the things in action.
These are non-electronic shooter's muffs, with a noise reduction rating (NRR) of 25dB.
I've tried these, and I don't think they're nearly as comfortable as the Leightnings, but they're not bad at all.
Everyone in the vicinity of loud noises should have ear protection.