Improvised silencers.

Homemade suppressors

I think the joke here is that any monitoring the FBI may be doing on the author is going to just end up being confusing for them, because the search makes no obvious sense, hence the goofy, smug grin from the character in the right side.

Edit: although other comments suggest the filters can be used for other, possibly illegal uses, which could also result in the goofy, smug grin