He Sells Silence -- A "gun" that shoots out sound - Douglas Smart fits a blank cartridge into a reflector. The noise of the shot can be accurately aimed. Restoring quiet in a noisy world is the unusual occupation of Mr. Douglas Smart. As one of the few architectural acoustical engineers in Britain, he often finds himself a keeper of the peace. Once he was called in to treat a council house after rebellious tenants, demanding privacy, had refused to pay rent. In his 20 years' experience, Mr. Smar