silencer

noun5

1. •
   Something that silences or (notionally) nearly silences.
2. •
   An attachment to a gun that reduces the sound it emits.
3. •
   The muffler of an internal combustion engine.

   • Before the fitting of silencers to these locomotives was completed by the end of 1961, they were very noisy in operation, but the effect of the silencer has been remarkable. — 1962
4. •
   Any of various devices to silence the humming noise of telegraph wires.
5. •
   Something that prevents or greatly reduces gene expression.