Silencer is an invention created by Dexter Alvin Dizznee, first introduced in Book 9: Stellarlune.
It creates a sound barrier prevented anyone from hearing sounds unless within the set perimeter, presumably made to stop people from eavesdropping on conversations.
When turned on it creates a steady crackling hum, one similar to static.
“That's my silencer. It guarantees that no one can eavesdrop - over here at least.”
—Dexter Alvin Dizznee, in Book 9: Stellarlune, page(s) 27, paperback