Use of small strips of fo.Q material is highly recommended. As the top plates of US spec receptacles are usually held by only one screw the edges will unavoidably resonate. Adding strips of fo.Q across the edges of the top plate to the wall or outlet frame will damp this resonance. Be careful not to overdamp however as this can deaden the sound. This phenomena may also be why slightly loosening the top plate screw can sound better the having it fully tightened