Every killer projects a terror radius: within it, survivors hear a heartbeat that grows louder as the killer gets closer. It is the primary way survivors know a killer is nearby without seeing them.
Some killers have a smaller radius or can suppress it entirely, which makes them much harder to detect and lets them start chases from a position of advantage.
In a 1v1 duel the terror radius matters less than usual, because both players already know exactly where the other one is - the chase starts by agreement rather than by discovery. It still shapes the moment-to-moment read on distance during the chase itself.