It turns out Amnesia: The Bunker has been quietly stressing players out using a well-known sound illusion employed in scary movies as well as Super Mario 64 to create suspense.
Amnesia: The Bunker creative lead Fredrik Olsson revealed that the latest entry in the venerable horror series uses a trick called a "Shepard Tone" to create the illusion of a sound that ascends or descends perpetually. The purpose of it is typically to heighten the player, viewer, or listener's level of tension, with just one example being the 'endless stairs' obstacle in Mario 64's Mushroom Castle.
As Olsson explained in a tweet, the Shepard Tone can faintly be heard in Amnesia: The Bunker at all times when the generator - itself one of the scariest innovations in horror in recent memory - is powered up and the lights are on. The desired effect is to create persistent tension even in times of relative safety. It's played at such a low volume that you probably don't even notice it but are nonetheless uneased.
"Our hope is that this detail helps adding tension for times when you are playing with the lights on. In addition we believe the contrast of no longer hearing the shepard tone helps adding punch to the moments when the lights suddenly go out," explained Olsson.
As Olsson pointed out, the illusion is commonly used in films, one of the best examples being Christopher Nolan's Dunkirk. I also recall it being used in The Exorcist, which is commonly considered one of the best horror movies of all time – not that that's attributable solely to the Shepard Tone, but it probably helped.
For more frights, check out our guide to the best horror games ever made.
