Hearing is the alarm for what you cannot see
Your eyes cover the space in front of you. Your ears cover everything, including the half of the world behind your head. That makes hearing our early-warning system: a sound from behind is, almost by definition, something we have not seen yet.
We are also worse at placing sounds that come from behind. Front and back can be confused, so a noise behind us is often less precisely located than one in front - and an uncertain threat is more unsettling than a clear one. The natural response is to turn around, which is exactly what the scene is daring you not to do.
Looming sounds
Studies of "looming" sounds - ones that grow louder as if approaching - have found that people tend to judge them as closer than they really are. The bias makes sense as a survival trick: overestimating how close something is gives you more time to react. Horror sound design exploits it with noises that swell from behind and stop just short of you.
Quiet beats loud
Loudness makes you jump; quietness makes you listen. A soft whisper or a footstep in a silent room forces you to strain to hear it, and that straining is a form of attention - fear needs attention to work. That is why the best horror soundtracks spend most of their time very quiet.
Getting sound behind the sofa at home
- A surround system with rear speakers behind the seating is the most effective. Make sure the rear channels are actually switched on and loud enough - many setups leave them too quiet.
- AirPods connected to Apple TV can place sounds around the listener with spatial audio on supported models - effective, but only for the person wearing them.
- TV speakers alone cannot put sound behind you. Lean on silence and sudden quiet sounds instead.
Using it at a party
Keep the ambient sound - rain, wind, a crackling fire - at a comfortable level, so the room feels calm. The contrast is what makes the quiet sound from behind land. More on building the evening in how to pull off a Halloween prank on the TV.
Haunted TV plays rain, wind and fire all evening, with soft music if you like, and its scares - footsteps behind you, a whisper over your shoulder - can come from behind with surround sound or AirPods.