HRTF and 3D audio: How games place sounds behind the player

When footsteps can be heard behind the player in a VR game even though the speakers sit directly at the ears, a technique called HRTF is behind it. The abbreviation stands for “Head-Related Transfer Function,” in German kopfbezogene Übertragungsfunktion. It describes how the head, torso, and pinnae change a sound before it reaches the eardrum. Games simulate this change computationally and thus create the impression that a sound comes from a specific place in the room.

How humans hear direction

Hearing uses three types of cues to locate a sound source.

1. Interaural time difference (ITD): A sound from the left reaches the left ear a fraction of a second earlier than the right. The brain evaluates this difference of less than a millisecond. The effect is especially important at low frequencies.
2. Interaural level difference (ILD): The head shields sound. The ear facing away hears a sound quieter, especially at high frequencies.
3. Spectral cues: The folds of the pinna reflect sound differently depending on the angle of incidence. This amplifies or attenuates certain frequencies. This coloration reveals whether a tone comes from the front, back, above, or below.

Time and level differences alone are not enough to distinguish front from back. A tone exactly in front of the head and one exactly behind it produce the same differences at both ears. Only the spectral cues of the pinna resolve this confusion.

What an HRTF is

An HRTF is essentially a set of filters. For every direction from which a sound can come, it describes how the sound changes on the way to the left and right ear. It is classically measured with small microphones in the ears of a person or a dummy head while speakers play test tones from many directions.

A game takes a dry sound, such as a footstep, and applies the filter for the desired direction. Over headphones or the speakers of a headset, the footstep then sounds as if it comes from there. Because each ear is shaped slightly differently, most systems use a generic HRTF that works well for many people but is not equally precise for everyone.

Why head tracking makes the difference

In VR and mixed reality, a decisive factor is added: The headset continuously measures how the head is oriented. If the player turns their head, the system recalculates the direction of each sound. A sound source thus stays in its place in the room instead of moving with the head.

This coupling also helps with front-back confusion. Even a small head movement changes the time and level differences in a clear direction, and the brain can assign the source. In everyday life, people do this unconsciously when they cannot immediately locate a sound.

Distance and space

Besides direction, spatial sound needs cues for distance and environment. These include volume, the loss of high frequencies over distance, and the amount of reverberation. A whisper right at the ear sounds dry and present, a call from another room muffled and reverberant. Many audio systems for XR also simulate reflections off walls to reinforce the impression of an enclosed space.

Why horror benefits especially

Horror works with what the player does not see. A sound behind the back forces a decision: turn around or not. With spatial sound, this becomes a game mechanic.

The mixed reality game HAUNTED: Are You Alone? for Meta Quest uses spatial audio in this way. According to the studio, sounds come from real spots in the scanned room: footsteps from behind, knocking from inside a wall, voices under the floor. In the prologue, footsteps approach from behind while a voice on the phone whispers that you should not turn around. In the chapter “The Hands” and the arcade mode Arm Cutter, knocking announces from which wall the next arm will break. Those who listen closely gain time. HAUNTED is published by ThreeFlavors LLC.

The studio does not specify which audio engine the game uses internally. But the effect relies on the same principles: direction via HRTF filters, stability via head tracking, and anchoring sounds to the scanned surfaces of the room.

Limits of the technology

• Generic HRTFs: For some people, localization above, below, or behind works less reliably.
• Speakers instead of headphones: The open speakers of the Quest headsets are convenient but let ambient noise through. In a quiet environment, spatial sound works much better.
• Computational cost: Many simultaneous, filtered sound sources cost processing power. Games for standalone headsets must make trade-offs here.

The bottom line

HRTF-based sound uses timing, volume, and the filtering effect of the pinna to place sounds in the room. Head tracking keeps these sounds in place and resolves the typical front-back confusion. For horror games, this is a central tool: A sound behind the back often has more impact than any image. Mixed reality titles like HAUNTED even anchor such sounds to the walls and floors of the real room.