How to Use FFmpeg SOFA Filter for HRTF Spatial Audio

This guide explains how to use FFmpeg's sofalizer filter to apply Head-Related Transfer Functions (HRTF) to multi-channel audio using standard SOFA (Spatially Oriented Format for Acoustics) files. You will learn the required prerequisites, the core command syntax, key filter parameters, and practical examples for rendering surround sound layouts into immersive binaural stereo for headphones.

Prerequisites

To use SOFA HRTF processing, your FFmpeg build must be compiled with libmysofa support. You can check if this library is available on your system by running:

ffmpeg -filters | grep sofalizer

If sofalizer appears in the list, your build is ready.

You also need a .sofa HRTF measurement file. High-quality open-source HRTF profiles can be downloaded from databases like the SOFA Conventions website or the Club Fritz database.

Basic Syntax

The sofalizer audio filter converts multi-channel audio tracks (such as 5.1, 7.1, or custom channel mappings) into a two-channel binaural audio track using spatial impulse responses.

The basic command structure is:

ffmpeg -i input_surround.wav -af "sofalizer=sofa=/path/to/hrtf.sofa" output_binaural.wav

Key sofalizer Parameters

You can fine-tune the spatial rendering using several parameters inside the filter string:

  • sofa: Specifies the absolute or relative path to the .sofa file.
  • type: Processing mode. Accepts time (time-domain convolution) or freq (frequency-domain processing). Frequency-domain (freq) is generally much faster and recommended for real-time or fast processing.
  • gain: Adjusts input/output gain in dB (e.g., gain=-6) to prevent digital clipping caused by acoustic summing.
  • radius: Distance in meters from the listener to the virtual sound sources (e.g., radius=1.5).
  • speakers: Allows manual definition of virtual speaker positions using azimuth and elevation coordinates if your source layout requires custom placement.

Practical Examples

1. Standard 5.1 to Binaural Stereo

Convert a standard 5.1 surround mix to 3D binaural stereo using frequency-domain processing and an attenuation gain of -4 dB:

ffmpeg -i movie_5.1.mkv -vn -af "sofalizer=sofa=dtf_nh2.sofa:type=freq:gain=-4" -c:a aac -b:a 320k binaural_audio.m4a

2. 7.1 Video Remuxing

Process a 7.1 audio stream from a video file into binaural stereo while copying the video stream without re-encoding:

ffmpeg -i input_7.1.mp4 -c:v copy -af "sofalizer=sofa=ClubFritz12.sofa:type=freq:gain=-6" -c:a libopus -b:a 192k output_binaural.mkv

3. Custom Speaker Placement

For non-standard audio channels, define the azimuth and elevation (in degrees) for each channel using the speakers parameter:

ffmpeg -i quad_surround.wav -af "sofalizer=sofa=default.sofa:speakers=45 0|-45 0|135 0|-135 0:type=freq" output.wav

In the speaker definition, each channel's target direction is separated by a pipe (|), where the first value is the azimuth (-180 to 180 degrees, relative to front-center) and the second value is the elevation (-90 to 90 degrees).