How to Use Custom SOFA Files in FFmpeg

FFmpeg allows users to create immersive, 3D binaural audio from multichannel sound sources using its integrated sofalizer filter. This guide covers how to locate and download custom Spatially Oriented Format for Acoustics (SOFA) database files containing personalized or standardized Head-Related Transfer Functions (HRTFs), verify your FFmpeg build requirements, and apply the files through proper command-line syntax.

1. Verify FFmpeg Requirements

To process custom SOFA files, FFmpeg must be compiled with the libmysofa library. Verify that your installation supports it by running:

ffmpeg -filters | grep sofalizer

If the output lists sofalizer, your FFmpeg build supports loading custom SOFA files. If it is missing, you must install an FFmpeg build configured with the --enable-libmysofa flag.

2. Download Custom SOFA Files

SOFA files contain measurement data that describes how sound reaches the eardrum from different angles, creating a 3D audio effect over stereo headphones. You can acquire free, scientifically validated SOFA files from several public databases:

  • SOFA Conventions Database: The official repository host files adhering to AES69-2015 standards, including measurements from various institutions.
  • ARI (Acoustics Research Institute): Offers high-resolution HRTF measurements for dozens of human subjects.
  • CIPIC HRTF Database: Contains individualized measurements across multiple human subjects and dummy heads (such as KEMAR).
  • SADIE II Database: High-quality studio measurements provided by the University of York, offering various dummy head and human HRTF datasets.

Download a .sofa file (for example, ClubFritz4.sofa or subject_003.sofa) and save it to an accessible folder on your system.

3. Configure the sofalizer Filter

The sofalizer filter accepts multichannel input (such as 5.1 or 7.1 surround sound) and maps the channels into virtual 3D space using the impulse responses from the designated SOFA file.

The basic syntax to map a 5.1 audio track to binaural stereo is:

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

4. Advanced Filter Configuration

You can customize how the SOFA file is processed using additional parameters within the filter string:

  • sofa: Specifies the absolute or relative path to the .sofa file.
  • type: Determines the processing mode. Set to freq for frequency-domain processing (faster, recommended for most systems) or time for time-domain convolution (higher precision, higher CPU usage).
  • gain: Adjusts the overall gain in dB to prevent digital clipping resulting from the convolution process (typically between -6 and -18).
  • speakers: Manually overrides virtual speaker azimuth and elevation positions using channel identifiers (e.g., FL|FR|FC|LFE|BL|BR).
  • rotation: Rotates the virtual listener's head in degrees horizontally (yaw).
  • elevation: Tilts the virtual listener's head up or down in degrees (pitch).

Example: Converting 7.1 Video to Binaural Stereo

To process a video file containing a 7.1 surround stream using frequency-domain convolution, custom speaker placements, and a -9 dB gain adjustment:

ffmpeg -i movie_7.1.mkv -c:v copy -af "sofalizer=sofa=/audio/HRTF/kemar.sofa:type=freq:gain=-9" -c:a aac -b:a 320k output_binaural.mp4

This configuration copies the original video stream without re-encoding, applies the HRTF data from kemar.sofa, and encodes the spatialized output into high-bitrate AAC stereo suitable for standard headphones.