Download and Configure FFmpeg SOFA Spatial Audio

Spatially Oriented Format for Acoustics (SOFA) files allow FFmpeg’s sofalizer filter to create immersive, 3D binaural audio from multi-channel audio inputs. This guide provides a direct walkthrough on where to download custom SOFA database files and how to configure them using FFmpeg commands to achieve high-quality spatial audio.

1. Where to Download SOFA Files

To use spatial audio in FFmpeg, you need a .sofa file containing Head-Related Transfer Functions (HRTF). These files map how sound waves interact with human ears to mimic 3D space. You can download standardized SOFA files from several reputable repositories:

• SOFA Acoustics Union: The official website (sofaconventions.org) hosts public databases containing HRTF measurements from various research institutions.
• IRCAM LISTEN Database: A widely used database containing high-quality HRTF measurements taken from both human subjects and dummy heads (like KEMAR).
• Acoustic Research Institute (ARI): Offers a comprehensive collection of HRTF measurements compatible with the SOFA standard.

Download your preferred .sofa file (e.g., KEMAR_HRTF.sofa) and save it to a dedicated folder on your computer.

2. Configuring the FFmpeg Sofalizer Filter

FFmpeg utilizes the sofalizer audio filter to apply the spatial acoustics of a SOFA file to multi-channel audio (such as 5.1 or 7.1 surround sound), rendering it into a 2-channel binaural stereo output.

The basic command syntax is:

ffmpeg -i input_51.wav -af "sofalizer=sofa=/path/to/your/file.sofa" output_spatial.wav

3. Advanced Configuration Parameters

The sofalizer filter supports several parameters to fine-tune the spatial rendering. Parameters are appended to the filter, separated by colons:

• sofa: The absolute path to your downloaded .sofa file. On Windows, use forward slashes (e.g., C:/audio/file.sofa).
• gain: Adjusts the output gain in dB (default is 0). Spatialization can sometimes reduce the perceived volume; a gain of 3 to 9 can help normalize the output.
• type: Sets the processing domain. Options are time (time-domain processing, which is the default) or freq (frequency-domain processing, which is faster for larger SOFA files).
• speakers: Allows you to manually define the layout coordinates of the input speakers if you are using a non-standard layout.

Example with advanced parameters:

ffmpeg -i input_71.mp4 -af "sofalizer=sofa='/usr/share/sofa/kemar.sofa':gain=6:type=freq" output_binaural.mp3

4. Testing the Output

To verify that the configuration worked, listen to the output file using stereo headphones. The multi-channel source audio should now sound as if the speakers are placed around you in a physical room, rather than inside your head.