How to Use FFmpeg SOFA Filter for Custom HRTF

This guide explains how to use the spatial audio filters in FFmpeg to apply custom Head-Related Transfer Function (HRTF) data to multi-channel audio. By utilizing Spatially Oriented Format for Acoustics (SOFA) files, you can convert surround sound into an immersive, 3D binaural audio experience designed for standard stereo headphones. This article covers how to verify your FFmpeg installation, run the basic command syntax, and customize spatial parameters for optimal playback.

Step 1: Verify FFmpeg Support for SOFA

To read .sofa files, your FFmpeg build must be compiled with the libmysofa library. Verify that your installation supports the spatial filter by running this command in your terminal:

ffmpeg -filters | grep sofa

If the output lists sofaspatial, your FFmpeg installation is ready to process SOFA files.

Step 2: Acquire a SOFA File

The sofaspatial filter requires a .sofa database file containing the HRTF measurements of a human or dummy head (such as the KEMAR model). You can download free, standardized .sofa files from open-source acoustics databases like the SOPRANO project, the LISTEN database, or the Genelec Immersive Science database.

Step 3: Use the sofaspatial Filter

The sofaspatial filter is the most direct way to apply HRTF data in FFmpeg. It automatically maps the input channels of your source audio to virtual 3D coordinates based on the SOFA file’s configuration.

Run the following command to apply a SOFA file to a multi-channel (e.g., 5.1 or 7.1 surround) audio file:

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

Advanced Adjustments

You can customize the spatialization by appending specific parameters to the filter:

• sofa: The path to your .sofa file.
• gain: Adjusts the volume level in dB (default is 0). Because spatialization can cause digital clipping, lowering the gain is often necessary.
• rotation: Rotates the virtual listener’s head in degrees. The value format is “yaw, pitch, roll” (e.g., rotation=90 turns the listener’s head 90 degrees to the left).

Example with custom gain and head rotation:

ffmpeg -i input_surround.wav -af "sofaspatial=sofa=/path/to/database.sofa:gain=-6:rotation=45" output_binaural.wav

Step 4: Alternative Method Using the headphone Filter

For more precise manual control over how specific input channels map to the virtual space, you can use the headphone filter. This filter also supports SOFA files but requires you to explicitly map the input channels.

Use this syntax for mapping a standard 5.1 surround sound file:

ffmpeg -i input_51.wav -filter_complex "headphone=map=FL|FR|FC|LFE|BL|BR:sofa=/path/to/database.sofa" output_binaural.wav

In this command: * The map parameter defines the exact sequence of channels to process: Front Left (FL), Front Right (FR), Front Center (FC), Low-Frequency Effects (LFE), Back Left (BL), and Back Right (BR).