How to Use FFmpeg SOFA Filter for HRTF Spatial Audio

This guide explains how to use the sofa audio filter in FFmpeg to apply a Head-Related Transfer Function (HRTF) to multichannel audio. By using Spatially Oriented Format for Acoustics (SOFA) files, you can convert surround sound formats (such as 5.1 or 7.1) into immersive, 3D spatial audio optimized for playback on standard stereo headphones.

Step 1: Obtain a SOFA File

To apply an HRTF, you need a .sofa file containing the acoustic measurements of how human ears perceive sound from different angles. You can download standardized SOFA files from public databases, such as the MIT KEMAR dummy head measurements or the SADIE II database.

Step 2: Basic FFmpeg Syntax

The basic syntax for applying the sofa filter requires a multichannel input file, the path to your downloaded .sofa file, and a stereo output destination.

Run the following command in your terminal:

ffmpeg -i input_51.wav -af "sofa=sofa=/path/to/kemar.sofa" output_binaural.wav

In this command: * -i input_51.wav specifies your multi-channel source audio (e.g., 5.1 or 7.1 surround sound). * -af invokes the audio filter graph. * sofa=sofa=/path/to/kemar.sofa points the filter to your specific HRTF dataset. * output_binaural.wav is the resulting 3D spatialized stereo audio file.

Step 3: Adjusting Filter Parameters (Optional)

The sofa filter offers several options to fine-tune the spatialization process. You can append these parameters using colons inside the filter arguments.

Adjusting Gain

Spatialization can sometimes cause audio clipping or volume drops. You can manage the output volume with the gain parameter (measured in dB):

ffmpeg -i input_51.wav -af "sofa=sofa=kemar.sofa:gain=-6" output_binaural.wav

Changing Processing Type

You can choose between time-domain and frequency-domain processing using the type parameter. Frequency-domain processing is typically faster for long impulse responses:

Example:

ffmpeg -i input_51.wav -af "sofa=sofa=kemar.sofa:type=freq" output_binaural.wav

Video and Audio Integration

If you want to apply the spatial filter to the audio track of a video file while copying the video stream without re-encoding, use the following command:

ffmpeg -i input_video.mp4 -af "sofa=sofa=kemar.sofa" -c:v copy -c:a aac output_spatial.mp4