Adjust Speaker Positions in FFmpeg Binaural Filter

This article provides a practical guide on adjusting virtual speaker positions in FFmpeg using the sofalizer audio filter. By utilizing custom azimuth and elevation coordinates alongside a SOFA (Spatially Oriented Format for Acoustics) impulse response file, you can precisely control the spatial placement of audio channels in a binaural mix to create an immersive 3D listening experience over headphones.

Prerequisites

To place virtual speakers in 3D space, FFmpeg requires a SOFA database file containing Head-Related Transfer Functions (HRTF). You can download standardized .sofa files freely from sources like the SOFA Acoustics website (for example, the Club Fritz or ARI databases).

The sofalizer Filter Coordinate System

Speaker coordinates in the sofalizer filter are expressed using a spherical coordinate system:

  • Azimuth: The horizontal angle in degrees. 0 is directly in front of the listener, positive values (up to 180) rotate to the left, and negative values (down to -180) rotate to the right. Alternatively, positive values from 0 to 360 can be used counter-clockwise.
  • Elevation: The vertical angle in degrees. 0 is at ear level, positive values (up to 90) move up towards the ceiling, and negative values (down to -90) move down toward the floor.
  • Distance: The distance from the listener's head in meters (default is typically embedded in the SOFA file).

Adjusting Speaker Positions with the speakers Option

The speakers parameter lets you define the exact coordinates for each input channel. The syntax takes channel position definitions separated by a vertical bar (|), where each entry specifies the azimuth and elevation:

speakers=CH_NAME AZIMUTH ELEVATION|CH_NAME AZIMUTH ELEVATION|...

Common channel names include: FL (Front Left), FR (Front Right), FC (Front Center), LFE (Low Frequency Effects), BL (Back Left), BR (Back Right), SL (Surround Left), and SR (Surround Right).

Example: Adjusting a 5.1 Surround Setup

By default, standard 5.1 placement positions the front speakers at ±30 degrees and surround speakers at ±110 degrees. If you want to widen the front stage, elevate the center channel, and push the surround channels directly to the sides at ear level, define custom angles like this:

ffmpeg -i surround_input.wav -af "sofalizer=sofa=/path/to/file.sofa:speakers=FL 45 0|FR -45 0|FC 0 15|LFE 0 -20|BL 90 0|BR -90 0" output_binaural.wav

In this command:

  • FL 45 0: Front Left is placed 45° to the left at ear level.
  • FR -45 0: Front Right is placed 45° to the right at ear level.
  • FC 0 15: Front Center is placed directly ahead, angled 15° upwards.
  • LFE 0 -20: Subwoofer audio is placed directly ahead, angled 20° downwards.
  • BL 90 0: Back Left is moved directly to the left side (90°).
  • BR -90 0: Back Right is moved directly to the right side (-90°).

Adjusting Global Rotation with rotation and elevation

If you want to maintain default speaker layouts but reorient the entire sound field (for example, head tracking simulation or creating an elevated mix), adjust the overall soundstage orientation using the rotation and elevation parameters:

  • rotation: Rotates the virtual listener's head horizontally (yaw).
  • elevation: Tilts the virtual listener's head vertically (pitch).
ffmpeg -i surround_input.wav -af "sofalizer=sofa=/path/to/file.sofa:rotation=15:elevation=5" output_binaural.wav

This tilts the entire virtual speaker rig 15 degrees horizontally and 5 degrees upwards relative to the listener.

Key Parameter Tuning Tips

  • Interpolation (type): If the custom coordinates do not exist precisely in your SOFA file's measurement grid, enable bilinear interpolation using type=b to avoid spatial artifacts.
  • Gain adjustment (gain): Summing multiple spatial channels into binaural stereo can cause clipping. Add gain=-3 (or lower) inside the filter argument to maintain dynamic headroom.