The main stage faced south. A warm southerly wind blew into the audience's faces at 20 km/h. The sound engineer mixed from 40 meters out and everything sounded perfect at his position. Fans at 100 meters reported the sound was quiet and thin. Meanwhile, residents 500 meters behind the stage called the police about excessive noise. Sound was traveling backwards.

Wind doesn't push sound — it bends it. Sound traveling into the wind encounters a velocity gradient: the wind is slower near the ground (friction) and faster at height. This makes sound waves curve upward, creating a shadow zone where SPL drops dramatically. Sound traveling with the wind (behind the stage) encounters the opposite gradient and curves downward, concentrating energy at ground level far from the source.

The result was a 15 dB difference between upwind and downwind at 200 meters. The audience in front of the stage (upwind) experienced progressively quieter sound past 80 meters. The neighborhood behind the stage (downwind) received focused bass energy with enough level to rattle windows. The sound engineer's mix position, at only 40 meters, was inside the direct-field zone and unaffected by refraction — he had no idea anything was wrong.

Wind refraction follows Snell's law applied to the atmospheric velocity profile. Even moderate winds (15-20 km/h) create significant effects at festival distances.

The Moral: Outdoor sound doesn't travel in straight lines when wind is present. Use SonaVyx Transfer Function at multiple audience positions throughout the event — conditions change, and so should your tuning.