The festival organizer had done everything right — or so he thought. Four subwoofers in a broadside array, centered on stage, aimed at a field of 5,000 people. Sound check at noon was glorious. By showtime at 8 PM, something had changed.

A steady 15 km/h wind had developed. Not enough to notice physically, but enough to refract low-frequency sound downward on the upwind side and lift it over the audience on the downwind side. The result: bass existed in a single corridor about 20 meters wide, slightly left of center. One group of fans was having the best bass experience of their lives. Everyone else thought the subs were broken.

To compound matters, the subs were elevated on scaffolding at 2 meters height. This eliminated the ground-plane coupling that doubles acoustic output — a free 6 dB at low frequencies that outdoor engineers depend on. The subs were also spaced far enough apart that their combined pattern had lobes and nulls across the field.

Wind gradients create temperature and velocity differentials that bend sound waves. At low frequencies where wavelengths are long, even modest wind refracts bass significantly over 50+ meter distances. Ground-coupling requires the sub to be on (or very near) the ground.

The Moral: Outdoor bass is fragile. Measure at the mix position with SonaVyx Transfer Function at showtime, not just during sound check. Conditions change — your tuning should change with them.