A specialized tunnel entrance featuring a series of vents or a flared, perforated structure. Fast-moving trains push a massive wave of air ahead of them upon entering an underground passage. The porous portal bleeds off this aerodynamic pressure gradually, preventing a loud sonic boom from exiting the far end of the tunnel and reducing structural stress on the vehicles.
Heard on the job
The planned high-speed alignment requires a 50-foot porous portal on the west approach to mitigate the pressure wave.