Methods of reducing tunnel boom include making the train's profile highly aerodynamic, adding hoods to tunnel entrances, installing perforated walls at tunnel exits, and drilling vent holes in the tunnel (similar to fitting a silencer on a firearm, but on a far bigger scale). The HS2 project in the United Kingdom has developed "porous portal" tunnel hoods to mitigate tunnel boom for residents, as well as minimising aural discomfort for passengers that could arise from in-train air pressure changes.