Field investigation on the high-speed train‑induced aerodynamic pressures and sonic booms in tunnels with different lengths

The wave steepness enables the initial compression wave (ICW) to coalesce into a weak shock wave and induce a sonic boom. Field tests were conducted in five tunnels longer than 10.0 km. Results imply that amplitude of ICW is not sensitive to the spatial location. Distortion enables weak shock waves occuring when the distortion rate exceeds 1.0 and keeps increasing. The maximum pressure gradient approaches 198.25 kPa/s. Sonic boom happens inside the tunnel and can be detected outside the tunnel depending on its magnitude of A-weighted sound pressure level Lp−A. Within the tunnel, Lp−A of sonic booms first increases and then decreases, with the peak value reaches 102.4 dBA. Sonic booms are classified into four typical levels, i.e. critical value 60 dBA, slight booms within 60 ~ 70 dBA, clear booms within 70 ~ 90 dBA, and explosive bangs beyond 90 dBA. Loudness of the explosive bangs in present tunnels is less than 80 phon.