Abstract
This paper addresses the issue of ultra-low frequency sound waves inside railway tunnels. The infrasound spectrum shows two main features: 1) a Kolmogoroff −5/3 slope above a critical frequency and a flat spectrum below; 2) distinct resonance peaks at the tunnel axial eigenmode frequencies. These features suggest that a turbulent velocity pattern is induced by the velocity shear produced by the entry and exit of the train. The turbulence acts as a source for the acoustic wave equation in a cylindrical cavity. The spectral peaks at the cavity eigenfrequencies are due to the development of stationary waves. A power law p ∝ vα has been used to represent the functional dependence of peak pressure wave amplitudes on average train velocities in tunnels. Our data only suggest a value of α between 1 and 1.5. Total exposure times above 140 dB are of the order of 10 minutes/day. Values are below the hearing impairment threshold for continuous exposures of similar duration. One pressure peak exceeding 150 dB has been measured in association with a train encounter inside the tunnel.