Piston effect platforms with a single tunnel, however, air quality worsens when relying on the piston effect alone for ventilation. This still allows for potential energy savings by taking advantage of the piston effect rather than mechanical ventilation where possible. Tunnel boom is a loud boom sometimes generated by high-speed trains when they exit tunnels. These shock waves can disturb nearby residents and damage trains and nearby structures. The perception of this sound by humans is similar to that of a sonic boom from supersonic aircraft, but unlike a sonic boom, tunnel boom is not caused by trains exceeding the speed of
Undercompressive shock wave Undercompressive shock wave An undercompressive shock wave is a shock wave that does not fulfill the Peter Lax conditions. Ordinary shock waves are stable, that is the speeds of characteristic behind the shock are greater than that of the shock itself, which is greater than the characteristics in front of the shock. The characteristic speed is the speed of small, travelling perturbations. These conditions are necessary for a shock wave to remain and not decay. If the peak of a wave moves faster than at its base, then the wave front becomes self-sharpening and eventually becomes a nearly discontinuous shock,