Definitions for "Infrasound"
People miles away from the Halifax Explosion reported "hearing" it. What they heard was infrasound, as the shock bounced off the atmosphere and back to earth. The phenomenon was reported over a hundred miles (160 km) away.
Sound waves with frequencies lower than those that elicit normal tonal perceptions in humans. In general, lower than about 20 Hz.
sound so low that humans cannot hear it