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

Sound produced below human hearing or 20 Hertz.

Sound in the frequency range of 0.02 to 4 Hertz. One category of CTBT IMS stations will monitor sound at these frequencies with the aim of detecting explosive events such as a nuclear test explosion at a range up to 5000 km.

Sound that is lower than the range of human hearing (i.e. lower in frequency than about 20 hertz).

Sound of such low frequency that it falls outside the human range of hearing.

Sound at infrasonic frequencies.

Infrasound is sound with a frequency too low to be detected by the human ear. The study of such sound waves is sometimes referred to as infrasonics, covering sounds from the lower limit of human hearing (about 16 or 17 hertz) down to 0.001 hertz. This frequency range is the same one that seismographs use for monitoring earthquakes.