Infrasound
Sound waves too low-frequency for human ears to hear, typically below 20 Hz. Often associated with natural phenomena like earthquakes, wind, or ocean waves.
Sound waves with frequencies below approximately 20 Hz, below the lower limit of human hearing. Infrasound occurs naturally from earthquakes, ocean waves, severe weather, and animal vocalizations, and can be detected only with specialized equipment. Though inaudible, it may produce physical sensations or physiological effects in humans at high intensities.
What this means in real life
When a distant thunderstorm rumbles so low you feel vibrations in your chest rather than hear a clear sound, that's infrasound—your body senses the pressure waves even though your ears cannot process them as sound.
What it isn’t
Infrasound is not ultrasound (high-frequency sound above human hearing). It's also not inaudible because it's quiet; it's inaudible because its frequency is fundamentally outside the human ear's detection range, regardless of volume.
Commonly misused online
Social media often claims infrasound causes unexplained fear or 'haunted house' sensations, conflating speculative physiological effects with proven scientific fact. While intense infrasound may cause discomfort, the link to paranormal experiences remains unproven.