The frequency of sound that humans typically cannot hear is known as infrasound. Infrasound refers to sound waves with frequencies below the lower limit of human audibility, which is generally considered to be around 20 hertz (Hz). Infrasound can be generated by various natural and man-made sources, such as earthquakes, volcanic eruptions, wind, and certain machinery. While humans may not consciously perceive infrasound, some studies suggest that it can still have subtle effects on our bodies and emotions.