The term for a sound with a frequency below the lower limit of human hearing is "infrasound." Infrasound refers to vibrations that are below 20 Hz, which is the threshold of human hearing. Such frequencies are produced by various natural and artificial sources, including earthquakes, volcanic eruptions, and heavy machinery. Although humans cannot hear infrasound, it can still have physiological effects and is often utilized in scientific research, including studies of animal communication and environmental monitoring.