infrasonic

Infrasonic, or infrasound, refers to sound waves with frequencies below the lower limit of human hearing, about 20 Hz. These waves have long wavelengths and can propagate over long distances through the atmosphere with relatively low attenuation, especially when atmospheric conditions favor ducting. Detection relies on sensitive instruments such as microbarometers and infrasonic arrays that record tiny pressure fluctuations.

Natural sources include ocean swells and storms, volcanic eruptions, earthquakes, meteor explosions, and large-scale avalanche or glacier calving. These events impart energy into the atmosphere and generate infrasonic waves that can be detected thousands of kilometers away under favorable conditions.

Propagation is strongly influenced by the atmosphere. Temperature, wind shear, and humidity create refraction and ducting, which can channel waves along layers or around obstacles. Infrasound can travel in air, sometimes via ground coupling, and is detected at distant locations with networked sensor arrays to determine source direction and timing.

Applications include hazard monitoring and scientific research. Infrasound is used to monitor volcanic eruptions and meteorological phenomena, and in international security, the Comprehensive Nuclear-Test-Ban Treaty Organization operates an International Monitoring System with infrasound stations to detect clandestine nuclear tests. Some animals, notably elephants and some whale species, communicate using very low-frequency infrasound.

Measurement and safety: typical infrasonic levels in the environment are below human perception, but high-amplitude events can cause felt vibrations or discomfort and may affect structures. Instruments like microbarometers measure pressure changes in pascals; data are often reported as sound pressure levels relative to a reference pressure.