Infrasound consists of sound waves with frequencies below the human hearing threshold, typically less than 20 Hertz (Hz). Though inaudible, these low-frequency waves carry substantial information. Infrasound is a valuable tool with applications across scientific and industrial fields. Its ability to travel long distances enables unique forms of monitoring and communication.
Understanding Infrasound
Infrasound encompasses frequencies below 20 Hz. These waves have long wavelengths, allowing them to propagate over vast distances with minimal energy loss. They can travel hundreds or even thousands of kilometers, unhindered by obstacles like mountains or buildings. Infrasound naturally originates from large-scale geophysical events such as volcanic eruptions, earthquakes, severe weather, ocean waves, and meteors. Human activities also generate infrasound from sources like large explosions, heavy machinery, wind turbines, and sonic booms.
Monitoring Earth and Atmosphere
Infrasound monitors Earth’s geological activity and atmospheric phenomena. Volcanic activity produces energetic infrasound signals that indicate eruptions, ash plumes, and internal magma movements. These signals are not affected by cloud cover, making infrasound a reliable tool for detecting and characterizing eruptions.
Infrasound also helps detect seismic events and track tsunamis. Atmospheric pressure waves from large oceanic earthquakes, which precede tsunamis, travel faster than the tsunami waves themselves. Detecting this infrasound provides 10 to 30 minutes of early warning, allowing for timely evacuation.
Atmospheric disturbances, such as severe thunderstorms and tornadoes, emit infrasound. Powerful storms generate infrasound that travels over a thousand kilometers, offering early detection and warning of hazardous weather. Infrasound also aids in detecting meteors and bolides as they enter and explode in the atmosphere. The blast waves from these events produce infrasound detectable thousands of kilometers away, providing data on their energy, location, and altitude.
Industrial and Scientific Applications
Infrasound has diverse applications beyond natural disaster monitoring. It is employed in nuclear test detection as part of the global monitoring system for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). This network detects clandestine explosions by analyzing their low-frequency acoustic signatures.
Atmospheric research utilizes infrasound to study the layers, winds, and temperature profiles of the atmosphere, including the stratosphere and thermosphere. Since infrasound propagation is influenced by atmospheric structure, scientists infer details about high-altitude conditions. In industry, infrasound monitors machinery and infrastructure. It detects malfunctions in heavy equipment, identifies pipeline leaks, or assesses the structural integrity of large facilities.
Some animal species use infrasound for communication. Elephants produce infrasonic rumbles to communicate over long distances, sometimes up to 10 kilometers. Whales also use infrasound to communicate across hundreds of miles underwater. Other animals that utilize these low-frequency sounds include:
Hippos
Rhinoceroses
Giraffes
Alligators
How Infrasound is Detected and Analyzed
Detecting and analyzing infrasound requires specialized equipment and sophisticated techniques. Infrasound sensors, called microbarometers, are highly sensitive microphones that register minute atmospheric pressure changes below 20 Hz.
To locate and characterize infrasound events, scientists deploy sensor arrays. These arrays consist of multiple microbarometers spread across an area. This configuration differentiates true infrasound signals from environmental noise, especially wind interference. The collected data undergoes advanced processing, including algorithms and filtering, to extract meaningful information. This includes frequency analysis, source localization, amplitude, and duration. This analysis allows scientists to interpret acoustic signatures, translating them into valuable insights for diverse applications.