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Infrasound

Updated Jun 14, 2026

Infrasound is defined as sound waves characterized by frequencies below the lower limit of human audibility, which is generally accepted as 20 Hz. While these waves remain imperceptible to the human ear as distinct tones, they possess unique physical properties that allow them to travel vast distances through the atmosphere, oceans, and solid earth with minimal attenuation.

On This Page

  1. Short Definition
  2. Detailed Explanation
  3. Key Formula / Syntax / Principle
  4. Important Characteristics
  5. Practical Example
  6. Common Confusions or Misconceptions
  7. Related Terms
  8. Why It Matters

Short Definition

Infrasound occupies the frequency spectrum below 20 Hz. Because of its long wavelengths, it is less susceptible to scattering and absorption by obstacles compared to higher-frequency sound, enabling it to propagate over thousands of kilometers. It is generated by both natural phenomena—such as volcanic eruptions and seismic activity—and anthropogenic sources, including industrial machinery and atmospheric explosions.

Detailed Explanation

The study of infrasound, known as infrasonics, focuses on the propagation and detection of these long-period waves. Sound waves are mechanical vibrations transmitted through a medium. In the infrasonic range, the wavelength is exceptionally long; for example, at 1 Hz, the wavelength in air is approximately 340 meters. This length allows the waves to diffract around large structures and travel through the atmosphere without losing significant energy.

Atmospheric infrasound often travels through "ducts" created by temperature and wind gradients in the upper atmosphere. This phenomenon allows infrasonic signals to be detected by sensitive barometric sensors thousands of miles from the original source. In the ocean, infrasonic waves can propagate through the SOFAR (Sound Fixing and Ranging) channel, a horizontal layer of water where the speed of sound is at a minimum, acting as a waveguide.

Key Formula / Syntax / Principle

The relationship between frequency (##f##), wavelength (##\lambda##), and the speed of sound (##v##) is governed by the fundamental wave equation:

###v = f \cdot \lambda###

For infrasound, because ##f < 20 \text{ Hz}##, the wavelength ##\lambda## is inversely proportional and becomes very large. The propagation of these waves is also influenced by the atmospheric density and the adiabatic gas law. The pressure fluctuations (##p##) associated with an infrasonic wave can be expressed as:

###p(t) = P_0 \sin(2\pi ft + \phi)###

Where ##P_0## is the amplitude of the pressure variation, ##f## is frequency, and ##\phi## is the phase constant.

Important Characteristics

Infrasound differs significantly from audible sound in how it interacts with the environment and biological organisms. The following table summarizes the primary physical distinctions.

Characteristic Infrasound Audible Sound
Frequency Range < 20 Hz 20 Hz – 20,000 Hz
Attenuation Very Low High
Primary Travel Path Atmospheric/Tectonic Line of Sight/Direct

Additional notable characteristics include:

  • Long-Distance Propagation: Due to low frequency-dependent attenuation, infrasound can traverse the globe.
  • Structural Resonance: Infrasonic waves can cause large structures, such as bridges or buildings, to vibrate if the wave frequency matches the structure's natural resonance.
  • Atmospheric Coupling: Infrasound effectively couples energy from large-scale atmospheric events into the ground.

Practical Example

A prominent example of infrasound detection is the global monitoring network used to verify the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The International Monitoring System (IMS) employs arrays of microbarometers—highly sensitive pressure sensors—to detect infrasonic signals generated by atmospheric nuclear tests. Because these waves travel around the world, the IMS can triangulate the location of an explosion by analyzing the time-of-arrival differences across multiple stations.

Source Type Examples
Natural Volcanoes, earthquakes, meteors, ocean waves (microbaroms)
Anthropogenic Nuclear tests, mining blasts, aircraft, heavy machinery

Common Confusions or Misconceptions

A persistent misconception is that infrasound is inherently harmful or "deadly." While high-intensity infrasound can cause discomfort, dizziness, or nausea—often attributed to the vibration of internal organs or the resonance of the middle ear—it does not possess the capacity to "shatter" objects or cause the extreme biological damage often depicted in popular media. Furthermore, infrasound is frequently confused with "low-frequency noise" (LFN), which typically refers to the range between 20 Hz and 200 Hz, where sound is still audible but difficult to localize.

  • Ultrasound: Acoustic energy with frequencies above the human hearing range (typically > 20 kHz).
  • Microbaroms: Continuous, low-frequency pressure oscillations in the atmosphere generated by ocean wave interactions.
  • Acoustic Resonance: The tendency of a system to oscillate at maximum amplitude at specific frequencies.
  • Seismic Waves: Vibrations that travel through the Earth; infrasound is often generated at the atmosphere-earth interface by these waves.

Why It Matters

Understanding infrasound is critical for both civil and scientific applications. In meteorology, it provides insights into severe weather patterns, such as the formation of tornadoes. In geophysics, it serves as a tool for monitoring volcanic activity, allowing scientists to detect eruptions before they become visible to satellites. Furthermore, advancements in infrasound sensing technology enhance our ability to monitor global security, ensuring compliance with international treaties regarding nuclear testing.

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