Infrasound: low-frequency sound below the range of human hearing

Characteristics

Because their wavelengths are long, infrasound waves travel far and can diffract around obstacles and follow terrain. Long wavelengths also mean they lose energy more slowly in the atmosphere than higher-frequency sounds. Detection typically uses sensitive pressure sensors or microbarometers rather than ordinary microphones. For human perception and thresholds see human hearing limits and for amplitude-related effects consult studies summarized at reference materials.

Sources and examples

Natural and artificial phenomena both produce infrasound. Common natural sources include:

• Geophysical events: earthquakes, volcanic eruptions, avalanches and large storms.
• Astronomical/atmospheric: meteors and bolides entering the atmosphere.
• Biological: some large animals use infrasound for long-range communication — notably elephants and some whale species.

Human-made sources include industrial machinery, large fans or turbines, explosions and supersonic aircraft; further reading on biological signals is available at whale communication and elephant communication.

Uses, detection and importance

Infrasound monitoring plays a role in scientific research and practical monitoring. Worldwide infrasound arrays help detect volcanic eruptions, atmospheric explosions and meteor events, and contribute to treaty verification systems for explosive testing. Instrument networks and signal processing methods are summarized in overviews such as instrumentation reviews and technical guides at monitoring resources.

Effects, research and notable facts

At high amplitudes infrasound can produce physical sensations — pressure in the ears, vibration or unease — but claims about specific psychological effects remain debated and require careful experimental support. Because infrasound propagates efficiently over distance, it is valuable for remote sensing and for studying long-range animal communication. For additional context and summaries of key studies see listed resources.