Infrasound
Infrasound refers to sound waves with frequencies below the lower limit of human hearing, typically below 20 Hz, including frequencies as low as 0.001 Hz for geophysical phenomena. Although inaudible to humans without specialised equipment, infrasound can travel enormous distances through air, water, and the earth's crust with very little attenuation, making it valuable for monitoring nuclear explosions, volcanic eruptions, earthquakes, and severe weather. Many large animals — including elephants, whales, rhinos, and alligators — communicate via infrasound over hundreds of kilometres.
Key Formula
v = f × λ
LaTeX: v = f \lambda
| Symbol | Meaning | Unit |
|---|---|---|
| v | Speed of sound in the medium | m/s |
| f | Frequency of the infrasound wave | Hz |
| \lambda | Wavelength of the infrasound wave | m |
Worked Example
Problem
An elephant produces infrasound at 14 Hz. The speed of sound in air is 340 m/s. What is the wavelength of this infrasound, and how does it compare to the height of a two-storey building (~7 m)?
Solution
Step 1: Use λ = v / f. Step 2: λ = 340 / 14 ≈ 24.3 m. Step 3: Compare: 24.3 m is about 3.5 times the height of a two-storey building. Conclusion: The extremely long wavelength means infrasound diffracts easily around obstacles and travels great distances.
Answer
Wavelength ≈ 24.3 m, about 3.5× taller than a two-storey building
Infrasound Sources and Characteristics
| Source | Frequency (Hz) | Wavelength (km) | Detectable Range | Detection Method |
|---|---|---|---|---|
| Nuclear explosion | 0.01 – 1 | 0.34 – 34 | Global (thousands of km) | Infrasound arrays (IMS) |
| Volcanic eruption | 0.1 – 10 | 0.034 – 3.4 | Regional (500–5000 km) | Seismo-acoustic sensors |
| Elephant call | 14 – 35 | 0.01 – 0.024 | Up to 10 km | Specialised microphones |
| Severe weather | 0.5 – 5 | 0.07 – 0.68 | 100–1000 km | Barometric sensors |
| Ocean waves (microseisms) | 0.05 – 0.2 | 1.7 – 6.8 | Global | Seismometers |
| Wind turbines | 0.5 – 8 | 0.04 – 0.68 | Up to 10 km | Low-freq microphones |
Interactive Tools
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Related Terms
Ultrasound
Ultrasound refers to sound waves with frequencies above the upper limit of human hearing, typically above 20,000 Hz (20 kHz), extending to several gigahertz in specialised applications. Because of its high frequency and corresponding short wavelength, ultrasound can resolve fine structural details and is strongly absorbed or reflected by tissue boundaries, making it invaluable in medical diagnostics (obstetric scans, echocardiography), industrial non-destructive testing, sonar navigation, and the sonication used in cleaning and chemical processing.
Sound Intensity
Sound intensity is the power carried by a sound wave per unit area perpendicular to the direction of propagation, measured in watts per square metre (W/m²). It quantifies how much acoustic energy passes through a given surface each second and decreases with the square of the distance from a point source — the inverse square law. Sound intensity is the physical basis for the decibel scale and is central to audiology, architectural acoustics, and occupational noise exposure standards.
Wave Diffraction
Wave diffraction is the bending and spreading of waves around obstacles or through openings, occurring most prominently when the wavelength of the wave is comparable in size to the obstacle or aperture. The phenomenon is a direct consequence of Huygens's principle, which states that every point on a wavefront acts as a source of secondary wavelets. Diffraction is exploited in X-ray crystallography to determine molecular structures, in radio communication to allow signals to travel around hills, and in optical instruments to understand resolution limits.
From Latin "infra" (below, beneath) + "sonus" (sound). The term distinguishes frequencies below the auditory range, just as "ultra-" marks those above. Scientific study of infrasound expanded significantly after the 1963 Partial Nuclear Test Ban Treaty required atmospheric monitoring systems.