Infrasound

An infrasound is a sound wave with a frequency below 20 Hz, below the audible limit for the human ear.

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How to measure it in class

With FizziQ, students can demonstrate infrasounds by using the smartphone accelerometer to detect very low frequency vibrations.

Steps:

Open FizziQ and select the Accelerometer instrument. Place the smartphone flat on a table.
Gently and regularly tap the table at a slow rhythm (approximately once per second). The smartphone captures the table vibrations through the accelerometer.
Observe the recorded signal: the oscillations have a frequency of about 1 to 5 Hz, well below the audibility threshold. Yet, the smartphone detects them perfectly.
Interpret: these vibrations are infrasounds. They are inaudible but very real. Students understand that the smartphone sensor detects phenomena beyond the reach of the human ear.

Scientific activities on this topic

Several experiments easily achievable with a smartphone, tablet, or computer allow exploration of the domain of infrasounds and low-frequency vibrations.

1: Meteorite Fall - Explore how energy propagates in the form of seismic waves during a meteorite impact: https://www.fizziq.org/en/activities/fall-of-a-meteorite/

2: Accelerocardiogram - Determine heart rate through chest movements: https://www.fizziq.org/en/activities/accelerocardiogram/

3: What is an accelerometer - Discover the concept of linear acceleration: https://www.fizziq.org/en/activities/what-is-an-accelerometer/

Learn more

A bit of history

The discovery of infrasound is attributed to the French physicist Vladimir Gavreau in the 1960s. He reportedly felt discomfort in his laboratory caused by vibrations from a fan producing infrasound. This observation opened a new field of study in acoustics.

Geophysical monitoring

The CTBTO global network uses infrasound stations to detect atmospheric nuclear explosions. These stations have also detected the explosion of the Chelyabinsk meteor in 2013 and the eruptions of the Hunga Tonga volcano in 2022, whose infrasounds traveled around the Earth several times.

Animal communication

Biologist Katy Payne discovered in 1984 that elephants communicate through infrasound. She felt vibrations in her chest while observing elephants at the Portland Zoo and confirmed the existence of vocalizations at 14-35 Hz, inaudible to humans but essential for herd social cohesion.

Additional experiments

The experiment Meteorite Fall explores how energy propagates in the form of waves, including in the infrasound domain, during the impact of a celestial body.

Formula

No specific formula applies uniquely to infrasounds.

Infrasounds obey the same laws as audible sounds:

v = lambda x f

Meaning:

v: wave velocity in the medium (m/s)

lambda: wavelength (m)

f: frequency, below 20 Hz

Example: an infrasound of 5 Hz in air has a wavelength of 343/5 = approximately 69 meters.

Application examples

Earthquakes generate infrasounds that propagate over thousands of kilometers
Elephants communicate through infrasound at frequencies around 14 Hz, perceptible at more than 10 km
Volcanic eruptions produce infrasounds detected by monitoring stations worldwide
Wind turbines can produce infrasounds felt as vibrations by nearby residents
Ocean waves generate a permanent infrasound noise called microseism, detectable everywhere on Earth

FAQ

Q: Can we feel infrasounds? A: Yes. Although inaudible, high-intensity infrasounds are felt as vibrations in the chest or abdomen. At very high intensity, they can cause nausea or discomfort.

Q: Which animals use infrasounds? A: Elephants communicate through infrasound (14 to 35 Hz) over distances of more than 10 km. Blue whales emit sounds at frequencies as low as 10 Hz to communicate over hundreds of kilometers.

Q: How are infrasounds detected? A: Microbarometers, seismometers, or sensitive accelerometers are used. The CTBTO network (nuclear test monitoring) has infrasound stations distributed worldwide.

Q: Are infrasounds dangerous? A: At normal intensity, no. But high-intensity infrasounds (over 170 dB) can cause internal damage. The levels encountered in everyday life are harmless.

Q: Why do infrasounds propagate so far? A: Infrasounds have very long wavelengths (tens to hundreds of meters). They are very weakly absorbed by air and easily bypass obstacles through diffraction, allowing them to propagate over thousands of kilometers.

Ultrasound - Frequency - Sound wave - Seismic wave - Wavelength - Accelerometer - Hearing