Infrasound: What Exists Below 20 Hertz

The common human hearing range is often described as 20 hertz to 20,000 hertz.

That description creates a misleading border.

Sound does not stop below 20 hertz.

Hearing does not switch off instantly.

The region below 20 hertz is called infrasound.

A conventional boundary

Infrasound is commonly defined as acoustic frequency below 20 hertz.

The boundary is useful.

It is not absolute.

As frequency falls, human hearing becomes much less sensitive.

A low-frequency tone must usually have a much higher sound-pressure level to be perceived.

At sufficiently high levels, humans can hear below 20 hertz.

What very-low-frequency sound feels like

The character of perception changes.

A higher tone feels continuous.

At very low frequency, a listener may notice:

• separate pressure cycles;
• pulsing;
• ear pressure;
• flutter;
• rattling in the environment;
• bodily vibration at higher levels.

The ear remains an important sensory organ.

Vibration can also be transmitted through floors, seats, walls, and structures.

Long wavelengths

In air, low frequency means long wavelength.

A 10-hertz wave has a wavelength of tens of metres.

Long waves interact differently with buildings and terrain.

They can bend around obstacles more effectively than short wavelengths.

Under suitable atmospheric conditions, infrasound can travel hundreds or thousands of kilometres.

Natural sources

Natural infrasound can come from:

• volcanic eruptions;
• meteors;
• ocean waves;
• earthquakes coupling into the atmosphere;
• avalanches;
• tornadoes and severe storms;
• lightning-related events;
• auroral and atmospheric processes.

The source moves a large amount of air or creates a large pressure disturbance.

Human-made sources

Human sources include:

• explosions;
• rocket launches;
• aircraft;
• heavy machinery;
• compressors;
• ventilation systems;
• mining;
• industrial processes;
• wind turbines.

The source label does not determine the risk.

Exposure at the listener matters.

A planetary monitoring tool

The Comprehensive Nuclear-Test-Ban Treaty Organization operates a global infrasound network.

Its primary purpose includes detecting atmospheric nuclear explosions.

The same sensors detect natural events.

Infrasound helped record:

• volcanic eruptions;
• meteors;
• large accidents;
• chemical explosions;
• severe atmospheric events.

A signal can be scientifically valuable even when humans do not perceive it.

Volcano monitoring

Volcanoes produce both seismic and atmospheric signals.

Seismometers record ground motion.

Infrasound sensors record pressure waves in air.

Combining the two helps distinguish:

• underground earthquakes;
• surface explosions;
• gas release;
• ash-producing events;
• debris flows.

Infrasound can monitor remote volcanoes when dense local networks are unavailable.

Does infrasound cause symptoms?

At sufficiently high levels, low-frequency noise and infrasound can be uncomfortable and harmful.

Possible experiences include:

• pressure;
• annoyance;
• vibration;
• sleep disturbance;
• concentration difficulty;
• pain at extreme exposure.

The critical words are level, duration, spectrum, and context.

A frequency below 20 hertz is not automatically dangerous.

Environmental exposure claims

Some communities report symptoms near wind turbines, industrial equipment, ventilation systems, or unexplained sources.

Investigation requires measurement.

Relevant questions include:

• What is the sound-pressure level?
• Which frequencies are present?
• Is audible low-frequency sound present?
• Does the building rattle?
• Is vibration transmitted through the structure?
• When does exposure occur?
• Does it differ from background?
• Are sleep and expectation involved?

Without measurement, the label infrasound can become an explanation for every symptom.

Expectation and the nocebo effect

Controlled experiments show that negative expectations can increase symptom reporting during infrasound exposure.

This is a nocebo effect.

It does not mean that a person is lying.

Expectation can change real attention, stress, and bodily experience.

It also does not prove that every complaint is caused by expectation.

A complete investigation considers both physical exposure and psychological context.

Detection is not exposure

A scientific sensor may detect a weak signal over a great distance.

That does not mean the signal is intense enough to be heard or to cause harm.

Sensitive instruments routinely measure phenomena below human thresholds.

The statement “infrasound was detected” is incomplete without level and location.

Infrasound and fear

Low-frequency sound can feel mysterious because the source may be hard to locate.

Buildings can amplify rattling.

The sound may be more noticeable at night.

Uncertainty increases attention.

A grounded approach combines:

• acoustic measurement;
• structural inspection;
• medical assessment where appropriate;
• sleep and stress context;
• transparent communication.

A clean evidence rule

A strong infrasound claim should define:

1. Source
2. Frequency spectrum
3. Sound-pressure level
4. Duration
5. Distance
6. Measurement equipment
7. Control condition
8. Health outcome

The word infrasound alone is not a dose.