The Infrasound Effect: Can Frequencies Below Human Hearing Influence Human Perception?

Imagine entering an abandoned building late at night.

There is no visible movement. Nobody is speaking. The corridors appear empty, and the surrounding environment seems unusually quiet.

Yet something feels uncomfortable.

You experience an unexplained sense of pressure, notice a faint vibration, or feel as though something is present nearby. You cannot identify a clear sound, but the atmosphere feels different.

Could an environmental factor be influencing your perception without producing a sound that you consciously recognize?

One possibility worth investigating is infrasound.

Infrasound refers to sound waves with frequencies below approximately 20 hertz (Hz). These frequencies are generally below the range of ordinary human hearing, but that does not mean they are physically irrelevant to the human body or the surrounding environment.

In recent decades, low-frequency sound has attracted interest from researchers studying acoustics, environmental noise, building vibrations, human physiology, and unusual sensory experiences.

For paranormal investigators, this raises an important question:

Can low-frequency environmental sound contribute to experiences that people interpret as unexplained or paranormal?

To investigate this responsibly, we must understand the physics first, examine the available research, and separate scientific findings from popular claims.

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1. Understanding Infrasound: What Exactly Are We Measuring?

Sound is a mechanical wave that travels through a medium such as air, water, or solid materials.

When an object vibrates, it produces pressure variations in the surrounding medium. These variations travel outward as sound waves.

Frequency describes how many complete cycles occur per second and is measured in hertz (Hz).

For example:

• 10 Hz means ten cycles per second.
• 100 Hz means one hundred cycles per second.
• 1,000 Hz means one thousand cycles per second.

The frequency of a sound influences how it is perceived.

The commonly used classification is:

| Frequency range | Classification | |---|---| | Below 20 Hz | Infrasound | | Approximately 20 Hz–20 kHz | Audible sound | | Above 20 kHz | Ultrasound |

These boundaries are approximate rather than absolute. Human hearing sensitivity varies with sound pressure level, age, individual physiology, and other conditions.

At sufficiently high sound pressure levels, some people may perceive very low-frequency sound even when its frequency is below the conventional hearing threshold.

This is an important distinction: inaudible does not necessarily mean undetectable by every physical or physiological mechanism.

2. Why Can We Sometimes Experience Sound Without Clearly Hearing It?

Human hearing is not equally sensitive to every frequency.

The ear responds differently across the frequency spectrum. Under ordinary listening conditions, sensitivity is greatest in the mid-frequency region, while substantially greater sound pressure is generally required to detect very low-frequency sounds.

This relationship is represented by equal-loudness contours.

These contours describe the sound pressure levels required at different frequencies for sounds to be perceived as equally loud.

For example, a sound at 100 Hz and another at 1,000 Hz may have the same physical sound pressure level but will not necessarily be perceived as equally loud.

At very low frequencies, the sensation may also differ from the familiar experience of hearing a conventional sound.

Depending on frequency and intensity, people may report:

• A faint rumbling sensation.
• Pressure-like sensations.
• Vibrations.
• A general feeling of discomfort.
• A barely perceptible humming sound.

However, these experiences are not specific to infrasound. Similar sensations can have other acoustic, mechanical, physiological, and psychological explanations.

A sensation alone cannot establish the presence of infrasound.

The difference between hearing and physical exposure

A sound wave can interact with the environment even when it is not consciously perceived.

For example, low-frequency acoustic energy can travel through buildings, interact with structural components, and produce vibrations under suitable conditions.

This means investigators should not rely exclusively on their ears when assessing environmental acoustics.

Instrument-based measurement is necessary.

3. Where Does Infrasound Come From?

Infrasound is not an unusual or exclusively human-generated phenomenon.

It occurs naturally and is also produced by various industrial and mechanical systems.

Understanding these sources is important because an unexplained low-frequency reading may have an ordinary environmental origin.

3.1 Natural sources

Natural infrasound can be generated by several processes.

Thunderstorms

Large atmospheric pressure changes and thunder-producing events can generate substantial low-frequency acoustic energy.

Volcanic activity

Explosive eruptions and other volcanic processes can produce infrasound that travels considerable distances.

Earthquakes

Earthquakes can generate ground vibrations and atmospheric disturbances. Some events produce detectable infrasonic signals.

Ocean waves

Large ocean waves and interactions between ocean systems and the atmosphere can generate low-frequency pressure fluctuations.

Wind

Wind interacting with terrain, vegetation, buildings, and other obstacles can produce low-frequency noise and pressure fluctuations.

These sources demonstrate that infrasound is part of the natural acoustic environment.

3.2 Artificial sources

Human activities can also generate low-frequency sound.

Common examples include:

• Industrial machinery.
• Large ventilation systems.
• HVAC equipment.
• Generators.
• Compressors.
• Heavy vehicles.
• Large fans.
• Pumps.
• Construction machinery.
• Electrical and mechanical infrastructure.

An old building may contain several potential sources, including damaged ventilation equipment, loose mechanical components, and nearby industrial installations.

A paranormal investigation that ignores these sources risks misinterpreting ordinary environmental activity.

4. Can Infrasound Influence Human Perception?

This is where the subject becomes particularly interesting.

Researchers have investigated how low-frequency sound influences human comfort, perception, and physiological responses.

However, it is important to distinguish between established acoustic effects and claims that have not been adequately demonstrated.

4.1 Discomfort and annoyance

Low-frequency noise can be disturbing even when it is not perceived as a conventional audible sound.

Depending on frequency, intensity, duration, and individual sensitivity, exposure may contribute to:

• Annoyance.
• Difficulty concentrating.
• Sleep disturbance.
• Perceived vibration.
• Discomfort.

These effects are influenced by the complete acoustic environment rather than frequency alone.

A low-frequency source operating continuously inside a building may therefore affect occupants differently from a brief, isolated sound.

4.2 Pressure and vibration sensations

At sufficiently high sound pressure levels, low-frequency acoustic energy can produce physical sensations.

Depending on the exposure conditions, people may perceive vibration or pressure-like sensations.

These experiences should not automatically be interpreted as psychological or paranormal events.

However, neither should every unusual sensation be attributed to infrasound without measurement.

The frequency and sound pressure level must be established before making that assessment.

4.3 Anxiety and fear: What does the evidence say?

One popular explanation in paranormal media suggests that infrasound can directly produce fear or the sensation of an unseen presence.

This idea has received attention partly because of research and historical discussions surrounding low-frequency sound and unusual experiences.

However, the scientific evidence does not justify treating infrasound as a proven explanation for ghost sightings, hallucinations, or a sensed presence.

Human emotional responses are complex.

They can be influenced by:

• Environmental conditions.
• Previous expectations.
• Darkness and visibility.
• Fatigue.
• Stress.
• Acoustic conditions.
• Individual differences.

Even when an acoustic environment is uncomfortable, that does not establish that it creates a specific paranormal-like experience.

AERM's position is that such claims should be investigated through controlled experiments rather than accepted from anecdotal reports.

5. The 18.9 Hz Question: Is There a Connection Between Infrasound and Ghost Sightings?

One frequently discussed frequency in paranormal literature is approximately 18.9 Hz.

The topic gained attention through discussions about possible relationships between low-frequency sound, visual disturbances, and unusual experiences.

Researcher Vic Tandy published an account in the Journal of the Society for Psychical Research in 1998 titled Ghosts in the Machine.

The article discussed an unusual experience in a laboratory environment and investigated whether an approximately 19 Hz standing wave might have contributed to visual disturbances.

The proposed explanation involved the possibility of resonance affecting the eye.

This account became popular in paranormal discussions because it offered a physical explanation for certain ghost-like experiences.

However, an important distinction must be maintained.

Tandy's account is not proof that 18.9 Hz produces ghost sightings.

It should be treated as a hypothesis-generating case rather than a universally established physiological mechanism.

There is no scientific basis for claiming that every reported shadow, apparition, or sensed presence is caused by an 18.9 Hz sound wave.

What should investigators learn from this?

The value of this example is not that it explains paranormal experiences.

Its value is that it demonstrates how an unusual experience can lead to a testable environmental hypothesis.

Instead of immediately accepting an extraordinary interpretation, investigators can ask:

1. Was a low-frequency acoustic source present?
2. Was the frequency measured accurately?
3. Was the sound pressure level sufficient to produce a relevant physical effect?
4. Could structural vibration or another environmental factor explain the observation?
5. Can the reported effect be reproduced under controlled conditions?

These questions transform speculation into an investigation.

6. How Does Infrasound Travel Through Buildings?

Low-frequency sound behaves differently from many higher-frequency sounds.

Its relatively long wavelengths can interact with large spaces, building structures, and environmental boundaries.

The wavelength of a sound wave can be calculated using:

Wavelength = Speed of sound / Frequency

Mathematically:

λ = c / f

Where:

• λ = Wavelength in metres.
• c = Speed of sound in metres per second.
• f = Frequency in hertz.

For an approximate speed of sound of 343 m/s:

| Frequency | Approximate wavelength | |---|---:| | 5 Hz | 68.6 m | | 10 Hz | 34.3 m | | 15 Hz | 22.9 m | | 20 Hz | 17.2 m | | 50 Hz | 6.86 m |

These values demonstrate why low-frequency sound can interact with building dimensions and structural systems in interesting ways.

Room modes and resonance

When sound waves interact with enclosed spaces, certain frequencies may produce standing-wave patterns known as room modes.

These patterns can create areas where pressure variations are stronger or weaker.

However, conventional room-acoustic calculations become less straightforward at very low frequencies, especially when wavelengths are much larger than the room.

Large halls, underground spaces, corridors, and interconnected building structures may exhibit complex low-frequency behaviour.

Structural vibration is another important consideration.

A vibrating machine may transmit mechanical energy into floors, walls, or support structures. These structures can then radiate sound into surrounding spaces.

This is one reason why an investigator should examine both airborne sound and building vibration.

7. How Can Investigators Detect Infrasound?

An ordinary paranormal investigation kit is not necessarily capable of measuring infrasound.

A typical handheld sound-level meter may not provide reliable measurements below its specified frequency range.

Similarly, a smartphone microphone should not be treated as a calibrated infrasound instrument.

A proper measurement system must be designed around the frequency range being investigated.

Equipment requirements

A suitable measurement setup may include:

1. Low-frequency measurement microphone

The microphone must have a documented frequency response extending into the intended infrasound measurement range.

2. Measurement preamplifier

The preamplifier should be compatible with the microphone and measurement system.

3. Data acquisition system

An appropriate recording interface should support the required sampling rate, dynamic range, and frequency response.

4. Acoustic calibration equipment

A suitable acoustic calibrator helps verify the measurement chain.

Calibration must be appropriate for the instrument and frequency range. A conventional 1 kHz calibration alone does not establish accurate response at 5 or 10 Hz.

5. Wind protection

Wind can produce substantial unwanted low-frequency pressure fluctuations at the microphone.

Suitable wind protection and careful microphone placement are essential.

6. Signal analysis software

Software capable of displaying frequency spectra and time-series data can help identify persistent low-frequency components.

The software itself does not compensate for an unsuitable microphone or an improperly configured recording system.

8. Understanding the Measurement: Frequency, Amplitude and Sound Pressure Level

Detecting a low-frequency signal is only the beginning.

Investigators must also understand how to interpret the measurement.

Three important parameters are:

Frequency

Frequency identifies how many cycles occur per second.

It helps determine whether the detected component falls within the infrasonic range.

Amplitude

Amplitude describes the magnitude of a signal.

For an acoustic pressure waveform, amplitude is related to the pressure variation around atmospheric pressure.

Sound Pressure Level

Sound pressure level (SPL) expresses acoustic pressure relative to a reference pressure.

For sound in air, the standard reference pressure is 20 micropascals.

The formula is:

SPL = 20 log10(p / p₀)

Where:

• p = Measured RMS sound pressure.
• p₀ = Reference sound pressure of 20 μPa.

SPL is expressed in decibels (dB).

An important point is that decibels use a logarithmic scale. A difference of 20 dB corresponds to a tenfold difference in RMS sound pressure.

For infrasound measurements, the frequency weighting and reporting method must also be specified.

A-weighting is commonly used for many environmental noise measurements, but it strongly attenuates very low frequencies. It is therefore not sufficient on its own when the objective is to characterize infrasound.

Depending on the purpose, investigators may need unweighted measurements or an appropriate low-frequency measurement method.

9. A Practical AERM Field Experiment: Investigating Low-Frequency Sound

The following experiment provides a framework for examining whether low-frequency acoustic activity exists at a location where occupants have reported unusual experiences.

The experiment is designed to investigate environmental conditions, not to prove paranormal activity.

Objective

Determine whether measurable low-frequency acoustic components are present at a selected location and whether their occurrence corresponds with documented environmental or witness observations.

Step 1: Select a suitable location

Choose a location where unusual experiences have been reported.

Obtain permission from the property owner or responsible authority.

Do not enter unsafe or abandoned structures without proper authorization and a safety assessment.

Step 2: Establish an environmental baseline

Before taking detailed measurements, record:

• Date and time.
• Weather conditions.
• Wind conditions.
• Indoor temperature.
• Relative humidity.
• Nearby machinery.
• Ventilation activity.
• Traffic conditions.
• Building occupancy.

These details help identify possible environmental influences.

Step 3: Position the microphone

Place the measurement microphone at a documented location.

Avoid placing it directly beside vibrating equipment unless that is the specific object being studied.

Record the microphone height, orientation, distance from walls, and measurement position.

Use suitable wind protection.

Step 4: Record acoustic data

Use a measurement system with a verified low-frequency response.

Record continuously for a predefined observation period.

A longer recording may help identify intermittent sources that would be missed during a short measurement.

Keep the instrument configuration unchanged throughout comparable measurements.

Step 5: Examine the frequency spectrum

Use spectral analysis to identify low-frequency components.

Look for:

• Persistent frequency peaks.
• Intermittent frequency peaks.
• Changes in sound pressure level.
• Periodic variations.
• Components associated with machinery operation.

A frequency peak alone does not identify its source.

Step 6: Compare with environmental activity

Compare the acoustic record with:

• HVAC operation.
• Generator activity.
• Traffic movement.
• Electrical and mechanical equipment.
• Wind conditions.
• Building vibrations.

If a low-frequency component repeatedly appears when a particular machine operates, that relationship deserves further examination.

Step 7: Correlate with witness reports

Compare the measurement timeline with independently documented witness observations.

For example:

| Time | Observation | Environmental data | |---|---|---| | 21:00 | Baseline recording started | Ventilation operating | | 21:20 | Low-frequency component detected | Machinery operating | | 21:35 | Witness reports discomfort | Acoustic recording continues | | 21:50 | Machinery switched off | Component decreases | | 22:10 | Follow-up observation | Background measurement |

Important: This is an illustrative example, not actual AERM field data or an experimental result.

A temporal relationship does not establish causation.

A controlled follow-up experiment would be required to investigate whether the acoustic condition contributes to the reported sensation.

10. Common Mistakes When Investigating Infrasound

Even technically equipped investigators can produce misleading results when measurement procedures are inadequate.

Mistake 1: Using an ordinary smartphone

Most smartphone microphones are not designed or calibrated for reliable infrasound measurement.

A displayed frequency spectrum cannot automatically be considered scientifically valid.

Mistake 2: Treating every low-frequency peak as infrasound

A peak must be interpreted using the actual frequency scale, microphone response, noise floor, and recording configuration.

Mistake 3: Ignoring wind noise

Wind can produce strong low-frequency disturbances at the microphone.

Without suitable protection and measurement controls, these disturbances may be mistaken for environmental acoustic signals.

Mistake 4: Confusing vibration with airborne sound

A floor vibration and an airborne acoustic wave are related but different physical measurements.

A geophone or accelerometer does not directly replace an acoustic microphone.

Mistake 5: Assuming a frequency peak explains a witness experience

Finding a 12 Hz component does not prove that it caused someone to feel watched or frightened.

Investigators must establish the signal's intensity, duration, location, timing, and plausible physiological relevance.

Mistake 6: Ignoring the measurement instrument's limitations

Every measurement system has a frequency response, noise floor, calibration requirement, and operating range.

Measurements outside the documented operating range should not be presented as reliable findings.

11. What Does the Scientific Evidence Actually Establish?

Infrasound is a recognized physical phenomenon, and its sources, propagation, measurement, and potential effects on human comfort have been investigated through acoustics and environmental research.

There is also scientific literature examining the relationship between low-frequency sound and human responses.

However, several claims commonly repeated in paranormal discussions require more careful treatment.

| Claim | Scientific assessment | |---|---| | Infrasound exists in natural and artificial environments | Established | | Low-frequency sound can travel through buildings and interact with structures | Established | | Sufficiently intense low-frequency sound can produce physical sensations | Supported by acoustic research | | Low-frequency noise can contribute to annoyance and discomfort | Supported by research | | Infrasound always causes fear | Not established | | A particular infrasonic frequency produces ghost sightings | Not established | | Detecting infrasound proves paranormal activity | Incorrect |

The distinction between physical evidence and interpretation is essential.

An instrument may identify an acoustic signal, but it cannot independently determine what a witness experienced or why.

The objective should be to establish the environmental conditions as accurately as possible.

12. The AERM Research Perspective

At Anomalous Evidence Research & Media (AERM), environmental measurements are considered an important part of investigating unusual reports.

Infrasound is particularly interesting because it demonstrates how an environmental factor can exist without being immediately obvious to human observers.

However, the presence of low-frequency sound should never be treated as an automatic explanation for unusual experiences.

A responsible investigation should follow a sequence:

1. Document the reported experience.
2. Establish the environmental baseline.
3. Identify potential acoustic sources.
4. Measure low-frequency sound using suitable equipment.
5. Examine the measurement data.
6. Compare observations with environmental activity.
7. Conduct controlled follow-up testing where justified.
8. Report findings and limitations transparently.

If no relevant acoustic signal is detected, that result should also be documented.

If a signal is detected but its source remains uncertain, the finding should remain inconclusive until further investigation.

The absence of an identified explanation does not establish the presence of a paranormal cause.

13. Conclusion: Investigating What We Cannot Hear

Infrasound provides an interesting connection between environmental physics and human perception.

It reminds us that the acoustic environment extends beyond the sounds we consciously recognize.

Low-frequency sound can originate from natural processes, industrial equipment, ventilation systems, vehicles, and structural interactions. Under appropriate conditions, it can produce measurable acoustic energy and physical responses.

Nevertheless, the suggestion that infrasound explains ghost sightings, sensed presences, or paranormal experiences remains a hypothesis requiring further controlled investigation.

For investigators, the real opportunity lies in measurement, reproducibility, and critical evaluation.

Rather than asking whether an invisible sound is responsible for an unexplained experience, a more productive question is:

What acoustic conditions were actually present, how were they measured, and can they be independently verified?

That is where a paranormal claim becomes an opportunity for scientific investigation.

"An unexplained experience is a reason to investigate the environment, not a reason to abandon scientific reasoning."

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References and Further Reading

The following publications provide a starting point for further research into low-frequency acoustics and human responses.

1. Leventhall, G. (2004). Low Frequency Noise and Annoyance. Noise & Health, 6(23), 59–72.

1. Møller, H., & Pedersen, C. S. (2004). Hearing at Low and Infrasonic Frequencies. Noise & Health, 6(23), 37–57.

1. Salt, A. N., & Hullar, T. E. (2010). Responses of the Ear to Low Frequency Sounds, Infrasound and Wind Turbines. Hearing Research, 268(1–2), 12–21.

1. Tandy, V. (1998). Ghosts in the Machine. Journal of the Society for Psychical Research, 62(851).

1. World Health Organization. (1999). Guidelines for Community Noise. WHO.

1. International Organization for Standardization. (1995). ISO 7196: Acoustics — Frequency-weighting Characteristic for Infrasound Measurements.

1. International Electrotechnical Commission. (2014). IEC 61260-1: Electroacoustics — Octave-band and Fractional-octave-band Filters — Part 1: Specifications.

These references cover different aspects of acoustic measurement, low-frequency hearing, environmental noise, and historical discussions of unusual experiences. They should not be interpreted as collectively establishing a paranormal explanation.

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About AERM

Anomalous Evidence Research & Media (AERM) is an independent research organization dedicated to investigating unexplained reports through scientific methodology, environmental analysis, critical thinking, and transparent documentation.

Through the AERM Research Journal, we aim to make scientific concepts accessible while encouraging responsible investigation and evidence-based reasoning.

Research Principle:

No anomaly is considered paranormal until environmental, psychological, technical, and scientific explanations have been systematically evaluated.

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Educational Disclaimer

This article is intended for educational and research purposes. It does not establish that infrasound causes paranormal experiences, hallucinations, or ghost sightings.

Environmental measurements should be conducted using appropriately calibrated equipment and documented procedures. Investigators should not deliberately expose themselves or others to potentially harmful sound levels.

Scientific conclusions must remain proportional to the quality and limitations of the available evidence.