E02.02 Low Frequency Noise: How Humans Hear Sound Beyond Decibels
Key Takeaways
- Sound is a physical pressure wave that exists independently of human hearing, while hearing is the biological process by which the brain interprets those pressure waves.
- The decibel (dB) is a logarithmic unit that can describe different physical quantities, including sound pressure and sound power, depending on what is being measured.
- Engineers distinguish between sound power (the acoustic energy emitted by a source) and sound pressure (the sound reaching a listener), because they serve different purposes in environmental noise assessment.
- Human hearing is far more sensitive to some frequencies than others, which is why frequency-weighted measurements such as dBA were developed.
- Conventional noise metrics were designed primarily for ordinary environmental sounds such as traffic, speech and industrial activity, and may not always represent the characteristics of low frequency noise accurately.
- Understanding low frequency noise requires combining acoustics, physiology and psychoacoustics rather than relying on a single measurement or noise limit.
Introduction
Almost everyone has experienced this phenomenon.
Standing beside a waterfall can feel surprisingly peaceful despite the considerable amount of sound it produces. Yet a faint mechanical hum from an air-conditioning unit, an electrical transformer or a distant cooling tower may become difficult to ignore, particularly during a quiet night.
Similarly, two sounds measured at exactly the same sound level may be experienced very differently by different people.
Why?
The answer lies in an important distinction that is often overlooked.
Sound and hearing are not the same thing.
Sound is a physical phenomenon. It exists as pressure waves travelling through the air, whether or not anyone is present to hear them.
Hearing, by contrast, is a biological process. The ear converts those pressure waves into electrical signals, which the brain then interprets as loudness, pitch, tone and meaning.
Understanding this distinction is essential for making sense of modern environmental noise assessment.
Engineers measure sound using instruments. Regulators develop noise limits using internationally recognised metrics. Communities experience sound through the remarkable complexity of the human auditory system.
These perspectives overlap, but they are not identical.
This difference becomes particularly important when discussing low frequency noise (LFN).
Most environmental noise standards were developed using decades of research into how people perceive ordinary environmental sounds such as road traffic, aircraft, industrial facilities and human speech. However, scientific understanding of very-low frequency sound continues to evolve, with recent research suggesting that the human auditory system may respond to these frequencies in ways that are more complex than previously understood.
This does not mean that existing engineering standards are incorrect.
Rather, it highlights why low frequency noise remains an active area of scientific research and why engineers, regulators and researchers continue to refine how it is measured, interpreted and managed.
This article therefore begins with the fundamentals.
Before discussing noise limits, frequency weightings or environmental regulations, we first examine what sound actually is, how it travels through the environment, and how the human auditory system transforms tiny fluctuations in air pressure into the rich sensory experience that we call hearing.
Only then can we understand why measuring low frequency noise is considerably more complex than simply reading a number from a sound level meter.
MDCO Insight: Sound exists in the physical world, but noise exists in human experience. Understanding low frequency noise therefore begins by understanding both acoustics and human hearing.
Before We Hear Sound, Physics Comes First
Sound Exists Whether Anyone Hears It
Long before sound reaches the ear, it already exists as a physical phenomenon.
Imagine dropping a stone into a still pond.
Ripples spread outward in every direction. The water itself does not travel across the pond; instead, each water molecule passes energy to the next, creating a travelling wave.
Sound behaves in much the same way.
Instead of water, however, sound travels through air.
When an object vibrates—whether a loudspeaker, a cooling tower fan, a transformer core or a person’s vocal cords—it repeatedly compresses and expands the surrounding air. These tiny changes in air pressure travel away from the source as waves at approximately 343 metres per second under typical atmospheric conditions.
As each wave passes, the air molecules move back and forth by only microscopic distances. They do not travel from the sound source to the listener. Rather, they transfer energy from one molecule to the next, allowing the disturbance to propagate through the atmosphere.
This distinction is important because it explains why sound can travel long distances without transporting air itself.
Everything that produces sound does so by creating these pressure variations.
A cooling tower fan generates sound as its rotating blades disturb the surrounding air.
A transformer produces sound because microscopic changes in its steel core cause the surrounding structure to vibrate.
Thunder results from the rapid expansion of superheated air following a lightning strike.
Human speech arises when vibrating vocal cords repeatedly compress and rarefy the air flowing from the lungs.
Although these sources appear very different, the underlying physics is exactly the same.
Each creates pressure waves that travel through the surrounding medium.
Whether those waves are eventually heard depends entirely on what happens when they reach the human ear.
Frequency Determines Pitch
Not all sound waves are alike.
One of their most important characteristics is frequency.
Frequency describes how many complete pressure cycles occur every second and is measured in hertz (Hz).
A frequency of 100 Hz means the air pressure oscillates one hundred times every second.
A frequency of 1,000 Hz oscillates one thousand times each second.
Higher frequencies are generally perceived as higher-pitched sounds, while lower frequencies are perceived as deeper sounds.
For example:
| Approximate Frequency | Typical Example |
| 20 Hz | Threshold of normal human hearing |
| 50 Hz | Electrical power frequency effects in many countries |
| 63 Hz | Common reference band for environmental low frequency noise assessment |
| 250 Hz | Deep male speech |
| 1,000 Hz | Region of greatest speech intelligibility |
| 4,000 Hz | Region of peak human hearing sensitivity |
| 20,000 Hz | Approximate upper limit of normal human hearing |
The range between approximately 20 Hz and 20,000 Hz is traditionally regarded as the audible range for healthy young adults.
In reality, hearing sensitivity varies considerably with age, health and individual differences. Many adults cannot perceive sounds near either extreme of this range, particularly at lower sound levels.
Low Frequency Means Long Wavelength
Frequency also determines another important property: wavelength.
Wavelength is the physical distance occupied by one complete cycle of the sound wave.
Because sound travels through air at a nearly constant speed, lower frequencies have much longer wavelengths than higher frequencies.
For example:
| Frequency | Approximate Wavelength |
| 20 Hz | 17 metres |
| 63 Hz | 5.4 metres |
| 125 Hz | 2.7 metres |
| 1,000 Hz | 34 centimetres |
| 4,000 Hz | 8.6 centimetres |
These enormous differences help explain why low frequency sound behaves differently from higher-frequency sound.
A 63 Hz sound wave is longer than many residential rooms.
A 20 Hz wave may extend across an entire house.
Such long wavelengths interact differently with buildings, terrain and the atmosphere. They bend around obstacles more readily, are absorbed less efficiently by the air, and may remain detectable over much greater distances than higher-frequency sounds.
These physical characteristics are one reason why low frequency noise has become an important topic in the study of large industrial facilities, including modern hyperscale data centres.
MDCO Insight: Low frequency sound is not simply “deeper” sound. Its much longer wavelength changes how it travels through the environment, how it interacts with buildings, and ultimately how it may be experienced by nearby communities.
Why Decibels Can Be Confusing
One of the most misunderstood concepts in acoustics is the decibel, commonly abbreviated as dB.
Many people assume that the decibel is simply a unit of loudness.
It is not.
The decibel is a logarithmic unit used to express the ratio between two quantities.
Because human hearing can respond to an enormous range of sound pressures—from the faintest audible sound to levels capable of causing immediate pain—using ordinary arithmetic would produce inconveniently large numbers.
A logarithmic scale compresses this enormous range into values that are easier to work with.
This has an important consequence.
A sound measuring 60 dB is not twice as loud as a sound measuring 30 dB.
Likewise, increasing the sound level by 10 dB does not mean that the sound has increased by only ten units. Instead, each increase of 10 dB represents approximately a tenfold increase in sound intensity and is commonly perceived as roughly twice as loud for many ordinary sounds. However, perceived loudness depends strongly on frequency, sound characteristics and individual hearing sensitivity.
This logarithmic behaviour explains why environmental acoustics relies heavily on logarithmic calculations.
It also explains why the sound produced by several identical machines cannot be added using ordinary arithmetic.
For example, if two identical cooling towers each produce a sound power level of 90 dB, the combined sound level is 93 dB, not 180 dB.
Similarly, ten identical cooling towers produce approximately 100 dB, not 900 dB.
Understanding this principle is essential for interpreting environmental noise assessments and explains why increasing the size of a data centre does not increase its sound level in direct proportion to its electrical capacity.
MDCO Insight: The decibel is a logarithmic measurement, not a linear one. This is why sound levels, equipment numbers and perceived loudness do not increase in simple proportion.
Sound Pressure and Sound Power Are Different
Another concept that often causes confusion is the distinction between sound power and sound pressure.
Although the terms sound similar, they describe fundamentally different aspects of sound.
Sound power describes the total acoustic energy emitted by a sound source.
It is an inherent property of the equipment itself.
A cooling tower, transformer or chiller produces essentially the same sound power regardless of where it is measured.
For this reason, engineers often describe equipment using its sound power level (Lw).
Manufacturers commonly publish sound power data because it allows engineers to compare different machines independently of the surrounding environment.
Sound pressure, on the other hand, describes the tiny fluctuations in air pressure that actually reach a particular location.
This is what a microphone measures and what people experience.
Unlike sound power, sound pressure changes continuously with distance and environmental conditions.
The same cooling tower may produce very different sound pressure levels:
- immediately beside the equipment;
- at the site boundary;
- inside a nearby house; or
- one kilometre away.
The sound source has not changed.
Only the location of the listener has changed.
For this reason, engineers typically begin environmental noise assessments by identifying the sound power of every significant item of equipment. They then use internationally recognised propagation models to estimate how that acoustic energy spreads through the surrounding environment and what sound pressure levels are likely to be experienced at different locations.
This distinction forms the foundation of modern environmental acoustics and underpins virtually every professional assessment of industrial noise.
In later sections of this article, we will see that an additional layer of complexity arises because the sound pressure measured by an instrument is still not the same as the sound perceived by a human listener.
That difference lies at the heart of why low frequency noise continues to present scientific and engineering challenges.
MDCO Insight: Engineers predict community noise by starting with the sound power emitted by equipment and then estimating how that energy becomes sound pressure as it travels through the surrounding environment.
How the Human Ear Converts Air Pressure into Sound
The Classical Understanding of Hearing
Every sound that humans hear begins as nothing more than a tiny fluctuation in air pressure.
A cooling tower fan, a passing motorcycle, a waterfall or another person’s voice all produce pressure waves that travel through the surrounding air. These waves carry no meaning by themselves. They are simply microscopic compressions and expansions of air molecules moving outward from their source.
The remarkable process of hearing begins only when those pressure waves reach the outer ear.
The visible part of the ear acts rather like a natural collector, gently gathering sound waves from the surrounding environment and directing them into the ear canal. Although often overlooked, its curved shape helps the brain determine the direction from which sounds originate, particularly those arriving from above or behind.
At the end of the ear canal lies one of the body’s most delicate structures—the eardrum.
As each sound wave reaches the eardrum, the changing air pressure causes the thin membrane to vibrate. Loud sounds produce larger movements, while quieter sounds produce movements so small that they are measured in billionths of a metre.
Yet even these extraordinarily small vibrations contain all the information needed to identify a familiar voice, recognise a favourite song or detect the distant hum of industrial equipment.
The eardrum itself does not communicate directly with the brain.
Instead, it transfers its motion through three tiny bones located within the middle ear.
These bones—the malleus, incus and stapes—form the smallest chain of bones in the human body. Working together like an intricate system of mechanical levers, they amplify the vibrations arriving from the eardrum before transmitting them into the fluid-filled inner ear.
Without this natural amplification system, much of the sound energy travelling through air would simply reflect away from the much denser fluid inside the inner ear.
The middle ear therefore acts as an efficient mechanical transformer, ensuring that acoustic energy is transferred with minimal loss.
The journey then reaches its most extraordinary stage.
Hidden deep within the skull lies the cochlea—a tiny spiral-shaped organ scarcely larger than a pea. Despite its modest size, it performs one of the most sophisticated forms of signal processing found anywhere in nature.
Inside the cochlea is a narrow membrane that extends from one end of the spiral to the other.
Rather than vibrating uniformly, different parts of this membrane respond to different frequencies.
High-frequency sounds cause the membrane to vibrate near its entrance.
Lower frequency sounds travel much further before producing their greatest movement.
Very low frequency sounds stimulate the membrane closer to its inner end.
In effect, the cochlea behaves like a natural frequency analyser.
Instead of using electronic filters, it separates different frequencies according to the mechanical properties of its own tissues.
Covering this membrane are thousands of microscopic sensory cells known as hair cells.
Despite their name, these cells do not resemble human hair. Each carries bundles of tiny projections that bend as the membrane moves beneath them.
When these microscopic structures bend, they open specialised ion channels within the cell membrane.
This converts the mechanical vibration into an electrical signal.
Only at this point does sound cease to be a physical pressure wave.
It has become information.
These electrical signals travel along the auditory nerve towards the brain, where they are interpreted as pitch, loudness, timbre, rhythm and speech.
What people ultimately perceive is therefore not the sound wave itself.
It is the brain’s interpretation of millions of tiny electrical signals generated inside the cochlea every second.
This classical model of hearing has formed the foundation of auditory science for more than a century and continues to explain how humans perceive most everyday sounds remarkably well.
MDCO Insight: The ear does not hear sound directly. It converts tiny fluctuations in air pressure into electrical signals, allowing the brain to construct the rich auditory world that people experience.
Why Different Frequencies Are Heard Differently
If the ear simply converted air pressure into electrical signals, every frequency would be heard equally well.
But that is not what happens.
Human hearing is remarkably selective.
People are naturally most sensitive to sounds between approximately 2,000 Hz and 5,000 Hz. This is no coincidence.
These frequencies contain much of the information needed to understand human speech, distinguish consonants and recognise warning sounds in the environment.
By contrast, both very low and very high frequencies require considerably greater sound pressure before they become audible.
A 63 Hz sound, for example, must generally possess much more physical energy than a 1,000 Hz sound before it is perceived as equally loud.
This difference does not arise because low frequency sound contains less energy.
Quite the opposite.
It reflects the way the human auditory system has evolved.
The mechanical properties of the ear canal, middle ear and cochlea collectively favour frequencies that are most important for communication and survival. As a result, the auditory system amplifies some frequencies more efficiently than others.
Imagine a piano keyboard stretching from one end of the cochlea to the other.
Each location along the membrane is naturally tuned to respond most strongly to a particular frequency.
When a sound enters the ear, different regions respond simultaneously according to the frequencies present within that sound.
The brain then reconstructs these countless individual responses into what we perceive as a single coherent sound.
This elegant biological process explains why a waterfall, birdsong and human speech all possess distinctive acoustic characters, even when their overall sound pressure levels are similar.
It also explains why low frequency sounds are often described as being “felt” as much as heard.
Because the auditory system is relatively insensitive at very low frequencies, considerably more acoustic energy is required before these sounds become consciously audible. Yet their long wavelengths and greater physical energy may still interact with the body and surrounding environment in ways that people notice.
Understanding this difference is fundamental to interpreting environmental noise measurements.
A sound level meter faithfully measures physical sound pressure.
Human hearing responds according to the characteristics of the auditory system.
The two are closely related, but they are not identical.
MDCO Insight: Human hearing is naturally tuned to certain frequencies. A low frequency sound and a higher-frequency sound with the same physical sound pressure may not be perceived as equally loud.
Why Loudness Is Created by the Brain
People often assume that loudness exists within the sound itself.
In reality, loudness exists only within the brain.
Outside the body, there are only pressure waves travelling through the air.
The ear converts those waves into electrical signals, but it is the brain that ultimately determines whether a sound is perceived as quiet, loud, pleasant, irritating or even threatening.
This interpretation depends on far more than sound pressure alone.
The brain continuously analyses numerous characteristics simultaneously, including:
- frequency content;
- tonal quality;
- duration;
- repetition;
- rhythm;
- direction of arrival;
- previous listening experience; and
- the surrounding acoustic environment.
Consider entering a quiet library.
A faint whisper immediately attracts attention because it contrasts strongly with the surrounding silence.
The same whisper would be almost impossible to notice inside a crowded airport terminal.
The physical sound may be identical.
The perceived loudness is not.
Likewise, people quickly become accustomed to many natural environmental sounds.
The continuous sound of rainfall, ocean waves or rustling leaves often fades into the background after only a short period because the brain classifies these sounds as familiar and non-threatening.
Mechanical sounds frequently behave differently.
A continuous electrical hum, the repetitive drone of industrial fans or a persistent low frequency tone often attracts attention because its regular pattern differs from the natural variability found in most environmental sounds.
Even when two sounds produce similar measured sound pressure levels, they may therefore evoke very different subjective responses.
This distinction forms the basis of an entire scientific discipline known as psychoacoustics—the study of how humans perceive sound rather than simply how sound behaves physically.
Psychoacoustics recognises that hearing is not merely a mechanical process.
It is also a neurological and psychological one.
This understanding explains why modern environmental noise assessment increasingly considers not only measured sound levels, but also tonal characteristics, frequency content, time of day and community context.
For ordinary environmental sounds, decades of psychoacoustic research have led to internationally accepted measurement methods and noise guidelines.
Whether these same approaches fully capture the experience of very low frequency sound remains an active area of scientific investigation, which we will examine in the next section.
MDCO Insight: Sound can be measured with scientific precision, but loudness is ultimately created by the brain. Human experience therefore depends on both the physical properties of sound and the way the auditory system interprets them.
How Humans Hear Sound — And Why Low Frequency Sound Is Different
The Ear Is Not Simply a Microphone
Many people imagine that hearing works like a microphone.
Sound enters the ear.
The ear converts it into electrical signals.
The brain recognises those signals as sound.
Although this description is broadly correct, it leaves out almost everything that determines what we actually hear.
Human hearing is an extraordinarily sophisticated biological system that has evolved over millions of years.
It does not measure sound pressure directly.
Instead, it continuously analyses:
- frequency;
- intensity;
- timing;
- direction;
- movement;
- repetition;
- changing patterns.
Only after all of this processing do we become consciously aware of “hearing” a sound.
This distinction is important because the ear is not designed to measure every frequency equally.
Instead, it is optimised for the frequencies that have mattered most throughout human evolution.
Those frequencies are overwhelmingly found within speech, natural environments and ordinary daily activities—not the persistent mechanical sounds produced by modern industrial infrastructure.
MDCO Insight: Human hearing is an active biological system that interprets sound rather than simply measuring it.
Following a Sound Through the Ear
Imagine standing outside on a quiet evening.
A distant cooling tower begins operating.
Pressure waves travel through the air towards you.
Nothing physical leaves the cooling tower.
Only tiny oscillations in air pressure move across the landscape.
When those pressure waves reach your head, an extraordinary sequence of events begins.
The Outer Ear
The visible part of the ear—the pinna—is not simply decorative.
Its curved shape acts like a natural acoustic collector.
Different folds reflect and redirect sound depending on where it comes from.
Higher frequencies are affected more strongly than lower frequencies because their shorter wavelengths interact more readily with the complex geometry of the ear.
This allows the brain to estimate whether a sound is coming from:
- in front;
- behind;
- above;
- below.
The sound then travels along the ear canal until it reaches the eardrum.
The Middle Ear
The eardrum is an extremely thin membrane.
As air pressure rises and falls, it vibrates back and forth.
These tiny movements are transferred through three of the smallest bones in the human body:
- the malleus;
- the incus;
- the stapes.
Together these bones form an ingenious mechanical amplifier.
Without them, most sound energy would simply bounce off the fluid-filled inner ear.
Instead, the middle ear increases pressure while reducing movement, allowing sound to enter the cochlea efficiently.
This amplification is particularly important for frequencies in the middle of the hearing range.
Very low frequencies, however, are transmitted much less efficiently.
The middle ear therefore behaves like a mechanical filter.
It naturally reduces our sensitivity to deep bass sounds.
This is one reason why low frequency sound must often reach much higher physical sound pressure levels before it becomes consciously audible.
MDCO Insight: The middle ear naturally attenuates very low frequencies, making human hearing much less sensitive below about 100 Hz.
Inside the Cochlea
Beyond the middle ear lies the cochlea.
The cochlea is a remarkable spiral-shaped organ filled with fluid.
Running through its centre is the basilar membrane.
This membrane is one of the most extraordinary biological structures in the human body.
It is not uniform.
Instead:
- it is narrow and stiff near the entrance;
- it becomes wider and more flexible towards the tip.
This gradual change allows different frequencies to vibrate different parts of the membrane.
High frequencies vibrate the stiff end.
Low frequencies travel further before producing their maximum movement.
Extremely low frequencies reach the flexible apex of the cochlea.
Rather than acting like a single microphone, the cochlea behaves more like thousands of tiny frequency detectors arranged side by side.
Each responds most strongly to a particular range of frequencies.
Together they perform a biological frequency analysis long before the sound reaches the brain.
Engineers often compare this process to a real-time spectrum analyser.
Nature invented it millions of years before electronic signal processing existed.
The Hair Cells
Sitting on top of the basilar membrane are approximately 16,000 sensory cells known as hair cells.
Despite their name, they are not hairs.
Each cell possesses microscopic projections called stereocilia.
As the basilar membrane moves, these stereocilia bend.
When they bend, tiny ion channels open.
This generates electrical impulses that travel along the auditory nerve towards the brain.
This conversion from mechanical vibration into electrical signals is known as mechanotransduction.
It is one of the most remarkable processes in biology.
Without it, hearing would be impossible.
Unlike many other cells in the human body, however, these hair cells do not regenerate once they are destroyed.
Excessive noise exposure can permanently damage them.
This irreversible loss explains why protecting hearing from excessive sound levels is so important.
Why Different Frequencies Sound Equally Loud Only at Different Sound Levels
A common misunderstanding is that a decibel always represents the same perceived loudness.
It does not.
Imagine two sounds measured at exactly 40 dB.
One is a 1,000 Hz tone.
The other is a 31.5 Hz tone.
Although both have the same physical sound pressure level, they will not sound equally loud.
The 1,000 Hz tone will usually be clearly audible.
The 31.5 Hz tone may be barely perceptible—or not consciously heard at all.
The difference arises because the cochlea responds far more efficiently to some frequencies than others.
Its sensitivity peaks roughly between 2 kHz and 5 kHz, where much of human speech occurs.
Sensitivity declines steadily towards lower frequencies.
This relationship has been demonstrated repeatedly through hearing experiments over many decades.
The results are known as equal-loudness contours.
Each contour represents combinations of sound pressure level and frequency that listeners perceive as equally loud.
These curves reveal that:
- high frequencies require relatively little sound pressure to be heard;
- mid frequencies are heard most efficiently;
- very low frequencies require much higher sound pressure before becoming audible.
This explains why a low frequency mechanical hum may contain considerable acoustic energy while still appearing relatively quiet.
It also explains why simply comparing decibel values without considering frequency can be misleading.
MDCO Insight: A decibel measures physical sound pressure, not perceived loudness. The human ear hears different frequencies with very different sensitivities.
Why the Ear Evolved This Way
This frequency-dependent sensitivity is not a flaw.
It is an evolutionary advantage.
Throughout human history, recognising speech, footsteps, animal calls and environmental warning sounds greatly improved survival.
These sounds occur primarily in the middle portion of the audible spectrum.
Natural selection therefore favoured ears that were especially sensitive within these frequencies.
Deep low frequency sounds were comparatively rare in nature.
Thunder.
Ocean waves.
Earthquakes.
Strong winds.
These events certainly existed, but they usually carried less immediate survival value than recognising speech or nearby movement.
Consequently, the auditory system evolved to prioritise frequencies that conveyed the most useful information.
Modern industrial environments present a very different acoustic landscape.
Large cooling towers.
Industrial compressors.
Electrical transformers.
Ventilation systems.
Heavy machinery.
These sources can generate continuous low frequency sound for hours or even days.
From an evolutionary perspective, this is a relatively new type of exposure.
Whether the human body responds to these sounds only through the classical hearing pathway—or also through additional biological mechanisms—is one of the most active areas of contemporary low frequency research.
The next section examines how decades of hearing science have shaped modern acoustic measurements, and why those measurements work well for most environmental noise but become increasingly uncertain when applied to low frequency sound.
How Engineers Measure Sound — And Why Different Weightings Exist
Measuring Sound Is More Complicated Than Measuring Temperature
At first glance, measuring sound appears straightforward.
A microphone detects pressure fluctuations in the air.
An instrument converts those fluctuations into decibels.
A single number is displayed.
In reality, measuring environmental sound is considerably more complex.
Unlike temperature or distance, sound has many dimensions simultaneously.
Every sound possesses:
- sound pressure level;
- frequency;
- duration;
- direction;
- temporal variation;
- tonal characteristics.
A single decibel value cannot describe all of these properties.
For example, two sounds may both measure 60 dB, yet be perceived very differently.
One may be a quiet conversation.
The other may be the continuous hum of industrial machinery.
Their physical sound pressure may be identical, but their frequency content, perceived loudness and potential to cause annoyance can differ substantially.
This is why acoustic engineers measure sound in several different ways, each designed to answer a different question.
MDCO Insight: A decibel alone does not fully describe a sound. Frequency and measurement method are equally important.
The Decibel Without Weighting
The simplest measurement is the unweighted sound pressure level.
Modern sound level meters often label this as:
- Z-weighting;
- Linear;
- Flat.
These terms all refer to essentially the same concept.
No attempt is made to mimic human hearing.
The instrument measures the physical sound pressure across its full operating frequency range as uniformly as practicable.
This measurement represents the actual acoustic energy arriving at the microphone.
It neither increases nor decreases particular frequencies because they sound louder or quieter to people.
For engineering purposes, this makes unweighted measurements particularly valuable when evaluating:
- machinery;
- industrial equipment;
- vibration;
- low frequency sound;
- infrasound.
Because no frequency adjustment is applied, Z-weighted measurements provide the closest representation of the physical sound field.
However, they do not indicate how loud that sound is likely to be perceived by people.
A powerful low frequency sound may produce a large Z-weighted reading while being only weakly audible.
Conversely, a moderate mid-frequency sound may produce a similar Z-weighted level yet appear much louder.
This distinction explains why additional frequency weightings were developed.
Why Frequency Weightings Were Invented
During the early twentieth century, researchers repeatedly observed that human hearing was not equally sensitive across all frequencies.
Listeners consistently required:
- much higher sound pressure to hear deep bass;
- relatively little sound pressure around speech frequencies;
- increasing sound pressure again at the highest audible frequencies.
The challenge therefore became clear.
If a sound level meter simply measured physical sound pressure, it would often report values that differed markedly from human perception.
Engineers therefore introduced frequency weighting networks.
These are mathematical filters built into sound level meters.
Instead of measuring all frequencies equally, they deliberately increase or reduce different parts of the spectrum to approximate how people hear.
Today the most widely used weightings are:
- A-weighting;
- C-weighting;
- Z-weighting.
Each serves a different engineering purpose.
None is universally correct.
Each answers a different question.
A-weighting
A-weighting is by far the most widely used environmental noise metric.
It is written as:
dB(A) or dBA.
The A-weighting curve was developed to approximate human hearing at relatively modest sound levels.
It strongly reduces very low frequencies.
For example:
- 20 Hz is reduced by approximately 50 dB;
- 31.5 Hz by approximately 39 dB;
- 63 Hz by approximately 26 dB;
- 125 Hz by approximately 16 dB.
By contrast, frequencies around 2–5 kHz receive very little correction because this is where human hearing is naturally most sensitive.
The resulting measurement therefore approximates perceived loudness much more closely than an unweighted measurement for most everyday environmental sounds.
Because of this, environmental regulations throughout the world—including those in Malaysia—primarily use dBA.
Typical examples include:
- road traffic noise;
- aircraft noise;
- industrial facilities;
- construction activities;
- community noise.
A-weighting works remarkably well for these situations because much of their acoustic energy lies within the frequencies where human hearing is most sensitive.
For ordinary environmental noise, it provides an excellent balance between engineering practicality and human perception.
MDCO Insight: A-weighting was developed because people do not hear every frequency equally. It remains the foundation of most environmental noise standards.
C-weighting
Some sounds contain substantial low frequency energy.
Examples include:
- explosions;
- heavy industrial machinery;
- music;
- large diesel engines;
- cooling towers.
For these situations, A-weighting may underestimate the contribution of lower frequencies.
C-weighting was therefore developed.
Written as dB(C) or dBC, it applies much less attenuation to low frequencies than A-weighting.
At 63 Hz, for example, the correction is only a few decibels instead of more than twenty.
Consequently, C-weighted measurements often remain much closer to the actual physical sound pressure.
Many acousticians compare both A-weighted and C-weighted measurements.
The difference between them provides a useful indication of how much low frequency energy is present.
When:
dBC − dBA
becomes large, the sound usually contains a substantial low frequency component.
Although no universal threshold exists, differences greater than approximately 15–20 dB often suggest that further investigation of low frequency sound may be appropriate, particularly if community complaints are reported.
This comparison is widely used in environmental investigations because it is simple while still providing useful diagnostic information.
Z-weighting
Unlike A-weighting and C-weighting, Z-weighting applies essentially no intentional frequency correction.
It is therefore the preferred measurement when engineers wish to know the actual acoustic energy reaching the microphone.
This makes Z-weighting particularly valuable for:
- laboratory measurements;
- machinery testing;
- acoustic research;
- spectrum analysis;
- low frequency investigations.
Modern acoustic standards increasingly recommend recording Z-weighted data alongside octave-band or one-third octave-band spectra because these measurements preserve information that weighted values may conceal.
For research into low frequency sound, Z-weighting has become increasingly important.
It allows investigators to determine whether apparently modest A-weighted levels actually contain significant acoustic energy at frequencies below approximately 100 Hz.
Comparing the Three Weightings
Although all three measurements use decibels, they answer different questions.
| Measurement | What it represents | Typical application |
| dBZ | Physical sound pressure across the frequency spectrum | Engineering analysis, machinery, research, low frequency investigations |
| dBC | Approximate loudness with relatively small low frequency correction | Industrial noise, music, heavy machinery, diagnostic comparison |
| dBA | Approximate human loudness at ordinary listening levels | Environmental regulations, occupational noise, community assessments |
None of these measurements is inherently superior.
Each was developed for a particular purpose.
Problems arise only when a measurement intended for one purpose is used to answer a different question.
Why Low Frequency Sound Creates Difficulties
Most environmental noise standards were developed using research focused on conventional hearing.
They assume that annoyance and potential health effects are closely related to perceived loudness.
For many common environmental sounds, this assumption has proved highly successful.
Road traffic.
Aircraft.
Railways.
Construction.
Urban environments.
In these situations, A-weighted measurements correlate reasonably well with community response.
Low frequency sound is different.
Because A-weighting substantially reduces frequencies below approximately 100 Hz, a sound dominated by deep bass may produce a relatively modest dBA value even when its physical sound pressure remains considerable.
Consequently, two environments may both satisfy the same A-weighted guideline while containing very different amounts of low frequency energy.
This does not necessarily mean existing standards are incorrect.
Rather, it reflects the fact that those standards were developed primarily for sounds whose dominant frequencies lie within the range of greatest human hearing sensitivity.
Whether these same measurements adequately characterise modern sources dominated by persistent low frequency sound remains an active area of scientific investigation.
This question has become increasingly relevant with the growth of large-scale infrastructure such as data centres, wind turbines and industrial cooling systems.
MDCO Insight: A-weighting is an excellent predictor of perceived loudness for most environmental sounds, but it intentionally discounts low frequency energy. Whether that remains sufficient for modern sources dominated by persistent low frequency sound continues to be investigated by researchers worldwide.
Beyond the Classical Model — The Emerging Understanding of Low Frequency Sound
A New Scientific Question
For more than a century, the science of hearing has been remarkably successful.
The classical model explains how sound travels through the ear, how the cochlea separates different frequencies, and why humans hear some sounds more easily than others.
It has shaped:
- hearing science;
- acoustic engineering;
- environmental noise standards;
- occupational noise regulations;
- hearing conservation programmes.
For most everyday sounds, it continues to provide an excellent description of human hearing.
However, during the past two decades, researchers studying persistent low frequency sound have encountered an intriguing observation.
Some people report perceiving or reacting to low frequency sound even when conventional hearing models predict that the sound should be barely audible or inaudible.
These observations do not overturn the classical understanding of hearing.
Instead, they raise a new question:
Can humans respond to low frequency sound through mechanisms that are not fully described by conscious hearing alone?
This question has become an active area of research in neuroscience, auditory physiology and environmental acoustics.
Hearing Is Not the Same as Perception
In everyday language, “hearing” usually means consciously detecting a sound.
Scientifically, however, the process is more complex.
A sound entering the body may produce several different outcomes.
It may be:
- consciously heard;
- barely noticeable;
- sensed as vibration;
- processed subconsciously by the nervous system;
- ignored completely.
These responses need not occur together.
For example, a person may not consciously hear a very low frequency sound while still noticing:
- a feeling of pressure;
- a slight vibration;
- difficulty relaxing;
- a vague awareness that “something feels different.”
Such experiences do not automatically demonstrate that the sound caused the sensation.
Many other environmental and psychological factors may also contribute.
Nevertheless, they illustrate that conscious hearing is only one part of how humans interact with sound.
The Brain Processes More Than We Notice
Modern neuroscience increasingly recognises that the brain continuously processes enormous amounts of sensory information without conscious awareness.
Vision provides a familiar example.
People often respond to movement in their peripheral vision before consciously recognising what moved.
Similarly, the auditory system continuously analyses sounds that never reach conscious awareness.
Examples include:
- maintaining balance;
- locating sound sources;
- filtering background noise;
- detecting approaching objects;
- monitoring environmental changes.
The brain therefore performs much more auditory processing than people consciously experience.
This raises an important scientific possibility.
If some components of low frequency sound are processed subconsciously, traditional hearing thresholds alone may not fully describe how humans respond to such sounds.
This possibility remains an active area of investigation rather than an established conclusion.
The Cochlea May Be More Sensitive Than We Once Thought
One of the most influential developments in recent years concerns the cochlea itself.
Classically, researchers assumed that sounds below the hearing threshold produced little meaningful activity within the inner ear.
Recent experimental studies suggest the situation may be more complicated.
Measurements indicate that certain structures within the cochlea may continue responding mechanically to very low frequency sound even when the listener reports hearing nothing.
In other words, the ear may detect more physical information than reaches conscious perception.
Researchers are now investigating whether this mechanical activity contributes to:
- subconscious neural processing;
- physiological responses;
- perception under particular conditions.
At present, no consensus exists regarding the significance of these findings.
However, they have encouraged scientists to re-examine assumptions that remained largely unchanged for decades.
MDCO Insight: Modern research suggests that the inner ear may continue responding mechanically to very low frequency sound below conventional hearing thresholds. Whether this contributes to human perception or health effects remains an important question under active investigation.
Beyond the Cochlea
The ear is only the first stage of hearing.
Once sound has been converted into electrical signals, the nervous system distributes that information throughout multiple regions of the brain.
Some researchers are therefore investigating whether low frequency sound influences systems beyond those traditionally associated with hearing.
Areas of interest include:
- autonomic nervous system activity;
- attention;
- emotional processing;
- stress responses;
- sleep regulation.
Importantly, investigating these possibilities does not imply that low frequency sound is necessarily harmful.
Instead, researchers are attempting to understand whether persistent exposure interacts with biological systems in ways not captured by traditional measures of audible loudness.
Much of this work remains exploratory.
Different studies often employ different exposure conditions, frequencies and measurement techniques.
Consequently, individual findings cannot yet be combined into a single unified explanation.
Why This Matters for Environmental Noise
Environmental noise standards are necessarily based on the best scientific knowledge available when they are developed.
Most existing standards evolved from decades of research linking:
- perceived loudness;
- speech interference;
- hearing damage;
- annoyance.
These remain highly relevant outcomes.
However, if future research confirms that low frequency sound interacts with the body through additional pathways, engineers and regulators may eventually need to consider factors beyond conventional hearing thresholds.
That does not mean current standards are obsolete.
Rather, it suggests they may not answer every scientific question relating to persistent low frequency sound.
This distinction is important.
Engineering standards are designed to provide practical guidance based on established evidence.
Scientific research, by contrast, continually tests whether existing knowledge is complete.
The two processes complement rather than contradict one another.
Why Scientific Debate Continues
Readers occasionally encounter headlines claiming that scientists have “discovered a new way humans hear.”
Such statements often exaggerate what the research actually shows.
The emerging evidence does not replace the classical understanding of hearing.
Instead, it expands the range of questions being investigated.
Current areas of debate include:
- whether subconscious responses occur consistently across individuals;
- which frequencies are most significant;
- whether exposure duration matters;
- how laboratory findings relate to real-world environments;
- whether physiological responses translate into meaningful health outcomes.
These are legitimate scientific questions.
They are also difficult to answer because low frequency sound varies greatly between:
- natural environments;
- industrial facilities;
- transportation systems;
- wind turbines;
- data centres.
Each produces different frequency spectra, operating patterns and exposure durations.
Consequently, conclusions drawn from one source cannot automatically be applied to another.
What This Means for Data Centres
Large data centres differ from many traditional environmental noise sources (→E02 How Data Centres Interact with the World).
Cooling systems may operate:
- continuously;
- throughout the day and night;
- for many years.
Their dominant acoustic energy often lies within the lower part of the audible spectrum, particularly where large cooling towers and slow-speed fans are used (→E02.01 How Data Centres Generate Low Frequency Noise).
This does not imply that all data centres create problematic low frequency sound.
Many modern facilities incorporate sophisticated acoustic design that substantially reduces emissions.
However, it does mean that low frequency sound deserves careful engineering assessment rather than being dismissed solely because overall A-weighted levels comply with existing limits.
As the scientific understanding of low frequency perception continues to evolve, engineers, regulators, operators and communities all benefit from maintaining an evidence-based approach grounded in measurement, transparent analysis and ongoing research.
MDCO Insight: The classical science of hearing remains fundamentally sound, but emerging research suggests it may not capture every way humans respond to persistent low frequency sound. This uncertainty strengthens, rather than weakens, the case for careful measurement, thoughtful engineering and continued scientific investigation.
Designing Better Data Centres
The discussion throughout this article points towards an important conclusion.
Low frequency sound should not be viewed as an unavoidable consequence of modern infrastructure.
It is also an engineering design challenge.
Many of the factors influencing acoustic performance can be addressed during the planning and design stages.
Examples include:
- selecting lower-noise cooling technologies;
- optimising fan diameter and operating speed;
- incorporating variable-speed control;
- improving equipment layout;
- increasing separation from sensitive receptors;
- providing appropriate acoustic treatment;
- minimising vibration transmission.
These engineering decisions often have a greater influence on community exposure than the overall capacity of the facility itself (→A09.01 Why Low-Frequency Noise Becomes a Community Conflict).
As discussed in previous MDCO articles, a larger data centre is not necessarily proportionally louder than a smaller one.
Good engineering design can significantly reduce environmental impacts while maintaining operational efficiency.
Looking Beyond Decibels
Environmental noise discussions often become dominated by a single question:
What was the measured dBA?
Although important, this question alone is insufficient.
Understanding environmental sound also requires asking:
- What frequencies dominate the sound?
- Is the sound continuous or intermittent?
- Is it tonal?
- How does it change with weather conditions?
- How is it experienced inside nearby homes?
- Does the measurement method adequately represent the characteristics of the source?
These questions become particularly relevant for large infrastructure whose acoustic signatures differ from those of conventional urban noise.
Low frequency sound reminds us that engineering measurements are tools for understanding reality rather than reality itself.
The choice of measurement should always reflect the question being asked.
The Next Question
This article has examined how humans detect and measure sound.
It has shown that:
- hearing is frequency dependent;
- engineering measurements use different weighting systems for different purposes;
- A-weighting remains an excellent predictor of perceived loudness for most environmental sounds;
- persistent low frequency sound presents additional scientific questions that remain under active investigation.
One question remains.
Even if two people experience the same low frequency sound, why might one person sleep comfortably while another reports persistent annoyance or disturbance?
Answering that question requires moving beyond acoustics into psychology, neuroscience and environmental health.
The future article (→A09.03 Low Frequency Noise Effects: What Research Shows) in this series will examine what current scientific research tells us about:
- annoyance;
- sleep disturbance;
- stress responses;
- long-term health outcomes;
- why individual responses to low frequency sound can differ so markedly.
The Observatory Perspective
Scientific understanding evolves.
Engineering practice evolves with it.
The history of acoustics demonstrates that improvements in measurement have repeatedly led to improvements in engineering design and environmental management.
The growing interest in low frequency sound should therefore not be interpreted as evidence that existing engineering principles have failed.
Rather, it reflects the continual refinement of scientific knowledge as new questions emerge and better measurement techniques become available.
For data centres, this has an important implication (→A09 Understanding Low-Frequency Noise from Data Centres).
As facilities become larger and increasingly integrated into urban and suburban environments, understanding low frequency sound becomes part of responsible infrastructure planning.
Careful measurement, transparent reporting, thoughtful engineering and open engagement with affected communities are all essential components of good governance.
The objective is not to create unnecessary concern, nor to dismiss legitimate questions prematurely.
It is to ensure that decisions are guided by the best available evidence while remaining open to new scientific understanding as it develops.
MDCO Insight: The science of hearing continues to evolve, but one principle remains constant: better engineering begins with better understanding. As data centres become a permanent part of modern communities, understanding how people perceive and experience sound is as important as measuring the sound itself.
Selected References
- International Organization for Standardization (ISO). ISO 226: Acoustics — Normal Equal-Loudness-Level Contours. Establishes the internationally recognised equal-loudness contours describing how human sensitivity to sound varies with frequency. This standard forms the scientific basis for understanding why humans perceive low frequency sound as less loud than mid-frequency sound at the same physical sound pressure level. https://www.iso.org/standard/83117.html
- International Organization for Standardization (ISO). ISO 532-1: Acoustics — Methods for Calculating Loudness — Part 1: Zwicker Method.
Provides a psychoacoustic method for estimating perceived loudness based on human auditory response rather than physical sound pressure alone. It illustrates the difference between measured acoustic energy and human perception of loudness. https://www.iso.org/standard/63077.html - International Organization for Standardization (ISO). ISO 1996-1: Acoustics — Description, Measurement and Assessment of Environmental Noise — Part 1: Basic Quantities and Assessment Procedures. Provides the framework for describing environmental noise, including sound pressure levels, weighting methods and assessment concepts used in environmental noise management. https://www.iso.org/standard/59765.html
- International Organization for Standardization (ISO). ISO 1996-2: Acoustics — Description, Measurement and Assessment of Environmental Noise — Part 2: Determination of Sound Pressure Levels. Provides guidance on environmental noise measurements, including frequency analysis, measurement procedures and assessment of sound levels at receiving locations. https://www.iso.org/standard/59766.html
- International Organization for Standardization (ISO). ISO 389-7: Acoustics — Reference Zero for the Calibration of Audiometric Equipment — Part 7: Reference Threshold of Hearing Under Free-Field and Diffuse-Field Listening Conditions. Defines reference hearing thresholds used in audiology and supports understanding of the relationship between physical sound levels and human hearing sensitivity across frequencies. https://www.iso.org/standard/77365.html
- Moore, B. C. J. (199\). An Introduction to the Psychology of Hearing. A comprehensive reference on auditory perception, including frequency selectivity, loudness perception, masking, auditory processing and the relationship between physical sound and human experience. Academic Press.
- Moore, B. C. J., & Glasberg, B. R. (2007). Modeling binaural loudness. Reviews psychoacoustic models used to explain how humans perceive loudness and why equal physical sound levels can produce different perceptual responses depending on frequency and sound characteristics. https://doi.org/10.1121/1.2431331
- Hudspeth, A. J. (2008). Making an Effort to Listen: Mechanical Amplification in the Ear.
Reviews the active mechanical processes within the cochlea that amplify and transform sound vibrations into neural signals, contributing to modern understanding of hearing sensitivity. https://doi.org/10.1016/j.neuron.2008.07.012 - Géléoc, G. S, Lennan, G. W, Richardson, G. P., & Kros, C. J. (1997). A Quantitative Comparison of Mechanoelectrical Transduction in Vestibular and Auditory Hair Cells of Neonatal Mice. Provides experimental insight into the cellular mechanisms responsible for converting mechanical vibrations into electrical signals within sensory hair cells. https://doi.org/10.1098/rspb.1997.0087
- Møller, A. R. (2013). Hearing: Anatomy, Physiology, and Disorders of the Auditory System.
Provides a comprehensive overview of auditory anatomy, including sound transmission through the outer ear, middle ear, cochlea and central auditory pathways. - Leventhall, H. G. (2004). Low Frequency Noise and Annoyance. Reviews the relationship between measured low frequency sound levels, human perception and reported annoyance. The paper remains one of the most frequently cited discussions of low frequency noise perception and community response. https://pubmed.ncbi.nlm.nih.gov/15273024/
- Moorhouse, A. T., Waddington, D. C., & Adams, M. D. (2005). Procedure for the Assessment of Low Frequency Noise Complaints. Prepared for the UK Department for Environment, Food and Rural Affairs (DEFRA). Provides practical guidance for investigating low frequency noise complaints, including measurement methods, frequency analysis and consideration of subjective response. https://doi.org/10.1121/1.3180695
- World Health Organization (WHO). Environmental Noise Guidelines for the European Region.
Reviews scientific evidence linking environmental noise exposure with health outcomes, particularly annoyance, sleep disturbance and environmental quality. The guideline provides the broader public health context for environmental noise assessment. https://www.who.int/europe/publications/i/item/9789289053563 - Basner, M., Babisch, W., Davis, A., et al. (2014). Auditory and Non-Auditory Effects of Noise on Health. Reviews evidence on both auditory effects and non-auditory responses to environmental noise, including sleep disturbance, stress responses and cardiovascular considerations. https://doi.org/10.1016/S0140-6736(13)61613-X
- Salt, A. N., & Hullar, T. E. (2010). Responses of the Ear to Low Frequency Sounds, Infrasound, and Wind Turbines. Discusses experimental observations regarding inner ear responses to very low frequency sound and explores whether conventional hearing thresholds fully describe all biological responses to low frequency acoustic energy. https://doi.org/10.1016/j.heares.2010.06.007
- Salt, A. N., & Kaltenbach, J. A. (2011). Infrasound from Wind Turbines Could Affect Humans.
Examines hypotheses regarding biological responses to infrasound and low frequency sound exposure. The work remains part of an ongoing scientific discussion regarding mechanisms beyond conventional hearing perception. https://doi.org/10.1177/0270467611412555 - Auerbach, B. D., Rodrigues, P. V., & Salvi, R. J. (2014). Central Gain Control in Tinnitus and Hyperacusis. Reviews how changes in central auditory processing can influence sound perception, sensitivity and responses that are not explained solely by peripheral hearing mechanisms. https://doi.org/10.3389/fneur.2014.00206
Citation
Malaysia Data Centre Observatory (MDCO). E02.02 Why Low Frequency Noise: How Humans Hear Sound Beyond Decibels. MDCO Explain Series.
MDCO Note
This article forms part of the Malaysia Data Centre Observatory (MDCO) Explain Series, which aims to improve public understanding of data centre development through evidence-based, accessible and balanced analysis. It is intended for educational and informational purposes only and does not constitute legal, engineering, planning, environmental or professional advice.
Malaysia’s rapidly evolving data centre ecosystem includes facilities developed, owned or operated by organisations such as AirTrunk, Amazon Web Services (AWS), Bridge Data Centres, DayOne, EdgeConneX, Google, K2 Data Centres, Microsoft, NTT Global Data Centers, Princeton Digital Group (PDG), ST Telemedia Global Data Centres (STT GDC), STACK Infrastructure, Vantage Data Centers, YTL Data Centre Park and many others. MDCO is independent of these organisations, as well as governments, regulators, utilities and advocacy groups. Its role is to facilitate transparency, structured understanding and equal access to information by presenting publicly verifiable evidence, relevant context and multiple stakeholder perspectives. MDCO does not endorse, oppose or advocate for any particular organisation, project or policy position.