E02.01 — How Data Centres Generate Low-Frequency Noise

Understanding the Engineering Behind One of Modern Data Centres’ Most Discussed Environmental Issues

Key Takeaways

• Low-frequency noise is an unintended by-product of the mechanical systems required to remove heat from continuously operating servers.
• Different cooling components generate different frequency characteristics because they rely on different mechanical and fluid-flow processes.
• Large, continuously operating fans are often the dominant source of persistent low-frequency noise in modern data centres.
• Cooling towers, chillers, pumps, transformers and other infrastructure each contribute distinct acoustic signatures to the overall sound environment.
• Low-frequency noise is influenced not only by equipment design, but also by operating conditions, structural transmission and local environmental factors.
• Understanding how sound is generated provides the engineering foundation for evaluating measurement methods, mitigation strategies and community impacts.

Introduction

This article forms part of the Malaysia Data Centre Observatory (MDCO) Explain Series and extends →E02 How Data Centres Interact with the World, which introduced how modern data centres exchange electricity, water, heat, telecommunications and other forms of interaction with their surrounding environment. It also builds upon →E03 Why Data Centres Are Built This Way and →E03.01 Understanding Data Centre Cooling Systems, which explained why continuous cooling is essential for reliable digital infrastructure.

One of the less visible, but increasingly discussed, interactions between data centres and their surrounding communities is low-frequency noise (LFN). Unlike electricity or water consumption, sound cannot be measured simply by the amount of energy consumed. It arises from the operation of numerous mechanical systems that work together to remove heat from continuously operating servers.

Understanding how this sound is generated requires a different perspective from the broader discussions presented in the MDCO Analyse Series. Rather than examining governance, planning, regulation or community experience, this article focuses on the engineering principles that produce low-frequency noise. It explains how mechanical components generate vibration, why certain equipment produces predominantly low-frequency sound, and why these characteristics differ between cooling technologies.

Readers interested in how low-frequency noise affects communities, how different jurisdictions regulate the issue, and how it has emerged as a subject of legal disputes and public policy should also refer to →A09 Understanding Low-Frequency Noise from Data Centres and the accompanying articles in the A09 series.

By understanding how low-frequency noise is physically generated, readers will be better equipped to interpret later discussions concerning measurement, mitigation, regulation and community engagement.

Why Cooling Inevitably Produces Sound

Modern data centres exist for one fundamental purpose: to provide reliable computing capacity.

Every digital transaction, artificial intelligence model, cloud application and online service ultimately depends on electronic components processing electrical signals within servers. In doing so, these components consume electricity, and under the laws of thermodynamics, nearly all of that electrical energy is ultimately converted into heat.

Heat is therefore not an occasional by-product of computing—it is an unavoidable consequence of computation itself.

If this heat were not continuously removed, temperatures inside servers would rapidly exceed their allowable operating limits. Excessive temperatures reduce equipment performance, shorten component lifespan and may ultimately lead to system failure. Reliable cooling is therefore not an optional feature but a fundamental engineering requirement for every modern data centre.

As explained in →E03.01 Understanding Data Centre Cooling Systems, removing this heat requires an extensive network of mechanical systems. Depending on the cooling architecture, these systems may include:

• computer room air handlers (CRAH);
• computer room air conditioners (CRAC);
• chillers;
• cooling towers;
• dry coolers;
• pumps;
• compressors;
• heat exchangers;
• ventilation fans;
• associated pipework and control systems.

Although these components perform different functions, they all share one common characteristic: they transfer heat by moving air, water or refrigerant.

Moving fluids requires energy.

Energy creates mechanical motion.

Mechanical motion produces vibration.

Vibration propagates through solid structures and the surrounding air as sound.

From an engineering perspective, this sequence is unavoidable.

Rather than viewing noise as an isolated environmental issue, it can be understood as part of a continuous chain of energy conversion:

Electrical energy → Heat → Fluid movement → Mechanical motion → Vibration → Sound

Each stage is a necessary consequence of the stage before it.

Consequently, the objective of cooling system design is not to eliminate sound completely—an impossible engineering objective—but to remove heat safely and efficiently while managing the acoustic impacts of the mechanical systems involved.

This distinction is important because it reframes how low-frequency noise should be understood. The sound does not originate from the computing process itself. Instead, it originates from the infrastructure required to keep computing operating safely, continuously and reliably.

As computing demand increases, cooling systems generally become larger, more complex and more powerful. Larger cooling systems require larger fans, greater airflow, increased pumping capacity and higher heat rejection rates. These changes may also alter the characteristics of the sound generated, particularly at lower frequencies where large rotating equipment naturally operates.

Understanding this relationship provides the foundation for the remainder of this article.

MDCO Insight: Low-frequency noise originates not from computing itself, but from the mechanical systems required to remove the heat that computing continuously generates.

How Mechanical Systems Produce Low-Frequency Noise

Understanding why data centre equipment produces low-frequency noise begins with understanding the relationship between vibration and sound.

Sound is a form of mechanical wave generated when an object vibrates. As the object moves, it creates alternating regions of compression and rarefaction within the surrounding air. These pressure fluctuations propagate outward as sound waves until they are absorbed, reflected or dissipated by the surrounding environment.

Every rotating machine within a data centre generates some degree of vibration.

Electric motors rotate shafts.

Fans rotate blades.

Pumps rotate impellers.

Compressors compress refrigerant.

Gearboxes transmit torque.

Even transformers experience microscopic dimensional changes as alternating magnetic fields act upon their steel cores.

In practice, no mechanical system is perfectly balanced. Small manufacturing tolerances, rotating masses, fluctuating fluid pressures and aerodynamic forces all introduce dynamic loads that cause components to vibrate during operation.

These vibrations may travel through two principal pathways.

The first is airborne noise, where vibrating surfaces radiate sound directly into the surrounding air. Cooling fans, cooling towers and air-cooled chillers are typical examples.

The second is structure-borne noise, where vibration travels through supporting structures such as steel frames, concrete slabs, pipework or building elements before being radiated elsewhere as audible sound. Pumps, compressors and transformers commonly contribute to this form of transmission.

In many situations, both pathways occur simultaneously.

A cooling tower fan, for example, radiates airborne sound from the rotating blades while also transmitting vibration through the tower structure and supporting foundations.

The resulting acoustic environment therefore depends not only on the equipment itself but also on how that equipment interacts with the surrounding structure.

Not all mechanical sound is the same.

Some equipment produces tonal noise, where energy is concentrated around one or more distinct frequencies. Transformer hum and rotating fan blades are familiar examples.

Other equipment produces broadband noise, where sound energy is distributed across a much wider range of frequencies. Turbulent airflow and falling water are typical examples.

Many data centre components generate both simultaneously.

For example, a cooling tower may produce a distinct fan tone together with the broadband sound of moving water. Likewise, a chiller may emit tonal frequencies from its compressor while also generating broadband noise associated with airflow through the condenser.

The overall acoustic signature is therefore the combined result of multiple independent mechanisms operating at the same time.

Understanding these mechanisms is important because different noise sources respond differently to engineering solutions. A measure that effectively reduces aerodynamic fan noise may have little influence on hydraulic pulsation within a pumping system or structural vibration transmitted through a building.

For this reason, acoustic engineers generally begin by identifying the dominant physical mechanisms responsible for the observed sound before selecting appropriate mitigation measures.

MDCO Insight: Low-frequency noise is generated through multiple physical mechanisms, each requiring different engineering approaches to understand and manage.

Understanding Frequency Signatures

One of the reasons low-frequency noise is often misunderstood is that people naturally describe sound using subjective terms such as humming, droning, rumbling or throbbing. Engineers, however, describe sound differently.

Rather than relying primarily on loudness, engineers examine the frequency signature of a sound.

A frequency signature describes how acoustic energy is distributed across different frequencies. It provides an acoustic “fingerprint” that often reveals the underlying physical process responsible for generating the sound.

Two sounds may have similar overall sound levels yet possess entirely different frequency signatures.

For example, the sound produced by a transformer is dominated by narrow tonal frequencies associated with the alternating electrical supply, typically centred around the fundamental mains frequency and its harmonics. In contrast, turbulent airflow through a large ventilation system produces broadband noise distributed across a much wider range of frequencies.

Similarly, the acoustic signature of a cooling tower differs from that of a pump because the underlying physical mechanisms differ.

Engineers therefore analyse frequency spectra rather than relying solely on overall decibel values.

Several engineering concepts are particularly important when interpreting these frequency signatures.

Rotational Frequency

Most mechanical equipment contains rotating components.

The rotational frequency depends on how fast the equipment rotates and is commonly expressed in revolutions per minute (RPM). This rotational speed establishes the fundamental vibration frequency generated by the machine.

Large industrial equipment often rotates relatively slowly compared with smaller machinery. Consequently, much of the vibration energy occurs within lower frequency ranges.

Blade-Pass Frequency

Fans introduce an additional characteristic known as the blade-pass frequency.

Each time a fan blade passes a fixed point, it generates a small pressure fluctuation. The repeated passage of multiple blades produces a characteristic tonal frequency determined by both the fan speed and the number of blades.

Because cooling fans are among the largest continuously operating rotating machines within many data centres, blade-pass frequencies often contribute significantly to the overall low-frequency acoustic environment.

Harmonics

Mechanical systems rarely vibrate at only one frequency.

Instead, additional frequencies known as harmonics often occur at integer multiples of the fundamental frequency.

A transformer supplied at 50 Hz typically produces a dominant hum at 100 Hz, together with higher harmonic frequencies such as 200 Hz and 300 Hz. Similarly, rotating machinery frequently generates multiple harmonics resulting from aerodynamic loading, structural resonance and mechanical interactions.

These harmonics contribute to the distinctive character of different equipment.

Resonance

Mechanical systems also possess natural frequencies determined by their size, geometry and structural stiffness.

When an operating frequency coincides with one of these natural frequencies, resonance may occur, causing vibration amplitudes to increase significantly.

Resonance does not create new sources of sound. Rather, it amplifies vibration already being generated by the equipment.

For this reason, two apparently identical cooling systems installed in different locations may produce noticeably different acoustic outcomes depending on how the supporting structures respond.

Recognising these engineering characteristics allows acoustic specialists to distinguish between different sources of sound and to identify which components contribute most significantly to the overall low-frequency environment.

The following sections examine how these principles apply to the major components found within modern data centre cooling systems.

Component	Primary Source of Sound	Typical Frequency Characteristics	Dominant Noise Type
Large axial fans	Blade rotation and airflow	Tonal with harmonics	Airborne
Cooling towers	Fan, water movement and gearbox	Tonal and broadband	Airborne and structure-borne
Pumps	Impeller rotation and hydraulic pulsation	Tonal with vibration harmonics	Structure-borne
Chillers	Compressors, fans and refrigerant flow	Tonal and broadband	Airborne and structure-borne
Transformers	Magnetostriction	Strong discrete tonal frequencies	Structure-borne and airborne
Standby generators	Engine combustion and exhaust	Broadband with low-frequency engine components	Airborne

MDCO Insight: Every major component within a data centre produces a characteristic frequency signature that reflects its underlying mechanical and fluid-flow processes.

Why Fans Deserve Special Attention

Among the many mechanical systems found within a modern data centre, fans are by far the most widespread and often the most significant continuous source of low-frequency noise.

Unlike standby generators, which operate only during testing or power interruptions, cooling fans operate continuously whenever heat must be removed from the facility. Whether the data centre uses direct air cooling, chilled water systems, liquid cooling or hybrid cooling architectures, fans remain essential because heat must ultimately be rejected to the surrounding atmosphere.

Consequently, a large hyperscale data centre may contain hundreds or even thousands of fans distributed throughout different cooling systems.

Examples include:

• CRAH unit fans circulating air within white spaces;
• CRAC condenser fans;
• air-cooled chiller condenser fans;
• dry cooler fans;
• cooling tower fans;
• ventilation and fresh-air fans;
• electrical room ventilation fans;
• standby generator radiator fans.

Although these fans perform different functions, they all operate according to similar aerodynamic principles.

They accelerate large volumes of air by converting the rotational energy of an electric motor into airflow. During this process, pressure fluctuations are generated around each rotating blade. These fluctuations become sound waves that propagate into the surrounding environment.

Because fans often operate continuously throughout the day and night, even relatively modest sound levels can become significant when experienced over extended periods.

In many community noise investigations, the dominant source is not a single exceptionally loud machine, but the cumulative effect of numerous fans operating simultaneously across a large facility.

This explains why acoustic engineers often begin investigations by examining fan systems before considering other mechanical equipment.

MDCO Insight: Fans are often the dominant continuous source of low-frequency noise because they are present throughout the cooling system and operate whenever heat is being removed from the data centre.

Types of Fans Used in Data Centres

Although commonly referred to simply as “fans”, several different fan designs are used within modern data centres. Each is optimised for different airflow requirements, pressure conditions and energy efficiency objectives, resulting in different acoustic characteristics.

Axial Fans

Axial fans are the most recognisable type of industrial fan.

They move air parallel to the rotating shaft, much like a household fan but on a much larger scale.

Axial fans are capable of moving enormous volumes of air efficiently and therefore dominate equipment such as:

• cooling towers;
• dry coolers;
• air-cooled chillers;
• rooftop heat rejection equipment;
• large ventilation systems.

Their large diameters—often between 2 and 5 metres—and relatively low rotational speeds make them particularly important contributors to low-frequency noise.

Because the blades pass a fixed point at regular intervals, axial fans often generate prominent tonal components associated with the blade-pass frequency discussed earlier.

Centrifugal Fans

Centrifugal fans operate differently.

Instead of pushing air straight through the fan, they draw air into the centre before accelerating it radially outward using centrifugal force.

This allows them to generate much higher static pressure than axial fans, making them well suited for overcoming the resistance created by filters, cooling coils and ductwork.

They are commonly found in:

• CRAH units;
• CRAC units;
• ventilation systems;
• indoor air-handling equipment.

Although centrifugal fans often produce less pronounced low-frequency tonal noise than large axial fans, they may generate broader frequency spectra because of the more complex airflow patterns within the fan housing.

Backward-Curved Centrifugal Fans

Many modern CRAH units employ backward-curved centrifugal fans.

The backward-curved blade profile improves aerodynamic efficiency by reducing airflow separation and turbulence.

Compared with older forward-curved designs, backward-curved fans generally:

• consume less energy;
• achieve higher efficiency;
• generate lower sound levels;
• produce smoother airflow.

For these reasons, they have become increasingly common in high-efficiency hyperscale facilities.

Electronically Commutated (EC) Fans

An important development in recent years has been the widespread adoption of electronically commutated (EC) fans.

Rather than operating at fixed speeds, EC fans use integrated electronic motor controls that continuously adjust rotational speed to match the cooling demand.

When server utilisation decreases, fan speeds can also decrease, reducing both power consumption and noise generation.

Variable-speed operation has therefore become one of the most effective methods of improving both energy efficiency and acoustic performance.

However, EC fans also introduce new acoustic characteristics.

As fan speeds vary continuously, the dominant frequencies generated by the fan also change continuously. Instead of producing one fixed tonal frequency, the acoustic signature may shift throughout the day as cooling loads fluctuate.

Propeller Fans

Propeller fans resemble simple axial fans but generally operate at lower pressure and are used where unrestricted airflow is sufficient.

They are commonly found in:

• equipment enclosures;
• electrical rooms;
• auxiliary ventilation systems;
• generator radiator systems.

Although individually less significant than cooling tower fans, numerous propeller fans operating simultaneously may contribute to the overall acoustic environment.

How Fans Generate Low-Frequency Noise

Despite the diversity of fan designs, the physical mechanisms responsible for generating sound are remarkably similar.

Rather than arising from a single source, fan noise is produced through several interacting aerodynamic and mechanical processes.

Blade-Pass Frequency

The most characteristic source of fan noise is the blade-pass frequency (BPF).

As each blade rotates past a stationary point, it creates a pressure pulse within the surrounding air.

Because this process repeats with every revolution, a periodic sound is produced.

The blade-pass frequency depends upon:

• rotational speed; and
• number of blades.

For example, a fan rotating at 300 RPM with six blades produces a blade-pass frequency of:

300 RPM ÷ 60 × 6 = 30 Hz

This falls squarely within the low-frequency range.

Consequently, very large, slow-moving fans naturally generate lower-frequency tonal sound than smaller, faster-rotating fans.

Aerodynamic Loading

Fan blades function as rotating airfoils.

As they generate lift to move air, pressure differences develop between the front and rear surfaces of each blade.

These pressure variations fluctuate continuously during rotation, creating additional sound energy.

Higher airflow generally requires greater aerodynamic loading, which increases acoustic output.

Wake Interaction

Each blade leaves behind a turbulent wake.

As following blades pass through these disturbed airflow regions, additional pressure fluctuations occur.

This interaction produces extra tonal components and contributes to harmonic frequencies beyond the fundamental blade-pass frequency.

Tip Vortices

Near the blade tips, high-pressure air leaks around the blade edges into lower-pressure regions.

This leakage forms rotating vortices that persist downstream.

Tip vortices are an important source of broadband aerodynamic noise and become increasingly significant as fan diameter and airflow increase.

Flow Separation

When airflow no longer follows the blade surface smoothly, flow separation occurs.

Separated flow produces unstable turbulence, increasing broadband noise while also reducing aerodynamic efficiency.

Poor blade design or unfavourable operating conditions can therefore increase both energy consumption and acoustic emissions simultaneously.

Mechanical Imbalance

No rotating machine is perfectly balanced.

Small differences in blade weight, shaft alignment or bearing condition introduce periodic forces that excite structural vibration.

Although modern industrial fans are carefully balanced during manufacture, wear and maintenance conditions can gradually alter vibration characteristics over time.

Motor and Bearing Noise

The electric motor itself also contributes to the overall acoustic signature.

Motor electromagnetic forces, bearing rolling elements and lubrication conditions generate additional vibration that may combine with aerodynamic noise.

In well-maintained equipment these contributions are often relatively small, but deteriorating bearings or misalignment can substantially increase low-frequency vibration.

Why Large Fans Produce Lower Frequencies

One of the most common public observations surrounding data centre noise is the persistent “deep humming” reported near some facilities.

This characteristic is closely related to the physical size of the fans used.

Large fans move significantly greater volumes of air with each revolution.

Consequently, they can achieve the required airflow while rotating much more slowly than smaller fans.

Lower rotational speeds directly reduce the blade-pass frequency.

For example:

Fan Diameter	Typical Speed	Approximate Blade-Pass Frequency*
500 mm	1,500 RPM	Higher frequency
1.5 m	700 RPM	Mid-frequency
4 m cooling tower fan	150–300 RPM	Low-frequency

*Actual values depend on blade number and operating speed.

This engineering relationship explains why industrial cooling equipment often produces lower-frequency sound than smaller commercial HVAC systems, even when both move comparable amounts of air.

Modern hyperscale data centres increasingly favour fewer, larger fans because larger fans generally achieve:

• higher aerodynamic efficiency;
• lower pressure losses;
• reduced power consumption;
• improved cooling performance.

However, these same characteristics may shift a greater proportion of the acoustic energy into lower frequencies.

This does not necessarily mean the equipment is louder.

Rather, the sound spectrum changes.

Understanding this distinction is important because low-frequency sound behaves differently from higher-frequency sound during propagation, a topic examined later in this article.

How Fan Design Influences Noise

Although fan noise cannot be eliminated entirely, engineering design strongly influences both the amount and characteristics of sound produced.

Several design parameters are particularly important.

Fan Diameter

Larger fans generally operate more efficiently because they move greater air volumes at lower rotational speeds.

However, lower rotational speeds also shift the dominant frequencies toward the low-frequency range.

Rotational Speed

Fan speed is one of the strongest determinants of acoustic output.

In general, increasing rotational speed produces disproportionately greater noise.

For this reason, variable-speed operation has become an important engineering strategy for balancing cooling demand, energy consumption and acoustic performance.

Number of Blades

Increasing blade number changes the blade-pass frequency and redistributes aerodynamic loading.

Depending on the overall design, this may alter both tonal characteristics and overall sound levels.

Blade Geometry

Modern fan blades are carefully optimised using computational fluid dynamics (CFD).

Design features such as:

• aerofoil profiles;
• swept blades;
• serrated trailing edges;
• winglet-style blade tips;
• variable pitch angles,

can reduce turbulence and improve aerodynamic efficiency, thereby lowering sound generation.

Fan Shrouds and Diffusers

The structures surrounding the fan also influence noise.

Properly designed shrouds reduce recirculation, improve airflow uniformity and minimise vortex formation.

Diffusers recover static pressure more efficiently, allowing lower operating speeds for equivalent cooling performance.

Variable-Speed Control

Perhaps the most significant recent development is the widespread adoption of intelligent variable-speed control.

Rather than operating continuously at maximum capacity, modern building management systems adjust fan speeds according to:

• server heat load;
• outdoor weather;
• water temperatures;
• cooling demand.

Reducing fan speed by even a modest amount can significantly reduce both energy consumption and sound generation.

Future articles within this MDCO series will examine these engineering mitigation strategies in much greater depth. Here, it is sufficient to recognise that the acoustic characteristics of a fan are determined not only by its size, but also by how it is designed, installed and operated.

MDCO Insight: The low-frequency noise produced by fans reflects a combination of aerodynamic design, rotational speed, operating conditions and mechanical performance rather than any single design parameter.

How Individual Cooling Components Generate Low-Frequency Noise

Although fans are the most common source of low-frequency noise in data centres, they are not the only contributors. Every major cooling component generates sound through its own combination of mechanical motion, fluid dynamics and structural vibration.

Understanding these differences is important because each component produces a distinct acoustic signature and may require different engineering approaches to reduce its impact.

Rather than treating the cooling system as a single source of noise, acoustic engineers analyse each component individually before considering how they interact as an integrated system.

Computer Room Air Handlers (CRAH)

Function

As explained in →E03.01 Understanding Data Centre Cooling Systems, Computer Room Air Handlers (CRAHs) circulate cooled air through the data hall using chilled water supplied from a central plant.

Unlike CRAC units, CRAHs contain no refrigeration compressor. Their principal mechanical components include:

• large centrifugal or EC fans;
• chilled-water cooling coils;
• dampers;
• filters;
• control systems.

Primary Noise Sources

Because CRAHs contain relatively few moving components, their acoustic output is dominated by the supply fans.

The principal noise mechanisms include:

• blade-pass frequency from rotating fans;
• airflow turbulence across cooling coils;
• turbulence through filters and dampers;
• motor vibration;
• casing vibration.

Among these, the fan remains the dominant source.

Modern EC fans generally reduce overall sound levels because they can continuously adjust speed according to cooling demand.

Why CRAHs Produce Low-Frequency Noise

CRAH fans are designed to move very large volumes of air at relatively low pressure.

Large fan diameters combined with moderate rotational speeds naturally generate blade-pass frequencies within the lower portion of the audible spectrum.

The long operating duration is equally important.

Unlike many commercial HVAC systems that cycle on and off, CRAHs often operate continuously throughout the day and night because server heat generation is continuous.

Even when operating at partial load, the persistent nature of the sound can make it more noticeable than short-duration equipment operating at higher sound levels.

Design Factors Influencing Noise

The acoustic characteristics of CRAHs depend upon several engineering parameters, including:

• fan diameter;
• fan speed;
• EC versus fixed-speed motors;
• airflow velocity;
• duct resistance;
• cooling coil configuration;
• equipment location within the building.

Because most CRAHs are located inside the data hall, the building envelope itself provides substantial attenuation. Consequently, CRAHs are generally less significant contributors to off-site environmental noise than external cooling equipment.

MDCO Insight: CRAHs generate relatively modest environmental noise because most of their acoustic energy remains contained within the building, although their continuously operating fans still contribute to the overall acoustic environment.

Computer Room Air Conditioners (CRAC)

Function

CRAC units resemble CRAHs in their air distribution role but differ fundamentally in their cooling method.

Instead of receiving chilled water, CRAC units contain direct-expansion refrigeration systems incorporating:

• compressors;
• condensers;
• evaporators;
• expansion valves;
• circulating fans.

The refrigeration cycle introduces additional moving machinery that influences both energy consumption and acoustic performance.

Primary Noise Sources

CRAC units produce sound from multiple independent mechanisms:

• supply fans;
• compressor operation;
• refrigerant flow;
• condenser fans (for air-cooled systems);
• structural vibration transmitted through the unit frame.

The compressor often becomes the most distinctive source because it generates periodic mechanical vibration during refrigerant compression.

Why CRACs Produce Low-Frequency Noise

Most industrial refrigeration compressors operate at rotational speeds considerably lower than many smaller commercial air-conditioning systems.

Their large rotating masses produce cyclic forces that excite vibration within the compressor housing.

These vibrations may then propagate into:

• pipework;
• equipment frames;
• supporting floors;
• surrounding building structures.

The result is a combination of airborne sound and structure-borne vibration.

Where multiple CRAC units operate simultaneously, their individual frequency spectra combine to create a more complex acoustic signature.

Design Factors Influencing Noise

Important design considerations include:

• compressor type (scroll, screw or reciprocating);
• compressor capacity;
• vibration isolation;
• refrigerant operating pressures;
• fan selection;
• equipment mounting.

Modern inverter-driven compressors generally operate more smoothly than older fixed-speed designs because they avoid repeated start-stop cycling and reduce mechanical loading during partial-load operation.

MDCO Insight: In CRAC systems, low-frequency noise originates not only from fans but also from the refrigeration compressor, introducing additional tonal and structural vibration components.

Chillers

Function

Chillers form the heart of most large chilled-water cooling systems.

Their role is to remove heat from circulating chilled water before rejecting that heat to the condenser system, either through cooling towers or air-cooled condensers.

Large hyperscale facilities may employ multiple chillers operating in parallel, each providing several megawatts of cooling capacity.

Primary Noise Sources

Chillers generate sound from several major components:

• refrigeration compressors;
• condenser fans (air-cooled chillers);
• refrigerant expansion;
• internal pumps;
• motor drives;
• structural vibration.

The dominant source depends upon the chiller design.

For water-cooled chillers, the compressor is usually the principal contributor.

For air-cooled chillers, condenser fans often dominate the overall sound profile.

Why Chillers Produce Low-Frequency Noise

Large industrial chillers employ compressors capable of moving enormous refrigerant mass flow rates.

Common compressor types include:

• centrifugal;
• screw;
• scroll;
• reciprocating.

Among these, centrifugal and screw compressors are most common in hyperscale facilities because of their high capacity and energy efficiency.

These compressors contain large rotating assemblies that generate periodic mechanical forces.

In addition, refrigerant compression itself creates pressure pulsations that excite vibration within:

• compressor casings;
• refrigerant piping;
• supporting structures.

These pressure pulsations often contain strong low-frequency components.

Where chillers are installed outdoors, compressor noise combines with condenser fan noise, creating a complex acoustic spectrum extending across both low and higher frequencies.

Design Factors Influencing Noise

Acoustic performance depends upon:

• compressor technology;
• compressor speed;
• refrigerant type;
• condenser design;
• vibration isolation;
• acoustic enclosure;
• plant layout.

Modern magnetic-bearing centrifugal compressors have attracted considerable interest because they eliminate mechanical contact within the bearings, substantially reducing vibration compared with conventional oil-lubricated systems.

MDCO Insight: Chillers combine multiple independent noise mechanisms, making their acoustic behaviour considerably more complex than equipment driven primarily by fans alone.

Cooling Towers

Function

Cooling towers are among the most recognisable components of water-cooled data centre infrastructure.

Rather than directly cooling servers, they reject heat from the condenser water loop by using evaporative cooling.

As explained in →E03.01, warm condenser water is distributed over fill media while large fans draw air upward through the tower. A small proportion of the water evaporates, carrying away significant quantities of heat through the latent heat of vaporisation.

This process allows cooling towers to approach the ambient wet-bulb temperature, making them substantially more energy efficient than dry cooling under suitable climatic conditions.

Primary Noise Sources

Cooling towers differ from most other cooling equipment because they generate sound from both air movement and water movement.

Major sources include:

• large axial fans;
• blade-pass frequency;
• fan gearbox (where fitted);
• fan motor;
• falling water;
• water impacting fill material;
• water distribution systems;
• airflow turbulence;
• structural vibration.

Consequently, cooling towers typically produce one of the most complex acoustic signatures within the entire cooling system.

Why Cooling Towers Produce Low-Frequency Noise

Several mechanisms contribute simultaneously.

The large axial fan generally produces the dominant low-frequency tonal component.

Because these fans often exceed several metres in diameter and rotate slowly, their blade-pass frequencies naturally occur within the low-frequency range.

At the same time, enormous quantities of circulating water generate broadband sound through:

• splashing;
• turbulence;
• droplet impacts;
• water collection basins.

Unlike the fan noise, these water sounds extend across a much broader frequency spectrum.

Gear-driven cooling towers may introduce additional tonal components associated with gearbox operation.

The combined result is a characteristic acoustic signature containing:

• low-frequency fan tones;
• harmonic frequencies;
• broadband water noise;
• structural vibration.

This combination explains why cooling towers frequently become the focus of environmental noise assessments near large industrial facilities.

Design Factors Influencing Noise

Cooling tower acoustics depend upon:

• fan diameter;
• rotational speed;
• direct-drive versus gearbox design;
• fill configuration;
• water distribution method;
• drift eliminator design;
• tower height;
• airflow rate.

Modern towers increasingly employ direct-drive variable-speed fans, improved fill media and quieter water distribution systems to reduce both sound generation and energy consumption.

Future articles within this MDCO series will examine these engineering approaches in greater detail.

MDCO Insight: Cooling towers generate one of the most distinctive acoustic signatures in a data centre because large rotating fans and moving water simultaneously contribute to the overall sound environment.

Other Sources of Low-Frequency Noise

Although cooling systems are generally the dominant source of continuous operational noise, they do not account for all of the low-frequency sound produced by a modern data centre.

A large data centre contains numerous supporting infrastructure systems required to distribute electricity, maintain reliable operation and protect critical equipment. Many of these systems also generate characteristic acoustic signatures through electromagnetic forces, fluid movement or mechanical vibration.

Individually, some may contribute only a small proportion of the overall acoustic environment. Collectively, however, they form part of the facility’s total sound profile and may become significant under certain operating conditions.

Pumps

Function

Pumps are responsible for circulating fluids throughout the cooling system.

Depending on the cooling architecture, separate pumping systems may circulate:

• chilled water;
• condenser water;
• hot water (in heat recovery systems);
• refrigerant;
• glycol mixtures;
• make-up water;
• fire protection water.

Large hyperscale campuses may operate dozens of continuously running pumps of different capacities.

Primary Noise Sources

Unlike fans, pumps do not generate sound primarily through moving air.

Instead, their acoustic output originates from:

• rotating impellers;
• hydraulic pressure pulsations;
• motor vibration;
• bearing vibration;
• pipe vibration;
• cavitation (where present).

Most of these mechanisms initially generate structure-borne vibration rather than airborne sound.

Why Pumps Produce Low-Frequency Noise

As the impeller rotates, it periodically transfers energy to the fluid.

This creates pressure fluctuations within the pump casing and pipework.

These pressure waves travel through the piping network, causing pipes, valves and supporting structures to vibrate.

Because large industrial pumps rotate relatively slowly while handling high flow rates, much of this vibration occurs within the low-frequency range.

Hydraulic pulsations may become particularly noticeable when several pumps operate in parallel or when resonance develops within long sections of pipework.

Unlike fan noise, pump noise often propagates indirectly through building structures before radiating into occupied spaces.

Cavitation

One important phenomenon is cavitation.

Cavitation occurs when local pressure falls below the vapour pressure of the liquid, causing microscopic vapour bubbles to form and collapse rapidly.

Although cavitation is usually associated with higher-frequency broadband noise, severe cavitation also introduces vibration that can excite lower-frequency structural resonance.

More importantly, cavitation indicates that the pump is operating outside its intended design conditions and can lead to rapid equipment damage if left uncorrected.

Design Factors Influencing Noise

Pump acoustics are influenced by:

• impeller diameter;
• rotational speed;
• pump type;
• hydraulic loading;
• pipe support design;
• flexible connectors;
• vibration isolation;
• operating flow rate.

Well-designed hydraulic systems minimise pressure fluctuations and isolate vibration before it reaches the surrounding structure.

MDCO Insight: Pump noise originates primarily from hydraulic forces and structural vibration rather than moving air, making its propagation characteristics very different from those of cooling fans.

Pipework and Valves

Function

Pipework connects every major component of the cooling system.

Large data centres may contain several kilometres of interconnected piping carrying chilled water, condenser water, refrigerant and other fluids throughout the facility.

Although pipes themselves contain no moving mechanical parts, they can become efficient pathways for transmitting vibration.

Primary Noise Sources

Pipework does not create vibration independently.

Instead, it transmits vibration originating from:

• pumps;
• compressors;
• water flow;
• valve operation;
• thermal expansion;
• hydraulic pulsations.

Improperly supported pipework may also amplify existing vibration through resonance.

Why Pipework Produces Low-Frequency Noise

Long pipe runs possess natural frequencies determined by:

• length;
• diameter;
• wall thickness;
• support spacing;
• material stiffness.

When hydraulic pulsations coincide with these natural frequencies, vibration amplitudes may increase significantly.

This phenomenon resembles a musical instrument, where a vibrating string or air column amplifies specific frequencies.

Similarly, long steel pipe runs may reinforce particular vibration frequencies already generated elsewhere within the cooling system.

Water Hammer

Another important phenomenon is water hammer.

Water hammer occurs when rapidly changing flow conditions create sudden pressure surges within the piping network.

These pressure waves travel at high speed and can generate substantial mechanical forces.

Although modern data centres are designed to minimise water hammer through careful valve control and variable-speed pumping, the phenomenon illustrates how fluid movement alone can generate significant vibration.

Design Factors Influencing Noise

Engineering measures include:

• flexible couplings;
• resilient pipe supports;
• expansion joints;
• proper support spacing;
• gradual valve operation;
• hydraulic balancing.

These measures seek to prevent vibration generated by one component from propagating throughout the entire cooling system.

MDCO Insight: Pipework often acts as a transmission pathway rather than a primary noise source, allowing vibration generated in one location to appear elsewhere within the facility.

Transformers

Function

Every data centre depends upon transformers to convert incoming electricity into the voltage levels required by IT equipment and mechanical systems.

Unlike cooling equipment, transformers contain no rotating machinery.

Nevertheless, they are well known for producing the characteristic electrical “hum” associated with high-voltage installations.

Primary Noise Sources

Transformer noise originates principally from:

• magnetostriction within the steel core;
• electromagnetic forces acting on windings;
• cooling fans (where fitted);
• oil circulation pumps (for larger transformers).

Among these, magnetostriction is the dominant mechanism.

Why Transformers Produce Low-Frequency Noise

Magnetostriction is a property of ferromagnetic materials.

As alternating current passes through the transformer, the magnetic field within the steel core changes direction fifty times each second in countries using a 50 Hz electrical supply, including Malaysia.

These changing magnetic fields cause the steel laminations to expand and contract by extremely small amounts.

Although the movement is microscopic, it occurs continuously and across the entire transformer core.

The repeated dimensional changes generate vibration that is transmitted to the transformer enclosure and surrounding air.

Because the magnetic field reverses every half-cycle, transformer hum commonly occurs at approximately 100 Hz, together with higher harmonic frequencies.

This explains why transformer noise is highly tonal and remarkably consistent throughout operation.

Design Factors Influencing Noise

Transformer sound levels depend upon:

• core construction;
• magnetic flux density;
• structural clamping;
• enclosure design;
• vibration isolation;
• loading conditions.

Modern low-noise transformers use improved core materials, tighter manufacturing tolerances and enhanced vibration isolation to reduce audible hum.

MDCO Insight: Unlike cooling equipment, transformers generate low-frequency sound through electromagnetic forces rather than rotating machinery, producing one of the most recognisable tonal signatures within a data centre.

Standby Generators

Function

Standby generators provide emergency electrical power during utility supply interruptions.

As discussed in →E02 How Data Centres Interact with the World, modern data centres rely on generators to maintain continuous operation during power failures.

Under normal operating conditions, however, generators remain idle except during periodic testing and maintenance.

Primary Noise Sources

Generator noise arises from several mechanisms:

• diesel engine combustion;
• exhaust gases;
• turbochargers;
• radiator cooling fans;
• alternator vibration;
• engine block vibration.

Unlike cooling equipment, generators combine mechanical, aerodynamic and combustion noise simultaneously.

Why Generators Produce Low-Frequency Noise

Large diesel engines generate substantial low-frequency pressure pulses associated with the combustion cycle.

The reciprocating motion of pistons, crankshafts and connecting rods creates cyclic mechanical forces that excite vibration throughout the engine structure.

Engine exhaust systems also radiate strong low-frequency sound if not properly silenced.

For these reasons, generator noise is typically among the loudest sound sources within a data centre during emergency operation.

However, because generators operate only intermittently, they are generally not the principal source of continuous environmental low-frequency noise experienced by nearby communities.

Instead, community discussions concerning persistent operational noise more commonly involve continuously operating cooling systems.

Design Factors Influencing Noise

Noise control measures commonly include:

• exhaust silencers;
• acoustic enclosures;
• vibration isolation mounts;
• radiator silencers;
• intake attenuators;
• building integration.

Most jurisdictions impose separate noise requirements for routine generator testing because these events differ significantly from continuous operational cooling noise.

MDCO Insight: Although standby generators produce substantial low-frequency sound during operation, their intermittent nature distinguishes them from the continuously operating cooling infrastructure that typically dominates long-term community noise exposure.

Bringing the Components Together

Individually, each component discussed in this article generates only part of the overall acoustic environment.

In practice, a modern data centre operates as an integrated mechanical system.

Cooling towers reject heat generated by chillers.

Pumps circulate water between equipment.

CRAHs distribute cooled air through the data halls.

Transformers supply electrical power.

Ventilation systems maintain building conditions.

Each component contributes its own frequency signature.

When these signatures overlap, they form the complex acoustic spectrum measured around an operating data centre.

Understanding this complexity is important because there is rarely a single “data centre noise.”

Instead, environmental measurements typically represent the combined contribution of numerous mechanical systems operating simultaneously under changing weather conditions and cooling loads.

The following section examines how these combined sounds propagate through the surrounding environment and why low-frequency noise behaves differently from higher-frequency sound.

How Low-Frequency Sound Behaves Differently

Understanding how low-frequency noise is generated explains only part of the engineering picture. Equally important is understanding how low-frequency sound propagates after it leaves the equipment.

Two sound sources producing the same sound power may be experienced very differently depending on their frequency content. This is because sound does not travel uniformly across all frequencies. Lower-frequency sound behaves differently from mid- and high-frequency sound when interacting with the atmosphere, buildings and surrounding terrain.

These characteristics help explain why some mechanical systems can be heard—or sensed—at considerable distances even when higher-frequency sounds have diminished (→E02.02 Low Frequency Noise: How Humans Hear Sound Beyond Decibels).

Long Wavelengths

The behaviour of sound is closely related to its wavelength.

Wavelength is determined by both the speed of sound and the sound frequency.

Because low-frequency sounds have lower frequencies, they possess much longer wavelengths.

For example:

Frequency	Approximate Wavelength
20 Hz	17 m
50 Hz	6.9 m
100 Hz	3.4 m
1,000 Hz	0.34 m

These long wavelengths interact with the environment very differently from shorter wavelengths.

Large wavelengths are less easily blocked by small obstacles such as fences, trees or ordinary walls. Instead of being stopped, they tend to bend around obstacles and continue propagating.

This property, known as diffraction, explains why low-frequency sound often remains detectable even when the source itself is no longer visible.

Lower Atmospheric Absorption

As sound travels through the atmosphere, some of its energy is gradually absorbed by the surrounding air.

However, atmospheric absorption is strongly frequency-dependent.

Higher-frequency sound loses energy relatively quickly because molecular interactions within the air convert part of the acoustic energy into heat.

Low-frequency sound experiences much less atmospheric absorption.

Consequently, a larger proportion of its original energy remains after travelling the same distance.

This does not mean that low-frequency sound travels without attenuation. Rather, it generally loses energy more slowly than higher-frequency sound.

Reflection and Diffraction

Buildings, terrain and other structures also influence sound propagation.

High-frequency sound tends to behave more like light, producing stronger acoustic shadows behind obstacles.

Low-frequency sound behaves differently.

Because of its longer wavelength, it more readily bends around buildings, embankments and other structures.

At the same time, large building surfaces may reflect portions of the sound field, producing complex interference patterns in which some locations experience higher sound levels while nearby locations experience lower levels.

The resulting sound environment therefore depends not only on the source but also on the surrounding landscape and built environment.

Building Penetration

Another characteristic of low-frequency sound is its interaction with buildings.

Walls, roofs and glazing generally provide excellent attenuation for high-frequency sound.

Low-frequency sound is more difficult to attenuate because long wavelengths interact with entire building elements rather than only the surface.

Consequently, improving insulation against low-frequency sound often requires:

• heavier construction;
• greater structural stiffness;
• vibration isolation;
• specialised acoustic design.

Simply increasing the thickness of conventional insulation materials may provide only limited improvement.

Indoor Resonance

Once low-frequency sound enters a building, room dimensions become important.

Every enclosed space possesses natural acoustic frequencies determined by its geometry.

When incoming sound coincides with one of these natural frequencies, room resonance may occur.

Resonance does not create new sound.

Instead, it amplifies particular frequencies already present within the incident sound field.

This is one reason why occupants of neighbouring buildings may report different experiences even when exposed to the same external sound source.

Differences in:

• room dimensions;
• wall construction;
• window openings;
• furnishings;

can all influence the indoor acoustic response.

MDCO Insight: Low-frequency sound behaves differently because its long wavelengths interact with buildings, terrain and the atmosphere in ways that differ fundamentally from higher-frequency sound.

Malaysia’s Tropical Climate

Malaysia’s tropical climate influences both how cooling systems operate and the environmental conditions through which sound propagates.

These two effects should be considered separately.

Cooling System Operation

Unlike temperate climates, Malaysia experiences consistently high temperatures and humidity throughout the year.

Typical daytime temperatures remain between approximately 30°C and 35°C across much of the country, while relative humidity frequently exceeds 70%.

These climatic conditions increase the cooling demand placed on data centres.

Higher outdoor temperatures reduce the temperature difference available for heat rejection.

Consequently:

• cooling equipment may operate for longer periods;
• fan speeds may increase during hotter conditions;
• condenser temperatures may rise;
• cooling towers may experience higher evaporative loads;
• pumps may operate at higher flow rates.

Although modern cooling systems continuously adjust to changing operating conditions, tropical climates generally require sustained cooling throughout the year rather than the seasonal variation experienced in cooler regions.

This contributes to the continuous operation of many mechanical systems that generate low-frequency noise.

Atmospheric Propagation

Weather conditions also influence sound propagation.

Factors such as:

• wind speed;
• wind direction;
• temperature gradients;
• atmospheric stability;
• humidity;

can all affect how sound travels between a source and a receiver.

For example, stable nighttime atmospheric stratification can reduce vertical air mixing and refract sound waves back towards the ground, allowing low-frequency sound to propagate more effectively than under well-mixed daytime conditions.

Similarly, wind blowing from the source toward a residential area may increase the sound received at that location, while wind in the opposite direction may reduce it.

Humidity also influences atmospheric absorption, although its effect varies with frequency and environmental conditions.

It is important, however, not to overgeneralise these effects.

Weather influences sound propagation in all climates, not only tropical ones.

At present, there is limited published research specifically examining whether Malaysia’s tropical climate produces systematically different low-frequency propagation characteristics around data centres compared with other regions.

Accordingly, while established acoustic principles remain applicable, individual site conditions—including local topography, surrounding buildings and prevailing weather—often play a greater role than climate alone.

MDCO Insight: Malaysia’s tropical climate primarily influences low-frequency noise by increasing year-round cooling demand, while day-to-day weather conditions influence how that sound propagates through the surrounding environment.

Why Low-Frequency Noise Cannot Be Eliminated Completely

Because low-frequency noise originates from fundamental physical processes, it cannot be completely eliminated without also eliminating the equipment that performs the cooling function.

Every cooling system requires energy transfer.

Every energy transfer requires mechanical equipment.

Every mechanical system generates some degree of vibration.

Consequently, the engineering objective is not silence but optimisation.

Engineers seek to minimise sound generation while simultaneously maintaining:

• reliable cooling;
• energy efficiency;
• operational resilience;
• economic viability;
• maintainability.

These objectives often involve engineering trade-offs.

For example:

• Larger fans generally improve energy efficiency but may shift sound towards lower frequencies.
• Lower fan speeds reduce noise but may require larger equipment.
• Acoustic barriers reduce sound transmission but occupy additional space.
• Vibration isolation improves acoustic performance but increases construction complexity.
• Cooling towers improve energy efficiency but introduce additional aerodynamic and water-related sound sources.

No single solution simultaneously optimises every objective.

Instead, cooling systems are designed by balancing multiple engineering considerations according to the operational requirements of each facility.

Future articles within the MDCO Low-Frequency Noise series will examine these mitigation strategies in greater technical detail.

MDCO Insight: Low-frequency noise cannot be eliminated entirely because it arises from the same physical processes that enable reliable and efficient heat removal.

Why MDCO Is Examining This Issue

Low-frequency noise has become an increasingly important topic in discussions surrounding modern data centre development.

As explained in →A09 Understanding Low-Frequency Noise from Data Centres, community concerns in several jurisdictions have prompted broader discussions involving engineering, environmental assessment, planning, governance, regulation and legal accountability.

Meaningful discussion of these topics requires a sound understanding of the underlying engineering principles.

Without understanding how cooling systems generate sound, it becomes difficult to interpret:

• acoustic measurements;
• environmental assessments;
• equipment specifications;
• mitigation proposals;
• regulatory standards;
• community experiences.

This article therefore provides the technical foundation upon which the remainder of the MDCO low-frequency noise series is built.

Subsequent articles will examine:

• why different cooling technologies generate different acoustic characteristics;
• how low-frequency noise is measured;
• why measurement remains technically challenging;
• engineering approaches for reducing noise;
• governance and regulatory frameworks;
• international case studies and emerging policy developments.

Together, these articles aim to provide a multidisciplinary understanding of an issue that increasingly intersects engineering, public policy and community engagement.

The Observatory Perspective

Low-frequency noise is not produced by a single machine or technology. It emerges from the combined operation of cooling systems, electrical infrastructure and supporting mechanical equipment that together enable reliable digital infrastructure. Understanding these engineering foundations provides the basis for informed discussion on measurement, mitigation, governance and community outcomes.

Selected References

• American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). Thermal Guidelines for Data Processing Environments. Guidance on thermal design, cooling technologies and environmental operating conditions for data centres. https://www.ashrae.org
• American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). ASHRAE Handbook – HVAC Applications. Comprehensive reference on HVAC system design, including mission-critical facilities and data centres. https://www.ashrae.org
• International Organization for Standardization (ISO). ISO 3746: Acoustics — Determination of sound power levels of noise sources using sound pressure. International standard for measuring sound power emitted by machinery and equipment. https://www.iso.org/standard/52056.html
• International Organization for Standardization (ISO). ISO 9613-2: Acoustics — Attenuation of sound during propagation outdoors. Widely used engineering method for predicting outdoor environmental noise propagation. https://www.iso.org/standard/74047.html
• International Electrotechnical Commission (IEC). IEC 60076 Series — Power Transformers. International standards covering transformer design, testing and acoustic performance. https://webstore.iec.ch/

Citation

Malaysia Data Centre Observatory (MDCO). E02.01 – How Data Centres Generate Low-Frequency Noise. 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.