Acoustic Noise from EMC Sources

Electronic equipment generates acoustic noise through various electromagnetic mechanisms. Transformers hum, inductors whine, capacitors sing, and motors buzz in response to the electrical signals they process. While these sounds may seem incidental, they represent a significant aspect of product quality and user experience. In consumer electronics, office equipment, and residential applications, acoustic noise often determines product acceptability regardless of electrical performance.

The mechanisms that generate acoustic noise from electromagnetic sources involve the same physical phenomena responsible for EMC concerns: alternating currents create alternating magnetic fields that produce mechanical forces in magnetic materials; electric fields create electrostatic forces in capacitive structures; and switching transients excite mechanical resonances in components and structures. Understanding these mechanisms enables engineers to design quieter products while maintaining electromagnetic performance.

Transformer Hum

The characteristic hum of power transformers is one of the most recognizable sounds in electrical engineering. This sound results from magnetostriction in the core material and from magnetic forces between core laminations and windings. While low levels of transformer hum may be acceptable in industrial environments, it becomes objectionable in residential, office, and audio equipment applications.

Magnetostriction as the Primary Noise Source

Magnetostriction is the dimensional change that occurs in ferromagnetic materials when magnetized. In transformer cores, the cyclic magnetization caused by AC current produces cyclic dimensional changes that vibrate the core at twice the power frequency (100 Hz for 50 Hz mains, 120 Hz for 60 Hz mains).

The magnetostriction of the core material depends on several factors:

Core material composition: Silicon steel (used in most power transformers) has lower magnetostriction than plain iron. Higher silicon content further reduces magnetostriction but increases material cost and brittleness.
Grain orientation: Grain-oriented silicon steel (GOSS) has very low magnetostriction when the flux is aligned with the rolling direction. Non-oriented steel has more uniform properties but higher overall magnetostriction.
Flux density: Magnetostriction increases with flux density. Operating a core at lower flux density reduces noise but requires a larger, heavier, and more expensive core.
Harmonic content: Distorted current waveforms contain harmonics that produce additional magnetostrictive components at multiples of twice the fundamental frequency.

The frequency doubling effect of magnetostriction is important: magnetostriction depends on the magnitude but not the polarity of the magnetic field, so both positive and negative flux peaks produce the same strain direction. A 50 Hz or 60 Hz magnetic field therefore produces mechanical vibration at 100 Hz or 120 Hz, with harmonics at 200 Hz, 300 Hz, and so on.

Maxwell Forces and Lamination Vibration

In addition to magnetostriction within the core material, electromagnetic forces between elements of the transformer structure contribute to noise:

Lamination attraction: Adjacent laminations experience attractive forces due to the magnetic flux passing through them. Loose laminations can vibrate under these forces, producing a buzzing or rattling noise.

Winding forces: Currents in transformer windings create magnetic fields that produce forces between turns and between windings. These forces are proportional to the square of the current, producing vibration at twice the current frequency plus harmonic components.

Core-to-winding forces: The interaction between winding currents and core flux creates forces on the windings that vary with the product of flux and current.

These force-related noises are often higher in frequency content than pure magnetostriction noise and may include components at harmonics of the power frequency.

Transformer Noise Reduction Techniques

Reducing transformer acoustic noise involves addressing both the sources of vibration and the transmission of that vibration to the environment:

Core material selection: Using low-magnetostriction materials such as amorphous metal cores or higher-grade silicon steel can significantly reduce noise at the source. These materials are more expensive but may be justified for noise-critical applications.

Core construction: Tight lamination stacking with proper clamping prevents lamination vibration. Wound cores generally produce less noise than stacked cores because they have fewer joints. Core joints in stacked cores should be precisely cut and carefully assembled to minimize gaps.

Operating flux density: Reducing the operating flux density reduces magnetostriction. This requires a larger core for the same power rating, increasing size, weight, and cost.

Impregnation: Vacuum impregnation with varnish or resin fills gaps between laminations and windings, damping vibration and providing a more rigid structure.

Mounting and isolation: Resilient mounting isolates transformer vibration from the chassis and enclosure. Proper isolation mounts have natural frequencies well below the fundamental vibration frequency (100 Hz or 120 Hz for line-frequency transformers).

Enclosure design: Sound-absorbing materials in transformer enclosures reduce radiated noise. The enclosure structure should avoid resonances at transformer vibration frequencies.

Special Considerations for Different Transformer Types

Different transformer applications have different noise characteristics and requirements:

Distribution transformers: Large distribution transformers on utility poles or in substations can produce significant noise that affects nearby residences. Noise specifications for these transformers are often regulated, and special low-noise designs may be required in sensitive areas.

Audio transformers: Transformers in audio equipment must produce extremely low noise levels to avoid affecting audio quality. Premium audio transformers use special core materials, careful construction, and effective shielding.

High-frequency transformers: Transformers in switching power supplies operate at frequencies from tens of kHz to MHz. While the fundamental frequencies are ultrasonic, subharmonics, beat frequencies, and modulation effects can produce audible noise.

Variable transformers: Variacs and other variable transformers may produce more noise when the winding is only partially energized, creating unbalanced magnetic forces.

Inductor Whine

Inductors in power electronics and other circuits can produce high-pitched whining or squealing sounds. This "coil whine" is particularly common in computers, graphics cards, monitors, and LED lighting systems, where switching frequencies create magnetic field variations at audible or near-audible frequencies.

Sources of Inductor Acoustic Noise

Inductors generate noise through the same magnetostriction mechanism as transformers, but with different characteristics due to their construction and operating conditions:

Core magnetostriction: The core material expands and contracts with the magnetic field. For inductors in switching converters, this vibration occurs at the switching frequency and its harmonics. Switching frequencies from 20 kHz to 200 kHz can produce audible noise through subharmonics, beat frequencies, or direct radiation if the frequency is in the audible range or just above it.

Winding vibration: Forces between turns and between the winding and core can cause winding vibration. This is particularly problematic if windings are not tightly wound or securely bonded to the core.

Air gap effects: Gapped inductors have magnetic fringing fields near the gap that create forces on nearby conductors. These forces can cause localized vibration.

Loose construction: Any loose elements in the inductor construction can vibrate independently, producing noise that may be worse than the inherent magnetostriction.

Factors Affecting Inductor Noise

Several factors influence the noise level produced by an inductor:

Load current: Inductor noise often increases with load current because the magnetic field and resulting magnetostrictive strain increase with current. This explains why coil whine often becomes noticeable only under high-load conditions.

Operating frequency: The relationship between switching frequency and audible noise is complex. Frequencies above 20 kHz are nominally inaudible, but subharmonics, modulation effects, and individual hearing variations can make higher frequencies audible to some listeners.

Core material: Different core materials have different magnetostriction characteristics. Ferrite cores generally have lower magnetostriction than iron powder cores, but material selection involves many tradeoffs.

Core construction: The physical structure of the core affects vibration behavior. Potted or encapsulated cores tend to be quieter than open cores.

Temperature: Core properties and mechanical characteristics can change with temperature, affecting noise production.

Inductor Noise Reduction Strategies

Addressing inductor noise requires attention to component selection, circuit design, and mechanical construction:

Component selection: Select inductors with low acoustic noise specifications for noise-sensitive applications. Some manufacturers offer "quiet" or "low-noise" versions of their inductor families.

Core material selection: Choose core materials with low magnetostriction for critical applications. Ferrites generally produce less noise than iron powder or other distributed-gap materials.

Potting and encapsulation: Potting inductors with a suitable compound damps vibration and constrains mechanical motion. The potting material should be selected for thermal conductivity, temperature range, and damping characteristics.

Frequency selection: Choose switching frequencies well above the audible range to avoid direct audibility. Consider spread-spectrum or frequency-dithering techniques to reduce tonal noise peaks.

Mounting: Secure mounting prevents the inductor from transmitting vibration to the PCB and enclosure. Resilient mounting materials can isolate vibration if direct coupling is problematic.

PCB design: Locate inductors away from areas where acoustic noise is most objectionable (such as near ventilation openings in the enclosure). Use stiffened PCB areas to reduce vibration transmission.

Capacitor Singing

Ceramic capacitors, particularly multilayer ceramic capacitors (MLCCs), can produce audible noise when subjected to AC voltage. This phenomenon, sometimes called "capacitor singing" or "capacitor noise," results from the piezoelectric and electrostrictive properties of the ceramic dielectric material.

Mechanism of Capacitor Acoustic Noise

Ceramic capacitor noise originates from the mechanical response of the dielectric to the applied electric field:

Electrostriction: All dielectrics experience some dimensional change in response to an electric field. This electrostriction is proportional to the square of the field, producing vibration at twice the applied frequency plus higher harmonics.

Piezoelectric effect: High-permittivity ceramic formulations (X5R, X7R, Y5V, and similar types) are based on ferroelectric materials that exhibit piezoelectric behavior. These materials change dimension in response to the applied voltage, producing vibration at the applied frequency.

PCB coupling: Capacitor vibration couples to the PCB through the solder joints. The PCB then acts as a sounding board, amplifying and radiating the sound.

Resonance effects: If the excitation frequency coincides with mechanical resonances of the capacitor or the PCB, noise production can be greatly amplified.

Factors Affecting Capacitor Noise

Several factors determine the noise level from a ceramic capacitor:

Dielectric type: Different ceramic formulations have different piezoelectric activity. C0G/NP0 capacitors use non-ferroelectric materials and produce very little noise. X7R, X5R, and Y5V capacitors use ferroelectric materials and can produce significant noise.

Capacitor size: Larger capacitors can produce more sound because they have more surface area to radiate acoustic energy.

Applied voltage: Noise increases with AC voltage amplitude. The relationship is not linear for ferroelectric materials.

Frequency: Frequencies in the audible range (20 Hz to 20 kHz) produce directly audible noise. Higher frequencies may produce audible noise through beat frequencies or subharmonics.

Mounting orientation: How the capacitor is oriented on the PCB affects coupling to the board and resulting sound radiation. Orientation effects are particularly significant for large capacitors.

PCB characteristics: The PCB construction, thickness, and support conditions affect how capacitor vibration is transmitted and radiated as sound.

Capacitor Noise Reduction

Strategies for reducing capacitor acoustic noise include:

Use low-noise dielectrics: C0G/NP0 capacitors produce minimal noise and should be used where noise is a concern, if their limited capacitance range is acceptable.

Parallel smaller capacitors: Using multiple smaller capacitors in parallel instead of one large capacitor can reduce noise because the smaller capacitors produce less sound individually and may not vibrate in phase.

Choose different package styles: Different package geometries couple to the PCB differently. Interdigitated capacitors (for example, IDC style) may produce less noise than traditional MLCCs.

Constrain capacitor motion: Applying a damping compound or underfill to the capacitor can reduce vibration. The material must be compatible with the operating temperature range.

Control excitation: Reducing the AC voltage across noise-producing capacitors reduces noise. This may involve circuit topology changes or adding damping components.

PCB design: Locating capacitors near PCB supports reduces PCB motion. Using thicker or stiffer PCB construction reduces sound radiation. Avoiding placement near enclosure openings reduces perceived noise.

Change orientation: Mounting capacitors with their long axis perpendicular to the PCB rather than parallel can reduce noise in some cases.

Motor Noise

Electric motors are significant sources of acoustic noise in electronic systems. While some motor noise is purely mechanical (bearings, airflow), much of it has electromagnetic origins. Understanding electromagnetic motor noise is essential for designing quiet products.

Electromagnetic Sources of Motor Noise

Several electromagnetic mechanisms contribute to motor acoustic noise:

Radial forces: The electromagnetic interaction between stator and rotor creates radial forces that vary with angular position and time. These forces cause radial vibration of the stator, which radiates as acoustic noise.

Torque ripple: Variations in electromagnetic torque during rotation create torsional vibrations that can couple to radial modes and produce sound.

Magnetostriction: Magnetic materials in the motor experience magnetostrictive strain as the magnetic field varies. This is similar to transformer magnetostriction but with more complex field patterns.

Slot harmonics: The discrete distribution of windings in stator slots creates spatial harmonics in the magnetic field. These harmonics interact with rotor position to produce forces at frequencies related to rotational speed and slot count.

PWM-related noise: Motors driven by PWM inverters experience high-frequency current components that create high-frequency magnetic forces. While the PWM switching frequency may be ultrasonic, modulation effects can produce audible components.

Motor Types and Noise Characteristics

Different motor types have different noise signatures:

DC motors: Brush-type DC motors produce noise from brush commutation in addition to electromagnetic sources. Brushless DC motors eliminate brush noise but may have significant electromagnetic noise components.

Induction motors: The rotating magnetic field in induction motors creates characteristic noise patterns related to pole count and rotor slot count. Rotor eccentricity or unbalance increases noise.

Permanent magnet motors: Strong permanent magnets create high magnetic forces that can produce significant noise. Proper rotor magnetization and stator design are essential for quiet operation.

Switched reluctance motors: SRM motors have highly pulsating torque and radial forces that tend to make them noisier than other motor types unless carefully designed.

Stepper motors: Stepper motors produce noise at stepping frequency and harmonics. Microstepping reduces noise by smoothing the current waveform.

Motor Noise Reduction

Reducing motor electromagnetic noise involves motor design, drive design, and system integration:

Motor design: Proper selection of pole and slot combinations minimizes problematic harmonics. Skewing the rotor slots reduces slot harmonics. Careful magnetic design minimizes radial force variations.

Structural design: Motor housings should be stiff to have high resonant frequencies and should avoid resonances at expected excitation frequencies. Damping treatments can reduce resonant amplification.

Drive optimization: PWM strategies can be designed to minimize acoustic noise. Spread-spectrum modulation reduces tonal peaks. Random PWM reduces discrete frequency components.

Mounting: Resilient mounting isolates motor vibration from the supporting structure. The mount natural frequency should be well below the lowest problematic motor vibration frequency.

Enclosure: Motor enclosures can include sound-absorbing materials to reduce radiated noise. Ventilation openings should be designed as acoustic baffles where possible.

Switching Noise

Power electronics switching creates various forms of acoustic noise beyond the inductor and capacitor noise already discussed. The abrupt transitions in current and voltage excite mechanical resonances and create forces that can produce audible sound.

Sources of Switching-Related Acoustic Noise

Switching power conversion creates acoustic noise through multiple mechanisms:

Transformer and inductor noise: As discussed, magnetic components vibrate at switching frequency and harmonics.

PCB vibration: Fast-changing currents in PCB traces create magnetic fields that interact with nearby conductors and magnetic materials. These forces can excite PCB vibration.

Capacitor noise: Ceramic capacitors in switching circuits experience AC voltage that produces piezoelectric vibration.

Semiconductor package noise: Power semiconductor packages contain bond wires and internal connections that experience forces from high currents. These forces can cause package vibration.

Mechanical resonance excitation: The broadband spectral content of switching transients can excite mechanical resonances in any system component, potentially producing noise at resonant frequencies that differ from the switching frequency.

Frequency Considerations

The relationship between switching frequency and audible noise is complex:

Sub-20 kHz switching: Switching frequencies in the audible range produce directly audible tones. This approach is generally avoided in consumer products but may be acceptable in industrial equipment.

20-30 kHz switching: Frequencies just above the nominal hearing threshold may be audible to some individuals, especially young people. These frequencies are common in older power supply designs.

Above 30 kHz switching: Higher switching frequencies move the fundamental further from audibility but increase switching losses. Modern power supplies often use frequencies from 50 kHz to several MHz.

Subharmonics and modulation: Even with high switching frequencies, load variations, pulse skipping, burst mode operation, and other modulation effects can produce audible frequency components.

Switching Noise Reduction

Strategies for reducing acoustic noise from switching power conversion:

Frequency selection: Choose switching frequencies well above the audible range, accounting for subharmonics and modulation effects.

Spread-spectrum techniques: Modulating the switching frequency over a range spreads the energy across a band rather than concentrating it at discrete frequencies. This reduces the perceived loudness of switching-related noise.

Soft switching: Resonant and soft-switching topologies reduce the abruptness of transitions, reducing high-frequency spectral content and mechanical excitation.

Component selection: Choose magnetic components and capacitors with low acoustic noise characteristics.

PCB design: Use ground planes and proper layout to minimize loop areas and current-carrying trace lengths. Stiffen the PCB in areas of high-current traces.

Mechanical design: Secure components to prevent rattling. Use damping materials to reduce resonant amplification. Design enclosures to avoid acoustic resonances at switching-related frequencies.

Corona Discharge and Associated Noise

Corona discharge is a partial electrical breakdown that occurs when the electric field near a conductor exceeds the ionization threshold of the surrounding medium. Corona produces a characteristic hissing or crackling sound along with ultraviolet light and ozone. It is a concern in high-voltage equipment and indicates conditions that may lead to complete breakdown.

Corona Formation and Characteristics

Corona occurs when the electric field intensity exceeds approximately 30 kV/cm in air at atmospheric pressure. This typically happens at sharp points, edges, or small-radius conductors where the electric field is concentrated:

Field concentration: Electric field strength is inversely related to the radius of curvature. Sharp points and edges create field concentrations that can exceed the ionization threshold even when the average field is well below it.

Ionization cascade: When the field exceeds the ionization threshold, free electrons accelerate and collide with neutral molecules, creating more free electrons. This cascade creates a plasma region around the discharge point.

Polarity effects: Corona characteristics differ for positive and negative polarity. Positive corona tends to be more stable and quieter; negative corona is more irregular and produces more acoustic noise.

AC corona: In AC systems, corona polarity alternates with the applied voltage. The acoustic noise has components at twice the power frequency and higher harmonics.

Acoustic Characteristics of Corona

Corona produces acoustic noise through several mechanisms:

Direct acoustic emission: The plasma in the corona region heats and expands the surrounding air, creating pressure waves that propagate as sound.

Electrostatic speaker effect: The charged particles in the corona region experience forces from the alternating electric field, moving the air directly and creating sound.

Streamers and partial arcs: Intense corona can develop streamers that propagate away from the discharge point, creating crackling sounds.

The acoustic spectrum of corona includes broadband noise (hiss) and tonal components related to the power frequency. At higher voltages or with contaminated surfaces, the noise becomes more impulsive (crackling).

Corona Prevention and Control

Preventing corona involves maintaining electric field stress below the ionization threshold:

Geometry design: Use rounded shapes with large radii of curvature. Avoid sharp edges and points. Use corona rings or shields on high-voltage terminals.

Voltage grading: Distribute voltage stress uniformly along insulators and through insulation systems to avoid local field concentrations.

Insulation: Encapsulate high-voltage components in solid or liquid insulation with higher breakdown strength than air. Ensure no voids or inclusions that could create internal corona.

Environment control: Higher gas pressure increases breakdown strength. Using SF6 or other high-dielectric-strength gases in sealed enclosures allows higher operating voltages without corona.

Surface treatment: Keep insulator surfaces clean and dry. Contamination and moisture reduce surface breakdown strength and promote corona.

Arc Noise

Electrical arcs produce intense acoustic noise as the plasma channel heats and expands the surrounding air. Arc noise occurs in switches, circuit breakers, welding equipment, and as a fault condition in various electrical systems.

Arc Acoustic Mechanisms

Arcs generate sound through thermal-acoustic coupling:

Rapid heating: The arc plasma can reach temperatures of several thousand degrees, rapidly heating the surrounding air and creating a pressure wave that propagates as sound.

Arc instability: Arcs are inherently unstable, with the plasma column moving and fluctuating in intensity. These variations produce time-varying acoustic output.

Contact separation dynamics: In switches and circuit breakers, the arc forms and evolves as contacts separate. The changing arc length and dynamics produce a characteristic acoustic signature.

Magnetic forces: The arc current creates magnetic fields that interact with the arc plasma, causing motion and oscillation that contribute to acoustic noise.

Arc Noise in Electrical Equipment

Different equipment types produce characteristic arc noise:

Relays and contactors: Contact arcing during opening and closing produces brief noise bursts. Severe arcing indicates worn or misaligned contacts.

Circuit breakers: Interrupting high currents creates intense arcing that produces significant acoustic energy. Arc chutes and other arc-extinguishing features affect the noise characteristics.

Arc welding: Welding arcs produce sustained acoustic noise with spectral content related to welding parameters and electrode motion.

Arc faults: Unintentional arcing from damaged insulation or loose connections produces noise that can be used for arc fault detection.

Arc Noise Considerations

Managing arc noise involves both preventing arcs where unwanted and controlling necessary arcs:

Arc suppression: In switches and contactors, arc suppression networks (snubbers) reduce arc duration and intensity, reducing both electrical interference and acoustic noise.

Solid-state switching: Replacing electromechanical switches with solid-state switches eliminates contact arcing and associated noise.

Enclosure design: Equipment with unavoidable arcing (circuit breakers, arc welders) should have enclosures designed to contain both the arc and the resulting acoustic noise.

Fan and Cooling System Noise

While fan noise is primarily mechanical and aerodynamic, electromagnetic effects can influence cooling system acoustics. Motor electromagnetic noise, fan blade resonances excited by motor magnetic forces, and EMI-induced motor control issues all contribute to the acoustic behavior of cooling systems.

Motor Contributions to Fan Noise

The motors driving cooling fans produce electromagnetic noise as described in the motor noise section:

Radial magnetic forces: Variations in the magnetic force between stator and rotor cause vibration that couples into the fan structure and radiates as sound.

PWM drive effects: Fans driven by PWM speed controllers experience high-frequency current ripple that can produce motor noise and excite structural resonances.

Cogging torque: Variations in motor torque with rotor position create pulsating forces that can excite fan blade vibrations.

EMI Effects on Fan Control

Fan speed control systems can be affected by electromagnetic interference:

Control loop disturbance: EMI affecting the fan speed control loop can cause speed variations that produce audible noise modulation.

PWM timing errors: Interference with PWM generation can create irregular switching that produces acoustic artifacts.

Sensor interference: Hall sensors or back-EMF sensing for motor control can be affected by external magnetic fields or conducted interference, affecting fan behavior.

Fan Noise Reduction

Reducing electromagnetic contributions to fan noise:

Motor selection: Choose fan motors with low electromagnetic noise characteristics. Consider brushless DC motors with sinusoidal drive for minimum noise.

PWM frequency: Use PWM frequencies well above the audible range. Avoid frequencies that coincide with fan blade resonances.

EMI filtering: Ensure fan drive circuits have adequate filtering to prevent conducted interference from affecting control behavior.

Mounting: Use resilient mounting to isolate motor vibration from the equipment structure.

Psychoacoustic Considerations

The subjective perception of acoustic noise depends on more than just sound pressure level. Psychoacoustic factors determine how objectionable a noise is perceived to be and influence specifications and design targets for noise control.

Frequency Weighting and Perception

Human hearing sensitivity varies with frequency:

A-weighting: The A-weighted sound level (dBA) is the most common single-number noise metric. A-weighting de-emphasizes low and high frequencies where human hearing is less sensitive, approximating perceived loudness.

Tonal content: Pure tones or narrow-band noise are perceived as more annoying than broadband noise of the same overall level. Standards such as ISO 7779 include tone corrections to account for this effect.

High-frequency sensitivity: Young people can hear frequencies above 15 kHz that older adults cannot. Products sold to diverse audiences must consider this variation in hearing ability.

Sound Quality Considerations

Beyond simple loudness, the character of sound affects its acceptability:

Temporal variations: Steady sounds are generally less annoying than fluctuating sounds. Modulated noise (such as from variable-speed fans or PWM artifacts) can be more objectionable than steady noise of the same average level.

Impulsiveness: Impulsive sounds (clicks, pops, thumps) are perceived as more annoying than steady sounds. Relay actuation and contact arcing create impulsive noise that draws attention.

Roughness: Amplitude modulation at frequencies from 15 Hz to 300 Hz creates a rough quality that increases perceived annoyance.

Sharpness: High-frequency content increases the sharpness of sound, which is perceived as more annoying. Inductor whine often has high sharpness.

Application-Specific Requirements

Noise requirements vary greatly by application:

Office equipment: Products used in quiet office environments must meet stringent noise limits. ISO 7779 and ECMA-74 specify measurement methods for IT equipment.

Home electronics: Consumer expectations for home equipment are high, particularly for devices used in bedrooms or living rooms.

Medical equipment: Noise can interfere with patient communication and staff concentration. Medical device standards include noise requirements.

Industrial equipment: Higher noise levels may be acceptable in factory environments, but must not exceed occupational noise exposure limits.

Transportation: Vehicle noise standards and customer expectations drive noise requirements for automotive and aerospace electronics.

Measurement and Specification

Accurate measurement of acoustic noise is essential for specification, design validation, and compliance testing. EMC engineers should understand the basics of acoustic measurement even when detailed noise analysis is handled by acoustic specialists.

Sound Measurement Fundamentals

Key concepts in acoustic measurement:

Sound pressure level: SPL in decibels is the logarithmic ratio of sound pressure to a reference pressure (20 micropascals). SPL = 20 log(p/p_ref).

Sound power level: The total acoustic power radiated by a source, independent of measurement distance and room acoustics. Calculating sound power from pressure measurements requires knowledge of the measurement environment.

Frequency weighting: A-weighting (dBA) is most common for general noise assessment. C-weighting (dBC) is less sensitive to frequency and is used for peak and impulse measurements.

Time weighting: Fast (125 ms time constant), slow (1 s), and impulse (35 ms rise, 1.5 s decay) time weightings affect how transient sounds are captured.

Measurement Environment

The acoustic measurement environment significantly affects results:

Anechoic chambers: Fully anechoic rooms have negligible reflections and allow direct measurement of radiated sound. They are expensive and may not represent real operating conditions.

Semi-anechoic chambers: These rooms have absorptive walls and ceiling but a reflective floor, simulating outdoor conditions or a large room. They are commonly used for equipment noise testing.

Reverberation rooms: Highly reflective rooms are used for sound power measurement by the comparison method. The diffuse sound field allows total sound power to be determined from pressure measurements at a few points.

Real environments: Measurements in real operating environments provide relevant data but are affected by background noise, reflections, and other sources that complicate interpretation.

Standards and Specifications

Various standards govern acoustic noise measurement and limits:

ISO 7779: Standard for measuring airborne noise from information technology and telecommunications equipment.

ECMA-74: Measurement of airborne noise emitted by information technology equipment, closely aligned with ISO 7779.

IEC 61672: Specification for sound level meters, defining accuracy classes and frequency and time weightings.

Product-specific standards: Many product categories have specific noise limits and measurement procedures defined in relevant product standards.

Conclusion

Acoustic noise from electromagnetic sources represents an important quality issue for electronic products. Transformers, inductors, capacitors, motors, and switching circuits all generate mechanical vibrations through magnetostriction, piezoelectric effects, and electromagnetic forces. These vibrations radiate as sound that can be objectionable to users and may even affect product functionality in sensitive applications.

Controlling electromagnetic acoustic noise requires attention throughout the design process. Component selection can minimize noise generation at the source. Circuit and system design can avoid problematic frequencies and operating conditions. Mechanical design can isolate and damp vibrations before they radiate as sound. Finally, proper measurement and specification ensure that noise performance meets application requirements.

As electronic products continue to proliferate in homes, offices, and other noise-sensitive environments, the importance of acoustic quality increases. Engineers who understand the electromagnetic origins of acoustic noise can design products that are not only electromagnetically compatible but also acoustically acceptable to their users.