Engineer Noise and Vibration key terms

A logarithmic measure of the root-mean-square pressure fluctuation of a sound wave relative to 20 micropascals, expressed in decibels. It describes the strength of sound at a point in space.

A logarithmic ratio used to compress the enormous range of sound pressures and intensities into manageable numbers. In acoustics it compares a measured quantity to a stated reference value, usually 20 micropascals for airborne sound.

A property of a sound source describing the total acoustic energy it radiates per unit time, referenced to one picowatt and expressed in decibels. Unlike sound pressure level, it does not depend on distance or room conditions.

The acoustic power passing through a unit area perpendicular to the direction of propagation, measured in watts per square metre. It is a vector quantity, so it can show net energy flow direction.

Frequency is the number of pressure cycles per second in hertz; wavelength is the distance the wave travels in one cycle. They are inversely related through the speed of sound in the medium.

A frequency interval in which the upper band limit is twice the lower limit. Bands are identified by their centre frequency, and octave or one-third-octave analysis is standard practice for rating noise and selecting control measures.

A frequency-dependent correction applied to measured levels to approximate the reduced sensitivity of human hearing at low and very high frequencies. A-weighted levels are widely used for hearing damage risk and environmental noise limits.

The steady A-weighted level that would deliver the same total acoustic energy over a stated period as the actual fluctuating noise. It is the primary metric for occupational and environmental noise assessment.

The accumulated sound exposure over a work period expressed as a percentage of the maximum permitted daily exposure. A dose of 100 percent corresponds to the exposure limit, and dose doubles for each 3 dB increase in level.

The decibel increase in sound level that halves the permitted exposure duration. Most occupational regimes use 3 dB, while some regulations historically used 5 dB, so dose results are not interchangeable between them.

The A-weighted equivalent continuous level normalized to a nominal eight-hour working day, accounting for actual exposure duration. It allows comparison of varied daily exposures against a single occupational limit value.

The maximum instantaneous C-weighted pressure level during an event, capturing short impulsive sounds such as impacts or gunshots. It is assessed separately from energy-averaged levels because brief peaks can damage hearing.

Sound characterized by a rapid rise and short duration, typically from impacts, explosions, or press operations. It is evaluated with peak levels and separate criteria because its hazard is not well described by energy averaging alone.

A structured set of measures to prevent occupational hearing loss, typically including exposure assessment, engineering and administrative controls, audiometric testing, hearing protection provision, training, and recordkeeping. It is triggered when exposures exceed defined action levels.

Hearing threshold measurement, usually by pure-tone audiometry, used to establish a baseline and detect changes over time. A significant standard threshold shift prompts review of exposure controls, protection use, and referral.

A reversible elevation of hearing threshold following short-term noise exposure, recovering over hours or days. Repeated shifts may indicate inadequate protection and can precede permanent noise-induced hearing loss.

A single-number laboratory rating of a hearing protector's attenuation, derived from mean attenuation minus two standard deviations. Field performance is usually lower, so practitioners apply derating and check real-world fit.

The difference in sound level measured at a point before and after a control measure, such as a barrier or enclosure, is installed. It describes actual field performance rather than the laboratory rating of a product.

The reduction in sound level when sound passes from one space through a partition to another, usually stated in decibels per band. It depends on mass, stiffness, sealing, and flanking paths, not on the source alone.

Sound reaching a receiving space by paths other than the intended separating element, such as ducts, gaps, or structural connections. It can dominate performance and make a high-rated partition behave poorly.

The total sound energy absorbed by surfaces and contents in a room, often expressed in sabins. Increasing absorption reduces reverberant buildup and lowers levels away from the source, though it does not reduce direct sound.

The time required for sound pressure level in a room to decay by 60 decibels after the source stops. It characterizes how live or dead a space is and strongly affects speech intelligibility and noise buildup.

A solid obstacle placed between a source and receiver that attenuates sound by blocking the direct path and forcing diffraction over its top edge. Effectiveness depends on wavelength, geometry, and the path difference.

The bending of sound waves around obstacles and through openings, allowing sound to reach regions without a direct line of sight. It explains why low frequencies pass barriers more readily than high frequencies.

The use of resilient mounts, springs, or pads to reduce transmission of vibration from a machine to its supporting structure. Performance depends on the natural frequency of the isolated system relative to the driving frequency.

A frequency at which a structure or system vibrates freely after disturbance, determined by its mass and stiffness. Excitation near this frequency produces amplified response and is generally avoided in design.

The condition in which a system is driven at or near its natural frequency, producing a large response for a relatively small input. It can cause excessive vibration, noise radiation, or structural fatigue.

The dissipation of vibrational energy over time, reducing the amplitude of oscillation and the sharpness of resonance. It is expressed as a ratio or loss factor and is important for controlling structural response.

An instrument that measures sound pressure via a microphone and applies frequency weighting and time weighting to display levels. Its accuracy class, calibration status, and settings determine whether results are fit for a given assessment.

Verification of instrument sensitivity against a known reference, usually a portable acoustic calibrator placed on the microphone before and after measurement. It confirms the measurement chain and documents traceability for reported results.

Standardized averaging time constants in sound level meters, with fast responding quickly to changes and slow providing a steadier reading. The chosen weighting affects displayed levels and must be reported with results.

An efficient algorithm that converts a sampled time-domain signal into its frequency-domain spectrum. It is used to identify tonal components, resonances, and vibration frequencies from measured data.

The number of discrete samples taken per second when digitizing a signal, in hertz. It must exceed twice the highest frequency of interest to avoid aliasing, which corrupts the analysed spectrum.

A quantified range around a measured value within which the true value is expected to lie, accounting for instrument, environmental, and procedural contributions. Reporting it helps determine compliance against limits near the threshold.

A computer calculation that predicts sound levels across an area from sources such as roads, rail, aircraft, or industry. It accounts for propagation effects, terrain, and buildings, and supports mapping and action planning.

The decrease in sound level as energy spreads over a larger area with distance from the source. Point sources lose about 6 dB per doubling of distance in free field, while line sources lose about 3 dB.

Reduction of sound energy as it travels through air, caused by molecular relaxation and viscous effects. It depends strongly on frequency, temperature, and humidity and is included in long-range propagation models.

An A-weighted annual average environmental noise indicator that adds penalties to evening and night periods to reflect greater annoyance at those times. It is used in strategic noise mapping and action planning.

A negative subjective reaction to noise, influenced by level, tonality, impulsiveness, time of day, and individual factors. It is assessed through surveys and is a key outcome in environmental noise policy.

The subjective magnitude of a sound as perceived by a listener, depending on level, frequency content, and duration. It is not proportional to sound pressure level and is described using scales such as sones and phons.

The masking of speech by background noise, reducing intelligibility and communication efficiency. It depends on noise spectrum relative to speech frequencies and is a common criterion for indoor noise limits.

The rate at which sound propagates through a medium, about 343 metres per second in air at 20 degrees Celsius. It increases with temperature and affects wavelength, resonance, and propagation calculations.

The professional obligation to provide accurate, unbiased assessments within one's competence, disclose limitations and conflicts, and avoid misrepresenting results. Reports should allow independent review of methods, assumptions, and data.