NCJ RENR key terms

A logarithmic measure of the effective pressure variation caused by sound, expressed in decibels as 20 times the base-10 logarithm of the ratio of measured pressure to a 20 micropascal reference.

A dimensionless logarithmic unit used to express ratios of acoustic quantities such as pressure, intensity, or power; equal changes in decibels correspond to equal multiplicative changes in the underlying physical quantity.

A frequency-dependent correction applied to measured sound levels that approximates the reduced sensitivity of human hearing at low and very high frequencies; results are reported in A-weighted decibels (dBA).

A flatter frequency weighting than A-weighting that retains more low-frequency energy, commonly used with A-weighting to estimate low-frequency content or to compute rating levels.

The steady sound level that would deliver the same total acoustic energy over a stated time period as the actual fluctuating sound; energy-averaged, so loud events dominate the result.

The A-weighted sound level normalized to a one-second duration, capturing both the loudness and duration of a single event such as an aircraft overflight or a blast.

A level exceeded for a stated percentage of a measurement period; L90 approximates background or residual sound, while L10 and L50 describe higher and median portions of the distribution.

A 24-hour energy-averaged A-weighted level in which nighttime sound (typically 10 p.m. to 7 a.m.) is increased by 10 dB before averaging to reflect greater community annoyance at night.

A 24-hour energy-averaged A-weighted level that adds 5 dB to evening sound and 10 dB to nighttime sound before averaging, a metric widely used in European environmental noise assessment.

The averaging time constant applied by a sound level meter to fluctuating signals; fast is about 125 milliseconds, slow about 1 second, and impulse uses a rapid attack with slower decay for transient sounds.

A property of a source describing the total acoustic energy it radiates, expressed in decibels relative to one picowatt; unlike sound pressure level, it does not depend on distance or surroundings.

The decrease in sound level with distance from a source due to spreading of acoustic energy; for a point source in free space it is about 6 dB per doubling of distance.

Loss of acoustic energy as sound travels through air, caused by molecular relaxation and viscous effects; it increases with frequency and is affected by temperature, humidity, and pressure.

The bending of sound waves around barriers and edges, which creates an acoustic shadow and reduces levels behind an obstacle; the effect is strongest at high frequencies and small path differences.

The sound level at a location when the specific source under investigation is not operating, used as a baseline for evaluating the source's contribution to the total measured level.

The total sound present at a location at a given time, including all contributing sources plus residual sound, usually characterized by an energy average and a statistical distribution.

An instrument that measures sound pressure, applies frequency and time weightings, and displays levels in decibels; performance is classified by standards such as IEC 61672 Class 1 or Class 2.

A pre- and post-measurement check of a sound level meter using a portable acoustic calibrator of known level and frequency, documented to verify instrument stability during the survey.

A reference acoustic calibrator that uses a reciprocating piston to generate a precisely known sound pressure level at a fixed frequency, typically used for laboratory verification of calibrators.

A change in instrument response between the pre-measurement and post-measurement calibration checks; a difference exceeding the accepted tolerance requires investigation and may invalidate the data.

A porous foam cover placed over a microphone to reduce wind-generated turbulence noise and protect the diaphragm, typically allowing valid measurement below a stated wind speed.

Spectral analysis that divides the audible frequency range into bands with upper-to-lower frequency ratios of 2:1 or approximately 1.26:1, used for frequency-specific limits and noise control design.

A document specifying measurement objectives, locations, duration, instrumentation, calibration procedures, weather criteria, and reporting requirements before a monitoring survey begins.

The rationale for choosing when, where, and how long to measure so that collected data represent the conditions relevant to a permit, complaint, or impact assessment, including worst-case periods.

A time interval selected so that measured levels characterize the source or condition of interest, accounting for cycles of operation, time of day, and variability in the source.

The process of flagging, excluding, or annotating measurements affected by weather, extraneous sources, calibration drift, or instrument malfunction, with the reason documented for each decision.

The quantified range within which the true value is expected to lie, arising from instrument tolerance, calibration, sampling variability, and environmental conditions; it affects compliance decisions near a limit.

The location where noise limits apply, such as a property boundary, a dwelling facade, or a designated receptor; its correct identification is often decisive in compliance determinations.

A regulatory maximum sound level measured at or beyond a property boundary, often varying by receiving land use and time of day, and typically compared with measured or modeled levels.

A numeric or descriptive restriction on sound levels, durations, or characteristics such as tonality or impulsiveness, imposed by statute, regulation, permit, or ordinance.

The difference between a measured or predicted level and the applicable limit; when this difference is smaller than measurement uncertainty, a conclusion of compliance or exceedance may be qualified.

Increments added to measured or predicted levels when sound contains audible tones or impulsive character, reflecting greater annoyance relative to steady broadband sound.

A legally binding requirement attached to an operating permit, such as a noise limit, monitoring obligation, or reporting deadline, enforceable by the issuing authority.

A computational method that predicts sound levels at receptors from source emission data and accounts for distance, ground effects, barriers, atmospheric conditions, and meteorology.

Measured or manufacturer-supplied sound power levels, spectra, and operating schedules used as model inputs; their accuracy strongly influences predicted receptor levels.

Comparison of predicted levels with field measurements to confirm that a model represents the site; discrepancies are investigated and documented before predictions are relied upon.

A documented, unbroken record of who collected, handled, transferred, and stored evidence or data, with dates and signatures, demonstrating integrity from acquisition to presentation.

Oral or written evidence given by a qualified witness explaining technical findings, methods, and opinions, subject to standards of reliability and relevance and to cross-examination.