Statistical noise analysis characterizes the time-varying nature of environmental noise using percentile levels (Ln) that describe the noise level exceeded for a given percentage of the measurement period. Unlike the equivalent continuous sound level (Leq), which provides a single energy-averaged value, statistical percentile analysis reveals the statistical distribution of noise levels — essential for assessing noise sources with variable character such as road traffic, industrial operations, and construction sites.
Statistical noise analysis has been a cornerstone of environmental noise assessment practice for decades. The UK's CRTN traffic noise assessment method, the Dutch RMR method, the German TA Lärm, and numerous other national standards use statistical noise indicators as primary assessment parameters. In planning applications and environmental impact assessments, L10, L50, and L90 values provide regulators with a complete picture of the noise climate — not just an average, but the distribution from quiet periods to noisy peaks.
Key Ln Percentile Parameters and Their Meanings
L90 — the noise level exceeded 90% of the measurement time. Represents the background noise level — the quietest persistent noise in the environment. Used in BS 4142 as the reference background level for industrial noise assessment.
L50 — the median noise level, exceeded 50% of the measurement time. A measure of the typical noise level during the period.
L10 — the noise level exceeded 10% of the measurement time. Characterizes typical traffic noise peaks and is used in CRTN and other traffic noise assessment methods as the primary indicator of traffic noise impact.
L5 / L1 — levels exceeded 5% and 1% of the time; characterize occasional noise peaks and events.
L95 / L99 — levels exceeded 95% and 99% of the time; near-background noise characterization for very quiet environments.
Lden — Day-Evening-Night noise indicator per EU Environmental Noise Directive (ISO 1996). Calculated from Lday (07:00–19:00), Levening (19:00–23:00, +5 dB penalty), and Lnight (23:00–07:00, +10 dB penalty). Used for EU strategic noise mapping.
Lnight — Night-time noise indicator (23:00–07:00) for sleep disturbance assessment per WHO guidelines and EU END.
Standards Referencing Statistical Noise Analysis
ISO 1996-1 / ISO 1996-2 — description, measurement and assessment of environmental noise; Lden, Lnight, Lday
EU Directive 2002/49/EC — Environmental Noise Directive; Lden and Lnight as primary noise indicators for strategic noise mapping
CRTN (Calculation of Road Traffic Noise) — L10 18hr as primary traffic noise indicator
BS 4142 — L90 background noise level measurement and comparison with specific noise level
TA Lärm — German noise assessment using assessment noise level and reference background
Instruments with Statistical Analysis Capability
Bedrock Elite i10 — full Ln percentile suite (L1/L5/L10/L50/L90/L95/L99), Lden, Lnight, Lday calculations, data logging with time-stamped statistical records, GPS position tagging.
Bedrock Elite i9 — Ln percentiles (L10/L50/L90/L95/L99) and data logging for environmental noise assessments.
Bedrock SM90 — statistical analysis (L10/L50/L90/L95/L99) with data logging for routine environmental surveys.