Level vs Time

The Level vs Time analysis tracks the RMS sound or vibration level as a continuous function of time. It is the primary tool for characterizing time-varying signals, transient events, and assessing compliance with time-dependent noise limits.

Theory

The instantaneous RMS level is computed over a sliding window whose effective length is determined by the time weighting constant τ:

L(t) = 10 · log₁₀[ (1/τ) ∫_−∞^t p²(s) · e^{−(t−s)/τ} ds / p_ref² ]

This exponential averaging produces a smoothed level trace whose responsiveness depends on the time weighting selected.

Time Weighting

IEC 61672-1 defines three standard time weighting constants:

Time Weighting	Time Constant (τ)	Rise Time	Application
Fast (F)	125 ms	~200 ms	General-purpose noise monitoring, NVH testing
Slow (S)	1000 ms	~1.5 s	Averaging fluctuating noise, community noise assessment
Impulse (I)	35 ms rise / 1500 ms decay	~50 ms	Impulsive noise events (hammering, gunshots, impacts)

The Impulse time weighting uses an asymmetric detector: fast attack (35 ms) to capture peaks and slow decay (1500 ms) to hold the reading for visual observation. It is not simply a shorter exponential average.

Derived Metrics

L_eq — Equivalent Continuous Level

The energy-equivalent level over a measurement period T represents the constant level that would produce the same total energy:

L_eq,T = 10 · log₁₀[ (1/T) ∫₀^T p²(t)/p_ref² dt ]

L_eq is the fundamental metric for environmental noise regulations, occupational exposure standards (ISO 9612), and building acoustics.

Peak Level (L_peak)

The true peak level captures the maximum instantaneous pressure without any time weighting. It is critical for assessing hearing damage risk from impulsive noise and for verifying that signal levels remain within the dynamic range of the measurement chain.

Parameters

Parameter	Options	Description
Time Weighting	Fast, Slow, Impulse	Exponential averaging time constant per IEC 61672.
Frequency Weighting	A, C, Z	Spectral weighting filter applied before level computation.
Update Rate	10 ms – 1000 ms	Time interval between successive level readings.
Measurement Duration	User-defined	Total recording length for L_eq computation.
Display Range	20 dB – 120 dB	Vertical axis range for the level-time plot.

Level vs RPM Variant

When a tachometer reference is available, the level trace is re-mapped from the time axis to the RPM axis. This is essential for run-up and coast-down tests where the goal is to identify speed-dependent noise phenomena. Each RPM bin accumulates the energy-averaged level from all time segments at that rotational speed.

Interpretation

Steady-state signals: Fast and Slow weightings converge to the same value. If they differ noticeably, the signal is fluctuating.
Impulse detection: A large difference between L_Impulse and L_Fast indicates impulsive content. Differences exceeding 6 dB typically trigger impulsive noise penalties in regulations.
L_eq stability: If L_eq continues to change after several minutes, the measurement duration is insufficient to capture the noise pattern.

Practical Tips

Use Fast weighting as the default for NVH work; switch to Slow only for long-term environmental monitoring where short-term fluctuations are not of interest.
Always record the peak level alongside L_eq to verify that no clipping occurred during the measurement.
For regulatory compliance, ensure the measurement duration covers at least one complete cycle of the noise source’s operating pattern.
When comparing Level vs Time traces from different channels, verify that all channels use the same time weighting and frequency weighting settings.

The Impulse time weighting is defined in IEC 61672-1 but has been deprecated for new measurements in some jurisdictions. Check local regulations before relying on L_I for compliance purposes.