Noise Limit Assessment: A-Weighted Limits and NR Curves

The problem it solves

NR comparison uses the unweighted octave-band spectrum and the chosen curve. For total A-weighted criteria, the source describes a comparison at every refresh. Treat those values as current-frame indications until the complete measurement and time-averaging requirements of the selected method have been met.

NR evaluation is particularly laborious: the NR curves are a family of curves that bend with frequency, and the measured octave spectrum must be compared with the family band by band. The module automates this. With the standard and period selected, every refresh calculates and judges all selected channels: the total A-weighted level is compared directly with the limit, and the NR fit is performed band by band.

Functions

Screen layout

1. Result table (top): one row per channel with measured value, limit, pass/fail (green/red) and margin.
2. 1/3-octave spectrum (middle): measured spectra; with an NR criterion the target NR curve is overlaid so exceeding bands are visible.
3. A-weighted level history (bottom): total level against time with a red line at the current limit — where the curve crosses the line, the limit is exceeded.

The left panel selects channels, the standard and the day or night period.

Day and night monitoring of acoustic environment zones

Compliance monitoring in residential, educational, mixed, industrial and traffic-side areas against GB 3096-2008 classes 0–4b. Select the zone class and period for automatic judgment; the history shows whether exceedance is continuous or intermittent.

Indoor acoustic acceptance of civil buildings

Indoor noise acceptance of bedrooms, classrooms, wards and operating theatres, hotel rooms and offices against GB 50118-2010, for completion acceptance and buyer-commissioned tests.

Industrial boundary noise enforcement

When one boundary faces housing and another an industrial park, different classes apply (GB 12348-2008 classes 1–4). Four boundaries, four points and four limits are judged at the same time — the value of synchronized multi-channel judgment.

Construction site boundary monitoring

The construction site boundary limit (GB 12523-2011) is 70 dB by day and 55 dB at night. Whether night work disturbs residents and how much margin remains is visible in the history with the limit line.

Workplace occupational health

Workplace rounds can identify high-noise tasks. An eight-hour occupational exposure result must be calculated from representative measurements and actual task durations using the applicable occupational method; a single frame or location reading is not that result.

NR evaluation of indoor equipment noise

Plant rooms, transformer rooms, lift machine rooms and clean rooms are commonly specified by NR curves in contracts and design codes (for example “plant room NR-45”). NR evaluation fits the Z-weighted octave spectrum to the curve family band by band and gives the measured NR value, avoiding the case of low-frequency rumble where the total level passes but the spectrum does not.

Training and demonstration

With the built-in example data, the complete sequence — select the standard, judge, read the result, locate the exceeding band — can be demonstrated without field measurement points.

1/3-octave spectrum (IEC 61260)

The current frame is windowed and transformed by FFT, and the range from 20 Hz to 16 kHz is divided into 30 standard 1/3-octave bands; each band level is the energy sum of its lines:

L_band = 10 · lg(Σ p² / p0²) [dB], p0 = 20 µPa

Empty bands are shown at 0 dB so the curve remains continuous.

A-weighting (IEC 61672 class 1)

rA(f) = 12194² · f⁴ / [(f² + 20.6²) · √((f² + 107.7²)(f² + 737.9²)) · (f² + 12194²)], LA = 20 · lg rA + 2.00

A-weighting models the ear's low sensitivity at low frequencies and is the statutory weighting for environmental and occupational noise.

Criterion 1: total A-weighted level (GB standards)

LAeq = 10 · lg Σ 10^((L_band + A(f)) / 10) [dB(A)]

Pass if LAeq ≤ limit. Margin = limit − LAeq: positive values are margin, negative values the exceedance. Day or night limits apply according to the selected period.

Criterion 2: noise rating curves (ISO R 1996 basis)

The NR curves are a family of spectral limit curves from NR-0 to NR-100. The module contains the standard limit table for NR-15 to NR-85 in steps of 5 for nine octave bands from 31.5 Hz to 8 kHz, with linear interpolation for any NR value. The evaluation has four steps:

Two details: the NR curves are not parallel across bands, so the target curve is recalculated band by band from the table for the selected rating rather than simply shifted; and the 20 Hz band is extrapolated linearly from the low-frequency trend, while the 12.5 kHz and 16 kHz bands are not defined in the NR system and are excluded.

Why NR evaluation is locked to Z-weighting

NR evaluation is based on Z-weighted (linear) octave spectra. Comparing an A-weighted curve with an NR target would push the low-frequency spectrum down and give a false impression of compliance. When an NR criterion is selected, the display is locked to Z-weighting, so the evaluation basis and the chart always agree.

Display

Octave spectra are drawn as steps from lower edge through center to upper edge. The NR target curve is overlaid on the spectrum. The A-weighted history scrolls frame by frame (8192 points), and the red limit line shows the moment of exceedance.

Preparation

1. Microphones: place them according to the task (zone monitoring points, sensitive boundary points, room center or 1 m from equipment), away from air outlets and strong reflecting surfaces.
2. Calibration: calibrate with a sound calibrator, refresh and check that the A-weighted reading matches.
3. Define the assessment: decide the applicable standard and criterion type first (environmental, building, boundary, construction, occupational → total A-weighted level; contractual indoor equipment requirement → NR curve).

Procedure

Quick reference

Source-display illustration using the GB 3096-2008 class 4a day preset of 70 dB(A). The measurement period, sampling and corrections are not verified, so this is not a formal environmental compliance result.

• Channel 1: total A-weighted level 102.6 dB, exceeding by 32.6 dB (red); channel 2 exceeds by 35.2 dB; channel 3 also exceeds — all measurement points exceed by a large amount.
• The history shows the level low at first, then a step up to above 100 dB that stays above the 70 dB line: continuous rather than intermittent exceedance.

Read the displayed status and margin, then inspect the bands and history to investigate the source. These readings illustrate the software display only. Confirm the measurement period and the full assessment method before a compliance conclusion or selection of noise-control measures.