IEC 61260-1 and IEC 61672-1: Octave-Band SPL Measurement
TL;DR
When a sound level meter includes octave-band analysis, it must conform to both IEC 61672-1 (for the broadband measurement chain) and IEC 61260-1 (for the filter bank). The combined system measures octave-band or 1/3-octave-band sound pressure levels with frequency weighting applied before or after filtering. For broadband weighted levels (LAeq), the IEC 61672-1 weighting filters are used alone. For per-band weighted levels (e.g., A-weighted 1/3 octave spectrum), the IEC 61260-1 filters are applied first, then the corresponding A-weighting correction for each band centre frequency.
Combined Instrument Architecture
A sound level meter with octave analysis has two parallel signal paths:
- Broadband path: Input → A/C/Z weighting → time weighting → LAF/LAS display
- Filter path: Input → IEC 61260-1 filter bank → per-band detector → octave bar display
Both paths process the same input signal simultaneously. The broadband path provides the overall weighted level; the filter path provides the spectral breakdown.
Frequency Weighting in Octave Analysis
There are two approaches to applying frequency weighting in octave analysis:
- Pre-filter weighting: Apply A/C/Z weighting before the filter bank. Each band then contains weighted energy. This is simpler but means the band levels cannot be "un-weighted" later.
- Post-filter correction: Apply the filter bank to the unweighted signal, then apply the A/C weighting correction for each band's centre frequency. This allows both Z-weighted and A-weighted spectra from the same data.
Most modern instruments use post-filter correction for flexibility. SonaVyx uses the FFT approach, which naturally provides the unweighted spectrum first, then applies weighting corrections.
Instrument Classification
The combined instrument must satisfy both standards simultaneously:
| Measurement | Standard | Class requirement |
|---|---|---|
| Broadband LAeq, LAF | IEC 61672-1 | Class 1 or 2 |
| Octave/1/3 octave bands | IEC 61260-1 | Class 0, 1, or 2 |
A "Class 1 sound level meter with 1/3 octave analysis" must meet IEC 61672-1 Class 1 for broadband AND IEC 61260-1 Class 1 for filters. The classes must be stated independently — they can differ (e.g., Class 1 broadband with Class 2 octave filters).
Octave-Band Leq
The time-average level per band (Leq,band) is computed by integrating the squared band-filtered signal over the measurement period, analogous to broadband Leq. The sum of all band Leq values (in energy) should equal the broadband Z-weighted LZeq within the instrument tolerance.
NC/NR Curves
NC (Noise Criteria) and NR (Noise Rating) curves are defined in octave bands. Measuring NC rating requires an IEC 61260-1 compliant octave filter bank producing per-band levels that are compared to the standardized curves. The SPL meter NC rating feature depends on accurate octave-band data.
Building Acoustics
Sound insulation measurements per ISO 717-1 require 1/3 octave band data from 100 Hz to 3150 Hz. Both source room SPL and receiving room SPL must be measured with IEC 61260-1 Class 1 filters. The sound insulation tool uses this octave-band data for Rw and STC calculations.
SonaVyx Integration
The RTA mode and SPL meter share the same FFT-based octave analysis engine. Results are available in both Z-weighted and A/C-weighted forms. Per-band Leq values feed into NC curve rating, RT60 per band, and STI per band calculations. See digital implementation for the FFT-to-octave processing chain.
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Last updated: March 19, 2026