IEC 61672-1 Frequency Weighting: A, C, and Z Networks
TL;DR
IEC 61672-1 defines three frequency weighting networks. A-weighting approximates the 40-phon equal-loudness contour, attenuating low frequencies (e.g., -26.2 dB at 125 Hz, -16.1 dB at 250 Hz) to reflect human hearing sensitivity. C-weighting is nearly flat with slight roll-off at extremes (-0.2 dB at 1 kHz ref), used for peak measurements and low-frequency assessment. Z-weighting (zero weighting) is flat ±1.5 dB from 10 Hz to 20 kHz, introduced in the 2002 edition to replace the informal "Linear" or "Flat" designation.
A-Weighting Network (Clause 5.4)
A-weighting is the most widely used frequency weighting in noise measurement. Defined in Clause 5.4 and Table 2 of IEC 61672-1, it approximates the inverse of the ISO 226 equal-loudness contour at 40 phons. Key nominal values relative to 1 kHz:
| Frequency (Hz) | A-weighting (dB) |
|---|---|
| 31.5 | -39.4 |
| 63 | -26.2 |
| 125 | -16.1 |
| 250 | -8.6 |
| 500 | -3.2 |
| 1000 | 0.0 |
| 2000 | +1.2 |
| 4000 | +1.0 |
| 8000 | -1.1 |
A-weighted measurements are denoted with subscript "A" (e.g., LAeq, LAF). Almost all occupational noise regulations worldwide reference A-weighted levels.
C-Weighting Network (Clause 5.4)
C-weighting is approximately flat across the audible range, with gentle roll-off below 31.5 Hz and above 8 kHz. It attenuates only -3.0 dB at 31.5 Hz and -3.0 dB at 8 kHz. C-weighting is specified for peak sound pressure level measurements (LCpeak) and is used alongside A-weighting to estimate low-frequency content: a large LC - LA difference indicates significant energy below 500 Hz.
Z-Weighting Network (Clause 5.4)
Z-weighting (zero weighting) was introduced in IEC 61672-1:2002 to replace the ambiguous "Linear" or "Flat" terms used informally. Z-weighting is defined as nominally flat (0 dB ±1.5 dB) from 10 Hz to 20 kHz. It provides an unweighted measurement of sound pressure level, useful when post-processing will apply custom weighting or when comparing to other standards.
Tolerance Limits
Class 1 instruments must match the nominal weighting curve within ±1.1 dB at frequencies from 63 Hz to 4 kHz, widening to ±2.5 dB at frequency extremes. Class 2 tolerances are approximately ±1.4 dB to ±3.5 dB over the same range. The exact limits are specified per frequency in IEC 61672-1 Table 2.
Implementation in SonaVyx
The SonaVyx SPL meter implements all three weighting networks using IIR biquad filter cascades derived from the analog prototype via bilinear transform. The A-weighting filter uses a cascade of second-order sections matching the standardized poles and zeros. You can switch between A, C, and Z weighting in real time during measurement. For octave-band analysis with weighting, see IEC 61260-1 and IEC 61672 integration.
For the US equivalent frequency weighting specifications, see ANSI S1.4 frequency weighting. For understanding how time weighting interacts with frequency weighting, see IEC 61672-1 time weighting.
Try It Now
Open this measurement tool in your browser — free, no download required.
Last updated: March 19, 2026