Frequency Weighting Curves
The typical human ear does not perceive sound equally across the frequency spectrum. For instances, a low-frequency sound would be perceived as quieter than a mid-frequency sound that is the same loudness. A set of standardised curves were created to ‘weight’ noise signals such that they imitate how a typical humans would perceive sound. The most common frequency weighting is A-weighting and C-Weighting.
A-weighting
A-weighting is the most common form of acoustic frequency weighting. This weighting is typically associated with low to mid noise levels. At low noise levels, the human perception of sound is more sensitive at high frequencies and much less sensitive at low frequencies.
C-weighting
C-weighting is most commonly used for higher noise levels. The noise at work regulation uses C-weighting when assessing high level, impulsive events. At higher noise levels, the human perception of sound becomes more even across the frequency spectrum.
Z-weighting
Z-weighting is when no frequency weighting is applied.
Descriptors
The frequency weighting is either included within the descriptor of the measurement type (LAeq, LAFmax, LCpeak) or as a suffix to the measurement unit (dBA, dBC, dBZ).
Weighting Values
The frequency weighting curves are defined in IEC 61672-1 ‘Electroacoustics – Sound level meters‘. Tabulated weighting levels are shown in single octave bands and third octave bands below.
| Frequency (Hz) | 31.5 | 63 | 125 | 250 | 500 | 1000 | 2000 | 4000 | 8000 |
|---|---|---|---|---|---|---|---|---|---|
| A-Weighting (dB) | -39.4 | -26.2 | -16.1 | -8.6 | -3.2 | 0.0 | +1.2 | +1.0 | -1.1 |
| C-Weighting (dB) | -3.0 | -0.8 | -0.2 | 0.0 | 0.0 | 0.0 | -0.2 | -0.8 | -3.0 |
| Z-Weighting (dB) | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
| Frequency (Hz) | A-Weighting (dB) | C-Weighting (dB) | Z-Weighting (dB) |
|---|---|---|---|
| 25 | -44.7 | -4.4 | 0.0 |
| 31.5 | -39.4 | -3 | 0.0 |
| 40 | -34.6 | -2 | 0.0 |
| 50 | -30.2 | -1.3 | 0.0 |
| 63 | -26.2 | -0.8 | 0.0 |
| 80 | -22.5 | -0.5 | 0.0 |
| 100 | -19.1 | -0.3 | 0.0 |
| 125 | -16.1 | -0.2 | 0.0 |
| 160 | -13.4 | -0.1 | 0.0 |
| 200 | -10.9 | 0.0 | 0.0 |
| 250 | -8.6 | 0.0 | 0.0 |
| 315 | -6.6 | 0.0 | 0.0 |
| 400 | -4.8 | 0.0 | 0.0 |
| 500 | -3.2 | 0.0 | 0.0 |
| 630 | -1.9 | 0.0 | 0.0 |
| 800 | -0.8 | 0.0 | 0.0 |
| 1000 | 0 | 0.0 | 0.0 |
| 1250 | +0.6 | 0.0 | 0.0 |
| 1600 | +1.0 | -0.1 | 0.0 |
| 2000 | +1.2 | -0.2 | 0.0 |
| 2500 | +1.3 | -0.3 | 0.0 |
| 3150 | +1.2 | -0.5 | 0.0 |
| 4000 | +1.0 | -0.8 | 0.0 |
| 5000 | +0.5 | -1.3 | 0.0 |
| 6300 | -0.1 | -2 | 0.0 |
| 8000 | -1.1 | -3 | 0.0 |
| 10000 | -2.5 | -4.4 | 0.0 |
