C-Weighting (dBC)

C-weighting was originally intended to model human hearing at high sound levels (around 100 phons), where equal-loudness contours are relatively flat. Unlike A-weighting, C-weighting passes most of the audio spectrum with minimal modification: it applies roughly -3 dB at 31.5 Hz, 0 dB from 63 Hz to 4 kHz, and rolls off above 8 kHz. This makes it far more responsive to low-frequency energy. The most important use of C-weighting is in peak measurements (LCpeak). The EU Directive 2003/10/EC sets peak exposure limits at 135 dBC (lower action), 137 dBC (upper action), and 140 dBC (absolute limit). Peak measurements use C-weighting because the flat response captures the true peak pressure regardless of frequency content. The C-A value (dBC minus dBA) is a quick diagnostic for low-frequency dominance. A difference greater than 4-6 dB indicates significant low-frequency content. Values above 10 dB suggest a potential low-frequency noise problem that A-weighted measurements alone would miss. This metric is invaluable when assessing HVAC noise, traffic, or music venues. C-weighting is also used in hearing protector evaluation. OSHA uses the NRR (Noise Reduction Rating) formula: protected level = dBC - NRR, which inherently accounts for the spectral mismatch between workplace noise and A-weighting. In live sound, C-weighted measurements better represent the total energy a system produces, including subwoofer output that A-weighting would suppress. When tuning systems, comparing dBA and dBC readings at the same point reveals whether low-end energy is dominating the mix.