C-Weighting (dBC)
Definition
C-Weighting (dBC)
C-weighting is a nearly flat frequency weighting curve defined by IEC 61672-1:2013 that provides minimal attenuation across the audible spectrum, with slight roll-off below 31.5 Hz and above 8 kHz. It is used for peak SPL measurement, low-frequency noise assessment, and evaluating the LCpeak exposure threshold. SonaVyx implements C-weighting as cascaded IIR biquad filters.
C(f) = 20 × log₁₀(RC(f)), with 0 dB at 1 kHz and -3 dB at ~31.5 Hz and ~8 kHz
How C-Weighting Is Applied
C-weighting is implemented as an analog filter per IEC 61672-1:2013 Clause 5.4.7, converted to digital domain using bilinear transform. Unlike A-weighting which heavily attenuates low frequencies, C-weighting is nearly flat from 31.5 Hz to 8 kHz with a tolerance of ±1 dB. SonaVyx applies C-weighting in real time through its Rust WASM DSP pipeline, enabling simultaneous display of dBA and dBC values for comprehensive noise assessment.
Practical Example
At a nightclub, the A-weighted level reads 95 dBA while the C-weighted level reads 108 dBC. The 13 dB difference (dBC minus dBA) indicates dominant low-frequency energy from the subwoofer system. The EU Noise at Work Directive 2003/10/EC uses LCpeak thresholds: 135 dBC lower action value, 137 dBC upper action value, and 140 dBC exposure limit — all measured with C-weighting to capture bass transient peaks.
C-Weighting vs A-Weighting
A-weighting attenuates low frequencies by up to 39 dB at 50 Hz, reflecting the ear's reduced sensitivity at moderate listening levels. C-weighting applies only 0.8 dB attenuation at 50 Hz, preserving the low-frequency content in the measurement. The difference between dBC and dBA readings (called the C-A difference) indicates the proportion of low-frequency energy. A C-A difference above 10 dB suggests significant bass content that A-weighting alone would underrepresent.
Peak Measurement
IEC 61672-1 specifies that peak sound pressure level (LCpeak) be measured with C-weighting. Peak detection captures the absolute maximum instantaneous pressure, which can be 10 to 20 dB higher than the RMS level for impulsive sounds. The C-weighted peak preserves low-frequency transients that could damage hearing through mechanical cochlear displacement, a risk not captured by A-weighted averaging.
Hearing Protector Assessment
The OSHA NRR (Noise Reduction Rating) derating method uses the C-weighted workplace level: estimated exposure = LCeq minus (NRR - 7) dB. C-weighting is used because hearing protectors typically provide less attenuation at low frequencies than the manufacturer's published NRR suggests. The NIOSH subject-fit method also uses dBC as the starting point for a more accurate estimate of protected exposure levels.
Music and Entertainment
Many entertainment venue noise regulations now include C-weighted limits alongside A-weighted limits to address bass levels. A concert producing 96 dBA might simultaneously produce 112 dBC. Some jurisdictions limit LCeq to 104 dBC for music venues to prevent structural vibration complaints and protect patrons from excessive low-frequency exposure that A-weighted measurements would miss.
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