C-Weighting

Same sub, two filters: A-weighting drops the bass and reads 82 dB(A); C-weighting keeps it and reads 99 dB(C).

What it is

A loudness measurement that keeps the deep bass in, so the meter reads the full low-end punch the crowd actually feels.

Key facts

• C-weighting is nearly flat: 0 dB at 1 kHz, only -3 dB at 31.5 Hz and -3 dB at about 8 kHz (defined in standard IEC 61672-1).
• A-weighting slashes bass hard: -39 dB at 31.5 Hz and -26 dB at 63 Hz, so it ignores most sub energy; both curves are pinned to 0 dB at 1 kHz.
• Units: meters read dB(C) for C-weighting, dB(A) for A-weighting, dB(Z) for unweighted (flat/zero) response - always state which.
• Heavy-bass program (kick, sub, EDM) reads HIGHER on dB(C) than dB(A); the gap can be 10-20 dB+, and that gap is an instant bass-content check.
• 0 dB SPL = 20 micropascals (uPa), the threshold of human hearing at 1 kHz.
• SPL formula: SPL = 20 x log10(p / p0) dB, where p = measured RMS pressure and p0 = 20 uPa reference.
• +6 dB SPL = double the sound pressure; +10 dB = roughly twice as LOUD to the ear; +3 dB = double the power; -3 dB = half the power (the 'half-power' point that sets C's roll-off corners).
• Speed of sound approx 343 m/s at 20 C; wavelength = speed / frequency, so 50 Hz = 6.9 m long.
• Inverse-square law: SPL drops 6 dB per doubling of distance from a point source in free field.
• LCpeak (C-weighted peak) caps impulsive sound at the 140 dB(C) AU/EU limit; the 8-hour workplace average limit is 85 dB(A) (LAeq,8h).

How it works

1. Sound hits the mic capsule and becomes a voltage.
2. The C-weighting filter passes bass and most treble almost untouched, gently rolling off only the extreme lows and highs.
3. The meter squares the signal, averages it (RMS), and converts to decibels referenced to 20 uPa.
4. It displays the result as a dB(C) number.
5. Flip to dB(A) and the same sound reads lower because A-weighting throws away the bass.

Real examples

• Standing in front of a sub at a doof: chest-thumping 50 Hz barely moves a dB(A) meter but spikes dB(C).
• Council noise permit written as a dB(C) limit at the boundary because the complaint is bass bleed, not vocals.
• EDM or hip-hop show: dB(C) sits 15-20 dB above dB(A), confirming a sub-heavy mix.
• A solo acoustic set: dB(C) and dB(A) read close together because there is little deep bass.
• Logging LCpeak to make sure a snare or pyro hit never breaches 140 dB(C) peak.

How it helps in live sound

• Read dB(C) when a venue or council limit is written in dB(C) - it targets the bass they care about.
• Watch the dB(C) minus dB(A) gap on your meter: gap over 15 dB means too much sub, pull the LF or hi-pass.
• Park a logging SPL meter (set LCeq + LAeq, Slow) at the mix position or boundary and log the whole show.
• Hi-pass every vocal and most inputs around 80-120 Hz so only kick and bass feed the subs and your dB(C).
• Use LCpeak to police impulsive hits (snare, pyro, FX) against a 140 dB(C) peak ceiling.
• Calibrate the meter with a 94 dB @ 1 kHz pistonphone before doors so your dB(C) numbers are trustworthy.