dBA vs dBC: Which Should You Measure? (With Decision Table)
Two toggles on almost every sound meter, and choosing between them changes the number by tens of decibels. Here is what each weighting does, a decision table for the common cases, and how to read the useful gap between the two readings.
By Bhagya Masalawala · Last materially reviewed:
Short answer: Measure dBA by default: nearly every noise limit, hearing-safety threshold and loudness comparison is defined on A-weighting, which mirrors how human hearing rolls off at low and high frequencies. Choose dBC when the sound genuinely is its bass — thumping music, machinery hum, cannon-like impulses — because C-weighting stays flat down to much lower frequencies. If both toggles are handy, read both: a C−A gap near 15 dB or more is the signature of bass-heavy noise.
- dBA is the default for hearing risk, OSHA/NIOSH limits, community noise and product comparisons.
- dBC stays nearly flat to ~100 Hz (−6.2 dB at 20 Hz vs −50.5 dB for dBA) — the bass-revealing weighting.
- C − A ≥ ~15 dB as a screening rule flags low-frequency dominance the dBA number barely shows.
- Impulse/pulse peak limits are defined in dBC peak: ~135–140 dBC depending on the standard.
The two weightings in one paragraph
Both dBA and dBC are IEC 61672 weighting filters applied to the same microphone signal — they differ only in how much each frequency band counts. A-weighting tilts the spectrum to match human ear sensitivity: heavy subtraction in deep bass, near-flat in speech frequencies, slight lift around 2–4 kHz where the ear canal resonates. C-weighting is close to the unweighted (dBZ, linear) curve at everyday levels: almost flat from about 100 Hz upward, with only small corrections at both ends. Neither weighting changes the meter's sensitivity — they change which part of the sound contributes to the display. A full tour of the whole family, including why dBZ exists, lives on the main dB vs dBA explainer; this page is the decision guide.
Weighting correction table
| Frequency | A-weighting | C-weighting | What this means |
|---|---|---|---|
| 20 Hz | −50.5 dB | −6.2 dB | Sub-bass: dBA all but deletes it, dBC keeps it |
| 63 Hz | −26.2 dB | −0.8 dB | Ceiling-rumble / bass-music region |
| 125 Hz | −16.1 dB | −0.2 dB | Low machinery and HVAC hum |
| 500 Hz | −3.2 dB | 0.0 dB | Lower midrange |
| 1,000 Hz | 0.0 dB | 0.0 dB | Both definitions pin 1 kHz as the 0 dB reference |
| 4,000 Hz | +1.0 dB | −0.8 dB | Ear-sensitivity peak region |
| 16,000 Hz | −6.6 dB | −8.5 dB | Both attenuate the top octave |
The asymmetry above 1 kHz is small and only matters for specialist work. The decisive difference is bass: from 60–250 Hz the two weightings divorce by 16–26 dB on the same physical sound.
Decision table: which weighting for your situation
| Situation | Measure | Why |
|---|---|---|
| Workplace hearing risk, OSHA/NIOSH dose | dBA | All occupational limits are written in dBA; peak limits in dBC |
| Neighborhood noise, sleep quality, all-night average | dBA | WHO environmental guidance and annoyance are defined on A-weighting |
| Chainsaw, mower, blender — product troubleshooting or comparing | dBA | Ratings are in dBA; hearing-relevant spectrum |
| Thumping party / nightclub bass through a wall | dBC (+ dBA for the record) | Bass dominates; dBC quantifies what the wall transmits |
| Impulses: fireworks, nail guns, crashes, sirens at close range | dBC (peak, LCpeak) | Peak limits are specced in C-weighted peak; fast dBA averages under-read impulses |
| Bass guitar PA tuning, subwoofer alignment | dBC or dBZ | You need true bass-band levels unaffected by ear tilt |
| Heavy machinery, compressors, HVAC plants | both | dBA for compliance, dBC to document the low-frequency nuisance component |
| Plain curiosity in normal rooms | dBA | Matches how loud it feels; compares with every chart |
The trick of reading both: the C−A gap
Devices offering both weightings hand you a free diagnostic many pros use without saying so: measure the same sound on dBC, then dBA, and read the difference.
- C − A small (≈ 0–5 dB): normal broadband sound — speech, traffic at distance, fans. The dBA number faithfully represents it.
- C − A medium (≈ 6–14 dB): meaningful low-frequency content — refinery-scale machines under load, wind noise at the microphone itself.
- C − A large (≥ 15 dB): bass-dominated source; expect wall penetration, neighbour complaints and residuals the dBA figure does not show.
Caveat: this diagnostic only works on the same Leq/time-base, averaged steadily (Slow response) — and the microphone position matters, as fresh air-draughts and body reflections shift the low end. If the gap surprises you, our frequency analyzer shows the spectrum live so you can see which band is carrying the energy instead of inferring from two single numbers.
Where each appears in the standards
- OSHA: 8-hour limits, dose and hearing conservation are dBA; the ceiling for impulse/impact noise is 140 dBC peak.
- NIOSH REL: exposure guidance in dBA with a 3 dB exchange rate.
- EU Directive 2003/10/EC: daily limits in dBA; peak ceiling 135–140 dBC by framework tier.
- WHO environmental guidance: community and night protection values framed in dBA.
- IEC 61672 Class 1/2 meters: required to offer A and C weightings (Z optional on many units); the standard defines exactly the response tolerances behind the table above.
As a quick rule of thumb for value-judgement numbers: if the question is about hearing or annoyance, the answer is an A-weighted number. If the question is about physical bass level or a regulation that says "peak", the answer is a C-weighted one. Our noise limits page lists the thresholds side by side so you can slot your reading into whichever framework applies.
Measuring dBA and dBC in the browser
This site's live meter computes dBA, dBC and dBZ from the same microphone stream, so the C−A gap is a one-tap comparison, no hardware needed. Two practical reminders before you read the gap anywhere: browser estimates are screening numbers until calibrated — the device-offset procedure makes your phone comparable to a reference meter — and position matters terribly at the low end: hold the phone where a person's ears actually are, not on the floor next to the subwoofer.
dBA vs dBC FAQ
Which is better to measure, dBA or dBC?
For regulations, hearing risk and everyday loudness comparisons, dBA — it is the default weighting for nearly every noise standard. Switch to dBC when the sound is dominated by real bass energy: thumping music, industrial machinery, artillery-style impulses.
Why does dBA ignore bass?
Because hearing does too. At low frequencies the ear needs far more energy to hear the same loudness, and A-weighting subtracts up to −50 dB at 20 Hz to model that. That suppression is correct for hearing decisions and wrong when you need to quantify the bass itself.
What does a big difference between C and A readings mean?
It flags low-frequency dominance. As a screening rule, C − A around 15 dB or more means the sound carries a heavy bass component a dBA number barely represents — typical of thumping music or heavy machinery.
Do I need dBC for hearing protection?
For impulse peaks, standards specify C-weighted peak limits: 135 to 140 dBC peak depending on the framework. Check the peak reading on dBC if your device reports it.
Measure dBA and dBC live now → · Full dB vs dBA explainer → · dBFS vs dB SPL scale difference → · Where the limits live →
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