What the Letters After dB Mean
A decibel meter measures the physical sound pressure, but the human ear does not hear all frequencies equally — it is most sensitive in the speech range (roughly 2–5 kHz) and much less sensitive to very low and very high tones. Frequency weighting is a filter applied to the measurement so that the result better matches what humans actually perceive (or, in some cases, the raw physical level).
The suffix tells you which filter was applied:
- dB — unweighted sound pressure level (in practice, usually means Z-weighting or unspecified).
- dBA — A-weighted; approximates the ear's response at moderate levels; the standard for noise exposure.
- dBC — C-weighted; flatter response that keeps more low-frequency content; used for loud, bass-heavy sound.
- dBZ — Z-weighting (zero); a flat response across the audible range, measuring raw sound pressure.
The same physical sound can therefore have three different numbers — for example 90 dB (Z), 88 dB (C), and 83 dB (A) — depending on the filter. None of them is "wrong"; they answer different questions.
dB — The Raw Sound Pressure Level
Unweighted dB is the physical sound pressure level measured across the full frequency range of the instrument, with no frequency correction. It tells you how much acoustic energy is present, which is useful for engineering questions: how much sound does a machine produce, how does sound propagate through a structure, what is the acoustic environment like in physical terms.
The catch: raw dB does not reflect perceived loudness. A 90 dB reading dominated by low-frequency rumble sounds much quieter than 90 dB of mid-frequency noise, even though the physical level is identical. For questions about people — loudness, annoyance, hearing risk — weighted readings are the right tool.
dBA — A-Weighting, the Standard for Noise
A-weighting applies a frequency filter that mirrors the sensitivity of the human ear at moderate listening levels: it attenuates very low and very high frequencies, where the ear is less sensitive, and passes the speech range largely unchanged. The result, dBA, is a close approximation of how loud a sound feels.
Because perceived loudness is what drives annoyance and hearing damage, A-weighting is the default for essentially all practical noise measurement:
- Environmental and community noise (most countries regulate in dBA).
- Occupational exposure limits (NIOSH, OSHA, EU directives express limits in dBA).
- Product noise labelling, HVAC and building acoustics, neighbor noise disputes.
The exposure numbers everyone quotes — 85 dBA, 8-hour limits, the 3 dB exchange rule — are all A-weighted. See thenoise exposure guide for how those numbers are used.
dBC — C-Weighting for Loud, Bass-Heavy Sound
C-weighting is a much flatter curve than A-weighting: it leaves most of the low-frequency content intact and mainly rolls off the very lowest octaves. At high sound levels, the ear's frequency response flattens out, so C-weighting better matches perception for loud sounds — typically above 90–100 dB.
Use dBC when low-frequency energy is the point:
- Music venues, nightclubs, and subwoofer systems.
- Engines, industrial machinery, and construction plant.
- Comparing a measurement with its A-weighted counterpart: if dBC is much higher than dBA, the sound is dominated by low frequencies.
- Some noise regulations use dBC for peak/impulse measurements (for example, peak limits of 140 dBC in workplaces).
dBZ — Zero Weighting, Flat Response
Z-weighting ("zero") is a flat frequency response across the audible range — effectively the raw physical measurement with modern equipment. It is used when no frequency correction is desired at all:
- Acoustic research and measurement comparisons.
- Audio engineering and equipment testing where the physical signal is the subject.
- Building acoustics work where engineers want unmodified levels.
For most everyday users, Z-weighting is less useful than A-weighting, because its numbers do not reflect how loud things feel. But it is the honest "no filter" mode — see dB above.
Which One Should You Use?
Practical rule of thumb:
- Almost always dBA. Room noise, street noise, office, home, speakers, general awareness — A-weighting matches what you hear and what the limits use.
- dBC when there is serious bass — concerts, machinery, engines — especially if you want to compare C vs. A to detect low-frequency dominance.
- dBZ/dB when you want raw physics — engineering, research, or when comparing against another instrument's unweighted mode.
Whichever you choose, say which one you used when you share a number: "70 dBA" is meaningful, "70 dB" is ambiguous. For context on what the values mean, see the sound level chart.
Switching Weighting in the Decibel Meter
The online decibel meter includes A, C, and Z weighting underAdvanced Controls → Frequency Weighting, alongside fast, slow, and impulse time weighting. The default is A-weighting, which is the right starting point for almost every measurement. Switch to C to check bass-heavy sources, or Z for an unfiltered reading, and note the suffix when you record the result.
Related: how to measure decibels,what affects accuracy, and thecommon sounds in dB chart.