What does dB(A) mean?
dB(A) is a decibel reading filtered to match human hearing, which is less sensitive to very low and very high frequencies. The A-weighting curve trims those extremes — a 60 Hz hum measures far lower in dB(A) than its raw level. Nearly all noise regulations and exposure limits are written in dB(A).
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Why weighting exists
A microphone hears all frequencies almost equally. Human ears don’t: at moderate listening levels we’re most sensitive between roughly 1–4 kHz (where speech lives) and much less sensitive to deep bass and the highest treble. An unweighted meter would tell you a 50 Hz transformer hum is as “loud” as a 1 kHz alarm of the same pressure — your ears disagree.
What the A-curve does
A-weighting applies a standard filter (defined in IEC 61672) derived from the ear’s equal-loudness response at moderate levels. Relative to 1 kHz, it cuts about 19 dB at 100 Hz, about 30 dB at 50 Hz, boosts slightly around 2–3 kHz, and rolls off again above 10 kHz. A reading taken through that filter is written dB(A) or dBA, and most measurements you encounter — NIOSH’s 85 dB limit, city noise ordinances, the readout on a building-site meter — are A-weighted.
Other weightings you’ll see
C-weighting (dB(C)) stays nearly flat across bass frequencies and is used for peak measurements of loud impulsive sound — gunfire, fireworks, concert subwoofers. Z-weighting is no filter at all. If a spec just says “dB” with no letter, it usually means dB(A) in a regulatory context, but it’s worth confirming.
Our tools
The online decibel meter has an A-weighting toggle — it’s an approximation built from standard biquad filters, and the readout label switches to dB(A) when it’s on. The Decibel Shield iOS app uses device-tuned level corrections rather than a formal weighting filter, so treat its readings as unweighted estimates when comparing against dB(A) limits.