A-weighting
A filter curve that rolls off bass and extreme treble before a sound level is measured, so the reading tracks how the ear hears quiet sounds. Results are quoted in dB(A).
The curve descends from the equal-loudness measurements of the 1930s and roughly matches the ear's sensitivity at low volume: heavy attenuation below a few hundred hertz, unity at 1 kHz, a slight bump around 2–3 kHz. It now lives in IEC 61672, the sound level meter standard, alongside C-weighting, which is nearly flat and used for peaks and loud sources.
A-weighting governs workplace rules, noise limits and the noise-floor specs of audio gear, where it is used enthusiastically to make the number a few decibels prettier. Its weakness is that it underrates bass at high levels, so a low thud from the neighbour's subwoofer can measure low and still keep the whole building awake.