The so-called A-weighted dB scale (abbreviated dBA) is based on the Fletcher-Munson equal-loudness curve for an SPL of 40 phons.F.8 Thus, a dBA weighting assumes a fairly quiet pure tone. Despite this assumption, the dBA weighting is often used as an approximate equal loudness adjustment for measured spectra.
An analog filter transfer function that can be used to implement an approximate A-weighting is given byF.9
where normalizes the gain to unity at 1 kHz.
The ITU-R 468 noise weightingF.10is said to perform better for measuring noise in audio systems.