dB(A) or dBA stands for decibels adjusted. It is the weighted absolute, calculated in dB (SPL) (decibels referenced to the sound pressure level SPL of 20 micropascals = 0 dBA).
Equal-loudness curves
The curves are based on the psophometric curves for human hearing; the equal-loudness contour. It is intended to approximate the level of human perception of noise. They are often used for determining the sound exposure of humans. The A indicates a certain frequency weighting representative for the noise sensitivity of the human ear. In Europe, the dBA is generally used in all sorts of noise regulations. dB(B) and dB(C) are similar measurements using different weighting curves. The curves approximate the human ear at different loudnesses, so only one of the curves should (theoretically) be chosen for a certain measurement, depending on the absolute signal level present.
The curves are defined by the following transfer functions [1]:
A
B
C
D
The k values are constants which are usually used to normalize the function to a gain of 1 (0 dB) at 1 kHz. For A-weighting, kA ≈ 7.39705.
Note: The use of F1A-line or HA1-receiver weighting must be indicated in parentheses as required. A sound level of 0.63 pascal 1000 Hz tone will read +90 dBA, but the same sound pressure as white noise, randomly distributed over a 3 kHz band (nominally 300 to 3300 Hz), will read +82 dBA, due to the frequency weighting.
Manufacturers give the dBA rating for each appliance. The lower the rating the quieter the appliance when it is operating.
The A-weighted noise floor of an audio system is often several dB lower than that measured with a flat-response device. In the field of audio electronics, it is therefore sometimes used in marketing literature to make systems seem better than they are.
Synonym: dBrn adjusted.
Source: from Federal Standard 1037C and from MIL-STD-188, plot of the filtercurve from J. Wolfe, University of New South Wales