A-weighted decibels, abbreviated dBA, or dBa, or dB(a), are an expression of the relative loudness of sounds in air as perceived by the human ear. In the A-weighted system, the decibel values of sounds at low frequencies are reduced, compared with unweighted decibels, in which no correction is made for audio frequency. This correction is made because the human ear is less sensitive at low audio frequencies, especially below 1000 Hz, than at high audio frequencies.
In computer systems, dBA is often used to specify the loudness of the fan used to cool the microprocessor and associated components. Typical dBA ratings are in the neighborhood of 25 dBA, representing 25 A-weighted decibels above the threshold of hearing. This is approximately the loudness of a person whispering in a quiet room.