A-weighting
A-weighting uses a weighting curve based on ‘equal-loudness contours’ that describe our hearing sensitivity to pure tones, but it turns out that the assumption that such contours would be valid for noise components was wrong. While the A-weighting curve peaks by about 2dB around 2 kHz, it turns out that our sensitivity to noise peaks by some 12dB at 6 kHz. Another weakness of A-weighting is that it is usually combined with an rms (root mean square) rectifier, which measures mean power, with no attempt made to account for proper hearing dynamics.
Read more about this topic: Audio Noise Measurement