So far we have talked about sound pressure levels and frequencies without regard to how the human ear perceives these. Although the normal, healthy human ear can hear frequencies between 20 and 20,000 hertz, it does not hear these frequencies with the same loudness. In fact, if each frequency were generated with the same sound pressure level, represented by the zero line in the illustration, the ear would perceive the sound pressure levels as shown by the red curve. The low frequencies below 1000 hertz would be sharply reduced, and the middle frequencies of 1000 to 4000 hertz would sound slightly louder. Above 4000 hertz, the sound pressure levels would be reduced again. This curve approximates the frequency response of the human ear. It's called the A-weighted curve. It has been generated from the results of tests performed on thousands of humans. All sound level meters have an A-weighting function, so that the resulting overall single-number sound measurement approximates what the human ear hears. A-weighted decibels are designated as dBA.