Human hearing deteriorates from the late teens on. High frequency response is lost, so that the elderly may miss the treble notes on a piano.
Between 500 Hz and 6 kHz, the human ear is very sensitive but less so at lower and higher frequencies. This is easily demonstrated by turning down the volume on a recorded piece of music – the bass seems to disappear. To hear low bass requires an adequate sound pressure level (SPL) level. To hear 25Hz requires a much higher SPL level than to hear 250 Hz.
So called A-weighting takes account of this variation in the sensitivity of the human ear to sounds of different frequencies. An A-weighting filter covers the full audio range, from 20 Hz to 20 kHz, but the shape is similar to the response of the human ear. A-weighted noise measurements approximate to how the human ear perceives the noise and is the most widely 'weighting' in sound level meters and the class of the meter reflects the accuracy of the meter including the filters.