Weighting networks. The majority of noise measurements are made using
sound level meters (IEC:60651, 2001), which give numerical levels as a
representation of the noise. For environmental noise it is normal to use the
sound level meter A-weighting, which gradually reduces the significance of
frequencies below 1000Hz, until at 10Hz the attenuation is 70dB. The C-
weighting is flat to within 1dB down to about 50Hz and then drops by 3dB at
31.5Hz and 14dB at 10Hz. Figure 4 shows the A and C weighting curves.