A-Weighting

A-Weighting can be found from the following formulae

For A-Weighting: A(f) =

                              12200^2 f^4
------------------------------------------------------------------
(f^2 +20.6^2) (f^2 +12200^2) (f^2 +107.7^2)^0.5 (f^2 +737.9^2)^0.5

The weighting in dB relative to 1000Hz is now given by

                  A(f)
          20 lg -------        where  A(1000) = 0.794
                A(1000)

It is convenient to list A-Weighting at nominal octave or 1/3-octave (“third-octave”) frequencies, for example 1250 Hz or 2500 Hz. Ideally weightings should be calculated for the exact frequencies which may be determined from the formula 1000 x 10^(n/10), where n is a positive or negative integer. Thus the frequency shown as 1250 Hz is more precisely 1258.9 Hz etc.

At these precise frequencies, the A- and C-Weighting values are as follows:

A, C & U Weighting Table (dB)

   Nominal         Exact
  Frequency      Frequency    A-Weight  C-Weight  U-Weight
                                                     *
     10             10.00     -70.4     -14.3       0.0
     12.5           12.59     -63.4     -11.2       0.0
     16             15.85     -56.7     - 8.5       0.0
     20             19.95     -50.5     - 6.2       0.0
     25             25.12     -44.7     - 4.4       0.0
     31.5           31.62     -39.4     - 3.0       0.0
     40             39.81     -34.6     - 2.0       0.0
     50             50.12     -30.2     - 1.3       0.0
     63             63.10     -26.2     - 0.8       0.0
     80             79.43     -22.5     - 0.5       0.0
    100            100.00     -19.1     - 0.3       0.0
    125            125.9      -16.1     - 0.2       0.0
    160            158.5      -13.4     - 0.1       0.0
    200            199.5      -10.9       0.0       0.0
    250            251.2      - 8.6       0.0       0.0
    315            316.2      - 6.6       0.0       0.0
    400            398.1      - 4.8       0.0       0.0
    500            501.2      - 3.2       0.0       0.0
    630            631.0      - 1.9       0.0       0.0
    800            794.3      - 0.8       0.0       0.0
   1000           1000.0        0.0       0.0       0.0
   1250           1259        + 0.6       0.0       0.0
   1600           1585        + 1.0     - 0.1       0.0
   2000           1995        + 1.2     - 0.2       0.0
   2500           2512        + 1.3     - 0.3       0.0
   3150           3162        + 1.2     - 0.5       0.0
   4000           3981        + 1.0     - 0.8       0.0
   5000           5012        + 0.5     - 1.3       0.0
   6300           6310        - 0.1     - 2.0       0.0
   8000           7943        - 1.1     - 3.0       0.0
  10000          10000        - 2.5     - 4.4       0.0
  12500          12590        - 4.3     - 6.2     - 2.8
  16000          15850        - 6.6     - 8.5     -13.0
  20000          19950        - 9.3     -11.2     -25.3
  25000          25120                            -37.6
  31500          31620                            -49.7
  40000          39810                            -61.8

* There is some reason to believe that a very low frequency rollover frequency of 4 Hz may be appropriate for instruments that are to be used to measure sound affecting humans.