IEC Acoustic Standard IEC 61400-11

One-Third Octave Levels
Concepts are the same but averaging and correcting on the onethird
octave bands.
– Take three minutes of one-third octave data closest to the integer
wind speed for turbine and background
– Arithmetically average wind speeds and energy average the onethird
octave spectra by band (i.e. 20, 25, 31.5 Hz)
– Extrapolate or interpolate to get integer values
– Background correct by band if there is a 6 dB separation. There is
a standard correction for differences between 3 and 6 dB. Cannot
background correct for differences lower than 3 dB. Note: There
can be negative numbers. There can be bands where the
background levels greater than the turbine levels.
WS bin 20 25 31.5 40 50 63 80 100 125 160 200 250 315 400 500 630 800 1000 1250
Turb 10.0 16.4 18.6 20.2 23.6 23.4 25.9 28.7 31.2 33.1 35.4 45.4 55.0 39.9 39.5 41.9 41.6 42.2 42.4 42.1
Back 10.1 16.4 18.7 20.2 24.1 22.2 24.3 28.7 30.0 30.9 32.8 36.9 34.8 35.3 34.6 34.6 35.0 34.4 34.7 33.8
Integer Turb 10 16.4 18.7 20.3 23.6 23.4 25.9 28.8 31.3 33.1 35.4 45.4 55.1 40.0 39.6 42.0 41.7 42.2 42.5 42.1
Integer Back 10 16.2 18.4 20.0 24.0 22.2 24.3 28.6 30.0 30.9 32.7 36.7 34.7 35.2 34.5 34.5 34.8 34.3 34.6 33.6
Difference 10 0.2 0.3 0.3 -0.4 1.3 1.6 0.2 1.3 2.2 2.7 8.7 20.4 4.8 5.1 7.5 6.8 7.9 7.9 8.5
Back Corr 10 44.8 55.1 38.7 38.3 41.1 40.7 41.5 41.7 41.5
NATIONAL RENEWABLE ENERGY LABORATORY