7.2.3.3 Determination of experimental constants a, b and LgThe experimental programme should include several sizes of fan from the series, including the smallest and largest that can be tested with accuracy. Each size should be tested over the widest possible range of speed.For all the tests, the value of ϕ shall be the same and the air conditions at fan inlet preferably within the limits given in 7.2.3.2. If ambient pressure covers a wider range, correct approximately to atmospheric pressure pa by subtracting 5lg(p/pa) from each observed LW.At each value of Dr and n the fan sound power level should be measured in the 24 third-octave bands from 50 Hz to 10 kHz. For calculation purposes, 8-octave band levels can then be evaluated by adding the three adjacent third-octave sound powers around each third-octave centre frequency from 63 Hz to 8 kHz.