The function Lg is derived from a programme of tests on members of the fan series covering the range of n and Dr and determining values for a and b. It has the form of a single generalized spectrum of octave-band sound power level plotted against χ and may be extrapolated by not more than one octave beyond the last plotted value of χ, following the general shape of the spectrum approaching each end, where χ = 10lg(fc/n).