Components of Blade Passing Frequency(BPF) are the important elements of fan noise.In the spectrum analysis of noise signal

it was discovered that different analysis bandwidth made a significant difference in the BPF component value.In this paper

the Fast Fourier Transform(FFT) processes with different bandwidth for the same record of noise were discussed.Because the result of SPL is the energy sum within a bandwidth

while the range of BPF variation due to the varying rotating speed exceeds the bandwidth

the variation of bandwidth will make the SPL different.Finally

a method for determining the fan noise spectrum was introduced.