(ii) Countermeasures for leakage error (Window Function) :
To decrease the leakage error due to discrepancy between the time duration measured and the period of the original signal, we must connect the repeated wave smoothly. For this purpose we multiply a weighting function that decrease gradually at both sides. This weighting function is called time window. Representative time windows: the Hanning window, Hamming window and Blackman-Harris window are shown in Figure 11.29. These windows are defined in the range: 
as
Hanning window 
Hamming window 
Blackman-Harris window 
and outside 
for all three cases. (11.37)