Spectral leakage
The Fourier transform of a function of time, s ( t ) {\displaystyle s(t)} , is a complex-valued function of frequency, S ( f ) {\displaystyle S(f)} , often referred to as a frequency spectrum. Any linear time-invariant operation on s ( t ) {\displaystyle s(t)} produces a new spectrum of the form H ( f ) ∗ S ( f ) {\displaystyle H(f)*S(f)} , which changes the relative magnitudes and/or angles (phase) of the non-zero values of S ( f ) {\displaystyle S(f)} .