Understanding Acoustic Emission Testing- Reading 1 Part B-A

Fourier-Transform (FT)
The Fourier theorem states that any waveform can be duplicated by the
superposition of a series of sine and cosine waves. As an example, the
following Fourier expansion of sine waves provides an approximation of a
square wave. The three curves in the plot show the first one term (black line),
four terms (blue line), and sixteen terms (red line) in the Fourier expansion.
As more terms are added the superposition of sine waves better matches a
square wave.
Charlie Chong/ Fion Zhang
http://www.tissuegroup.chem.vt.edu/chem-ed/data/fourier.html