'What do those letters mean?'

摘要

Fast Fourier Transform - is a mathematical technique for converting a signal from the time domain into the frequency domain. Signals on a flaw detector, on an oscilloscope or on an acoustic emission display are typically time domain signals showing how the amplitude varies with time. When transformed into the frequency domain the display shows how the amplitude varies with the frequency. This display is often referred to as the frequency spectra of the signal. A sine wave of a single frequency in the time domain will give a single line at that frequency in the frequency domain. A spike in the time domain will give a spread of frequencies in the frequency domain. See the Figure. The spike is made up by adding the sine waves of all the different frequencies - at one point in time they all add together to give the spike whilst at all other times they all cancel out to give zero signal. The inverse of the FFT converts back from the frequency domain to the time domain.