Detection was initially achieved utilising filters to separate the normal background noises from the sound of a tube leak. With the advent of modern digital sound analysis techniques - namely Fast Fourier Transforms (FFT) - frequency spectrums are generated for each audio sensor. The analysis of these spectrums for a combination of frequencies determines the value of the alarm in percentage when a sensor channel is above a pre-set alarm value for a pre-set time when the leak alarm relay is activated. The computer scans up to 32 sensor points situated around the boiler. Each point is analysed for the sound made by escaping steam. A detected leak is graphically displayed on screen in the form of an audio spectrum and an alarm Bar Graph.