1. A method of detecting an abnormal signal in a compound sampled signal recorded in the time domain, the method comprising: dividing the sampled signal recorded in the time domain into a plurality of sample segments; transforming each of the plurality of the sample segments from the time domain into the frequency domain to determine a plurality of transformed sample segments, each transformed sample segment having a plurality of frequency points, each frequency point having an amplitude associated with a certain frequency; for a frequency point in a given transformed sample segment, determining a ratio by dividing the amplitude of the frequency point by a value indicative of an average of the amplitudes of the frequency points at the same frequency across a plurality of transformed sample segments; repeating the determination of a ratio for a plurality of frequency points in each transformed sample segment to determine a plurality of ratios for each transformed sample segment; repeating the determination of a plurality of ratios for a plurality of transformed sample segments; and using the ratios to detect the abnormal signal in the compound sampled signal.