A simple theoretical model has been developed to describe the acoustic energy generated when a fine crack grows in a fatiguing rolling-element steel-bearing in the gearbox of a helicopter during flight. The acoustic-impulse＇s amplitude (at the bearing) and duration increase from 9.8 x 10(-12) to 29.0 x 10(-12) m, and from 0.1 to 1 mu s, respectively, as the crack growth becomes critical. The detection and measurement of this acoustic energy can lead to the prediction of the associated crack-growth.