The characteristics of two types of ultrasonic acoustic emissions observed in laser material processing involving melting, vaporisation and plasma generation are compared. These acoustic emission sources are: (a) ultrasonic airborne acoustic emissions due to plasma/vapour generation (acoustic nozzle) and (b) acoustic emission on the back of laser beam guide mirrors (acoustic mirror) due to laser beam back reflection. Both experimental work and theoretical analyses have been carried out to clarify the nature of the signals. A new interpretation of the "acoustic mirror" signal generation is given. (C) 2002 Elsevier Science B.V. All rights reserved.