Linda Brunet (asker) May 23, 2010:

Excellent explanation, thanks to you both.

chris collister May 22, 2010:

Signature Every machine has a "signature", which may be vibrational, acoustic, related to elements found in oil analysis, etc. Submarine and aircraft detection and classification in particular relies heavily on individual acoustic signatures. Similarly for turbomachines, where the (usually vibrational) signature when new is compared with the actual signature throughout the life of the machine.