a. Acoustic signature12

¹² Philippe Eberlin. "Underwater acoustic identification of hospital ships''. IRRC, No. 267, November-December 1988, pp. 505-518.

This method is widely used by naval forces to identify ships belonging to their own forces or to friendly forces. Its basic principle relies on the monitoring and analysis of the sound produced when a ship is under way, especially by its main and auxiliary engines, the propeller revolving, and so forth. The combination of these noises constitutes the ship's "acoustic signature". Each ship theoretically has its own unique acoustic signature, a sort of sonic fingerprint which can be used for identification purposes.

However, ships of identical design, built by the same shipyard but sailing under different flags, may have almost identical characteristics and thus acoustic signatures which are so similar as to be very easily confused.

Moreover, a ship's acoustic signature is not immutable. When a ship's load changes, so does its draught; this alters the acoustic signature, as does the ship's age and any damage or modifications made to it. Some experts believe that the acoustic signature should be measured and recorded every six months to make reliable identification possible. The identification is made by comparing the signal recorded by means of hydrophones with a pre-recorded specimen signature.

Acoustic signature is established by recording the noises produced by a ship when it performs manoeuvres in a basin especially fitted for the purpose. The operation requires complex installations and sophisticated measuring and recording instruments. This kind of installation is usually available only in countries which have a well-developed navy and are familiar with the technology involved. In time of war it might therefore be extremely difficult if not impossible for a ship from a small, neutral country, being used as a medical transport, to have its acoustic signature recorded and then to communicate it to the belligerents.

Given the complexity of establishing an acoustic signature and the uncertainties involved in its propagation at sea, only well-trained specialists with sophisticated equipment can make a reliable identification.