Tv-Holography Used to Study the Mutual Acoustic Coupling Between Neighbouring Elements in a Passive Underwater Acoustic Array

In an underwater acoustic array, the piezoelectric transducer elements will always interact.There will be a mutual acoustic coupling between the elements through the acoustic medium, and through a possible acoustic window.The mutual acoustic coupling is dif cult to calculate and it is therefore also hard to predict the behaviour of the array.The coupling has, however, been calculated for Special cases , most often for active transducers with relative simple shapes and few elements.A rigid and infinite baf e is also often assumed.In this paper we will use experimental methods to study the mutual acoustic coupling of a passive four-element array at different frequencies.The coupling between pairs of elements will be computed, based on TV-holographic measurements of the surface displacement.TV-holography, or Electronic Speckle Pattern Interferometry (ESPI),is a full eld interferometric measuring technique used to observe and detect small mechanical vibrations 7 in "real time" on a video monitor.The monitor image of a test object will be covered by a fringe pattern providing information about the vibration.When sinusoidal vibrations are studied in standard operation mode (time average recording), the fringe patterns are directly dependent on the vibration amplitude.