Presentation of the company Marine acoustics Access plan - Principle - Operation of underwater acoustic systems - Future developments       Principle Most telecommunication systems (telephone, radio, television) use the propagation of electromagnetic waves, of the same characteristics as light, as a means of transmitting information. However the underwater environment almost completely negates the use of electromagnetic waves, because water, due to its dispersing characteristics linked to its high conductivity, causes such a high attenuation that they are rendered useless. The only vector enabling information to be transmitted underwater that has reasonably exploitable characteristics is acoustic waves, these are mechanical vibrations in the propagation environment. These vibrations, characterised by their frequency (number of vibrations per second, expressed in Hertz) propagate in sea water in a very favourable manner, water being an almost uncompressible material ; it is to be noted that the absorption of sound increases very rapidly with the frequency. The ease with which acoustic waves propagate in water has been established for a long time, but the use of this characteristic is much more recent, as the first viable, practical applications only appeared at the beginning of the last century with Paul Langevin’s experiments. In 1917 he established the basis of modern underwater acoustics, recommending the use of ultra-sound waves : mechanical vibrations of materials at frequencies inaudible to the human ear (>20,000 Hz). Ever since, their applications have continued to develop, and one can say that underwater acoustics today allows the ocean to cover the essential role played by radar waves in the atmosphere, and radio waves in space. • Detection and location of obstacles or targets : this is the primary function of sonar systems, particularly concerning military applications in the hunt for submarines and mines, and also for the fishing industry. • Measurement of the characteristics of the marine environment • Signal transmission UNDERWATER ACOUSTICS : Techniques using sound waves to transmit and receive information in the marine environment SONAR : Sound Navigation and Ranging                    Navigation and Acoustic distance measurements The detection and the location of a target using a sonar system is based on the reception : • Either of the echo of a signal transmitted by the system and reflected on the target (active sonar) :    → the reception occurs on an antenna, often made up of several sensors (hydrophones) • Or directly, using the noise radiating from the target (passive sonar)   Operation of underwater acoustic systems For transmission as for the reception of underwater acoustic signals, electro-acoustic transducers are indispensable to convert (as microphones and loud-speakers do in air) an electrical energy into acoustic energy, or the opposite. The ASM transducers use piezoelectric (or sometimes magneto-strictive) characteristics of certain crystals existing in nature or obtained artificially (piezoelectric materials). An electrical (or magnetic) field applied to these materials enables deformations to be imposed and their modulations reproduce those of the stimulator; these deformations generate an acoustic wave in the propagation environment. The opposite effect is used in reception. In general one tries to use piezo-electric ceramics at around their frequency of resonance to obtain the best possible output. The technology that is used the most in underwater acoustic transducers is that of Tonpilz : a stack of piezoelectric rings are prestressed under high static pressure imposed by a pre-stress bolt ; this stack is part of a head (balanced with a rear mass at the other end) which transmits the vibrations induced by the application of a stimulatory electrical field. With very high resonance, these systems enable high transmission levels to be reached but in return the bandwidths are low. In the area of components, magneto-strictive materials, that obtain better results but are expensive, have never managed to be incorporated other than in very particular applications. However the recent emergence of receivers in PVDF, a piezoelectric material, facilitates the manufacture of surface antennae of diverse forms and dimensions, well adapted for installation onto the hulls of submarines and ships, and which have found many interesting applications in the field of passive sonars.   Future developments The key factor in the current advance in underwater acoustics is due to the extraordinary development of computer technology : numerical treatment of signals enables the capacity of sonars to progress considerably, due to incomparably high calculation capacity and the outstanding tools software. The industrial and scientific applications of underwater acoustics are thus becoming more and more numerous, diverse and outstanding. The technology used for ocean exploration and exploitation has today become indispensable : the tool most selected for communication in the underwater world. These technologies, originally used by the military, are to be found more and more in both professional and civil fields.     A subsidiary of Thales Underwater Systems © Thales Pons | Legal Mentions | Credits