Application of the Odeon program in underwater acoustics

A cooperation between the Department of Acoustic Technology (AT) and the Institute of Naval Research in Norway was arranged in the autumn of 1997. The issue was to apply the ODEON program, which has been developed for room acoustic calculations, to underwater acoustic modelling of Norwegian fjords. Introducing a sound velocity of 1500 m/s and a frequency dependent absorption coefficient associated with the other medium is the only change made in ODEON. The Norwegian fjords are approximately
500-1000 m wide, and the maximum depths can range from 100 to 600 m, which in an
underwater acoustic context can be regarded as shallow water conditions. Although the medium is not homogeneous (temperature and salinity vary with depths and position and therefore the speed of sound will do the same), the relatively small distances in the
geometry make it acceptable to omit refractive effects.
AT has received raw data from acoustic measurements made in some fjords. However, because the original excitation was not available, it was necessary to solve the particular problem of determining the impulse response of a system from the response of the system to a swept sine signal that is not available. A method of synchronising the excitation and the response by maximising the coherence was developed, tested on various known systems, and finally applied on real data from the fjord. The resulting impulse responses made it possible to calculate the reverberation time.
The last phase of the project concentrated on modelling the fjord with the room acoustic calculation model Odeon. Since Odeon has been developed for a quite different purpose, the main objective was to determine the Odeon parameters that give the best agreement with the experimental results (number of rays, reflection order etc).