Source level and Urick

Abstract

Source level (SL) is the first term in Bob Urick’s sonar equation, and it represents the root-mean-square pressure of sound radiated at the target, in decibels. The fundamental measure of source level is the signal-energy level (SEL) represented by the time integral of sound pressure squared, to which the detection index is proportional, in decibels. Despite using a form of sonar equation more natural for long-pulse transducers than for impulsive ones, he only described explosives in detail in his chapter on generation of underwater sound. When ‘‘Principles of Underwater Sound for Engineers’’ was published, there was a tendency to use impulsive sound sources to study sound interactions with the sea and to use narrow-band long-pulse transducers in sonar systems. The diminution of this tendency has been a very important development since Bob’s book was written, and it has occurred because of improvements in both kinds of transduction. Transducer bandwidth has been increased, both by materials with larger electromechanical coupling coefficients and by innovative designs. These include ‘‘hybrid’’ transducers that combine magnetostrictive and electrostrictive actuators and projector arrays designed to have no mechanical reactance. Improved impulsive sound sources such as air guns, sparkers, and explosive arrays now find sonar use.