This method is fairly standard for tracing non-metallic water lines and for leak detection. A pipe under rapidly varying mechanical stress may deform and generate an acoustic emission, which can be monitored by pickup transducers. The basic premise is that the sound will be loudest directly over the pipe because the elastic wave travel distance is the shortest at this point. However, the type of surface material, density of the in-situ soil and interferences from external sources can distort the sound distribution. Application of acoustic-based technology to utility locating includes active sonics, passive sonics and resonant sonics. Active sonics includes the induction of a sound onto a pipe. Tracing the utility is accomplished by marking the loudest points at the ground surface. Most of these devices were originally developed for the water leak detection industry, but have been utilized by some in the utility detection business. On the other hand the passive sonic technique relies upon the product being able to escape the pipe. The resulting vibration is carried along the pipe allowing detection. Low ambient noise, shallow pipes, smooth stiff ground surface, and high fluid pressure represent optimal conditions for this technique. The resonant sonic techniques rely upon the generation of a pressure wave in the pipeís fluid to form vibrations in the pipe that can be detected. It has the advantage of being able to tune the oscillatorís frequency to the resonant frequencies of the pipe. It is limited to non-compressible fluids and relatively small wave intensities to avoid joint damage.