with a sensitive transducer for the rasping sound produced by the organ- 

 isms as they bored into the wood. As a follow-on to this work, NCEL 

 sponsored a project to determine the feasibility of using this technique 

 to detect the presence of marine borers in timber piling. In the first 

 phase of the work, isolated specimens of Bankia and Limnoria were col- 

 lected and their characteristic sound spectra recorded in the laboratory. 

 It was hoped that the sounds made while boring would be unique in spec- 

 tral content and could, therefore, be distinguished from ambient back- 

 ground noise present in all waterfront environments. 



Test results revealed that the sonograms of isolated mollusks and 

 gribbles were almost identical to the ambient noise recorded in Monterey 

 Harbor. In his final report (Ref 5), Professor Haderlie concluded that 

 "...the natural sound of barnacles and other foulers on a piling are so 

 diverse and complicated that they mask any borer sounds coming from 

 within the piling and we have been unable to filter out the extraneous 

 sounds which might make it possible to detect borers in a wooden harbor 

 structure." 



Low Frequency Ultrasonics 



In an early state-of-the-art survey (Ref 6) low frequency ultrasonic 

 NDT was identified as having the greatest potential for improving the 

 Navy's ability to accurately evaluate the integrity of wooden waterfront 

 structures. This NDT method was selected for further evaluation because 

 it is known to penetrate through wood, is not hazardous to work with, 

 and can be readily used in an underwater environment. 



Low frequency ultrasonic inspection is based upon the influence of 

 the test specimen on the propagation of a known sound wave. In flaw 

 detection, the transit time of an ultrasonic pulse traveling through a 

 test specimen with a fixed path length is measured. Dividing the path 

 length or the separation distance between two transducers by the transit 

 time determines the acoustic velocity. Solid homogeneous materials have 

 a constant acoustic velocity. Therefore, uncharacteristic changes in 

 the pulse velocity in these types of materials are due to defects, such 

 as cracks or voids, which either delay or accelerate the received signal. 



In nonhomogeneous materials, acoustic velocity varies locally due 

 to natural changes in the microstructure such as grain orientation in 

 wood. Although nonhomogeneous materials do not have a constant acoustic 

 velocity, an average acoustic velocity can be obtained, for instance, 

 for a given wood grain direction in a given specimen. A deviation from 

 the average acoustic velocity greater than the deviations caused by the 

 nonhomogeneity of the material itself signifies an "uncharacteristic" 

 change in pulse velocity and, therefore, material properties. Use of 

 ultrasonics is based on relating the uncharacteristic sonic signal to 

 the condition of the structure. 



Low frequency ultrasonic inspection of nonhomogeneous materials 

 (wood) uses two transducers in a through-transmission mode, with one 

 transducer acting as the transmitter and the other as a receiver. In 

 contrast, high frequency ultrasonic inspection of homogeneous materials 

 (metals) uses one transducer that acts as both transmitter and receiver 

 in a pulse echo mode. 



