The transducers operate at a frequency of 54 kHz and were water- 

 proofed for underwater use. Two holders were designed and fabricated to 

 align and secure the transducers in the correct orientation for direct 

 and indirect transmission (Figure 8). Direct transmission requires that 

 the transducers be placed on opposite sides of the element. Indirect 

 transmission requires that the transducers be placed on the same side of 

 the specimen (Figure 9) . Both frameworks maintain the transducers a 

 fixed distance apart. In recent ultrasonic tests, only the indirect 

 transducer holder was used because the acoustic velocity of the sonic 

 signal across the wood grain is so similar to that across water.* 



Laboratory Test Preparations . Prior to each series of laboratory 

 tests, the following preparations were made: 



• The circumference of the test pile was divided and marked into 

 four quadrants. 



• The length of the pile was divided and marked into 3- to 6-inch 

 increments depending on the amount of damage. 



• A drawing was made of the test pile showing any external damage, 

 internal damage, or other pertinent information (damage measurements, 

 pile diameter, etc.). 



Description of Piles . Indirect ultrasonic laboratory tests were 

 conducted on piles with simulated and actual biological damage. Piles 

 with actual borer damage were obtained from San Diego and Hawaii (see 

 Figure 10). Those piles with actual borer damage were numbered: 5x, 

 4x1, 8335D and 8323E. The test pile with simulated borer damage was 

 referred to as the "standard" pile. Initially, the standard pile had 

 multiple small diameter (3/8-inch) holes drilled 21 inches deep in two 

 quadrants. However, these holes became enlarged and blocked with debris 

 after a relatively short period of time. 



Procedures. Following initial preparations, the transducer holder 

 (containing the TX and RX transducers) was placed against the timber 

 pile in the desired location. The transducer holder is designed to allow 

 the arms to be adjusted to the pile diameter. Although the transducer 

 holder is placed directly against the pile, the transducers themselves 

 remain approximately 1 inch away. Consistency between each series of 

 tests was achieved by beginning each test at the first marked line in 

 quadrant 1 and proceeding down the pile to the last line for each of the 

 quadrants. 



*This was explained previously in the section on Low Frequency 

 Ultrasonics and is shown in Figure 2. 



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