0 1 2 3 4 5 6 7 8 9 10 10 9 8 7 6 5 4 3 2 1 0 0 1 2 3 4 5 6 7 8 9 10 10 9 8 7 6 5 4 3 2 1 0 Through transmission ultrasonic (TTU) hand held Pulse echo–normal Pulse echo–delamination DEPTH SIGNAL STRENGTH DEPTH SIGNAL STRENGTH Through transmission ultrasonic (TTU) water yoke 7-17 Figure 7-25.  Ultrasonic testing methods. Through Transmission Ultrasonic Inspection Through transmission ultrasonic inspection uses two  transducers, one on each side of the area to be inspected. The  ultrasonic signal is transmitted from one transducer to the  other transducer. The loss of signal strength is then measured  by the instrument. The instrument shows the loss as a percent  of the original signal strength or the loss in decibels. The signal  loss is compared to a reference standard. Areas with a greater  loss than the reference standard indicate a defective area. Ultrasonic inspection has proven to be a very useful tool  for the detection of internal delaminations, voids, or  inconsistencies in composite components not otherwise  discernable using visual or tap methodology. There are many  ultrasonic techniques; however, each technique uses sound  wave energy with a frequency above the audible range.  [Figure 7-25]  A high-frequency (usually several MHz) sound  wave is introduced into the part and may be directed to travel  normal to the part surface, or along the surface of the part, or  at some predefined angle to the part surface. You may need  to try different directions to locate the flow. The introduced  sound is then monitored as it travels its assigned route through  the part for any significant change. Ultrasonic sound waves  have properties similar to light waves. When an ultrasonic  wave strikes an interrupting object, the wave or energy is  either absorbed or reflected back to the surface. The disrupted  or diminished sonic energy is then picked up by a receiving  transducer and converted into a display on an oscilloscope or  a chart recorder. The display allows the operator to evaluate  The repair technician must realize that the concepts outlined  here work fine in the repetitious manufacturing environment,  but are likely to be more difficult to implement in a repair  environment given the vast number of different composite  components installed on the aircraft and the relative  complexity of their construction. The reference standards  would also have to take into account the transmutations that  take place when a composite component is exposed to an  in-service environment over a prolonged period or has been  the subject of repair activity or similar restorative action. The  four most common ultrasonic techniques are discussed next. the discrepant indications comparatively with those areas  known to be good. To facilitate the comparison, reference  standards are established and utilized to calibrate the  ultrasonic equipment.  to the magnitude of the impact force; however, this duration  changes as the stiffness of the structure is altered. Therefore,  the signal from an unflawed region is used for calibration,  and any deviation from this unflawed signal indicates the  existence of damage.  Ultrasonic Inspection