Figure 7-23.  Corrosion of aluminum lightning protection mesh. Figure 7-24.  Tap test with tap hammer.  7-16 Automated Tap Test Audible Sonic Testing (Coin Tapping)  supports. Again, inherent in the method is the possibility  that changes within the internal elements of the structure  might produce pitch changes that are interpreted as defects,  when in fact they are present by design. This inspection  should be accomplished in as quiet an area as possible and  by experienced personnel familiar with the part’s internal  configuration. This method is not reliable for structures with  more than four plies. It is often used to map out the damage  on thin honeycomb facesheets.  [Figure 7-24] Sometimes referred to as audio, sonic, or coin tap, this  technique makes use of frequencies in the audible range  (10 Hz to 20 Hz). A surprisingly accurate method in the hands  of experienced personnel, tap testing is perhaps the most  common technique used for the detection of delamination  and/or disbond. The method is accomplished by tapping  the inspection area with a solid round disk or lightweight  hammer-like device and listening to the response of the  structure to the hammer.  [Figure 7-24]  A clear, sharp, ringing  sound is indicative of a well-bonded solid structure, while a  dull or thud-like sound indicates a discrepant area.  The tapping rate needs to be rapid enough to produce enough  sound for any difference in sound tone to be discernable to the  ear. Tap testing is effective on thin skin to stiffener bondlines,  honeycomb sandwich with thin face sheets, or even near  the surface of thick laminates, such as rotorcraft blade  This test is very similar to the manual tap test except that a  solenoid is used instead of a hammer. The solenoid produces  multiple impacts in a single area. The tip of the impactor  has a transducer that records the force versus time signal  of the impactor. The magnitude of the force depends on the  impactor, the impact energy, and the mechanical properties  of the structure. The impact duration (period) is not sensitive  Tap hammer Panel surface 38 mm (1.50 in) (approximately) 25 – 38 mm (1.00 – 1.50 in) (approximately)