Defect identification using TNAH: An exploration

The main objective of this paper is to investigate the possibilities to identify defects on structures using the newly developed TNAH method (Transient Near-field Acoustical Holography, or Transient NAH). TNAH is a specific NAH formulation for transient cases. It uses a Laplace transform in the time domain instead of a Fourier transform: the Laplace transform handles delays more efficiently and eliminates time aliasing problems. The new formulation provides precise observation of transient acoustic fields and opens new possibilities for the technique. The method was applied to 50cmx30cmx6mm Plexiglas plates with macroscopic defects. The defects considered were a local added mass and a small hole. It is possible to observe differences in the sound fields radiated from the modified plate and the reference plate. The comparison of time pressure signals of points close to the defects and symmetric points shows amplitude differences and delays. Although it is not possible so far to precisely localize the defects on the plate, the possibility of improving the detection capacity to the microscopic level is discussed.