Feasibility of ultrasonic spectral analysis for detecting insect damage in wooden cultural heritage

In cultural heritage, insect damage is one of the most serious damages, but conventional ultrasonic methods could not detect the early stage of insect damage because it is too small. In order to detect such small size damages, the analysis of attenuation is required, but elastomeric couplant and inconsistent contact condition of transducer make it difficult, in which only the elastomeric couplant is allowed to be applied to cultural heritage because of the paintings on the surface and the pressure employing transducers make effects on the attenuation measurement. Therefore, this study was aimed to investigate if the ultrasonic spectrum analysis can detect internal small hole, in which ultrasonic test was conducted with varied contact pressure. In this study, the diameter of drill hole was only 3 mm and this experiment was carried out under severely varied contact pressure. Nevertheless, spectral analysis with 2nd derivative pretreatment (Root mean square error of prediction, RMSE: 1.609) predicted the number of holes with much higher accuracy than the conventional methods (RMSE: 5.925). This result indicates that the spectral analysis has a high possibility in detection of insect damage in cultural heritage, even though contact condition is not consistent.