###### Abstract
The most promising methodologies are the non-invasive investigations based on the extent of the cover's surface vibrations, adopting different modalities both to excite surfaces and to measure the signal of interest [1; 2; 3]. One of the techniques of detection based on the vibration of the covers is the impact test [4; 5]: the surface is hit with a small \"hammer\" that can measure the intensity of the pulse, using a force sensor; this method is used to measure the impact force and the vibration rate of the surface, and examining differences in mobility due to localized damage, it is possible to diagnose the defect. Tap testing (also called the coin tap test, CTT) [6; 7] is a simple but effective impact-based NDT (Non-Destructive-Technique) technique that gives results from local excitation. This technique has been in use for a long time in the aerospace industry for the testing of composite materials. Over the years, it has evolved from hearing-based coin tap testing to an instrumented testing capable of producing \"images\" with quantitative detection of sub-surface discontinuities (delamination, unbound, and voids) in thin composite laminates, sandwich structures, and bonded joints [8; 9]. Coin tap testing involved the tapping of a coin or a hammer on the surface of the test object whose condition was assumed depending on the type of sound it made. Solid high-pitch sound would suggest that the point is in a good condition whereas a \"dull\" sound suggested a bad condition. Although simple and cost-effective, this method could only provide qualitative data and was entirely dependent on the operator's hearing ability and was influenced by the background noises. To eliminate these drawbacks, the technique was developed into instrumented devices [8]. Three major commercial devices to perform digital tap testing (DTT) were developed thereafter: the Rapid Damage Detection Device (RD3) by WichiTech Industries [9], the Woodpecker developed by Mitsui Industries [10], and the Computer Aided Tap Tester (CATT) by Iowa State University [11]. The techniques used in these DTTs involves tapping the test surface with an impact hammer or tapper, for which the contact time (impact duration) is measured. The contact time can be measured by mounting an accelerometer or a force transducer into the tapping mass. The force-time pulse received by the accelerometer after each tap is converted into a voltage pulse. The width at half-amplitude, in other words the contact time, is measured by a logic circuit and displayed in the numerical form [9]. In this case, the half amplitude width is taken because the force-time curve is not defined at the bottom for the full amplitude width due to inherent noise. The presence of defects will decrease the local stiffness of a structure and produce a wider force-time pulse or show a longer contact time. The test surface is analyzed by comparing the contact time or determining the deviation of the force-time pulse width from the nominal width obtained from a reference undamaged surface. Hence, a sound of a rigid area will give lower reading whereas a damaged area will give a higher reading. It is generally understood that [12] the impact sounds can be broadly divided into two areas: (a) sounds generated by the initially high surface acceleration and deceleration of impacting bodies found during the time of contact, and (b) sounds arising from the free-vibration of the bodies following the impact. We use the same method, but instead of measuring only the duration of the force-time pulse (as said, the width at half-amplitude), we try to model the same method used by professional restorers, recording the noise produced by the impact and trying to correlate it with a reference one. They make the surface under examination slightly vibrate by knocking on it with the knuckles of the fingers, producing a sound, and mentally perform a kind of correlation between the sound signals respectively of a point that they judge in good condition compared to another that may not be.