Time of arrival discrimination of interfering waves for thickness measurements in composite media

Ultrasonic pulse-echo techniques used to measure the thickness of rather inaccessible structures give poor results in the case of composite materials because of the very low working frequency. A technique based on the higher-order derivatives of the received signal, together with a proper choice of the central frequency of the driving tone burst in relation to that of the transducer, seems capable of discriminating the time of arrival of a wave masked by another. This technique can be applied in all cases in which more interfering waves are present. Only the time of arrival of the second wave (as well as that of the first one) can be detected unambiguously. A relevant role seems to be played by the phases of the higher spectral components of the received signal. A more accurate choice of the central frequency of the driving burst must be performed in order to optimize the ratio between the amplitudes of the spectral components of the received signal