A novel technique for the measurement of the acoustic properties of a thin linear-viscoelastic layer using a planar ultrasonic transducer

Abstract

Thin linear-viscoelastic layers such as films and coatings have many applications. Simultaneous measurement of the multiple acoustic properties of a thin layer is of great importance in ensuring its quality. In this paper, a novel technique is proposed for the simultaneous determination of the three acoustic properties of a thin linear-viscoelastic layer, namely the acoustic impedance Z2, the time-of-flight ?t2 and the attenuation coefficient ?2. A planar ultrasonic transducer, at normal incidence, is used to interrogate the thin layer, and all the reflections are received by the same transducer. Firstly, an optimal estimate of Z2?is obtained by subtracting the echo from the front surface of the thin layer from all the received echoes. After this determination of the acoustic impedance, optimal estimates for ?t2?and ?2?can be found if the echo from the back surface of the thin layer is subtracted without any remainder. This technique avoids the convergence problem that is frequently encountered in the traditional measurement techniques based mainly on fitting the experimental reflection spectrum to a theoretical model. The effectiveness of the new technique is firstly confirmed by numerical simulation and then by experimental application to four thin linear-viscoelastic layers. Experimental results show that Z2?and ?t2?can be obtained accurately while the error in determining of ?2?is relatively large due to the insensitivity of the ultrasonic echoes to that property. The relative uncertainties in determining the three acoustic properties Z2, ?t2?and ?2?in our experiments are around 1.2%, 1.5%, 5%, respectively, mainly arising from the principle of this measurement technique.