Ultrasonic thickness measurement of nonporous membranes with long wavelengths

A new thickness measurement technique is described in which air-coupled ultrasound is used to interrogate nonporous membranes at relatively long wavelengths. The thru-transmission technique compares a reference signal taken with the membrane absent to a material signal acquired when the membrane is present in the transducer's wave field. The technique uses both amplitude and phase information from the signals' frequency spectra to find the membrane's thickness along with its surface density. No a priori knowledge of the membrane's material properties is needed. The technique is experimentally verified by estimating the thicknesses of a number of metallic and non-metallic sheet-like materials. Thicknesses are typically measured to within 6% of their nominal values and the technique spans a range of roughly 10-100 μm.