A method and apparatus is provided to analyze a dual wall structure having at least one hollow core therein and having the same unknown material sound velocities in walls and in post as well. A transducer is located in at least two different positions with respect to the structure, whereby the transducer propagates an ultrasonic wave toward the dual wall structure. A portion of the ultrasonic wave is reflected back to the transducer. One wave traverses a portion of the dual wall structure that has a core filled with a medium having a known material acoustic property. A second wave traverses a portion of the post that has an unknown material sound velocity. From time of flight measurements of the above mentioned waves, the wall thickness and core shift, if any, can be determined.