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In this paper, we propose a new method that can inspect visual and non-visual features of objects simultaneously by using image and sound signal processing techniques. A method for discriminating a property of an object with the use of generated sound when striking it with a hammer is called a hammering test. This method can investigate non-visual features of objects such as inner structure of objects, e.g., the existence of defects and cracks inside objects. However, this method depends on human experience and skills. In addition, if we perform this test over a wide area of objects, it is required to manually record hammering positions one by one. To solve these problems, this paper proposes a hammering test system consisting of two video cameras that can acquire image and sound signals of a hammering scene. The shape of the object (visual feature) is measured by the image signal processing from the result of 3-D measurement of each hammering position, and the thickness or material (non-visual feature) is estimated by the sound signal processing in time and frequency domains. The validity of proposed method is shown through experiments
Yamashita et al. (Sun,) studied this question.