The defect free area in concrete allows sound waves to propagate in a straight line with the shortest distance traveled; When encountering defects such as holes, cracks, and loose layers, sound waves cannot directly pass through (defects are mostly gas/liquid media, with a large difference in acoustic impedance from concrete, which can cause reflection and refraction), and can only pass through the intact concrete around the defect, with the actual propagation path far greater than the geometric distance measured.
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