A change in the cross-sectional area of a duct or pipe due to the presence of an obstruction will modify the eigenfrequencies of that system. A technique is presented for determining the blockage area function of a pipe or duct based upon the concept of these eigenfrequency shifts but using resonance and antiresonance frequencies determined from a single measurement of sound pressure within a duct. The technique is shown to be capable of revealing the location and size of single and multiple obstructions to a high degree of accuracy. The technique has also been successfully applied to the detection, location and sizing of small holes in duct walls. The position of a hole in the duct wall is revealed as the beginning of an apparent gradual expansion in the reconstructed blockage area function emanating from an initial positive dc shift. Once the hole has been located, an impedance model of the duct incorporating the size and location of the hole and the measured wave number at the first-order duct resonance allows the determination of the area of the side hole from a simple quadratic equation.