Abstract
A new method utilizing sound is proposed for a non-destructive test of pipes. Random noise is transmitted from one end of a pipe and the sound pressure is picked up by a microphone at the other end. The output signal of the microphone is pre-whitened by an inverse filter and processed to calculate the partial auto-correlation (PARCOR) coefficients; the inverse filter which compensates the effects of the sound source and the microphone can be obtained from the PARCOR coefficients which are calculated in advance from the microphone signal for a straight pipe. Since the PARLOR coefficients give the set of reflection coefficients of a nonuniform acoustic tube represented as a series of sections with uniform area and equal length, the cross-sectional area function of the pipe is determined from the calculated PARCOR coefficients. An experimental apparatus was built and model experiments were performed for several pipes with an artificial defect, including a pipe with a partial plugging, a pipe with an extension of cross-sectional area and a pipe with an orifice in the wall. The results show that the cross-sectional area functions of pipes can be estimated by the proposed method.
