This paper presents an acoustic method to characterise a blockage of an arbitrary shape in an air filled pipe. Two sets of experiments are conducted using the transient pulse signals. For the first experiment, the frequency range extends beyond the plane wave regime so that three first cross-sectional modes are excited and analysed with a horizontal microphone array in the absence and presence of a blockage. The 2D Fourier transform is then applied to transfer the acquired sound pressure data into the frequency and wavenumber space from which the modal reflection and transmission coefficients can be determined. For the second experiment, the microphone array is placed at a single position and the frequency range is below the first cross-sectional mode which makes the analysis relatively simple and robust. A set of equations is derived and employed to calculate the area ratio and the length of a blockage making use of a reflection coefficient from a blockage and a phase change between incident and reflected signals.