This work is concerned with sound propagation, reflection and absorption in air-filled pipes with inhomogeneities such as an open end, lateral connection, cross-sectional change or porous termination. The focus in this study is made on the sound intensity which is a vector quantity. Because the sound intensity is treated as a complex vector, it gives more information about the local net transport of energy (real part) and local oscillatory transport of energy (imaginary part) and its direction. This provides a possibility to obtain more information on the nature of inhomogeneities in the pipe and their extent. An attempt is made to study the sound intensity in a pipe transmitted through higher-order modes. The results of this work are applied to measure the acoustical properties of porous media and living plants. It is shown that the frequency range of the standard ISO 10534-2 method can be extended significantly to measure the absorption and reflection from a large, inhomogeneous material specimen such as a living plant. The presented experimental data are supported by the hybrid numerical method which is based on the normal modal decomposition and finite element modelling, analytical methods which are based on normal mode decomposition alone and models for the acoustical properties of porous media. It is shown that accurate measurements of the sound intensity in a pipe are problematic. Therefore, this work presents experimental and theoretical evidence together with a sensitivity analysis and discussion on the applicability of this technology for the characterisation of conditions in air-filled pipes.