<p>A fundamental computational and experimental study of a monopole in an aluminium duct in water is presented. This is a precursor study before investigating a propeller in a duct that can be modelled as a rotating dipole, e.g. Tomko et al. (2019). Updates to our previously published method of images duct model (Fischer et al., 2022) are detailed along with a new transmission matrix model that captures solely the longitudinal modes of the duct. Predictions from these analytical models are compared with predictions from boundary element and finite element numerical models and also experimental results. The hydroacoustic experiment is conducted in the water tank located at Defence Science and Technology Group, Melbourne. The numerical simulations and experiments demonstrate that the vibro-acoustic coupling of the duct wall with the fluid leads to a change in the effective speed of sound in the duct and consequently the resonant frequency of the system. Therefore, using a rigid duct approximation would lead to an over prediction in the effective speed of sound in the duct and, for the example presented in the paper, an over prediction in the relative gain in sound power from placing a monopole in the duct. The method of images and transmission matrix models are shown to accurately predict the relative gain in sound power for a rigid duct. The current experimental set-up requires a source with greater acoustic power to allow future investigation of duct cross modes and dipole sources.</p>