The Transmission Line Matrix (TLM) method can be used to solve the wave equation numerically. It has been applied very widely in electromagnetics, less in acoustics. Propagation is modelled by pulses travelling through a grid or matrix of ideal transmission lines, thereby discretising the space of interest. During each time increment the pulses are assumed to travel from node to node in the grid, where they scatter according to a simple pattern. If the mesh is fine enough, the process models the wave equation. The focus of this paper is on the TLM modelling of acoustic devices such as microphones, in which mechanical-acoustic interaction plays a central role. A computer model of the device is easily built up by combining rectangular TLM building blocks or arrays. Along each array boundary there is either a further TLM array, interacting with it, or a defined boundary condition, passive or active. Passive boundaries range from total reflection to open space, or total absorption. An active boundary, such as a microphone membrane, has its own inherent dynamics which interact with the acoustic field; thus it is both driving the acoustic field and is driven by it. This coupling is successfully modelled. To allow modelling of finer details where necessary without excessive increase in the computational burden, the TLM mesh density in some arrays can be increased while still interacting smoothly with neighbouring arrays. The ultimate aim is a flexible design tool which would predict the effect of design changes on device performance. Considerable progress has been made in developing a useful model and verifying it using analytically solvable cases. 1 The TLM method for modelling wave propagation