A time-domain numerical model of brass instrument sound production is proposed as a tool to predict their brassiness. It is based on the generalized Burger’s equations which can be used to model weakly nonlinear wave propagation in non uniform ducts, as described in [1] where a frequency domain tool has been presented. The relevance of the present time-domain tool is evaluated by simulating idealized cases for which the results are theoretically known, and by doing preliminary simulations compared with previous results obtained for periodic regimes of brass instruments using the frequency domain tool.