Validation of brass wind instrument radiation models in relation to their physical accuracy using an optical schlieren imaging setup

The complexity of brass instrument radiation theory has led to a large number of hypotheses and simplifications, which allow for the development of computable models. Most of these, used in the calculation of input impedances, sound simulation and instrument auralisation amongst others, are yet to be validated experimentally. Schlieren imaging techniques permit visualisation of pronounced gradients in the refractive index of air, caused in turn by sharp changes in the density of air, such as the ones which occur during non-linear sound propagation in some loudly played brass wind instruments. In this study, a Schlieren optical setup is used to visualise the shape of the wave fronts radiated by a group of brass instruments with different degrees of flaring in their bells. The geometry of these wave fronts, and its evolution after leaving the instrument, is characterised and related to the flaring parameters for each instrument, providing grounds for evaluating the physical accuracy of existing models.