ON THE OUTPUT IMPEDANCE OF HORNS

In order to calculate the acoustic impedance at the input of a horn, t he impedance at the output has to be known. The purpose of this study is to determine the output impedance of real horns. To this end, measu rements were carried out on various exponential and catenoidal horns u sing a standing wave tube to give the complex input impedance as a fun ction of frequency. Starting from Webster's horn equation, transformat ion formulae are obtained which link the impedances at the output and input. In this way, the unknown output impedance may be calculated fro m the measured input impedance. The results obtained from this procedu re are compared to theoretical values for a piston in a plane or spher ical baffle. In addition, a simple mathematical description of the ave raged experimental data is given. This approximation can be considered a useful reference point for the boundary conditions at the terminati ons of horns which do not differ significantly in shape from those exa mined.