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.