THE ACOUSTIC POWER RADIATED FROM THE OUTLET OF A HARD-WALLED CIRCULAR DUCT THEORY AND MEASUREMENT

The paper presents an approach to the phenomena occuring in a hard-walled cylindrical duct when more than one mode is excited, by means of a model assuming equipartition of energy between the consecutive modes and phases being random variables. The excitation is harmonic and there is no mean flow. The power output and its distribution in the far field described by the power-gain function is discussed. The expected value, the variance and the normalized standard deviation for the power radiated outside are derived and compared with the experimental data obtained by means of three different methods: the intensity measured on the duct cross-section, the intensity measured in the far field outside the duct and the mean square pressure measured as previously. The diffraction parameter Ica was chosen so as to admit at least two modes in the incident wave. The experimental data agree well with predictions of the theoretical model and so in a way that confirms its correctness.