Validation of a Method for Determining the Modal Composition of a Tonal Sound Field in a Cylindrical Duct Based on Synchronous Measurements in the Duct and the Far Field in Absence of Flow

A method has been developed for determining the modal composition of correlated sound modes propagating in a cylindrical duct, the peculiarity of which is that the effect of sound reflection from the open end of the duct is taken into account by numerical or analytical calculation of the reflection coefficients. This method was validated in the TsAGI AC-2 anechoic chamber, in which the sound field was created in the duct of a small-scale air intake model using 12 speakers at fixed frequencies; the modal composition was determined using 48 microphones installed flush with the duct walls, and synchronous measurements of the directivity of sound emission from the open end of the duct in the far field were carried out. In this case, on based on the found amplitudes of sound modes in the duct, the radiation characteristics in the far field were calculated using an analytical solution for sound emission from a semi-infinite cylindrical duct, the results of which were compared with the measurement results. It is shown that the amplitudes of sound modes found according to the developed method make it possible to reconstruct the far sound field characteristics with high accuracy in cases where the number of generated propagating modes in the duct does not exceed the number of installed microphones in the cylindrical array.