Validity of the Point Source Assumption of a Rotor for Farfield Acoustic Measurements with Shielding

Abstract

Many theoretical models of shielding by hard surfaces assume the source to be a point monopole source. If one is interested in shielding the noise of a rotor system by interposing a hard surface between the rotor and the observer, can the rotor system really be considered to be a monopole? If rotating noise sources are under consideration what is the effect of configuration and de- sign parameters? Exploring the validity of point source assumption alluded to above for a rotor for far field acoustic measurements with and without shielding forms the backbone of the present work. This paper will investigate the validity of acoustic shielding models that are developed under the assumption that the source is a monopole, compared with the shielding of real rotor noise sources. To the author's knowledge, the present research is the first ever work that seeks to answer the question of whether rotor noise from a propeller or a fan can really be approximated by a point source (monopole) as is done in theoretical calculations when characterizing the effectiveness of acoustic shields. The effect of sound scattered and diffracted by the shield placed around or near the actual source of rotor noise is investigated and compared with data acquired using a point source. Shielding configurations consist of rectangular plates and ducts of various dimensions.