Measurement of the Sound Reduction Index as a Function of the Incidence Angle by Two Different Methods

Abstract

Generally sound reduction index R is measured for an incident diffuse field, and does not take into account the angular dependency. However, the variation of reduction index with direction of incoming wave can be significant, and it may be necessary to consider the angular influence in acoustical transmission problems, especially when considering directional noise sources. This paper presents two experimental methods, developed to measure the sound reduction index as a function of the incidence angle. Two set-ups, implemented in standard acoustical transmission laboratories, have been tested. In the first system, the vibrating wall is excited by a sound source which simulates a plane wave, with a given incidence angle. This incident field is generated by an acoustical horn. The reduction index R(θ) is obtained sequentially, for different angular positions of the source, from both the measurement of the outside wall pressure level and the transmitted pressure level. In the second system, the studied structure is excited by a diffuse field, and a wave by wave decomposition of the radiated intensity yields the reduction index as a function of the angle of incidence. The decomposition of the radiated intensity is based on the use of an acoustical holography technique, NAH-Phonoscopy. The mathematical formulation of the two methods is detailed. Experimental results are presented and compared to theoretical data. Then, the advantages and limits of each system are discussed.