Anechoic Duct Termination Development Using Scale Model Theory

Abstract

A 10 000 cfm aerodynamic and acoustic fan test facility required development of an anechoic section to facilitate in-duct sound testing. The resulting design incorporates sound absorbing material to reduce the plane-wave resonance effects within the test duct while permitting efficient air flow passage in either axial direction. This anechoic section is located between the end of the test duct and a 180° full radius elbow. The aerodynamic measuring section and auxiliary exhaust supply are connected to the elbow and physically located above the test duct. Using a one-to-four scale ratio, the 18-in. diam test duct was simulated by a 4.5-in. diam pipe. Conical, exponential, and catenoidal horn flaring geometries were reviewed as possible candidates for the termination's external configuration. The design value for the ratio of the maximum to minimum sound pressure occurring in the model test section as a result of the standing wave formed by incident and reflected plane waves was specified as 1.5 bB for the model frequency range of 240–1600 Hz. Results of the model test study indicate that an anechoic section having a catenoidal horn profile will provide satisfactory performance. The optimum acoustical performance (minimum SWR) was obtained with a sound absorbing material filling the termination cavity except for an aerodynamic passage through the center of the horn. Performance of a full scale design is compared with the model.