Measuring the Sound Attenuation of Space Absorbers for Jet-Engine Test Cells

Abstract

The space absorbers were long hollow cylinders of perforated metal, filled with glass fiber; their profile was streamlined for airflow parallel to the axis of the cylinders. The absorbers were placed in a hollow concrete pipe which was then used like an impedance tube except that it was rigidly terminated at the end, and the variable measured was the absorption of sound waves in passing along the tube. The direction of propagation of sound was parallel to the axis of the cylinders so that absorption occurred at grazing incidence. For octave bands of noise, the interference pattern given by theory for the mean squared pressure is 〈P2〉Av = cosh 2ax + (cos3kxsinkx)/kx, where k is the wave number and x distance along the pipe measured from the reflecting end. Experimental results were obtained which followed this pattern, and the required sound attenuation factor a was then obtained by a graphical method. Results of reverberation room absorption measurements on some of the cylinders will also be presented.