A wall-mounted source array for the excitation of incoherent broadband sound fields with prescribed modal distributions in ducts

In the analysis of broadband sound fields in ducts, for example, turbofan engines, large exhaust stacks, and exhaust mufflers, the assumption of ‘Equal Energy per Mode’ (EEpM) is frequently made. The practical realization of such a sound fields is valuable as a means of, for example, allowing liner attenuation measurements obtained from measurements on different test rigs to be compared directly, or for allowing measurements results to be compared with computer predictions in which the assumption is made. This paper describes a technique in which arrays of sound sources at the wall of a duct are driven by white noise signals to generate a sound field of prescribed modal energy distribution and modal coherence. The number of sources required for effective mode synthesis and the source geometry are also discussed. An example is presented in which ‘EEpM’ broadband sound field is generated up to a maximum non-dimensional frequency of ka=20 using 152 sources.