The Importance of Microphone Placement with Respect to the Test Article for Direct Field Acoustic (DFA) Testing – A Case Study

Paper presents Direct Field Acoustic (DFA) test results from three tests with Test Article (TA) and one without TA. Each used a microphone grid containing 25 equally spaced microphones, along its rows and columns, at three heights of 32”, 52” and 72” to measure the acoustic response in order to determine how well DFA testing compares to Reverberant Field Acoustic testing by determining the degree of diffuseness of the achieved DFA test’s acoustic field. The grid measured its uniformity, how well its SPL spectral responses agreed with its reference spectrum and how well the measured coherence between microphone pairs in the grid separated by 8” matched the ideal sinc2 characteristic of a diffuse acoustic field. DFA fields were created by MSI-DFAT’s 6x6 circularly arranged set of 8’ tall stacks of speakers and associated amplifiers that were controlled by a Jaguar MIMO system. Between the speaker stack circle and the Test Article (TA), there was a circle of 18 control microphones with randomized heights, angles, and average radial distances of 36” to the TA, which largely decouples them from the TA’s influence. The microphone grid was mainly located between the control microphones and TA at 10” away, whose near field was measured by 4 monitor microphones. The main subject of this paper is to characterize the effects of the TA’s near field on the grid’s response.