Schematic of the test-section and detailed view of the resonator neck. (a) Schematic of the experimental setup with source-side microphones (A₁, A₂, A₃, A₄) and transmission-side microphones (B₁, B₂, B₃, B₄). The entire flow system is the broadband noise source. There is no steady flow along the resonator in the main duct. (b) Schematic of the T-junction between the neck of the resonator and the main duct. The termination of the transmission side of the main pipe is either a closed end at the end of the steel tube (with closed ball-valve 1) or the PU tube termination (with closed ball-valve 2).