The pressure-neutral acoustic wind tunnel is a closed-loop, open section wind tunnel. The facility has recently undergone a series of major upgrades which include:

• a new motor and control system for more accurate control of a larger free-stream velocity range (5m/s to 35m/s)
• a re-designed settling chamber
• a rectangular nozzle part with a cross-section measuring 1.0m in width by 0.7m in height
• two sets of large splitter silencers to significantly reduce flow noise in the tunnel
• new flow conditioning devices with a honeycomb structure and woven mesh screens of various porosity to reduce the turbulence intensity of the uniform flow to approximately 0.2%, suitable for most aerodynamic studies.

In addition to the component upgrade and acoustic treatment, the open-jet section has been transformed into an anechoic chamber (3.8m long by 4.2m wide) to facilitate a range of aeroacoustic projects. There are four rectangular openings on both sides of the chamber room, above and below the collector and nozzle. This key feature of the upgrade helps to quickly restore the pressure balance in the chamber, thus ensuring the chamber is pressure neutral. The bottom openings are connected to ultra-quiet suction fans while the top openings allow natural air inflow. This unique feature prevents ingestion of self-turbulence and contamination of the aerodynamic and aeroacoustic measurement for proprotor experiments.

The pressure-neutral acoustic wind tunnel is equipped with numerous measurement capabilities including two far-field microphone arrays; a large beamforming array for far-field noise quantification and localisation; a 6-axis force plate; miniature load cells; 128-channel pressure scanners; a range of data acquisition and I/O modules; conventional and time-resolved 2D and 3D particle-image velocimetry (can be phase-locked to propeller systems); hot-wire and hot-film anemometry.