Straight GFIT

The Impedance Tube is a 4m long, with a 50.4mm x 50.4mm square cross-section. The setup can be used both with or without airflow. A 3m long diffusing section can be added to the facility to reduce air velocity and minimize acoustic reflections back into the test section. The diffuser section is treated with acoustic foam behind a perforated sheet. The acoustic source consists of two BMS 4592ND compression drivers, capable of generating sound pressure levels of up to 130dB in the test section. The loudspeakers are excited via a Tektronix function generator. The acoustic liner samples of up to 50cm length can be installed in the text section, with 16 microphones installed over the test section to measure the SPL reduction over the length of the test section. Two pairs of microphones are also placed before and after the test section for the measurement of acoustic transmission and reflection. Data acquisition is carried out using a National Instruments PXIe-1082 data acquisition system consisting of a PXIe-4499 sound and vibration module.

Curved GFIT

The curved duct test rig is 3500 mm long, 225mm x 75mm rectangular cross section tube. The test section is 50cm long and the acoustic liners can be installed on both a straight or different curved surfaces. The test section is equipped with an array of microphones to monitor the SPL variation as a result of the implementation of acoustic liners on the duct wall. The setup can also be used with and without airflow and it is therefore an ideal facility for the evaluation of the performance of acoustic liners in the presence of pressure gradient. The set-up can be installed on to the University of Bristol Aeroacoustic facility, reaching a velocity of 100m/s. When used with airflow, a 3000mm long diffusing section can be added to the facility which expands on the vertical axis to a length of 1.35m, to reduce air velocity and minimize acoustic reflections back into the test section. The acoustic source consists of several BMS 4592ND compression drivers, capable of generating sound pressure levels of up to 130dB in the test section. Two large arrays of microphones, up to 128 microphones, can also be used before and after the test section for the modal decomposition of the sound field before entering and after leaving the test section.