Soundproofing Aircrafts

The noise levels experienced by passengers within the aircraft cabin during cruise are also a significant issue, especially for long range aircraft. During cruise, jet mixing noise is relatively weak, yet the pressure ratio of the engine propulsion nozzles are supercritical. The engine exhaust flow becomes supersonic and quite strong shock noise can be generated.

To satisfy the cruise noise requirements of airlines and aircraft manufacturers, methods of reducing or alleviating this shock noise, which of relatively high frequency, must be pursued. To aid research, a special 'cruise' tunnel has been procured which utilises the air supplies in the Noise Test Facility.

The tunnel is attached to the existing flight stream duct work and has been designed to simulate cruise Mach numbers of over 0.8 in a 1.05 metre square working section. The working section, approximately 2 metres long, contains a fuselage model attached to its floor, while the turbofan engine exhaust is simulated at model scale by a coaxial jet on the axis of the test section. A linear array of microphones flush mounted in the side of the fuselage is used to measure the impinging jet noise under the simulated cruise conditions.

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