There are three principal sources of noise in the cabin of a typical twin-engine turboprop aircraft. The first is a result of engine vibration transmitted through the wing structure, which causes the cabin walls to vibrate. Secondly, cabin noise is generated by the propeller slip-stream, coming into contact with the sides of the fuselage. Lastly, this slipstream causes vibration in the rear of the fuselage where it strikes the aircraft's empennage.
These aircraft are subject to a "fundamental" noise frequency, which can be determined by multiplying the rotation speed of the propeller by the number of propeller blades. Harmonics (or noise peaks at multiples of this frequency) also occur in the frequency spectrum.
ANC systems attempt to eliminate this noise by generating a canceling waveform equal in amplitude and frequency, but exactly opposite in phase. They employ microphones in the cabin to acquire sound data, which is analysed by a digital processor, and then generating a canceling signal through an array of loudspeakers. Tachometers linked to the propellers are used to monitor the synchronisation of the cancellation signal with propeller speed.
ATVA-based systems work in a similar way, but with accelerometers in place of microphones and "shakers" instead of loudspeakers. Active structural control uses ATVAs controlled by accelerometers and microphones.
Active isolation-control employs active mounts to isolate vibration sources such as fuselage-mounted turbofan engines. Actuators are embedded in the mounts and generate control forces. Microphones and accelerometers are used to monitor noise and vibration respectively.
Passive-tuned dampers can also be used to reduce noise and vibration, but they operate at a fixed frequency and are therefore normally tuned to the fundamental prop-wash frequency in the cruise condition. They cannot provide any attenuation during other flight conditions or at higher harmonics. Synchrophasing, an essential part of noise control, is used to maintain a predefined phase angle between the propellers, typically to within an accuracy of plus or minus 1°. This allows the periodic "beating" against the fuselage caused by the propellers to be minimised.
Source: Flight International
