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Noise Control (Suppression)

Jet engine noise suppression has become one of the most important fields of research due to airport regulations and aircraft noise certification requirements. These govern the maximum noise level aircraft are allowed to produce. Although airframe generated noise is a factor in an aircraft's overall noise signature, the principal source of the noise in in the engine.
 

The significant noise sources originate in the fan or compressor, the turbine, and the exhaust jet or jets. The generation of the noise from these components increases with greater relative airflow velocity. Exhaust jet noise varies by a larger factor than that of the compressor or turbine, so a reduction of exhaust velocity has a stronger influence than equivalent reductions in the others.
	Jet exhaust noise is caused by the violent turbulent mixing of the exhaust gases with the atmosphere and is influenced by the shearing action caused by the relative speeds between the exhaust jet and the atmosphere. Turbulence created near the exhaust exit causes a high frequency noise (small eddies) and further downstream of the exhaust, turbulence causes low frequency noise (large eddies). In addition, a shock wave is formed when the exhaust velocity exceeds the speed of sound. A reduction in noise level can be accomplished when the mixing rate is accelerated or the exhaust velocity relative to the atmosphere is reduced. This can be achieved by changing the pattern of the exhaust jet as shown.
Compressor and turbine noise results from the interaction of pressure fields and turbulence for rotating blades and stationary vanes. Within the jet engine, the exhaust jet noise is of such high level that the turbine and compressor noise is insignificant during most operating conditions. However, low landing-approach thrusts cause a drop in exhaust jet noise and an increase in low pressure compressor and turbine noise due to greater internal power handling. The introduction of a single stage low pressure compressor significantly reduces the compressor noise because the overall turbulence and interaction levels are diminished. Also, the combustion chamber is another source of noise within the engine. However, because it is 'buried' within the engine's core, it does not have a predominant contribution.
	Mentioned earlier, the exhaust jet is the major source of jet engine noise. This can be suppressed by inducing a rapid or shorter mixing region. Where this reduces the low frequency level, it may increase the high frequency noises, which are quickly absorbed by the atmosphere. Thus the noise that does reach the listener is outside the audible range. This is achieved by increasing the contact area between the exhaust gas stream and the atmosphere by using a nozzle incorporating a corrugated or lobe-type noise suppressor. Deep corrugations, lobes, or multi-lobes give the largest reduction in noise level, but performance penalties limit the depth or number of corrugations or lobes. The same overall area as the basic nozzle must be kept, so when using this method, the final diameter of the suppressor may have to be increased causing excessive drag and weight results. Again, the noise reduction principle is to minimize the exhaust jet velocity while still maintaining performance objectives. The most successful method used is to mix the hot and cold exhaust streams within the engine and expel the lower exhaust gases through a single nozzle.
	Research has produced a good understanding of noise generation and comprehensive noise design rules exist. These are founded on the need to minimize turbulence levels, reduce the strength of interactions between rotating blades and stationary vanes, and optimizing the use of acoustically absorbent linings. Noise absorbing 'lining' material converts acoustic energy into heat. These linings normally consist of a porous skin supported by a honeycomb backing and provide a separation between the facesheet and the engine duct. For optimum suppression, the acoustic properties of the skin and the liner are carefully matched to the noise character. A disadvantage of these lining materials is the slight increase in weight and surface drag, resulting in an increase in overall fuel consumption. However, they do provide a very powerful noise suppression technique.

Thrust Reversing : Turbojet