Noise Suppression in Aircraft Design:

It has been seen that the first step towards noise suppression is at the design stage of the rotating and static parts of the engine. Thereafter, further reduction in the noise level emanating from a particular engine may be achieved by the incorporation of special materials and innovations during its construction. These additional methods of noise suppression are briefly described as:

a) Absorption by acoustic linings.
b) Turbine, compressor and fan noise alleviated by control of nozzle area and shape.
c) Reduction of exhaust jet noise by mixing.
d) Fan duct cowling design

Acoustic Linings

One method of suppressing the noise from the fan stage of a high by-pass ratio engine is to incorporate a noise absorbent liner around the inside wall of the by-pass duct. The lining comprises a porous face-sheet, which acts as a resistor to the motion of the sound waves and is placed in a position such that it senses the maximum particle displacement in the progression of the wave. The depth of the cavity between absorber and solid backing is the tuning device, which suppresses the appropriate part of the noise spectrum. The figure shows two types of noise absorbent line; the figure shows the location of a liner to suppress fan noise from a high by-pass ratio engine and also the use of a liner to suppress the noise from the engine core. The disadvantage of using liners for reducing noise are the addition of weight and the increase in specific fuel consumption caused by increasing the friction of the duct walls.