Acoustic linings in Aircraft engine:
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. Figure 19.7. shows two types of noise absorbent liner. Figure 19.8. 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.