Aircraft engine acoustic liner and method of making the same

A method of making a low resistance acoustic liner for scattering low mode order noise into a range of mode orders including high and low mode orders, the method comprising the steps of:

• (a) providing a partitioned cavity middle layer having a bottom side and a top side;
• (b) attaching an imperforate sheet to the middle layer bottom side;
• and
• (c) attaching a perforate sheet to the middle layer top side, the perforate sheet having a distributed open area of at least 15%, the low resistance acoustic liner having a resistance coefficient in the range of about 0 to 0.5 ρc.

A method of making a low resistance acoustic liner for scattering low mode order noise into a range of modes including high and low mode orders, the method comprising the steps of:

• (a) providing a support layer having top and bottom sides, the support layer being formed of an easily-cut material;
• (b) placing an uncured imperforate septum on the support layer top side;
• (c) placing a partitioned cavity middle layer on top of the uncured imperforate septum, the middle layer having a bottom side adjacent to the uncured imperforate septum and a top side opposite the bottom side;
• (d) pressing the middle layer through the uncured imperforate septum and into the support layer until the uncured imperforate septum is positioned at a desired position internally in the middle layer;
• (e) stabilizing the uncured imperforate septum until the septum is self-supporting within each cavity of the middle layer and then removing the support layer, thereby forming a permanent internal septum;
• and
• (f) attaching a perforate sheet to the middle layer top side, the perforate sheet having a distributed open area of at least 15%.

An improvement to an aircraft engine having an airflow duct with inboard and outboard walls, the improvement comprising:

• (a) an inner duct lining comprising at least one absorptive liner located near the inboard wall of the airflow duct, the absorptive liner for absorbing high mode order engine noise;
• (b) an outer duct lining comprising at least one absorptive liner located near the outboard wall of the airflow duct, the absorptive liner for absorbing high mode order engine noise;
• and
• (c) a splitter located approximately midway between the inner and outer duct linings, the splitter including at least one low resistance acoustic liner for scattering low mode order engine noise into both high and low order noise;
• the low resistance acoustic liner including a middle layer having a multiplicity of partitioned cavities, an imperforate sheet having a surface attached to one side of the middle layer, and a perforated sheet having a surface attached to the opposite side of the middle layer.

A method of reducing noise in an aircraft engine, the noise initially comprising at least low mode order noise, the method comprising the steps of:

• (a) scattering the low mode order noise into a combination of both high and low mode order noise with a low resistance acoustic liner attached to the engine, the low resistance acoustic liner including a middle layer having a multiplicity of partitioned cavities, an imperforate sheet having a surface attached near one side of the middle layer, and a perforated sheet having a surface attached to the opposite side of the middle layer;
• the low resistance liner having a resistance coefficient of about 0 to 0.5 ρc;
• and
• (b) absorbing the scattered high mode order noise with an absorptive acoustic liner positioned near the low resistance liner, the absorptive liner having an absorption coefficient of about 0.7 to 1.