The present invention relates to noise dissipation panels, and more particularly to acoustic liners and arrangements of liners in aircraft engines and surrounding surfaces.
Aircraft engine noise is a significant problem in high population areas and other noise controlled environments. Attempts currently focus on lining the aircraft engine nacelle and surrounding engine areas with acoustic liners to reduce the amount of noise radiating to the community.
As background information regarding general engine acoustic theory, there exists a linearized wave equation that describes the acoustic pressure distributions present in an airflow duct. This wave equation has a general solution given by a superposition (i.e., discrete summation) of eigenfunctions. Eigenfunctions vary with the boundary conditions at the duct wall, i.e., the wall""s impedance. There are an infinite number of such eigenfunctions, each with an associated eigenvalue, that are referred to as the xe2x80x9cmodes of propagationxe2x80x9d, or xe2x80x9cmodesxe2x80x9d for short. In general, low order modes have eigenvalues that are low in absolute value. High order modes have eigenvalues that are high in absolute value. Typical low mode order values for aircraft engine noise are 0 to 5, though the range will change depending on frequency, duct size, etc. Typical high mode order values for aircraft engine noise are 8 to 15, though, these will also vary.
As used in the discussions below, the term xe2x80x9clow mode order noisexe2x80x9d is meant to describe noise waves that are represented mathematically by relatively low absolute value eigenvalues with respect to the range of noise modes present in a given application. When viewed physically, the order of mode corresponds generally to the angle of wave propagation in the duct relative to the duct walls. As shown in FIG. 1, engine noise wavefronts 12 propagate along the duct 13 at various angles xcex8 relative to the duct walls 14. If the angle is zero, the wave is said to be a fundamental wave 15, i.e., xcex8=xcex8f=0. Fundamental waves have wavefronts that travel in the axial direction of the duct and have a uniform pressure distribution at any particular duct cross-section.
In addition to fundamental waves, there are non-fundamental noise wavefronts which reflect back and forth between the duct walls as the wavefronts travel along the duct. These non-fundamental wavefronts create a non-uniform pressure distribution across the duct cross-section. The non-fundamental waves are generally classified according to their angular directions relative to the duct walls. Low order modes of noise propagation have wavefronts 16 oriented at small angles as measured relative to the duct walls, i.e., xcex8=xcex8lxe2x89xa6approximately 30 degrees. High order modes of noise propagation have wavefronts 17 oriented at relatively larger angles as measured from the duct walls, i.e., xcex8=xcex8hxe2x89xa7approximately 60 degrees. A wide range of noise frequencies exists for each mode order, low or high. For further discussion of theoretical considerations, see Aeroacoustics of Flight Vehicles, by Harvey H. Hubbard, published for the Acoustical Society of America through the American Institute of Physics, 1995. See also, Theoretical Acoustics, by Philip M. Morse et al., McGraw-Hill Book Company, dated 1968.
In a given aircraft application, an engine will generate both high and low mode order noise. Current design practice focuses on reducing this noise through the use of absorptive acoustic liners. Absorptive liners are known in various configurations, including the use of a honeycomb core sandwiched between an imperforate sheet and a perforate sheet having a small amount of open surface area. This particular combination is sometimes referred to as a single degree of freedom absorptive acoustic liner.
Absorptive liners are successful because pressure waves cause air to pass into and out of the openings of the perforate sheet and to experience a sufficient amount of friction, or resistance, which is dissipated as heat energy. The overall impedance of an acoustic liner is a complex number, given by a real part, the resistance, and an imaginary part, the reactance. Resistance relates to the liner""s ability to dissipate noise energy as heat. Reactance relates to the liner""s tendency to react noise energy back onto itself Absorptive liners provide moderate resistance and low reactance for high mode order noise waves.
FIG. 2 illustrates the theoretical effect of using absorptive liners for a hypothetical case. A given total noise energy 18 is initially comprised of a combination of one low and one high mode order noise 19, 21, each having equal energy. Starting at the beginning of the duct at position 0, the total noise energy 18 encounters an absorptive liner that quickly reduces the high mode order noise 21 and more slowly reduces the low mode order noise 19. Since high mode order noise attenuates quickly in the duct, only a relatively short duct length is needed to dissipate most of the high mode order noise present. In FIGS. 2 and 3, the vertical axis is logarithmic. A change of about xe2x88x923 dB, for example, refers to a reduction in noise energy of about half The horizontal axis is normalized to be dimensionless. The exact values of the information shown in FIGS. 2 and 3 will vary according to the characteristics of a particular application, and in general there will be energy in more than two modes.
As is evident by FIG. 2, absorptive liners are very effective for absorbing high mode order noise 21, but are inefficient for reducing low mode order noise 19, i.e., those noise wavefronts traveling along the duct at a low angular displacement relative to the duct walls. Propagating at low angles, these low order modes strike the absorptive liners fewer times in a given length of duct. Therefore, to reduce all of the low mode order noise requires a greater length of acoustic lining than is typically possible in the space-limited regions of aircraft engines. Noise reduction from use of absorptive liners is thus practically limited to higher mode order noise.
Thus, a need exists for an acoustic liner, or arrangement of liners, that effectively reduces both high and low mode order noise. The present invention is directed to fulfilling this need.
The present invention provides a new type of acoustic liner and arrangement of liners specifically for use in dissipating low mode order noise. This new liner is termed a low resistance acoustic liner and includes a middle layer, or core layer, having partitioned cavities. The cavities aid in scattering a large amount of low mode order noise into higher mode order noise. An imperforate sheet is attached to one side of the middle layer. A perforate sheet having a large open surface area is optionally attached to the middle layer, opposite the imperforate sheet, so that the partitioned cavities of the middle layer are substantially sandwiched between the imperforate and the perforate sheets. The perforate sheet stops the whistling effect caused by high speed air flowing into the cavities and minimizes airflow drag as may be required for some applications, e.g., in aircraft engines. The preferred middle layer material for high-speed commercial aircraft engines is honeycomb core having cavities with axes preferably oriented perpendicular to the central plane of the core.
In accordance with further aspects of the invention, the preferred middle layer cavity depth is approximately one quarter the noise wavelength sought to be reduced. The middle layer cavities may also be comprised of cavities having varying depths. The preferred average diameter of each middle layer cavity is equal to or less than approximately one tenth the noise wavelength. For noise having wavelengths from 3 to 12 inches, the cavity depth is between roughly 0.75 and 3 inches, and the cavity average diameter is between roughly 0.3 to 1.2 inches.
In accordance with other aspects of the invention, the perforate sheet includes uniformly distributed openings. The percent open area of the perforate sheet is preferably in an amount of at least 15% the entire perforate sheet surface area. These openings are preferably holes that have diameters less than the average cavity diameter. The low resistance acoustic liner has an absorption coefficient of about 0 to 0.5, the preferred value being less than 0.5. The low resistance acoustic liner has a resistance coefficient of about 0 xcfx81c to 0.5 xcfx81c, the preferred value being 0.3 xcfx81c.
In accordance with yet further aspects of the invention, a preferred method of manufacturing a low resistance acoustic liner having cavities of constant depth includes attaching the imperforate sheet to one side of the middle layer using an adhesive. The perforate sheet is attached to the opposite side of the middle layer also using an adhesive.
In accordance with yet other aspects of the invention, a preferred method of manufacture for creating cavities of varying depth includes providing a support layer of hardened wax-like material, placing an uncured imperforate septum on top of the support layer, placing a middle layer formed of a partitioned cavity material (e.g., honeycomb core) on top of the uncured imperforate septum, and using a uniform force to press the middle layer through the septum and the support layer until the septum is located at a desired cavity position. The septum is cured to form the imperforate sheet. After curing, the support layer is removed, such as by melting, and a perforate sheet is optionally attached to the side of the core opposite the cured septum.
In accordance with still further aspects of the invention, the desired imperforate septum material is a thermoplastic, resin, rubber, or rubber-like film material with which the middle layer can be used to xe2x80x9ccookie-cutxe2x80x9d during pressing and that which can be later cured to form a seal within each cavity. The preferred support layer material is wax. The support layer is of varying cross-sectional thickness in order to cause the imperforate sheet to be located at different heights within the middle layer cavities. A mating layer may be optionally used to aid in supporting the middle layer as it is forced through the uncured imperforate septum and support layer. The mating layer is placed between the uncured imperforate septum and the middle layer prior to pressing. The surface shapes of the mating layer and the support layer are matched. The method of making a low resistance acoustic liner may optionally include the step of warming the support layer prior to pressing so that the support layer may be more easily cut by the middle layer.
In accordance with still other aspects of the invention, a first embodiment of acoustic liners includes inner and outer linings formed of alternating low resistance liners with absorptive liners. The liners are positioned side-by-side and extend within an airflow duct for a length as necessary or as space allows. It is preferable that an absorptive liner initially precede the first low resistance liner. The liners may be formed to replace duct walls or may be attached directly to the existing duct walls. All liners are placed with their imperforate sheets located farthest from the airflow. The low resistance acoustic liner surface area adjacent the airflow is sized approximately 5% to 25% the size of the absorptive liners. For circular engine locations such as in the bypass duct surrounding an turbofan engine core, the liners are formed as annuluses.
In accordance with yet additional aspects of the invention, a second embodiment of acoustic liners includes inner and outer linings with a splitter positioned generally mid-way between the linings. The splitter may be formed entirely of low resistance acoustic liner or may formed as a combination of both low resistance liner and absorptive liner. For circular engine locations, it is preferable to use annular liners attached to surrounding engine structures using conventional attachment methods.