US 3819009 A
A duct wall is provided with acoustic sound suppressing treatment comprising a panel including a plurality of cavities. The cavities are of various volumes and are intermixed according to volume within the panel along the duct wall. Each cavity is connected to the duct by means of a neck passage which provides substantially direct communication therebetween. Preselected of the cavities are spaced from the duct by predetermined non-uniform distances so that a variety of frequency bands are suppressed. Generally, larger volume cavities are spaced from the duct by greater distances and are associated with longer neck passages.
Description (OCR text may contain errors)
United States Patent 1 1 Motsinger June 25, 1974 41 DUCT WALL ACOUSTIC TREATMENT 3,726,359 4/1973 Dierl et a1 181/33 G X  Inventor: RussellE. Motsinger, Cincinnati, OREIGN PATENTS 0R APPLICATIONS Ohlo 348,808 10/1960 Switzerland 181/33 G Assignee: General Electric Company, Great Brltam G Cmcmnatl Ohlo Primary ExaminerRichard B. Wilkinson  F1led: Feb. 1, 1973 Assistant Examiner-John F. Gonzales  APPL 328,713 iilgtzgzfgAgent, 0r FirmJames M. Kipling; Derek P.
 US. Cl 181/33 G, 161/68, 161/139,  ABSTRACT Int Cl 1 3 A duct wall is provided with acoustic sound suppress- I o a s 6 6 s v s s 1 s a a a 6 6 s I a a  F of Search 181/33 33 33 of cavities. The cavities are of various volumes and are 181/33 l6l/68 139 intermixed according to volume within the panel along the duct wall. Each cavity is connected to the duct by  References C'ted means of a neck passage which provides substantially UNITED STATES PATENTS direct communication therebetween. Preselected of 2,887,173 5/1959 Boschi 181/33 G UX the cavities are spaced from the duct by predeter- 3,159,236 12/1964 Akerson 181/33 G UX mined non-uniform distances so that a variety of fre- 3,l66,l49 l/l965 Hulseet al 181/33 G UX queney bands are suppressed, Generally, larger volg 'ume cavities are spaced from the duct by greater disary e a 3,353,626 11/1967 Cremer et a1 181/48 x tances and are assoclated longer neck passages 3,688,866 9/1972 Kong 181/33 K 5 Claims, 3 Drawing Figures .54 4 J34 Q 1 c 7 LL! 1 1 I U I In L 11 1 j DUCT WALL ACOUSTIC TREATMENT BACKGROUND OF THE INVENTION This invention relates to sound suppression and, more particularly, to acoustic treatment of duct walls in gas turbine engines.
The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the United States Department of the Army.
Various means have been used in the past to suppress noise generated within gas turbine engines. Generally, noise treatment has been directed primarily toward high frequency noise, and one form of this treatment has utilized the well known Helmholtz resonator disposed in pluralities within a duct wall treatment of the engine.- Generally, these devices comprise a plurality of cavities connected to the duct by means of apertures which form entrances for generated sound waves.
Each individual cavity is capable of attenuating sound wave propagation within discrete frequency bands by means of energy dissipation within the cavity resulting from pressure losses of the flow passing in and out of the cavities. The frequency bands-within which an individual cavity is effective are closely limited about a resonant frequency which is established by the Helmholtz resonator equation:
W resonant frequency, H
C speed of sound in the medium, feet per second;
S area of the resonator neck, feet squared;
l the effective length of the resonator neck, including an end correction, feet; and
V contained volume in the resonator cavity, feet cubed.
In order to effectively remove objectionable sound phenomena from gas turbine engines, it is desired to treat sound of both high and low frequencies. Prior attempted solutions, not based upon Helmholtz resonators, have included the utilization of a thin porous layer over a subdivided air space. In another attempt, a solid blanket of porousmaterial has been used to treatduct walls. However, both of these attempts have suffered from the fact that each material suppresses sound as a function of the material thickness, so that low frequency attenuation requires high thickness. Particularly with respect to gas turbine engines used to power jet aircraft, overall nacelle thickness is of prime importance with respect to drag performance. Hence, sound attenuation based upon increasing duct wall thickness is undesirable.
Alternatively, duct walls have been treated with discrete layers of different sized Helmholtz cavities separated from the duct and from one another by thin porous face sheets so that sound of higher frequency is retained within the duct-bordering smaller cavities, while lower frequency sound penetrates these smaller cavities and is dissipated within radially outward larger cavities. However, once again, this approach suffers from affects on overall duct thickness.
Still another approach has been to align axially several panels of Helmholtz resonator cavities, each panel housing cavities of different size. This treatment has efficiently lessened sound propagation from the duct, but
at the substantial negative affect of requiring extension of the duct length. Duct length is another parameter which is preferably minimized in most gas turbine engines. Hence, this treatment also has met with limited usefulness.
The present invention deals successfully with the contemporaneous problems of wide frequency range sound suppression, duct length minimization, and nacelle thickness minimization. The basic concepts involved can be elucidated with reference to the foregoing Helmholtz resonatorequation. Referring to the denominator of the radial function, it can be seen that resonator frequency (W is inversely proportional to the product of the effective length (l') of the resonator neck and the contained volume (V) in the resonator. Hence, elongation of the neck passage permits reduction of the cavity volume without affecting resonant frequency. The present invention makes use of this concept by disposing a plurality of neck passages within an acoustic panel in such a way as to provide a plurality of relatively large volume cavities for the suppression of low frequency sound along with smaller volume cavities without the necessity of overly lengthening or thickening the panel itself. This is accomplished, in general, by utilizing structure already present in the panel to form various cavities and neck passages.
Stated in another way, the present concept permits combining several different size cavities into a configuration which inherently provides a variety of neck lengths. Thus, this invention provides for the use of several different effective frequency suppression bands without the necessity of adding substantially to the length or thickness of the duct which must be treated.
BRIEF DESCRIPTION OF THE INVENTION Consequently, it is a primary object of the present invention to provide for improved suppression of a wide range of noise frequencies, including low frequency noise, by the utilization of duct acoustic treatment which does not require substantial enlargement either in the length of duct treated or thickness of treatment.
Stated briefly, the present invention performs this and other objectives by the utilization of a sound suppressing panel comprising a plurality of cavities having a variety of cavity volumes and spaced from the duct within the panel by a variety of distances. A plurality of interspersed, various length neck passages is provided for providing substantially direct communication between the various cavities and the duct. The neck passages vary in length depending upon the disposition of the associated cavities within the panel, and their separation from the duct. In order to save weight as well as to minimize thickness and length, wall portions defining smaller cavities also combine to form the neck passages for larger cavities. In this way, a pyramiding effect is realized whereby a broad spectrum of sound frequencies may be absorbed and dissipated within the single sound suppressing panel.
BRIEF DESCRIPTION OF THE DRAWING The present invention may be more clearly understood by reference to the appended specification as well as the drawings wherein:
FIG. 1' is a cross-sectional view of a simplified gas turbine engine;
FIG. 2 is an enlarged cross-sectional view of a portion of the treated duct wall of FIG. 1 according to the present invention; and
FIG. 3 is an enlarged cross-sectional view, similar to that in FIG. 2, but representing another embodiment of the present invention.
DETAILED DESCRIPTION In FIG. 1, the simplified gas turbine engine designated generally includes an annular duct inlet 12, a bladed fan 14, a compressor 16, combustion chambers 18, and turbine along with exhaust nozzle .22. This engine operates in a fashion similar to typical engines of this variety. Atmospheric air enters inlet 12 to the left in FIG. 1 tobe operated upon and compressed by fan 14 and compressor 16, a portion of the flow passing through a fan duct 24 and the remainder through the compressor 16 and into conbustors 18. Within the combustors, the air is mixed with fuel and ignited, whereupon rapid expansion of the fuel occurs and a high velocity stream of products of combustion exits the combustors to the right and engages the rotatable bladed stages of turbine 20 to impart torque thereto for the operation of fan 14 and compressor 16. The gas stream exiting the turbine 20 is combined with the fan stream within fan duct 24 and expelled through the exhaust nozzle 22 to provide a substantial thrust toward the left in the Figure. v Objectionable engine noise has been determined to be generated in several portions of the engine: the fan blades rotating at high tip velocities generate a broad spectrum of noise frequencies; compressors and turbines generally high'frequency noise; and the combustors are also a noise source. As a means for reducing the noise propagated from the engine, the present invention provides a sound suppressing treatment for the wall 26 defining the duct 12 (or for similar duct surfaces throughout the engine).
More specifically, referring to FIG. 2, a first embodiment of the present invention is depicted in application to wall 26, as an example of typical wall applications. This figure shows an enlarged view of the duct wall 26 whereupon a panel 27 incorporating a plurality of Helmholtz resonator type cavities is disposed. Small cavities 30 having a first predetermined volume and disposed immediately adjacent the duct 12 are partially defined by a number of radially extending side walls 32 and an axially extending backing wall 34. Each of the first cavities 30 is separated from the duct by means of the thickness of axially extending duct wall 26, and in which a plurality of apertures 36 are disposed. Each of the small cavities 30 opens directly into the duct 12 by means of one of the apertures 36. (More than one aperture can serve each cavity, but for the sake of simplicity, a one-for-one relationship is maintained throughout this discussion.) A second plurality of cavities 40 is also incorporated into the panel 27. The cavities 40 have a second predetermined volume larger than the volume of cavities 30. Each cavity 40 is partially defined by an axially extending wall 44, along with walls 34 (which also bounds cavities 30, as stated). Moreover, it can be appreciated from the figure that the previously mentioned walls 32 and radially extending walls 42 (by which cavities 40 are further defined) are common walls to the extent of the length of walls 32. Such double usage of walls results in substantial weight savings due to the absence of a requirement for adding redundant walls.
Each cavity 40 is connected to duct 12 by means of one (or more, as above) radially extending neck passage 46 providing substantially-direct communication between the cavity 40 and the duct 12. It can be seen from the figure that neck passages 46 are defined by opposed pairs of the walls 32 of adjacent first cavities 30. Hence, a further weight savings.
The interrelationship between the first and second plurality of cavities 30 and 40, respectively, is such that cavities 30 are disposed generally in a layer (owing to axial colinearity of walls 34) which is disposed to the radial interior of a second layer of cavities 40 (produced by a similar colinearity of walls 44). Each cavity 40, in this first embodiment, substantially circumscribes at least one pair of cavities 30. Moreover, the cavities 30 lie substantially between the duct and cavities 40.
Proper functionality of the cavities as Helmholtz resonators requires that there be an entrance to each cavity for sound waves. The entrance with respect to each of cavities 30 is an aperture 36. The entrances to. cavities 40 are neck passages 46. Axially, it can be seen that the first and second neck passages 36 and 46, respectively, are substantially interspersed with respect to one another so that sound waves occurring within a given length of duct 12 will encounter both varities of neck passages and enter both types of cavities.
The functional importance of this particular configuration will now be described. As previously stated, the gas turbine engine noise emanating forward through duct 12 comprises a plurality of frequencies. Furthermore, the sound dissipation capacities of individual Helmholtz resonator type cavities are narrowly limited about predetermined resonant frequencies, determined, in part, by the product (lV) of cavity volume and neck passage length. As sound waves of various frequencies advance forward through duct 12 and encounter the plurality of interspersed neck passages 36 and 46, portions of the sound waves and associated sound energy enter each neck passage. Higher frequency waves will be effectively dissipated within cavities 30. Lower frequency waves will be dissipated within cavities 40; Consequently, the configuration as thus far described will effectively dissipate sound energy in bands centered about two discrete resonant frequencies, one characteristic of each combination of cavities (30, 40) and neck passages (36, 46) described.
To further enhance the sound dissipation capability of the embodiment depicted in FIG. 2, a third plurality of cavities 50 are defined by a number of radially extending walls 52 and an axially extending wall 54 along with a second weight-saving usage of walls 44. A neck passage 56 is defined by side walls 42 of adjacent cavities 40. The volume of cavities 50 is larger than the volume of cavities 40 (which is in turn larger than the volume of cavities 30). Furthermore, the length of neck passages 56 is.longer than, that of neck passages 46 (which isin turn longer than that of neck passages 36). Hence, the product lV is largest with respect to cavities 50, smaller with respect to cavities 40 and smallest with respect to cavities 30. Moreover, the entrances to neck passages 36, 46 and 56, are all interpersed with one another. As a consequence, the acoustic panal of the present invention, according to this first embodineck passage locations, the present invention makes possible the treatment of wide ranges of sound frequencies along a single length of acoustic panel without the necessity for adding additional lengths. Contemporaneously, the utilization of the cavity walls within the panel 27, to both define individual cavities as well as the various walls and neck passages for other cavities, results in a lightweight configuration. Moreover, the fact that the neck passage length for the lower frequency cavities are relatively long results in an ability to treat low sound frequencies with relatively small cavity volumes. (This fact can be appreciated by recalling that resonant frequency is a function of the product of neck passage length and cavity volume). Hence the overall panel thickness is advantageously minimized.
In general, then, this first embodiment'of the present invention results in a duct wall sound suppressing treatment including an intermixed plurality of resonant cavities having non-uniform volumes, and a plurality of neck passages providing substantially direct communication between the duct and the cavities. Furthermore, preselected of the cavities are spaced from the duct; and the neck passages are of non-uniform length, preselected longer neck passages being associated with cavities at relatively greater distances from the duct. The larger volume individuals of the cavities are associated with the longer neck passages in order to maximize the product of 1 'V, and hence to achieve effective sound treatment of low frequency sound waves.
The present invention is not, however, limited to this embodiment, since the concepts thereof are subject to broad application. For example, referring to FIG. 3, a second embodiment of the present invention is disclosed. For simplicity, this embodiment is identical with the first but for the addition of a porous face sheet 58. The face sheet partially defines duct 12 and enhances the aerodynamic efficiency of the duct as contrasted with the plurality of neck passage openings in FIG. 2. In this embodiment, the neck passages 36, 46 and 56 terminate at the face sheet, but the nature of the sheet is such that sound waves freely pass therethrough for effective sound wave entrance to the various cavities 30, 40 and 50. In other words, substantially direct communication between the cavity interiors and the duct is maintained despite the application of face sheet 58. In operation, the embodiment of FIG. 3 performs substantially similarly to that of FIG. 2.
Numerous other variations of the concepts of the present invention will be apparent to those skilled in the art. For example, the interrelationship between the cavities disclosed in the preceding embodiments can be substantially varied without varying the overall operation of the acoustic panel. Furthermore, in order to further expand the sound frequency dissipating range of the present invention, further cavities and associated neck passages can be interspersed having different l'V products and thus, difierent frequency range characteristics. As another variation, the relative volumes of the various cavities could be adjusted with similar adjustments to neck passage lengths while maintaining characteristic frequency suppression. This would allow the panel to be narrowed and elongated or shortened and thickened depending upon appropriate limitations in given applications, due to the equalizing interaction between neck passage length and-volume in the l 'V product. Such variations and those similar to it are intended to be interpreted as falling within the present invention.
What is claimed as new and desired to be secured by Letters Patent of the United States is:
1. A sound suppressing panel for use in ducts, the panel comprising:
a first plurality of cavities having predetermined first volumes;
a second plurality of cavities having predetermined the first cavities by a smaller distance than the sec ond cavities; said first means including a plurality of first neck passages, and said second means including a plurality of second neck passages, the second neck passages of greater length than the first; wherein the plurality of second neck passages is interspersed among the plurality of first neck passages; and said first cavities include first walls of predetermined first length, and opposed pairs of said first walls of adjacent first cavities, in their lengthwise dimension, partially define said second neck passages. 2. A sound suppressing panel for use in ducts, the panel comprising:
a first plurality of cavities having predetermined first volumes; a second plurality of cavities having predetermined second volumes; first means for providing substantially direct communication between said first cavities and said duct; and second means for providing substantially direct communication between said second cavities and said duct; said first and second cavities separated from the duct, the first cavities by a smaller distance than the sec ond cavities; said first means including a plurality of first neck passages, and said second means including a plurality of second neck passages, the second neck passages of greater length than the first; wherein the plurality of second neck passages is interspersed among the plurality of first neck passages; and further including a third plurality of cavities having predetermined third volumes, the third cavities separated from the duct, and a plurality of third neck passages for providing substantially direct communication between the third cavities and the duct; wherein said first cavities include first walls of predetermined first length, opposed pairs of said first walls partially defining said second neck passages; and said second cavities include second walls of predetermined second length, opposed pairs of said second walls partially defining said third neck passages.
7 8 3. The panel of claim 2 wherein said second length 5. The panel of claim 4 wherein: 18 greater thanl safidlfirst lze gg said first and second walls of preselected first and T e pane c mm W erem' second cavities are common Walls to the extent of preselected pairs of said first cavities are substantially circumscribed by preselected individuals of said 5 second cavities.