US3087574A

US3087574A - High acoustic transmission loss panel and the like

Info

Publication number: US3087574A
Application number: US851099A
Authority: US
Inventors: Bill G Watters
Original assignee: Bolt Beranek and Newman Inc
Current assignee: Bolt Beranek and Newman Inc
Priority date: 1959-11-05
Filing date: 1959-11-05
Publication date: 1963-04-30
Anticipated expiration: 1980-04-30
Also published as: BE590089A; AT282898B

Description

April 30, 1963 B. G. wATTERs HIGH ACOUSTIC TRANSMISSION LOSS PANEL AND THE LIKE 2 Sheets-Sheet 1 Filed NOV. 5, 1959 1000 FfQL/EA/c Y c ycLEs/sfcaA/o) INVENTOR. B/LL G. WTEPS BY Mm! OI. O O 1 April 30, 1963 B. G. WATTERS HIGH ACOUSTIC TRANSMISSION LOSS PANEL AND THE LIKE Filed Nov. 5, 1959 y JJ 2 SheetsfSheet 2 INVENTOR. /l 6. ffl/72996 Eau/w 76;,

3,@87574 Patented Apr. 3), 19h15 3,087,574 HIGH ACOUSTHC TRANSMISSIN LOSS PANEL AND THE LiKE Bill G. Watters, Nahant, Mass., assigner to Beit Beranek d; Newman, Inc., Cambridge, Mass., a corporation of Massachusetts Filed Nov. 5, 1959, Ser. No. 851,099 17 Claims. (Cl. ISI-33) The present invention relates to walls, partitions and other surfaces, hereinafter generically referred to as panels and the like, and more particularly, to structures for providing a high acoustic transmission loss over a wide band of acoustic frequencies. The terms acoustic, so-und, and vibrationa as herein employed, are intended to embrace all kinds of vibrations including audible, sub-audible and super-audible frequencies.

As is explained in my copending application, Serial No. 801,199, tiled March 23, 1959, for Panel and the Like of High Acoustic Transmission Loss, and in an article entitled New Nall Design for High Transmission Loss for High Damping, lappearing in the Journal of the Acoustical Society of America, volume 3l, No. 6, pages 739- 748, panels and the like heretofore employed for structurally separating spaces, such as rooms, radiate acoustic energy, particularly :at the high sound frequencies, as a result of the setting-up of transverse waves in the panel of velocity comparable to that of acoustic energy in the air or other medium surrounding the panel. The said article land copending application describe a new and improved design or construction of panels and the like that completely overcomes. this disadvantage by imbuing such panels and the like with a mechanism for preventing the elfective shear-wave velocity `Within the interior medium of the panel and the like from becoming comparable with the velocity of acoustic energy in the aixor other `surrounding medium; preferably confining the same so that the shear-wave velocity in the inner panel medium, as modified by the mass of the panel surfaces or facings, should not be greater than approximately 0.7 of the velocity of sound in the air `or other surrounding medium.

Such panels and the like have been found to be highly satisfactory. In some cases, however, it is desirable markedly to reduce the cost of manufacture of panels and the like embody-ing the constructional and performance characteristics above discussed. In addition, it is desirable in some applications to simplify the constructional |details of the interior medium of the panel and the like.

An object of the present invention, therefore, is to provide a new and improved panel Vand the like of the character described that is relatively inexpensive to manufacture and of relatively `simple construction, and yet is designed to take advantage of the high acoustic transmission-loss phenomenon attain-able in accordance with the teachings of the said copending application and the said article.

An :additional object is to provide a novel panel and the like of more general utility, also.

Other and further objects will be explained hereinafter and will be more particularly pointed out in connection with the appended claims.

The invention will now be described in connection with the accompanying drawing, FIG. l of which is a fragmentary longitudinal ysection of .a panel and the like constructed in accordance with the present invention;

FIGS. 2 and 3 are similar views of modified structures;

FIG. 4 is a view similar to FIG. 3 of still a further modification;

FIG. 5 is a fragmentary longitudinal section similar to FIG. 4 of a preferred embodiment of the invention;

FIGS. 6 and 7 are similar views of still additional modified structures;

FIG. 8 is a graph presenting experimentally obtained performance characteristics of la panel constructed in accordance with the embodiments of FIGS. 4 and 5 and FIGS. 9 to l2 are fragmentary views of further mod-i- Iications illustrated in the act of bending.

In the various gures of the drawings parts having gene-ral correspondence or similarity are designated by reference numbers having the same last digit or digits, the initial digit or digits designating the figure where appropriate.

rllhe criteria set forth in the above-mentioned cepending application and article for producing the phenomena required in order to achieve high transmission loss, are several Ifold. First, the velocity of the effective shear wave propagating in the interior medium of the panel between the outer surfaces or facing-s, must not exceed approximately `0.7 of the velocity C0 of sound in the air or other mediu-m surrounding the panel; preferably not greater than two-thirds such velocity. This requires a sumciently low ratio of shear modulus-to-density of the interior medium or core of the panel and the like, where density is defined as the density of the interior medium, as loaded with the panel structure. Secondly, the effective longitudinal stiffness of the inte-rior medium, defined as the longitudinal stiffness of the shearable or eifectiveiy shearable part of the interior medium, must be less than the longitudinal stiffness (Youngs modulus) of the inner and outer facing :surfaces in order to `set up the shear-wave boundary condition essential to the `attainment of the above-described phenomenon. Thirdly, if it is to be assured that the acoustic transmission loss be not reduced within the desired band of acoustic `frequencies by the phenomenon of mechanical double-wall resonances, set up transversely vbetween the inner and outer panel surface facings, the thickness of the panel and the parameters of the interior medium must be such as to provide a transverse mechanical resonant frequency outside the band of acoustic frequencies that are to be prevented from transmission through the panel and the like.

Referring to FIG. 1, a panel or the like that satisfies the above-mentioned criteria is constructed of relatively inexpensive materials and is of simple structural design. The panel and the like has inner and outer surfaces or facings 3 and 3 bounding an interior medium or thickness b. That interior medium is shown comprising inwardly extending transversely rigid supports 5 and 5', integral with the inner surfaces of the inner and outer panel facings 3 .and 3', though, as later explained, an integral construction is not essential, even if, in some cases, it is preferred. The rigid inwardly extending supports 5, 5 are shown longitudinally spaced from one another along the length of the panel and the like, the spaces being illustrated as cavities or openings 2. The .length of the rigid supports is illustrated by the dimension r and the length of the spaces 2 is illustrated by the dimension l1.

Sandwiched between the

rigid supports

5, 5 and in contact therewith is a relatively thin shearable elastic layer 4, as of rubber, synthetic elastomers, appropriately plasticized plastic materials such as polyvinyl chloride and similar soft but incompressible materials. 'The layer 4 may be adhered to the

rigid supports

5, 5 in any conventional manner, as with the aid of a bonding adhesive, not shown. The reason for the requirement of substantial incompressibility resides in the fact that it is desired to maintain a high transverse stiffness in the layer 4, without impairing its ability to shear in response to movement of the surfaces 3, 3', `thereby to avoid a detrimental doublewall transverse resonance that would otherwise be produced in the useful audible spectrum by panels and the like of convention dimensions.

spargere In order to attain the 0.7 velocity criterion above discussed, the following approximate relationship has been found to determine -the necessary value of the shear modulus p. and total layer thickness b2 of the shearable layer 4, the mass m per unit area of the panel, and the percent of the surfaces area AT of the panel surfaces 3 or 3' that is internally supported by the plurality of spaced relatively rigid portions of panel surfaces 5, 5', the regions adjacent the spaces 2 being internally unsupported:

relationship:

pbzrz Vmb2o-+m250-7 C0 (2) The shearable medium 4 need not be constituted of a single layer but may be constructed of multi layers. Materials appropriately laminated together may be used which will present the effective result of shearing and of providing high transverse stiffness. In FIG. 2, for example, a pair of layers 24 is shown laminated with a relatively thin stiff layer or membrane 26, as of metal, glass, mica or the like to assist in the provision of high transverse stiffness without impairing the shearing or effective shearing qualities of the composite shearable layer means 24-26.

Instead of placing the shearable layer or its equivalent in the position shown in FIGS. 1 and 2, it may bel placed closer to one of the surfaces and, indeed, in the embodiment of FIG. 3, the shearable layer 34 is shown adjacent the surface 33, with only the rigid supports 35 extending from the surface 33' and separated vby spacers 32.

The invention is not limited to the use of a single elastic layer or of a single composite elastic layer. As shown in the preferred embodiment of FIG. 4, for example, a pair of shearable layers 44" is employed, one adjacent the inner boundary of each of the inner and outer panel surfaces 43, 43', with the plurality of rigid supports 45 separated by spaces 42 interposed between the inner surfaces of the layers 44. In order to provide performance equivalent to that obtainable with the shearable layer of thickness b2 of FIGS. 1 and 3, the layers 44 need each be only the thickness substantially 1/2 b2. The supports 45, moreover, could also be divided by one or more intermediately extending sheets similar to the sheet 2d of FIG. 2, preferably disposed substantially parallel to the faces 43, 43'. In effect, this is illustrated in FIG. 10, where the central solid region 1050 of the spacer means 105 acts as an intermediate stiff dividing member, as of gypsum, metal or the like,

reference numbers

102, 103 and 103', and 104 designating the spaces, surfaces, and elastic layers previously described.

Still a different arrangement is shown in FIG. 9, where the shear layers 94 are disposed laterally between the rigid supports 950, the latter being of cross-shape to define the spaces or cavities 920 between thel surfaces 93 and 93.

In FIG. 8, an experimentally obtained graph is presented showing the relationship between the transverse or bending wave velocity propagated along a panel constructed in accordance with the embodiment of FIG. 4 (plotted along the ordinate in units of feet per second) and the frequency of the acoustic energy (plotted in units of cycles per second along the abscissa). The dimensions of the experimental structure were are follows: inner and outer surfaces 43 and 43' were lS-guage steel plates; the

shearable layers 44 were of gum rubber, 35 Durorneter, secured to the surfaces 43, 43 and the supports 45 with an epoxy adhesive and of thickness 1/2 b2 equal to 0.018 inch; rigid supports 45 of aluminum of thickness b1 equal to 1 inch; and support length r equal to 1/2 inc-h and space length h equal to 0.1 inch. It will be observed 'from FIG. S that a substantially constant transverse-wave velocity is achieved, well below the velocity of sound in air, over a broad band of acoustic frequencies, void of double-wall resonance effects. High transmission loss over this band can be obtained, that is comparable to the transmission loss obtainable with the structures described in the said copending application and in the said article.

It is to be understood that the rigid supports need not assume the particular geometric configuration shown and described, but, as explained in the said copending application, and in the said article, may take any of a variety of forms including, for example, appropriate spaced cell types tof structures. Thus, in FiG. 7, a spaced cell supporting structure 75 is illustrated extending inwardly from the outer surface '73', to sandwich a shearable layer 4 against the inner boundary of the opposite panel surface 3. In order to illustrate the versatility of the invention the supports 75 of FIG. 7 are shown kerfed to provide an even smaller contact area with the shearable layer. The resulting further spaces 72' are illustrated as of length g, and the more limited regions of contact with the shearable layer 74 are illustrated as of length j. `Equation 2 then reduces, for this case, to the approximate relation In some cases it will not be necessary to kerf the spacer material 75 into spaced blocks separated by spaces 72, provided the material of the cellular walls has a low longitudinal stiffness such that the walls can give longitudinally, as by stretching and compressing, in response to bending :of the panel surfaces, and yet with substantially no resulting transverse movement of the cell walls. Placing slack, somewhat as in FIG. 12, or perforated portions in the cell walls may achieve this result, as well as the above-described keriing. In all cases, however, the core sections must be of materials relatively stiff in the transverse direction, such as the metal blocks before referred to, or a cellular construction, or materials such as gypsum or the like, to mention Ibut a few materials. The total transverse compliance should be so sufficiently low that the mechanical resonant frequency lies well outside the useful band of acoustic frequencies, as before explained. Preferably, an appreciable, if not the principal, transverse compliance may reside in the shearable layer.

An effective shear-like performance can also be obtained by having cantilever-like resilient spacers 1250" that, like the above-mentioned slack or perforated spaced cell walls, can deform transversely, while the surfaces 123, 123' remain substantially undistorted, as shown in FIG. 12, in response to bending vibrations, thereby producing an eective shear-wave velocity of the required value.

As another example, a substantially homogeneous shearable medium 64, as of rubber or the like, is shown in FIG. 6 sandwiched between the inner and outer surfaces 63, 63', and containing relatively uniformly distributed rigid spheres or other spaced supporting members 65 therein.

Instead of employing the spherical configuration 65 of FIG. 6, the modification of FIG. 11 shows block supports 1050 with the longitudinally extending regions A of the elastic material 114 adjacent surfaces 113, 113' in shear, and the regions B between the blocks, bending, as shown, to contribute to the shear stilness.

In all of the above-described embodiments, any acoustic energy striking either of the panel surfaces can reflect back into the room or other space whence the acoustic energy originated. If it is desired to provide acoustic absorption at such surfaces, one or both of the outer panel surfaces 53, 53' may be perforated as shown in the case of the upper panel 53 of FIG. 5, having the perforations S7 therein. The perforated surface 53 may then be covered with an acoustically resistive layer or layers such as a fibrous iblanket or covering 58 to achieve the abovedescribed result of absorbing incident acoustic energy, in view of the action of the resistive layer or layers in combination with the air-permeable spaces 52 separating supports 55 within the core. Resistive material (not shown) could also be placed in such permeable spaces 52, access to the spaces being provided through the elastic layer 54 shown paired With another elastic layer 54. If the resistive layer or layers 58 is or are applied to the outside surface of the perforated member 53, rather than on the inside thereof, where it or they would work equally Well for acoustically absorptive purposes, the perforations will be concealed and an attractive wall-paper-like covering (not shown) may be provided. It is to be understood that this modification may also be incorporated into all of the other embodiments of the invention for the purpose described.

All of the embodiments of the invention may also be formed into small panels or blocks that, when assembled, provide the required sound-shearing wall of the present invention. The term panel, moreover, is also intended to embrace thin strips, such as beams and the like.

Further modifications will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the invention as dened in the appended claims. The terms steel-like, metal-like, and rubberlike as employed in the claims are intended to embrace various materials of the type described in the specification which are like steel, like metal, or like rubber, with respect to the functions performed by the designated elements in the acoustic panel of the invention.

What is claimed is:

r1. An acoustic panel for providing high acoustic transmission loss for a band of acoustic frequencies, comprising stiff, thin steel-like layers spaced apart by a core, said core having between the layers a plurality of spaced rigid metal-like blocks and at least one lamina of elastic rubberlike material, the thickness b2 of said lamina being substantially less than the thickness b of said core, said layers having a longitudinal stiffness greater than the effective longitudinal stiffness of said core, said panel having a mass Iper unit area m and said core having a shear modulus n related to the velocity C of acoustic waves in the medium surrounding said panel substantially by the eX- pression ubzAs 'm $0.7 C'o where As is the eective cross-sectional area substantially parallel to said layers of the blocks cumulatively, and AT is the surface area of a side of one of said layers.

`2. The panel of claim 1, wherein said blocks have a dimension r parallel to said layers and are spaced apart a distance h, and wherein the ratio As AT is given substantially by the expression ,z r+h 2 3. The panel of claim 1, wherein said blocks have spaced portions contacting said lamina, said portions deining cells therebetween.

4. The panel of claim 1, wherein said blocks have a dimension r parallel to said layers and are spaced apart a distance h, wherein said blocks have spaced portions contacting said lamina with a dimension j parallel to said lamina, said portions deiining cells therebetween with a dimension g parallel to said lamina, and wherein the ratio Ae AT is given substantially by the expression 5. The panel of claim l, wherein said core has a sui ciently high transverse stiffness to prevent transverse mechanical resonance within said band.

6. The panel of claim 1, wherein the acoustic frequency for which the bending velocity of acoustic waves along said panel is comparable to said velocity C0 lies outside Ithe band.

7. The panel of claim 1, wherein said elastic material is soft and incompressible.

8. The panel of claim 1, wherein said blocks are arranged on both sides of said lamina.

9. The panel of claim 1, wherein said blocks are a1'- ranged on one side of said lamina and the other side of said lamina is adjacent one of said layers.

10. The panel of claim 1, wherein the distance between `said blocks is small compared to the dimensions of said blocks.

11. The panel of claim 1, wherein said blocks have spaced cells therein.

12. The panel of claim 1, having at least two of said laminae with one lamina arranged adjacent each of said layers, respectively, and with said blocks between said laminae.

13. The panel of claim l, wherein at least one of said layers is perforated and has associated acoustically resistive means exposed to said perforations.

14. The panel of claim 13, wherein said resistive means comprises -a resistive sheet adjacent said at least one layer.

15. The panel of claim 1, wherein said core comprises a pair of said laminae on opposite sides of a relatively stiff sheet.

16. An acoustic panel for providing high acoustic transmis-sion loss for a band of acoustic frequencies, comprising stiff, thin steel-like layers spaced apart by a core, said core having between the layers a plurality of spaced rigid metal-like blocks, said layers having a longitudinal stiffness greater than the eiective longitudinal stiffness of said core, at least one of said layers having perforations therethrough in communication with the spaces between said blocks to permit the entry of acoustic energy into said spaces, and acoustically resistive means adjacent said perforations to absorb said acoustic energy, the ratio of the shear modulus to the density of the core providing an acoustic shear wave velocity 4for the propagation of said band of acoustic frequencies along said panel which is less than substantially seven-tenths the velocity of acoustic energy in the medium surrounding said panel.

17. The panel of claim 16, wherein said resistive means comprises sound absorbing material on the side of the perforated layer opposite said core.

References Cited in the le of this patent UNITED STATES PATENTS 2,184,482 Austin et al. Dec. 26, 1939 2,579,324 Kock Dec. 18, 1951 2,744,042 Pace May 1, 1956 2,806,509 Bozzacco etal. Sept. 17, 1957 2,838,806 Sabine June 17, 1958 FOREIGN PATENTS 533,200 Italy Sept. 19, 1955 754,299 Great Britain Aug. 8, 1956 810,505 Great Britain Mar. 18, 1959 OTHER REFERENCES Cyril M. Harris, Handbook of Noise Control (Mc- Graw-Hill Book Company, Inc., New York, 1957), pages 22-6 through 2212.

Claims

1. AN ACOUSTIC PANEL FOR PROVIDING HIGH ACOUSTIC TRANSMISSION LOSS FOR A BAND OF ACOUSTIC FREQUENCIES, COMPRISING STIFF, THIN STEEL-LIKE LAYERS SPACED APART BY A CORE, SAID CORE HAVING BETWEEN THE LAYERS A PLURALITY OF SPACED RIGID METAL-LIKE BLOCKS AND AT LEAST ONE LAMINA OF ELASTIC RUBBERLIKE MATERIAL, THE THICKNESS B2 OF SAID LAMINA BEING SUBSTANTIALLY LESS THAN THE THICKNESS B OF SAID CORE, SAID LAYERS HAVING A LONGITUDINAL STIFFNESS GREATER THAN THE EFFECTIVE LONGITUDINAL STIFFNESS OF SAID CORE, SAID PANEL HAVING A MASS PER UNIT AREA M AND SAID CORE HAVING A SHEAR MODULUS U RELATED TO THE VELOCITY C0 OF ACOUSTIC WAVES IN THE MEDIUM SURROUNDING SAID PANEL SUBSTANTIALLY BY THE EXPRESSION