US 2363068 A
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Nov. 21, 1944. R. L. LEADBETTER SOUND BARRIER WALL'OR DOOR CONSTRUCTION Filed May 4, 1944 2 SheetS-Sheetl 1 V l l Nov. 21, 1944. R. L'. LEADBETTER 2,363,068
SOUND BARRIER WALL OR DOOR CONSTRUCTION Filed may 4, 1.944 2 sheetwheet 2 v Patented Nov. 2l, 1944 SOUND BARRIER WALL OR DOOR CONSTRUCTION Ralph L. Leadbetter, Wheaton, Ill., assignor to Burgess-Manning Company, Chicago, Ill., a corporation of Illinois Application May 4, 1944, Serial No. 534,041
Claims. (Ol. 20-4) This invention relates to building construction, more particularly to doors and partitions which form barriers to the passage of sound from or into an enclosure such as a room or ofce. The specification herein forms a continuation-in-part application, the invention being described in the co-pending application of Ralph L. Leadbetter, Serial No. 372,310 illed December 30, 1940, now Patent No. 2,350,513, patented June 6, 1944.
Sound is the sensation produced by vibrations of air particles at the organs of hearing. The waves whichtravel from the source of sound to the tympanic membrane consist of alternating zones of pressure (condensations) and partial vacuum (rarefaction). These pressure waves, which are known as sound waves, possess appreciable kinetic energy and are capable of doing work.
Sound waves progressing in an elastic medium, such as air, and meeting anobstacle, such as the partition forming a wall of a room, are either reflected, absorbed and dissipated in the form of heat, or transmitted to structures connected with the obstacle or to the space adjacent the further side of the obstacle. More accurately, such waves are' generally reflected in part, and absorbed in part, the remaining energy being transmitted in the form of sound waves or vibrations. As a practical matter, no wall is a perfect rellector of sound, nor is any partition known which will absorb all of the energy of the sound waves incident upon its surface.
It follows from the foregoing that in order to acoustically insulate a room from the space surrounding it as, for example, to prevent the transmission of sounds from one room to an adjoining room through a common partition between them, the partition should be designed to .reflect and absorb as much of the energy of the sound waves impinging upon its surfaces as possible. However, since it is generally desirable in such circumstances to minimize -the sound level in one or both of the rooms, and since sound reilected from the partition would contribute to the maintenance of an elevated sound level, it is desirable in a sound barrier partition to absorb as much as possible of the sound energy rather than to reflect the sound waves back into the source room.
The primary object of this invention is to provide a sound barrier or insulating partition which willabsorb a large partof the energy of the sound waves impinging upon its surfaces with resulting minimum transmission land reflection of the sound. Secondary objectives include the provision of lightweight but stii and strong doors and partitions having these acoustical qualities.
Unless a partition is pervious to air in such a way that the oscillation of the air particles which make up the sound waves may continue from one side of the partition through openings therein to the other side, transmission of sound through the partition must be the result of forced vibration of the structure caused by the alternating pressures of the sound waves on the incident side of the partition and resulting in the formation of corresponding alternating pressures on the further side of the partition which, in turn, are manifested as sound waves. Thus, the acoustical energy of the soundwaves is converted into mechanical energy of vibration of the structure, the latter being reconverted into acoustical energy by the generation of sound waves in the receiving room, Various devices whereby attempts have been made to minimize the amplitude of such structural vibrations by increasing i, the mass of the structure as, for example, by
the use of masonryA walls, or by increasing the flexural stiffness, although effective to some extent, have proved to be generally undesirable from the cost or weight standpoints and necessarily result in increased reflection of the sound Waves into the source room. Attempts have also been made to increase the mechanical damping of the vibrating structure to thereby decrease the transmissivity without at the same time increasing the reiiectivity of the partition. Hollow walls have been packed with a bulky lling material, such as mineral wool, cotton linters, etc., but such lling material has a tendency to improve the coupling between the wall panels and is therefore ineifective as an acoustic insulating means. In one case, a plurality of spaced nonvibratile damping sheets are spaced between the outer panels of a, door;
A relatively complete description of known structural expedieni's intended to minimize transmission of sound through partitions may .be found in the National Bureau of Standards Report BMS 17 entitled` Sound Insulation of Wall and Floor Constructions, dated March 28, 1939.
The present invention proceeds on the assumption that it is impossible, as a practical matter, to prevent forced vibration of at least some element of a partition or door upon the surfaces of which sound waves impinge. The structure of the invention includes means for (1l-minimizing the amplitude of the forced vibrations.
(2) damping such vibrations, (3) preventing reilection of sound waves from the partition or door, and (4) absorbing sound within the source room to thereby minimize the sound level therein. A fifth component which is entirely unique with this invention comprises means for minimizing the coupling between the vibrating structure and the atmosphere adjacent the further surface of the partition; that is, means for preventing the generation of sound waves by the vibrating structure within the space adjacent the partition on the side thereof remote from the sound source. The structural expedients intended to perform these various functions are illustrated in the accompanying drawings, in which Fig. 1 is a perspective view, partly in section, of a portion of a partition and a door embodying the invention; V
Fig. 2 is an enlarged vertical .section of the door illustrated in Fig. 1; and
Figs. 3 to 5 are similar sectional views illustrating certain modifications of the invention.
Contrary to the heretofore universal practice whereby the outer panels of walls intended as sound barriers were made as air-tight as possible, all forms of the present invention have apertured surfaces. The surface member may be artificially perforated with sufficient small distributed perforations to render the member pervious to sound, or this member may be composed of a material having sucient inter-connected pores to admit of entrance of the sound waves without substantial reection thereof. In Figs. l and 2, apertured panels l and 2 comprise the exposed surfaces of partition 3 and door 4. As is more fully pointed out in Norris Patent No. 1,726,500, a panel which would ordinarily reflect a large part of the sound waves striking it may be made to transmit substantially all of the sound by perforating the panel as shown in Fig. 1. For example, panels l and 2 are made sound pervious and suitable for the purposes of the invention by providing them with'approximately fourteen sas inch holes per square inch throughout their areas.
Each form of the invention also includes air impervious inner members which are referred to herein as diaphragms because they may be set into vibration by the action of sound Waves. 'Ihe diaphragm 5, for example, is arranged between apertured panels I and 2 and is so mounted within the structurethat air particle movement on one side of the diaphragm may not be transmitted to the other side thereof except as a result of the vibration of the diaphragm itself. A stiiening grid 6 of intersecting strips may be cemented in place between apertured panels l and 2 to form a large number of cavities within the structure. creased stiffness resulting from the use of this grid limits the amplitude of vibration of the partition or door and for this reason added stiffness is effective. in preventing-transmission of sound waves. The cavities between the diaphragms 5 and the facing panels are filled with an intersticed mass 1, materials such as metallic, mineral or cellulosic wool, ceramic nodules, exfoliated vermiculite, or the loose fibrous material available under the trade name Cellufoam being suitable for this purpose. A material which has sufficient internal strength to support itself and not pack solidly in the lower portions of the several cavities is preferred.
Although experimental investigation has conclusively demonstrated the effectiveness of the It has been found that the ini above-described partition or door construction as a sound barrier, it is exceedingly dimcult to analyze the action and demonstrate the manner in which this desirable over-all result is accomplished. The various data available point to a plausible explanation of the action of the barrier. In order that the invention may be clearly understood, but without limitation to the particular action described, thisexplanation is given here. It is believed that the intensity of sound waves approaching the side of the partition adjacent the space indicated at A, Fig. 2, is substantially undiminished as the waves travel through the perforations of the apertured panel I and the interstices of the filling material 1. The alternating pressures applied to the diaphragm 5 by these sound waves cause the diaphragm to vibrate. The process thus far involves the transformation of the acoustical energy of sound Waves into mechanical energy of vibration of the diaphragm. If the structure on the further side of diaphragm 5, that is, on the side adjacent the space indicated by the letter B, were omitted, the mechanical energy of the vibrating diaphragm would be reconverted into acoustical energy as sound waves emanating from the diaphragm. However, it has been found that this reconversion is minimized by the presence of the intersticed masses l at the further surface of diaphragm 5. Apparently the presence of this filling material prevents the formation of sound Waves or at least minimizes their manifestation in the space B. Stated in a diiferent way, the presence of the filling material 1 results in poor acoustic coupling of the vibrating diaphragm 5 with the atmosphere in the space B. It is likely that apertured panel 2 itself contributes in some measure to this decoupling effect. 1
It should be pointed out that the structure on the incident side A of diaphragm 5 apparently operates in this same manner to prevent return of acoustical energy in the form of sound waves into the space A from the vibrating diaphragm. Once the diaphragm is set into vibration. whetherI from pressure waves approaching from A or from B, it tends to generate sound waves which would, except for the decoupling structure of the invention, travel in both directions from the two surfaces of the diaphragm. It will thus be seen that the sound levels in the spaces A and B are depressed .by the action of the barrier wall because (1) substantially no sound is reflected fromthe partition back into the vsource space and (2) substantially no sound is transmitted through the partition from the source space to the receiving space.
The efficiency of 'the barrier structure may be improved somewhat by providing means for internally damping the vibration of diaphragms 5 and thus dissipating the energy to some extent. For this' purpose, the diaphragms may be made of a suitable non-elastic material having substantial internal vdamping qualities, such as ordinary roofing felt. Such materials are referred to herein as non-rigid, non-vibratile materials. It should be recognized that the terms vibratile and non-vibratile as used herein are relative terms, it being understood that all materials will vibrate to some extent. The former term refers to such materials as sheet iron, plywood, etc.,
which tend to sustain vibration when struck and the latter term refers to relatively non-elastic In order to further limit vibration of the structure assembly, stiffening grids 6 are provided. It has been found that such grids increase the barrier efficiency of the partition, particularly in the lower frequency ranges.
In the embodiment illustrated in Fig. 3, each cavity formed ty grid 8 is provided with .a diaphragm 9 comprising two or more coextensive vibratile or non-vibratile sheets in contact with each other. It is advantageous to provide these sheets in different thicknesses, as this results in interference and damping of vibration. These diaphragms 9 should fit snugly between the strips forming the grid 8 but are free to move laterally to some extent so that the coupling between these vibrating members and the grid may be as poor as possible in view of the frictional engagement with the grid. The laminated diaphragm structure resulting from the use of two or more sheets introduces an additional damping factor resulting from interference between the individual sheets and frictional losses resulting from the sliding of the edges of the sheets against the gridwork and of one sheet against the adjacent sheet. As in the other forms of the invention, the spaces Il) and Il between the diaphragms 9 and the apertured outerpanels l2 and i3 are loosely packed with a filling material as described above.
The structures illustrated in Figs. 4 and 5 are similarto other embodiments of the invention except that the diaphragms I4 and I 4a are arranged diagonally within the cavities between the strips forming grids I and apertured panels I6 and Il. These diaphragms are preferably, although not necessarily, rigid. They may be arranged in paralle] orientation, as shown in Fig. 4, or alternate diaphragms Ila may span opposite diagonals of the cavities as illustrated in Fig. 5. Employment of such members greatly increases the stiffness of the structure with corresponding substantial improvement in acoustical performance. Furthermore, the diagonal diaphragms tend to diffuse the sound waves and apparently contribute to improve the over-all efficiency of the barrier and provide a more uniform action throughout the audible frequency range.
As used in the claims, the term wall" is intended to include permanent building partitions, panels constructed as separate units, doors, and similar structural elements.
1. A sound barrier wall construction comprising a pair of spaced substantially parallel apertured panels forming the exposed surfaces of said wall construction, a grid of intersecting strips between said panels and engaging the internal surfaces thereof and defining cavities therebetween, a. diaphragm within each said cavity and arranged between said apertured panels in spaced relation thereto, and intersticed masses in the spaces between said diaphragm and said apertured panels within each said cavity.
2. A sound barrier wall construction in accordance with claim 1 characterized in that each said diaphragm is arranged in substantial parallelism with said apertured panels and is floated between said intersticed masses without attachment to said grid.
3. The construction of claim l in which said diaphragms are non-vibratile.
4. The construction of claim l in which the edges of said diaphragms frictionally engage said grid.
5. The construction of claim 1 in which said diaphragms are non-vibratile and the erigeQ thereof fri'ctionally engage said grid.
6. A sound barrier wall construction comprising a pair of spaced substantially parallel apertured panels forming the exposed surfaces of said wall construction, a grid of intersecting strips between said panels and engaging the internal surfaces thereof and defining cavities therebetween, a plurality of diaphragms each substantially coextensive and in contact with adjacent; diaphragms and arranged within each cavity between said apertured panels in spaced relation thereto, and intersticed masses in the spaces between said diaphragms and said apertured panels within each said cavity.
7. The construction of claim 6 in which at least one of said diaphragms is non-vibratile.
8. A sound barrier wall construction comprising a pair of spaced substantially parallel apertured panels forming the exposed surfaces of said wall construction, a grid of intersecting strips between said panels and engaging the internal surfaces thereof and defining cavities therebetween, a diaphragm arranged diagonally within each said cavity to engage seid apertured panels and said grid at diagonally opposed lines of juncture of said grid with said panels.
9. A sound barrier wall in accordance with claim 8 in which the diaphragms in said cavities are arranged in substantially parallel orientation.
10. A sound barrier wall in accordance with claim 8 in which the diaphragms within alternate cavities are similarly oriented while the diaphragms in remaining cavities are oppositely oriented.
RALPH L. LEADBE'IIER.