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This invention pertains to floor structures having sound attenuation properties, and more specifically it pertains to a floor structure having vibration dampers incorporated therein.
At the present time, the National Building Code of Canada asks for a sound attenuation of 50 decibels through the walls separating apartments in a multi-apartment residential building. This standard is now under review, however. The Canada Mortgage and Housing Corporation for example, recently published sound attenuation objectives of over 55 decibels through walls and hard floors separating residential apartments, and over 65 decibels through carpet-covered floors. These objectives apply to sounds originating away from the floor, referred to as airborne sounds, and sound originating from the floor surface, referred to as impact sounds.
While several known floor structures can meet the requirement for airborne sound attenuation, the objective for impact sound attenuation has been a serious challenge in the construction industry.
The only prior art found disclosing a floor structure for minimizing impact noise transmission is the U.S. Pat. No. 3,270,475 issued to M. J. Kodaras on Sep. 6, 1966. This structure comprises a base layer made of low density material, fastened to the floor joists. Spaced-apart nailing strips are laid on the base layer perpendicularly to the floor joists and are retained to the base layer by spacer strips which are nailed to the base layer. The spacer strips have bevelled edges and define with the base layer spaced-apart dovetail slots in which the nailing strips are held without nail. The top flooring strips are nailed to the nailing strips with the nails not traversing the nailing strips. As the nails which secure the flooring strips to the nailing strips are completely isolated from the joists, there is no direct transmission of sound energy to the joists.
Although this document does not mention specific impact sound attenuation measurements, it is believed that this type of floor structure has great merits. This particular floor structure, however, is difficult and expensive to build by modern-day construction practices. It is believed that this difficulty constitutes a main reason, basically, why this method has not enjoyed a lasting commercial success.
Also, there is a trend in the construction industry to use 24 inch joist spacings as opposed to the long lasting standard of 16 inch spacings. The larger spacing requires more rigid floor and sub-floor layers. This trend motivates builders to combine rigidity and sound transmission attenuation performances in building systems.
As such, there is a need in the construction industry for a floor structure having acceptable sound attenuation characteristics without imposing a burden on existing construction trends and practices.
In the present invention, however, there is provided a floor structure that is compatible with modern-day construction practices with a joist spacing of 24 inches. The floor structure according to the present invention has an airborne sound attenuation of 65 decibels and an impact sound attenuation of 56 decibels. These sound transmission measurements were confirmed by the Acoustic Institute of the National Research Council of Canada.
More specifically, the present invention comprises a floor structure having spaced-apart floor joists, a base layer fastened to the joists, a resilient layer laid over the base layer, and a top layer mounted over the resilient layer. The top layer has a stiffness that is much greater than a stiffness of the base layer. The top layer is fastened to the base layer by wood screws which are placed substantially along a median between two adjacent floor joists, and each wood screw has a threaded portion extending simultaneously in both the top layer and the base layer.
The installation of the wood screws through both the top and the base layers causes a backlash in the advance of the screws upon entering the base layer, thereby causing the occurrence of a larger gap between the top layer and the base layer as compared with an installation using nails for example. The threaded portions of the screws being engaged simultaneously in both the top layer and the base layer act as spacers between the top layer and the base layer. The larger gap relaxes a pressure on the foam layer to reduce a transmission of sound and noise energy between the top layer and the joists.
In another aspect of the present invention, the top layer is made of balsam fir boards having a thickness of about 2 inches, and the base layer is made of oriented fibre boards having a thickness of ¾ inch. The balsam fir boards have a width of 24 inches, a length of 16 feet, and tongue-and-groove edges. The floor structure according to the present invention is very strong as compared with common floor structures, and is particularly appropriate for use with low-deflection flooring surfaces such as ceramic tiles.
This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiment thereof in connection with the attached drawings.
One embodiment of the present invention is illustrated in the accompanying drawings, in which like numerals denote like parts throughout the several views, and in which:
FIG. 1 is a perspective view of a floor structure according to the preferred embodiment of the present invention;
FIG. 2 is a perspective view of a low-density wood board used in the floor structure according to the preferred embodiment;
FIG. 3 is a schematic cross-section view of the preferred floor structure illustrating a general concept of a sound attenuating joint between floor layers in the preferred floor structure;
FIG. 4 illustrates the tip of a common wood screw;
FIGS. 5 and 6 illustrate two common floor structures of the prior art;
FIG. 7 is a cross-section view of the floor structure according to the preferred embodiment;
FIG. 8 is a table illustrating sound attenuation properties of the preferred floor structure as compared with floor structures of the prior art;
FIG. 9 is a cross-section view through the floor structure at a fastening point, with a fastener partly installed;
FIG. 10 is a cross-section view through the floor structure at a fastening point, with the fastener fully inserted through the floor layers, as seen in circle 10, in FIG. 7;
FIG. 11 is a cross-section view of the preferred floor structure at a fastening point, in a loaded condition, with the gap between the layers shown in an exaggerated mode.
While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will be described in details herein one specific embodiment of a floor structure with improved sound attenuation properties. The present disclosure is to be considered as an example of the principles of the invention and is not intended to limit the invention to the embodiment illustrated and described. For example, precise dimensions are used herein for convenience only to provide a better understanding of the structure of the present invention. Such dimensions should not be considered as being absolute and limiting.
Referring to FIGS. 1 and 2, the preferred floor structure comprises floor joists 20 spaced 24 inches apart. A base layer 22, is fastened to thejoist using nails or screws in any usual way. The base layer is made of plywood sheets or OSB™ (Oriented Strand Board) panels, having a thickness of ⅝ or ¾ inch, and more preferably ¾ inch.
A second layer 24 made of resilient material is laid on the base layer 22. The second layer is made of foam sheeting, geo-textile, rubber, felt or a similar material and has a thickness of about ⅛ inch. This second layer 24 is fastened to the base layer 22 in a conventional way, with staples for example. For economical reasons, the second layer 24 in the preferred embodiment is made of foam sheeting and is referred to herein as the foam layer 24 for convenience. The purpose of this second layer 24 is to prevent hard contact region or direct contact point between the base layer 22 and the top layer 26.
The top layer 26 is laid on the foam layer 24, and is fastened to the base layer 22 with wood screws 28 extending along medians 30 between adjacent joists 20 such that there is no direct transmission of sound energy from the top layer to the joists.
The top layer 28 is made of wood boards 32 having a thickness of 2 inches; a width of 24 inches and a length of 8 to 16 feet. The wood boards 32 are made of balsam fir and have a tongue-and-groove profile along their edges. The fir boards 32 are made of three plies with a different fibre alignment in the middle ply, as it is customary in plywood boards. This type of wood board is described in Applicant's Canadian patent application #2,434,248, filed on Jul. 3, 2003.
The two inch thick fir boards 32 have a moment of inertia which is about 32 times greater than a ⅝ inch sheet, and about 18 times greater than a ¾ inch panel (proportional to the cube of the thickness). The stiffness of the fir boards 32 is therefore greater than the stiffness of the base layer 22 by about the same proportions.
The principal contributing feature to obtain the sound attenuation properties of the floor structure according to the preferred embodiment of the present invention will now be explained while making reference to FIGS. 3 and 4. This feature is explained briefly using FIGS. 3 and 4, and in greater details when making references to FIGS. 9, 10, and 11.
In FIGS. 3 and 4, the screws 28 that are used to fasten the balsam fir boards 32 to the base layer 22 are three-inch common wood screws having threaded portions of about two inches long. In the installed position the threaded portion of each screw extends in both the balsam fir board 32 and in the base layer 22. Because of the nature of these screws 28, and its relatively blunt tip 34, the pressure on the screw 28 when exiting the balsam fir board 32 and entering the base layer 22 causes a thread backlash to occur, as the tip 34 of the screw digs into the surface of the base layer 22. This pressure also causes the base layer 22 to move away from the top layer 26 before the screw 28 can resume its advance into the base layer 22.
It will be appreciated that because of the engagement of the threaded portion in both the fir board 32 and the base layer 22, the screw cannot pull the base layer 22 against the fir board 32 at the end of its insertion. The screw segment ‘A’ traversing the foam layer 24 remains in compression to retain the top layer 26 at a distance from the base layer 22.
The combination of the thread backlash, the foam layer 24 and the long threaded portion of the screw 28, causes the occurrence of a joint 36 that has spacing and vibration-absorbing properties as illustrated schematically in FIG. 3. For visual description purposes, the spacer 38 represents the thread backlash or gap ‘A’ between the top layer 26 and the base layer 22, and the spring 40 represents the flexibility of the base layer 22 relative to the top layer 26.
Because of this type of joint 36, the base layer 22 is pre-stressed at every screw 28. Under no load condition, the base layer 22 springs back straight and pushes the top layer 26 upward, to release a compression in the foam layer 24. Because of this type of joint 36, it is believed that an impact force on the floor surface is partly absorbed by the deflection 42 in the base layer 22. It is also believed that a load on the top layer 26 is partly absorbed by a deflection 42 in the base layer 22 before a pressure is applied to the foam layer 24 in a region 44 above each joist 20 for example.
It is believed that sound transmission is effected primarily along these regions 44 above each joist 20 when the floor is loaded. It is also believed that the relaxation of pressure on the foam layer 24 due to the thread backlash or gap ‘A’ in each joint 36 contributes significantly to obtain the sound attenuation properties observed in the floor structure according to the preferred embodiment.
The sound attenuation properties referred to herein will be better understood when making reference to FIGS. 5-8. In FIG. 5, there is illustrated a floor structure made of floor joists 20 that are spaced 16 inches apart. Spaces between the joists 20 are partly filled with fibre-glass insulation 50. The floor portion is made of two layers of plywood sheets or OSB™ boards 52 laid over each other. Two layers of gypsum boards 54 are suspended to the joists 20 by suspension mouldings 56, which are common in the industry. The gypsum boards 54 constitute a ceiling for the apartment below the floor structure. The sound attenuation properties of this structure has been found to be 57 dB for airborne sounds and 50 dB for impact sounds, as shown in FIG. 8.
Another common type of floor structure, as illustrated in FIG. 6, has a 1- 1/2 inch thick cement slab 60 laid over a base layer 62 made of ⅝ or ¾ inch thick plywood sheets or OSB™ boards. The floor joists 20 are also spaced at 16 inches, and the insulation and ceiling arrangements are the same as in the first-described example. The sound attenuation characteristics of this second common floor structure has been found to be 69 dB for airborne sounds and 44 dB for impact sounds. The high density of this type of structure does not allow for wide span between joists.
Tests on the floor structure according to the preferred embodiment, however, have demonstrated that the sound attenuation properties of this preferred structure are 65 dB for airborne sounds and 56 dB for impact sounds. It will be appreciated that the sound attenuation properties of the floor structure according to the preferred embodiment exceeds the proposed requirement of 55 dB for both sound sources. It will also be appreciated that the high stiffness and relative low density of the floor structure according to the preferred embodiment allow for wide span of 24 inches or more between joists.
Referring now to FIGS. 9-11, the formation of screw joint 36 will be explained in greater details. As mentioned, a common 3-inch wood screw 28 has two inches of threads. When this screw is inserted in the fir board 32, whether it is inserted at right angle with the surface of the fir board 32, or at a slight angle in the tongue of the fir board 32, there is always a substantial portion of the thread length which remains engaged into the fir board 32. The tip 34 of a common wood screw 28, as shown in FIG. 4, does not have much axial grip or pull in a wood surface. The tip 34 normally extends to a length ‘B’ of about 0.050 to 0.080 inch. When the screw 28 reaches the surface of the base layer 22, the tip 34 drills into the surface of the base layer 22 for a few degrees or even a full turn or more before the thread starts pulling itself into the OSB™ or plywood layer 22.
During this initial drilling of the surface of the OSB™ or plywood layer 22, the engagement of the thread into the fir board 32 causes the screw 28 to continue to advance at a constant rate of speed. Consequently, a pressure is applied on the tip 34 of the screw and against the base layer 22.
Because the stiffness of the fir board 32 is much greater than the base layer 22, the base layer 22 is caused to move away from the fir board 32, one or few thousands of an inch or maybe more. When the screw 28 resumes its advance into the base layer 22, a small gap ‘A’ remains between the base layer 22 and the fir board 32.
Because of such screw backlash between the fir board 32 and the base layer 22, and because the screw 28 has thread engagement in both the fir board 32 and the base layer 22, the screw 28 constitutes a spacer for separating the fir board 32 from the base layer 22. Such a spacer means is represented by a block-type spacer 38 in FIG. 3. The gap ‘A’ define by this spacer 38 is perhaps very small but nonetheless contributes to relaxing a compression of the foam layer 24 to some extent. Such gap would not be formed in an installation using nails for example.
Because the fir boards 32 are much more rigid than the base layer 22, a loading on the floor structure deflects the base layer 22 before the fir boards 32, and before the fir boards 32 can apply a pressure on the foam layer 24 above the joists 20, as indicated by the regions 44. The base layer 22 acts as a shock absorber or a suspension system to support the fir boards 32 in a floating mode above the foam layer 24 and the joists 20.
FIG. 11 illustrates in an exaggerated manner the gap ‘A’ caused by the screw backlash mentioned before, and an initial deflection of the base layer 22, as in a leaf spring, when the floor structure is loaded. It is believed that such a suspension system contributes greatly to obtaining the sound attenuations properties described herein.
As to other manner of usage and operation of the present invention, the same should be apparent from the above description and accompanying drawings, and accordingly further discussion relative to the manner of usage and operation of the invention would be considered repetitious and is not provided.
While one embodiment of the floor structure according to the present invention has been illustrated and described herein above, it will be appreciated by those skilled in the art that various modifications, alternate constructions and equivalents may be employed without departing from the true spirit and scope of the invention. For example, it is known that similar advantageous results can be obtained with screws other than common wood screws, as long that their threaded portions extend simultaneously in the base layer and the fir boards. Therefore, the above description and the illustrations should not be construed as limiting the scope of the invention.