Patent Publication Number: US-2016220031-A1

Title: Mattress assembly

Description:
FIELD OF THE INVENTION 
     This application is a continuation of U.S. application Ser. No. 14/102,633, filed Dec. 11, 2013, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to body support assemblies, and more particularly to mattresses and other body supports having spring elements. 
     Body support assemblies are typically used in bedding, seating, and other applications to support a user&#39;s body or a portion thereof (e.g., head, shoulders, legs, etc.) while the user is at rest. With reference to mattress assemblies by way of example, many mattress assemblies include multiple foam layers. Conventional mattress assemblies are typically adapted for different firmnesses and comfort feel by adjusting the number, properties, and thickness of the constituent foam layers. However, due to the fact that inherent limitations exist in the design of body supports relying on these methods of firmness control, advancements in this area of technology are welcome additional to the art. 
     SUMMARY OF THE INVENTION 
     The invention provides, in one aspect, a mattress assembly including a first layer of viscoelastic foam defining an upper surface, and a second layer of non-viscoelastic foam supporting the first layer. The mattress assembly also includes a plurality of spring elements positioned beneath the upper surface for enhancing a firmness of the combined first and second layers. Each of the plurality of spring elements includes a first spring having a first spring rate and a second spring having a second spring rate different than the first spring rate. 
     Other features and aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a mattress assembly in accordance with an embodiment of the invention. 
         FIG. 2  is a cross-sectional view of the mattress assembly of  FIG. 1 , taken along line  2 - 2  in  FIG. 1 . 
         FIG. 3  is a cross-sectional view of the mattress assembly of  FIG. 1 , taken along line  3 - 3  in  FIG. 1 . 
         FIG. 4  is a cross-sectional view, similar to that of  FIG. 2 , of a mattress assembly in accordance with another embodiment of the invention. 
         FIG. 5  is a cross-sectional view, similar to that of  FIG. 3 , of the mattress assembly of  FIG. 4 . 
         FIG. 6  is a cross-sectional view, similar to that of  FIG. 2 , of a mattress assembly in accordance with a further embodiment of the invention. 
         FIG. 7  is a cross-sectional view, similar to that of  FIG. 3 , of the mattress assembly of  FIG. 6 . 
         FIG. 8  is a cross-sectional view, similar to that of  FIG. 2 , of a mattress assembly in accordance with yet another embodiment of the invention. 
         FIG. 9  is a cross-sectional view, similar to that of  FIG. 3 , of the mattress assembly of  FIG. 8 . 
     
    
    
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a mattress assembly  1  for use in a bed. The mattress assembly  1  includes a first layer  4  of viscoelastic foam defining an upper surface  8  of the mattress assembly  1  and having a thickness T 1  ( FIG. 2 ). Viscoelastic foam is sometimes referred to as “memory foam” or “low resilience foam.” Coupled with the slow recovery characteristic of the viscoelastic foam, the first layer  4  can at least partially conform to the user&#39;s body or body portion (hereinafter referred to as “body”), thereby distributing the force applied by the user&#39;s body upon the viscoelastic foam layer  4 . The viscoelastic foam layer  4  can provide a relatively soft and comfortable surface for the user&#39;s body. In other embodiments, the first layer  4  comprises another type of foam suitable as a mattress top layer. 
     In some embodiments, the viscoelastic foam layer  4  has a hardness of at least about 20 N and no greater than about 80 N for desirable softness and body-conforming qualities. Alternatively, the viscoelastic foam layer  4  may have a hardness of at least about 30 N and no greater than about 70 N. In still other alternative embodiments, the viscoelastic foam layer 4 may have a hardness of at least about 40 N and no greater than about 60 N. Unless otherwise specified, the hardness of a material referred to herein is measured by exerting pressure from a plate against a sample of the material to a compression of 40 percent of an original thickness of the material at approximately room temperature (e.g., 21 to 23 degrees Celsius). The 40 percent compression is held for a set period of time, following the International Organization of Standardization (ISO) 2439 hardness measuring standard. 
     With continued reference to  FIG. 1 , the viscoelastic foam layer  4  can also have a density providing a relatively high degree of material durability. The density of the viscoelastic foam layer  4  can impact other characteristics of the foam, such as the manner in which the viscoelastic foam layer  4  responds to pressure, and the feel of the viscoelastic foam layer  4 . In the illustrated embodiment, the viscoelastic foam layer  4  has a density of no less than about 30 kg/m 3  and no greater than about 150 kg/m 3 . Alternatively, the viscoelastic foam layer  4  may have a density of at least about 40 kg/m 3  and no greater than about 135 kg/m 3 . In still other alternative embodiments, the viscoelastic foam layer  4  may have a density of at least about 50 kg/m 3  and no greater than about 120 kg/m 3 . 
     The viscoelastic foam layer  4  can be made from non-reticulated or reticulated viscoelastic foam. Reticulated viscoelastic foam has characteristics that are well suited for use in the mattress assembly  1 , including the enhanced ability to permit fluid movement through the reticulated viscoelastic foam, thereby providing enhanced air and/or heat movement within, through, and away from the viscoelastic foam layer  4  of the mattress assembly  1 . Reticulated foam is a cellular foam structure in which the cells of the foam are essentially skeletal. In other words, the cells of the reticulated foam are each defined by multiple apertured windows surrounded by struts. The cell windows of the reticulated foam can be entirely gone (leaving only the cell struts) or substantially gone. For example, the foam may be considered “reticulated” if at least 50 percent of the windows of the cells are missing (i.e., windows having apertures therethrough, or windows that are completely missing and therefore leaving only the cell struts). Such structures can be created by destruction or other removal of cell window material, or preventing the complete formation of cell windows during the manufacturing process. 
     With reference to  FIG. 1 , the mattress assembly  1  also includes a second layer  12  of non-viscoelastic foam supporting the viscoelastic foam layer  4 . The non-viscoelastic foam layer  12  has a thickness T 2  that is greater than the thickness T 1  of the viscoelastic foam layer  4 . Alternatively, the thickness T 2  of the non-viscoelastic foam layer  12  may be the same or less than the thickness T 1  of the viscoelastic foam layer  4 . The non-viscoelastic foam layer  12  may be a latex foam or a high-resilience (HR) polyurethane foam, by way of example only. Such a latex foam has a hardness of at least about 30 N and no greater than about 130 N for a desirable overall mattress assembly firmness and “bounce.” Alternatively, the latex foam may have a hardness of at least about 40 N and no greater than about 120 N, or at least about 50 N and no greater than about 110 N. The latex foam has a density of no less than about 40 kg/m 3  and no greater than about 100 kg/m 3 . In still other alternative embodiments, the latex foam may have a density of at least about 50 kg/m 3  and no greater than about 100 kg/m 3 , or at least about 60 kg/m 3  and no greater than about 100 kg/m 3 . In other embodiments, the second layer can comprise other types of foam as desired. 
     In embodiments of the mattress assembly  1  in which the non-viscoelastic foam layer  12  includes HR polyurethane foam, such a foam can include an expanded polymer (e.g., expanded ethylene vinyl acetate, polypropylene, polystyrene, or polyethylene), and the like. In some embodiments, the HR polyurethane foam has a hardness of at least about 80 N and no greater than about 200 N for a desirable overall cushion firmness and “bounce.” Alternatively, the HR polyurethane foam may have a hardness of at least about 90 N and no greater than about 190 N, or at least about 100 N and no greater than about 180 N. The HR polyurethane foam has a density which provides a reasonable degree of material durability to the non-viscoelastic foam layer  12 . The HR polyurethane foam can also impact other characteristics of the non-viscoelastic viscoelastic foam layer  12 , such as the manner in which the non-viscoelastic foam layer  12  responds to pressure. In some embodiments, the HR polyurethane foam has a density of no less than about 10 kg/m 3  and no greater than about 80 kg/m 3 . In still other alternative embodiments, the HR polyurethane foam may have a density of no less than about 15 kg/m 3  and no greater than about 70 kg/m 3 , or no less than about 20 kg/m 3  and no greater than about 60 kg/m 3 . 
     With reference to  FIGS. 2 and 3 , the mattress assembly  1  further includes multiple static spring elements  16  positioned beneath the upper surface  8  of the mattress assembly  1  for enhancing a firmness of the combined viscoelastic and non-viscoelastic foam layers  4 ,  12 . Particularly, the spring elements  16  are embedded into the second layer (i.e., the non-viscoelastic foam layer  12 , in the illustrated embodiment) using a molding process, and the viscoelastic foam layer  4  is attached to the upper surface  20  of the non-viscoelastic foam layer  12  (e.g., using adhesives, etc.). In the illustrated embodiment, the spring elements  16  are aligned with a thickness T 3  of the mattress assembly  1  and are entirely encased within the non-viscoelastic foam layer  12  ( FIG. 2 ). In other words, each spring element  16  is separated or isolated from adjacent spring elements  16  by the non-viscoelastic foam layer  12 . The spring elements  16  may be partially encased within the non-viscoelastic foam layer  12  and covered by the viscoelastic foam layer  4  such that the spring elements  16  may be positioned between the viscoelastic and non-viscoelastic foam layers  4 ,  12 . 
     The spring elements  16  of the illustrated embodiment are arranged in an array having multiple rows and multiple columns ( FIG. 3 ). The array can be in the form of a grid, in which the spring elements  16  are spaced across a portion or all of the width and length of the mattress assembly  1 . In such cases, consecutive spring elements  16  extending in width-wise and length-wise directions along the mattress assembly  1  can extend substantially parallel to the width and length of the mattress assembly  1 . Alternatively, consecutive spring elements  16  may extend diagonally with respect to the width and length of the mattress assembly  1 . In other words, each row may be offset or shifted relative to the preceding and/or following row. In still other alternative constructions, the spring elements  16  may be arranged randomly, in a single row, in a single column, in arcs, rings, concentric rings, or other geometric shapes and patterns, or in combinations thereof. 
     With continued reference to  FIGS. 2 and 3 , the spring elements  16  are made of a polymeric material, and more specifically, a thermoplastic material (e.g., TPEE, SBS, SEBS, TPV, etc.). The spring elements  16  are configured as coil springs having the same length. Alternatively, the spring elements  16  may be configured as leaf springs, for example, or any of a number of different types of springs. In still other alternative constructions, the spring elements  16  may include different lengths. For example, a first spring element  16  may have a different length than a second spring element  16  or a first group of spring elements  16  may have a different length than a second group of spring elements  16 , and so forth. In the illustrated embodiment of the mattress assembly  1 , the spring elements  16  have the same spring rates. Alternatively, the spring elements  16  may have different spring rates. For example, a first spring element  16  may have a different spring rate than a second spring element  16  or a first group of spring elements  16  (e.g., located in a first region of the mattress assembly  1 , such as a torso region of the mattress assembly) may have a different spring rate than a second group of spring elements  16  (e.g., located in a second region of the mattress assembly  1 , such as a buttocks and/or legs region of the mattress assembly), and so forth. 
     The spring rate of the spring elements  16  can be a constant spring rate or a variable spring rate. Spring elements  16  including a constant spring rate often have the same or a constant spacing between coils of the spring element  16  as compared to a variable spring rate, in which the spacing between the coils is often different or variable. 
     In some embodiments of the mattress assembly  1 , the firmness of the combined viscoelastic and non-viscoelastic foam layers  4 ,  12  can be enhanced substantially uniformly across the width and length of the mattress assembly  1 . Alternatively, the firmness of the combined viscoelastic and non-viscoelastic foam layers  4 ,  12  can be enhanced non-uniformly across the width and length of the mattress assembly  1 . For example, the non-uniform firmness of the mattress assembly  1  may be tuned (e.g., by using different spring elements, different rate spring elements, a combination of constant and variable rate spring elements, etc.) in accordance with the locations or regions of the mattress assembly  1  normally associated with certain portions (e.g., head, shoulders, legs, etc.) of the user&#39;s body that require different support. In other words, the spring elements  16  may be selected to enhance the firmness of the combined viscoelastic and non-viscoelastic foam layers  4 ,  12  a greater amount in regions of the mattress assembly  1  associated with a reclined user&#39;s lower legs, posterior, and head/neck, for example. 
     With continued reference to  FIGS. 2 and 3 , the spring elements  16  have the same material thickness (i.e., the thickness of the material shaped into the spring elements  16  show by way of example in the illustrated embodiment), winding density, shape, and diameter. However, in alternative embodiments of the mattress assembly  1 , the material thickness, winding density, shape, diameter, or combinations thereof may be altered to more or less enhance the firmness of the combined viscoelastic and non-viscoelastic foam layers  4 ,  12 . 
     When using the mattress assembly  1 , the user&#39;s body contacts the upper surface  8  of the mattress assembly  1 . In turn, the spring elements  16  enhance the firmness of the combined viscoelastic and non-viscoelastic foam layers  4 ,  12  to provide comfort to the user. By replacing a portion of the non-viscoelastic foam layer  12  with the spring elements  16 , the mattress assembly  1  can have a lower weight as compared to conventional mattress assemblies, and can provide a firmness and pressure responsiveness that is more desirable for particular users. Additionally, the mattress assembly  1  can be readily altered with respect to the comfort and feel provided to the user by altering the spring elements  16  to have a different spring rate, material thickness, shape, and the like. In other words, the mattress assembly  1  can be manufactured in a cost-effective manner to provide users with different mattress assemblies  1  having different properties (e.g., firmness, feel, etc.) by altering the spring elements  16  as compared to a conventional mattress assembly in which an entire layer or more would need be redesigned to provide a different mattress assembly to the user. 
       FIGS. 4 and 5  illustrate a second embodiment of the mattress assembly  1   a  used in connection with beds. Like components to those of the embodiments described above in connection with  FIGS. 1-3  are identified with like reference numerals with the letter “a,” and will not be described again in detail. Rather than embedding the spring elements  16  into the non-viscoelastic foam layer  12  as shown in  FIGS. 2 and 3  and described above, the mattress assembly la illustrated in  FIGS. 1-3  include spring elements  16   a  positioned within discrete cavities  24  within the non-viscoelastic foam layer  12   a.  The cavities  24  can be formed in the non-viscoelastic foam layer  12   a  by a drilling process or a cutting process, for example. The spring elements  16   a  are placed or positioned within the cavities  24 , and the viscoelastic foam layer  4   a  is attached or fastened to the upper surface  20   a  of the non-viscoelastic foam layer  12   a  (e.g., using adhesives, etc.). 
     The mattress assembly  1   a  can be used in an identical fashion as the mattress assembly  1  shown in  FIGS. 2 and 3 . 
       FIGS. 6 and 7  illustrate another embodiment of the mattress assembly  1   b  used in connection with beds. The mattress assembly  1   b  is similar to the mattress assembly  1  described above in connection with  FIGS. 1-3 . Like components to those of the embodiments described above in connection with  FIGS. 1-3  are identified with the letter “b,” and will not be described again in detail. 
     With reference to  FIGS. 6 and 7 , the mattress assembly  1   b  includes multiple static spring elements  16   b  positioned beneath the upper surface  8   b  of the mattress assembly  1   b  for enhancing a firmness of the combined viscoelastic and non-viscoelastic foam layers  4   b,    12   b . Particularly, the spring elements  16   b  are embedded into the non-viscoelastic foam layer  12   b  using a molding process, and the viscoelastic foam layer  4   b  is attached to the upper surface  20   b  of the non-viscoelastic foam layer  12   b  (e.g., using adhesives, etc.). The spring elements  16   b  are configured as multi-rate spring elements and include a first spring  28 , a second spring  32 , and a third spring  36  arranged in series (i.e., one atop the next). Alternatively, the spring elements  16   b  may include a single spring or any other number of springs (e.g., two springs, four springs, etc.) arranged in series. The first spring  28  is supported on the second spring  32 , and the second spring  32  is supported on the third spring  36 . The spring elements  16   b  include dividers  40  positioned between adjacent springs (i.e., between the first and second springs  28 ,  32 , and between the second and third springs  32 ,  36 ) to facilitate and/or enhance force transfer between the springs  28 ,  32 ,  36 . The dividers  40  may be formed of a polymeric material, such as non-viscoelastic foam or thermoplastic material. In some embodiments, the dividers  40  may be omitted. As a further alternative, the springs  28 ,  32 ,  36  and the dividers  40  may be integrally formed together as a single piece or may be formed as separate pieces. 
     Each of the springs  28 ,  32 ,  36  in the illustrated embodiment of  FIGS. 6 and 7  has a different spring rate to give the mattress assembly  1   b  a different firmness or feel depending on the weight of a user&#39;s body supported by the mattress assembly  1 . In the illustrated embodiment of  FIGS. 6 and 7 , the first spring  28  has the lowest spring rate, the second spring  32  has an intermediate spring rate, and the third spring  36  has the highest spring rate. In other words, the first spring  28  includes the lowest stiffness of the springs  28 ,  32 ,  36 , while the third spring  36  includes the highest stiffness of the springs  28 ,  32 ,  36 . For example, the first spring  28  can have a spring stiffness rate between 150 lb/in and 200 lb/in, the second spring  32  can have a spring stiffness rate between 200 lb/in and 250 lb/in, and the third spring  36  can have a spring rate between 250 lb/in and 300 lb/in. In other embodiments, the first spring  28  stands up to a maximum weight of 200 lb human body, the second spring  32  stands up to a maximum weight of 250 lb human body and the third spring  36  stands up to a maximum weight of 300 lb human body. These numbers are for illustration purposes only and can be adjusted and modified by changing the stiffness spring rates for each spring  28 ,  32  and  36 . Alternatively, the springs  28 ,  32 ,  36  can have other spring rates or relative spring rates to tune the mattress to any desired firmness or feel. 
     With continued reference to the illustrated embodiment of  FIGS. 6 and 7 , as a relatively light weight (e.g., the weight of the user&#39;s body) is applied to the mattress assembly  1   b , the spring elements  16   b  exhibit a relatively low effective spring rate because a substantial amount of the compression of the spring element  16   b  occurs in the first spring  28  in each of the elements  16   b.  As the weight applied to the spring elements  16   b  increases (e.g., when a heavier individual is supported upon the mattress assembly  1   b ), the first springs  28  become fully compressed (or at least substantially more compressed), and the spring elements  16   b  transition to an intermediate spring rate because a substantial amount of the compression of the spring element  16  occurs in the first and second springs  28 ,  32  in each of the elements  16   b.  As the weight applied to the spring elements  16   b  increases further (e.g., when an even heavier individual is supported upon the mattress assembly  1   b ), the second springs  32  also become fully compressed (or at least substantially more compressed), and the spring elements  16   b  transition to their maximum effective spring rate because each of the springs  28 ,  32 ,  36  undergoes compression. Thus, the spring elements  16   b  provide a variable firmness or feel depending on the weight of the user&#39;s body supported by the mattress assembly  1   b . The springs  28 ,  32 ,  36  may be selected so that the low, intermediate, and maximum effective spring rates of the spring elements  16   b  correspond with particular weights supported by the mattress assembly  1   b . For example, the spring elements  16   b  may exhibit the relatively low effective spring rate for a user&#39;s body weighing between about 100 lbs. and about 150 lbs. The spring elements  16   b  may exhibit the intermediate effective spring rate for a user&#39;s body weighing between about 150 lbs. and about 220 lbs. The spring elements  16   b  may exhibit the highest effective spring rate for a user&#39;s body weighing between about 220 lbs. and about 350 lbs. In other embodiments, the springs  28 ,  32 ,  36  may be selected so that the spring elements  16   b  transition between effective spring rates at other weights. 
     Although the springs  28 ,  32 ,  36  of the spring elements  16   b  just described are selected with spring rates that are larger with increasing depth within the mattress assembly  1   b , this is not necessarily the case in other embodiments. The “staged” reaction of each spring  23 ,  32 ,  36  in a spring element  16   a  (i.e., one spring  23 ,  32 ,  36  of the spring  16   b  exhibiting compression at higher forces than at least one other spring element  23 ,  32 ,  36  of the spring  16   b ) can be achieved in cases where an overlying spring (e.g., spring  28 ) has a higher spring rate than an underlying spring (e.g., spring  32  and/or  35 ), in which case the underlying spring would exhibit compression before the overlying spring in a staged manner as described above. Although higher spring rates for underlying springs provide unique advantages in some embodiments, any combination of spring rates corresponding to different stacked positions of two or more springs in a spring element  16   b  is possible, and falls within the spirit and scope of the present invention. 
     In the illustrated embodiment, the spring rates of the respective springs  28 ,  32 ,  36  are constant. Alternatively, the spring rates of one or more of the springs  28 ,  32 ,  36  may be variable. Springs  28 ,  32 ,  36  having a constant spring rate often have the same or a constant spacing between coils as compared to a variable spring rate, in which the spacing between the coils is often different or variable. 
     With continued reference to  FIGS. 6 and 7 , each of the springs  28 ,  32 ,  36  is made of a polymeric material, and more specifically, a thermoplastic material (e.g., TPEE, SBS, SEBS, TPV, etc.). In the illustrated embodiment, the spring material is thermally conductive, and the springs  28 ,  32 ,  36  can therefore function as heat sinks to dissipate heat away from the viscoelastic foam layer  4   b  (and from the body of a user supported on the mattress assembly  1   b ). Alternatively, in other embodiments only the first spring  28  is thermally conductive, or less than all of the springs  28 ,  32 ,  36  are thermally conductive. In other alternative embodiments, the springs  28 ,  32 ,  36  may not be thermally conductive, and may not function as heat sinks. 
     As shown in  FIG. 6 , the springs  28 ,  32 ,  36  are each configured as coil springs having the same length. Alternatively, the springs  28 ,  32 ,  36  may be configured as leaf springs, for example, or any of a number of different types of springs. Alternatively, the springs  28 ,  32 ,  36  may include multiple different spring types. Accordingly, the springs of at least some spring elements  16   b  can all be of the same types of spring, or the springs of at least some spring elements  16   b  can have different spring types stacked atop one another). In still other alternative embodiments, the springs  28 ,  32 ,  36  may include different lengths. For example, a spring element  16   b  may include a first spring  28  having a different length than a second spring  32 , and may include a third spring  36  having a different length than the first and second springs  28 ,  32 . In the illustrated embodiment of the mattress assembly  1   b , the spring elements  16   b  have the same effective spring rates (i.e., the first springs  28  have the same spring rates, the second springs  32  have the same spring rates, and the third springs  36  have the same spring rates). It will be appreciated that the spring elements  16   b  may have different spring rates. For example, a first spring element  16   b  may have a different effective spring rate than a second spring element  16   b  or a first group of spring elements  16   b  may have a different effective spring rate than a second group of spring elements  16   b,  and so forth. In such an embodiment, the first spring element  16   b  or first group of spring elements  16   b  may have first, second, and third springs  28 ,  32 ,  36  that have different respective spring rates than first, second, and third springs  28 ,  32 ,  36  of the second spring element  16   b  or second group of springs elements  16   b,  and so forth. 
     In some embodiments of the mattress assembly  1   b , the firmness of the combined viscoelastic and non-viscoelastic foam layers  4   b,    12   b  can be enhanced substantially uniformly across the width and length of the mattress assembly  1 . Alternatively, the firmness of the combined viscoelastic and non-viscoelastic foam layers  4   b,    12   b  can be enhanced non-uniformly across the width and length of the mattress assembly  1   b . For example, the non-uniform firmness of the mattress assembly  1   b may be tuned (e.g., by using different spring elements  16   b,  different rate springs, a combination of constant and variable rate springs, etc.) in accordance with the locations or regions of the mattress assembly  1   b  normally associated with certain portions (e.g., head, shoulders, legs, etc.) of the user&#39;s body that require different support. In other words, the springs  28 ,  32 ,  36  of the spring elements  16   b  may be selected to enhance the firmness of the combined viscoelastic and non-viscoelastic foam layers  4   b,    12   b  a greater amount in regions of the mattress assembly  1   b  associated with a reclined user&#39;s lower legs, posterior, and head/neck, for example. 
     When using the mattress assembly  1   b , the user&#39;s body contacts the upper surface  8   b  of the mattress assembly  1   b . In turn, the spring elements  16   b  enhance the firmness of the combined viscoelastic and non-viscoelastic foam layers  4   b,    12   b  to provide comfort to the user. When supporting a relatively lightweight user, the spring elements  16   b  provide a relatively low firmness corresponding with compression of the first, softest springs  28 . When supporting a heavier user, first springs  28  of some or all of the spring elements  16   b  may become fully compressed, such that the spring elements  16   b  provide increased firmness corresponding with compression of the second, intermediate springs  32 . Similarly, when supporting an even heavier user, the first springs  28  and the second springs  32  may become fully compressed, such that some or all of the spring elements  16   b  provide even greater firmness corresponding with compression of the third, stiffest spring  36 . Therefore, due to the multi-rate design of the spring elements  16   b,  the mattress assembly  1   b  is able to self-adjust to provide an optimum firmness as a function of the weight of the user&#39;s body. 
       FIGS. 8 and 9  illustrate another embodiment of the mattress assembly  1   c  used in connection with beds. The mattress assembly  1   c  is similar to the mattress assembly  1   b  described above in connection with  FIGS. 6 and 7 . Like components to those of the embodiments described above in connection with  FIGS. 6 and 7  are identified with like reference numerals with the letter “c,” and will not be described again in detail. 
     Rather than embedding the spring elements  16   c  into the non-viscoelastic foam layer  12   c  like that shown in  FIGS. 6 and 7  and described above, the mattress assembly  1   c  includes spring elements  16   c  having first springs  28   c,  second springs  32   c,  and third springs  36   c  positioned in series within discrete cavities  24   c  within the non-viscoelastic foam layer  12   c.  The cavities  24   c  can be formed in the non-viscoelastic foam layer  12   c  by a drilling process or a cutting process, for example. The spring elements  16   c  are placed or positioned within the cavities  24   c,  and the viscoelastic foam layer  4   c  is attached or fastened to the upper surface  20   c  of the non-viscoelastic foam layer  12   c  (e.g., using adhesives, etc.). 
     The mattress assembly  1   c  is operable in an identical manner as the mattress assembly  1   b  shown in  FIGS. 6 and 7  and described above. 
     Various features of the invention are set forth in the following claims.