Patent Publication Number: US-9423149-B2

Title: Methods and apparatuses to attenuate acoustic waves

Description:
FIELD 
     The disclosure herein relates to a method and an apparatus (such as a muffler) that can be used to attenuate acoustic waves. More specifically, the disclosure herein relates to a method and an apparatus that can be used to attenuate acoustic waves generated by, for example, a compressor in a heating, ventilation, and air conditioning (HVAC) system. 
     BACKGROUND 
     A muffler is often used to attenuate acoustic waves (e.g., sound) generated by a machine, such as for example, an engine or a compressor. In an HVAC system, an HVAC compressor is one of the major acoustic wave sources. A muffler can be used with the compressor to attenuate the acoustic waves generated by the compressor, which may help reduce an operational sound of the HVAC system. Reducing operational sound may be desirable, for example, in a school, during nighttime, or in other situations. 
     SUMMARY 
     Methods and apparatuses (such as a muffler) configured to attenuate acoustic waves are described herein. The embodiments as disclosed herein are generally configured to provide a different phase shift(s) to acoustic waves and merge the acoustic waves after the phase shifting. Because the acoustic waves may have different phases after the phase shifting, the acoustic waves can be attenuated when merged. 
     In some embodiments, a muffler may include a first acoustic path and a second acoustic path. The first and/or the second acoustic paths may be external or internal to a housing of the muffler. The first acoustic path and the second acoustic path may be configured to provide different phase shifts to acoustic waves, and the first acoustic path and the second acoustic path are configured to direct acoustic waves to merge after traversing the first and second acoustic paths. 
     In some embodiments, the first acoustic path and the second acoustic path may have different lengths, which may help provide different acoustic impedance when an acoustic wave passes through the first and second acoustic paths. In some embodiments, the first acoustic path and the second acoustic path may be configured to have different acoustic impedance by other suitable configurations (e.g., different cross-sectional areas, sudden expansion/contraction, and/or baffle structures, etc.). When the acoustic waves merge after passing through the first and second acoustic paths, the difference in the acoustic impedance may result in a destructive interference between the acoustic waves from the first and second acoustic paths. 
     In some embodiments, the muffler may include a muffler housing, and the first acoustic path may be internal to the muffler housing, and the second acoustic path may be external to the muffler housing or vice versa. In some embodiments, both or neither of the first acoustic path and the second acoustic path may be internal to the muffler housing. 
     In some embodiments, the muffler may include a third acoustic path. The third acoustic path can be configured to provide a different acoustic impedance (e.g., a phase shift, etc.) than one or both of the first and second acoustic paths. In some embodiments, the first and second acoustic paths may be internal to the muffler housing and the third acoustic path may be external to the muffler housing. In some embodiments, the first and second acoustic paths may be external to the muffler housing and the third acoustic path may be internal to the muffler housing. 
     In some embodiments, a method of attenuating acoustic waves may include providing a first degree of phase shift to a first portion of the acoustic waves; providing a second degree of phase shift to a second portion of the acoustic waves; and merging the first portion of the acoustic waves and the second portion of the acoustic waves. In some embodiments, the first degree of phase shift and the second degree of phase shift may be different. 
     Other features and aspects will become apparent by consideration of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the drawings in which like reference numbers represent like parts throughout. 
         FIG. 1  illustrates a schematic diagram of a muffler, according to some embodiments. 
         FIG. 2  illustrates a muffler that is coupled to a compressor, according to some embodiments. 
         FIGS. 3A to 3E  illustrate side section views of exemplary acoustic paths, according to some embodiments.  FIG. 3A  illustrates an acoustic path that includes two contracted portions arranged in parallel and an expanded portion in communication with the contracted portions, according to some embodiments.  FIG. 3B  illustrates an acoustic path that includes two contracted portions and expanded portions arranged in series, according to some embodiments.  FIG. 3C  illustrates an acoustic path that includes an acoustic absorbing or phase shifting material, according to some embodiments.  FIG. 3D  illustrates an acoustic path that includes a perforated plate, according to some embodiments.  FIG. 3E  illustrates an acoustic path that includes a plurality of baffles, according to some embodiments. 
         FIG. 4  illustrates a muffler that includes a plurality of internal muffler paths. 
         FIG. 5  illustrates a muffler that includes a plurality of external muffler paths. 
     
    
    
     DETAILED DESCRIPTION 
     A compressor (such as, but not limited to, a screw compressor) of an HVAC system is one major source of acoustic waves, causing operational sound of the HVAC system. Reducing the sound is often desirable, for example, in a school, at nighttime, or in other situations. A muffler can be added to the compressor to attenuate the acoustic waves of the compressor, which may help reduce the operational sound of the HVAC system. Traditionally, a muffler may be configured to trap acoustic energy into resonances inside the muffler, which may help reduce the acoustic energy passing through. The traditional muffler, for example, may be configured to direct the acoustic waves through a contracted structure and an expanded structure. The acoustic waves can be attenuated when directed between the contracted structure and the expanded structure, which helps trap the acoustic energy. However, such a muffler may be relatively large in size. 
     The embodiments disclosed herein are directed to methods and apparatuses to attenuate acoustic waves. A method of attenuating acoustic waves may include directing the acoustic waves into a plurality of acoustic paths. Each of the acoustic paths may provide a different acoustic impedance (e.g., a phase shift(s), etc.) to portions of the acoustic waves, so that the portions of the acoustic waves may have different phases after passing through the plurality of acoustic paths. The method can also include merging the portions of the acoustic waves after the acoustic waves have passed through the plurality of acoustic paths. Because different portions of the acoustic waves may be out of phase, merging the portions of the acoustic waves back can result in destructive interference, which can help attenuate the acoustic waves. In some embodiments, a muffler incorporating the method of attenuating acoustic waves may include more than one acoustic path, each of which may have a different acoustic impedance (e.g., lengths, cross-sectional areas, sudden expansion/contraction, or baffles, etc.). The muffler can include an external acoustic path, which can be modified relatively easily for the purpose of, for example, optimizing the performance of the muffler at a working site. The muffler may be relatively compact and easy to be modified compared to a conventional muffler design. 
     Generally, a method to attenuate acoustic waves may include directing portions of acoustic waves into a plurality of acoustic paths, where the plurality of acoustic paths may be configured to provide phase shifts in the portions of the acoustic waves. This can result in the plurality portions of the acoustic wave to be out-of-sync relative to each other after passing through the acoustic paths. The method may also include merging the plurality of portions of the acoustic waves after passing through the acoustic paths. Because the acoustic wave portions are out-of-sync relative to each other, merging the plurality of acoustic wave portions may attenuate the acoustic waves (e.g., reduce an amplitude of the acoustic waves compared to the original amplitude of acoustic waves prior to phase shifting caused by being directed through the acoustic paths). In some embodiments, to help provide phase shifts in the portions of the acoustic waves, the acoustic paths may be configured to, for example, have different acoustic impedance (e.g., lengths, cross-sectional areas, sudden expansion/contraction, and/or impeding structures such as for example materials and baffles). 
     An “acoustic path,” as used in this specification, generally refers to a structure that can conduct acoustic waves or allow acoustic waves to pass therethrough. 
     A “phase shift,” as used in this specification, generally means that phases of acoustic waves may be changed when, for example, passing through an acoustic path. 
     The term “attenuate an acoustic wave,” as used in this specification, generally means that an amplitude of an acoustic wave is reduced. 
     The term “acoustic impedance,” as used in this specification, is generally referred to as acoustic properties of an acoustic path. More specifically, “acoustic impedance” is generally referred to as the acoustic characteristic of the corresponding acoustic path related to the external loading, absorption, and/or internal resonances that exist in the acoustic path, which may alter the phase of an acoustic wave as the acoustic wave travels through the acoustic path. 
     The embodiments disclosed in this specification can be used with a compressor of, for example, an HVAC system. The compressor can be, for example, a screw compressor, a rotatory compressor, a scroll compressor, or the like. The embodiments as disclosed herein can also be used with other machinery that may generate acoustic waves in operation, such as, but not limited to, a combustion engine. 
     References are made to the accompanying drawings that form a part hereof, and which illustrate embodiments in which the methods and apparatuses described in this specification may be practiced. It is to be understood that the terms used herein are for the purpose of describing the figures and embodiments and should not be regarded as limiting the scope. 
       FIG. 1  illustrates a schematic diagram of a muffler  100  configured to attenuate acoustic waves  110   a , according to some embodiments. The schematic diagram illustrates a general principle of configuring the muffler  100  and attenuating acoustic waves  110   a.    
     The muffler  100  has an inlet  102  and an outlet  104 . The muffler  100  is also configured to have more than one acoustic path  121 ,  122 , and  123 . It will be appreciated that the number of the acoustic paths illustrated is exemplary and that the number of paths can vary. The number of acoustic paths can generally be at least two. The acoustic paths  121 ,  122 , and  123  are in communication with the inlet  102  and the outlet  104 . The arrows in  FIG. 1  generally indicate the directions and distribution of the acoustic waves in operation. As shown, the acoustic waves  110   a  can be directed into the inlet  102 . Portions  111 ,  112 , and  113  of the acoustic waves  110   a  can then be directed into the acoustic paths  121 ,  122 , or  123  respectively. 
     The acoustic paths  121 ,  122 , and  123  are generally configured to provide different acoustic impedances, which may help provide phase shifts to the portions  111 ,  112  and  113  of the acoustic waves  110   a.    
     In the illustrated embodiment, the acoustic waves  110   a  can be divided into three portions  111 ,  112 , and  113 , which are directed into the acoustic paths  121 ,  122 , and  123  respectively. After the portions  111 ,  112 , and  113  of the acoustic waves  110   a  pass through the acoustic paths  121 ,  122 , and  123  respectively, the portions  111 ,  112 , and  113  can be merged back at the outlet  104 . 
     Because of, for example, different phase shifts that can be present when the portions  111 ,  112 , and  113  of the acoustic waves  110   a  pass through the acoustic paths  121 ,  122 , and  123 , the portions  111 ,  112 , and  113  of the acoustic waves  110   a  may have different phases when merged (e.g., the portions  111 ,  112 , and  113  of the acoustic waves  110   a  are out-of-sync). The out-of-sync portions  111 ,  112  and  113  can have destructive interference and cancel each other out when merged back, which can help attenuate the acoustic waves (e.g., compare the amplitude of the acoustic waves  110   a  to the amplitude of the acoustic waves  110   b ). In some embodiments, for example, when the portions  111 ,  112  and  113  may have at or about ¼ to at or about ¾ of wavelength difference in their phases at the outlet  104 , the acoustic waves may be attenuated. In some embodiments, the wavelength difference may be at or about ½ of wavelength. 
     The acoustic paths  121 ,  122 , or  123  may be configured to have different acoustic impedances (e.g., lengths, cross-sectional areas, sudden expansion/contraction, or baffles, etc.), which can help provide different phase shifts in the portions  111 ,  112 , and  113  of the acoustic waves  110   a . Examples of configurations that can help modify acoustic impedance of an acoustic path are illustrated in  FIGS. 3A to 3E . 
       FIG. 2  illustrates a muffler  200  that can incorporate the configuration as illustrated in the schematic diagram of  FIG. 1 , according to some embodiments. The muffler  200  may be coupled to a compressor  210  to, for example, help attenuate acoustic waves produced by the operation of the compressor  210 . The muffler  200  can help reduce operational sound of the compressor  210 . 
     As illustrated, the muffler  200  may include a plurality of acoustic paths: a main muffler path  201 , an internal muffler path  202  and an external muffler path  203 , each of which are in communication with the compressor  210  and a discharge line  204 . The muffler  200  may include a muffler housing  220 . The main muffler path  201  and the internal muffler path  202  are housed in the muffler housing  220 . The external muffler path  203  is configured to direct a portion of acoustic waves out of the muffler housing  220 . The terms “internal” and “external” are relative to a muffler housing (e.g., the muffler housing  220 ) of a muffler. The term “internal” indicates that a structure is generally enclosed within the muffler housing. The term “external” indicates that a structure is generally not enclosed within the muffler housing. It is to be appreciated that the configurations as illustrated are exemplary. 
     As illustrated in  FIG. 2 , the main muffler path  201 , the internal muffler path  202  and the external muffler path  203  are generally configured to direct portions of acoustic waves generated by the compressor  210  toward the discharge line  204 . The different portions of the acoustic waves can then be merged back in the discharge line  204 . 
     The main muffler path  201 , the internal muffler path  202 , and the external muffler path  203  may generally be configured to have different acoustic impedances. For example, in some embodiments, the main muffler path  201  may have a contracted portion  230 , a first expanded portion  240   a , and a second expanded portion  240   b . In some embodiments, the main muffler path  201 , the internal muffler path  202 , and the external muffler path  203  may be configured to have different lengths. When acoustic waves pass between the contracted portion  230  and the expanded portion  240   a  or  240   b , the phase of the acoustic waves can change. 
     In operation, the compressor  210  can produce acoustic waves, which are directed into the muffler  200 . The main muffler path  201 , the internal muffler path  202 , and the external muffler path  203  can be configured to receive a portion of the acoustic waves from the compressor  210 . Because, for example, the main muffler path  201 , the internal muffler path  202 , and the external muffler path  203  have relatively different acoustic impedance, the portion of the acoustic waves passing through these paths can be out-of-sync relative to each other. When the out-of-sync acoustic waves are merged back, the acoustic waves can be attenuated. The acoustic waves can be merged, for example, in the discharge line  204 . 
     It is to be appreciated that the out-of-sync portions of the acoustic waves can also be provided by configuring the main muffler path  201 , the internal muffler path  202 , and/or the external muffler path  203  to have different lengths. When the portions of the acoustic waves passing through different lengths, the phases of the portions of the acoustic waves can be out-of-sync (e.g., at or about ½ of a wavelength of the acoustic waves) when merged back. When using lengths of the acoustic paths to make the portions of the acoustic waves out-of-sync, the lengths of the acoustic paths may be relatively long, resulting in a relatively large muffler. 
     In some embodiments, for example, when the compressor  210  is a screw compressor, the main acoustic waves produced by the compressor  210  are at or about 200 Hz to at or about 400 Hz. The lengths of the main muffler path  201 , the internal muffler path  202 , and/or the external muffler path  203  may be at or about 8 to at or about 10 inches. 
     It is to be appreciated that it is not necessary for the acoustic waves generated, for example by the compressor  210 , to be directed into a plurality of acoustic paths at the same time. The acoustic waves can be directed into the plurality of acoustic paths at different times. For example, as illustrated in  FIG. 2 , the acoustic waves generated by the compressor  210  are initially directed into the internal muffler path  202  and the main muffler path  201 . A portion of the acoustic waves directed into the main muffler path  201  is then directed into the external muffler path  203 . 
     It is also to be appreciated that it is not necessary for the acoustic waves to merge back at the same time, after passing through the plurality of acoustic paths. Different portions of the acoustic waves can be merged at different times. For example, as illustrated in  FIG. 2 , the portion of the acoustic waves passing through the internal muffler path  202  and the portion of the acoustic waves passing through the main muffler path  201  can be merged first; and then can be merged with the portion of the acoustic waves passing through the external muffler path  203  in the discharge line  204 . 
     Generally, after the portions of the acoustic waves generated by compressor  210  is directed through the main muffler path  201 , the internal muffler path  202  and/or the external muffler path  203 , the phases of the portions of the acoustic waves may be shifted relative to each other. As a result, the acoustic waves may be attenuated when the portions of the acoustic waves are merged. 
     There are other ways to provide phase shifts to portions of the acoustic waves by providing different acoustic impedance. Acoustic impedance of an acoustic path can be modified by various configurations. The plurality of acoustic paths, which for example can include the main muffler path  201 , the internal muffler path  202 , and the external muffler path  203 , can incorporate various configurations to modify the acoustic impedance of the acoustic paths. As illustrated in  FIG. 2 , for example, the main muffler path  201  can include the contracted portion  230 , and the expanded portions  240   a  and  240   b  to modify the acoustic impedance of the main muffler path  201 . 
       FIGS. 3A to 3E  illustrate exemplary embodiments of configurations that can be used for an acoustic path (e.g., the main muffler path  201 , the internal muffler path  202 , and/or the external muffler path  203 ) to modify acoustic impedance of the acoustic path. 
     As illustrated in  FIG. 3A , an acoustic path  301 A can include more than one contracted portion  331 A,  332 A that are arranged in parallel. The contracted portions  331 A,  332 A may be configured to have different sizes/dimensions (e.g., diameters, cross-sectional areas, etc.). The contracted portions  331 A,  332 A can be in communication with an expanded portion  340 A. Generally, contracted portions (e.g.,  331 A,  332 A) and expanded portion (e.g.,  340 ) can help modify the acoustic impedance of the acoustic path  301 A. 
     As illustrated in  FIG. 3B , an acoustic path  301 B can include more than one contracted portion  331 B,  332 B that are arranged in series. The contracted portions  331 B,  332 B are in communication with a first expanded portion  340 B and a second expanded portion  341 B. It is to be appreciated that the size (e.g., cross-sectional area, etc.) of contracted portion  331 B may be different from contracted portion  332 B. It is also to be appreciated that the size of the first expanded portion  340 B may be different from the second expanded portion  341 B. 
     As illustrated in  FIG. 3C , an acoustic path  301 C may include a material that can help absorb acoustic waves and/or provide phase shifts to acoustic waves. 
     As illustrated in  FIG. 3D , an acoustic path  301 D can include a perforated plate  331 D that include a plurality of apertures  332 D. Phase shift can happen when the acoustic waves passes through the apertures  332 . 
     As illustrated in  FIG. 3E , an acoustic path  301 E may include one or more baffles  331 E. The baffles  331 E can direct acoustic waves within the acoustic path  301 E and can provide phase shifts to the acoustic waves. 
     By modifying the acoustic impedance of the acoustic paths (e.g., the main muffler path  201 , the internal muffler path  202  and/or the external muffler path  203  in  FIG. 2 ), out-of-sync acoustic waves may be provided with relatively short acoustic paths. It is appreciated that a combination of any or all of embodiments illustrated in  FIGS. 3A to 3E  can make up a suitable acoustic path to attenuate the acoustic waves. 
     A compressor (e.g., the compressor  210  in  FIG. 2 ) can be a fixed speed compressor or a variable capacity (e.g., variable speed) compressor. When the compressor has a variable capacity, the acoustic waves produced by the compressor may have a relatively wide range of wavelengths compared to, for example, a fixed speed compressor. A muffler may be optimized for different ranges of wavelengths. 
     For example, in some embodiments, a fixed speed compressor may have main acoustic waves with frequencies at or about 200 Hz to at or about 400 Hz. A variable speed compressor may have main acoustic waves with frequencies at or about 200 Hz to at or about 16,000 Hz. The muffler may be optimized for attenuating acoustic waves for a relatively wide range of frequencies. 
       FIGS. 4 and 5  illustrate two exemplary embodiments of mufflers  400 ,  500  respectively that may be configured to attenuate acoustic waves with a relatively wide range of wavelengths. The mufflers  400  and  500  may be used with, for example, a variable capacity (e.g., variable speed) compressor. It is to be appreciated that the muffler  400  and  500  can also be used to optimize acoustic wave attenuation for a fixed speed compressor. 
     As illustrated in  FIG. 4 , the muffler  400  may include a plurality of internal muffler paths  410 ,  420 , and  430 . The muffler  400  also includes an external muffler path  440 . The internal muffler paths  410 ,  420 , and  430  and the external muffler path  440  are configured to direct portions of acoustic waves from an inlet  402  toward an outlet  404 . 
     Each of the internal muffler paths  410 ,  420 , and/or  430  may be optimized for different ranges of wavelengths. That is, the internal muffler paths  410 ,  420 , and/or  430  may be configured to provide relatively high degree of phase shift to different ranges of wavelengths. When acoustic waves of a certain range of wavelengths are generated, one or more of the internal muffler paths  410 ,  420 , or  430  can provide phase shifts to portions of the acoustic waves, so that the acoustic waves can be out-of-sync with the portion of the acoustic waves directed through the external muffler path  440 . 
     It is to be appreciated that other design considerations may be taken into account when configuring the muffler  400 . For example, a pressure drop when passing through a muffler path may limit how much a cross-sectional area of a contracted portion can be reduced in the muffler path. 
     In operation, for example, when the compressor varies its operation speed, the range of wavelength of the acoustic waves generated by the compressor can also vary. The acoustic waves may be directed into the muffler. A portion of the acoustic waves may be directed through the external muffler path  440 . Other portions of the acoustic waves may be directed through the plurality of internal muffler paths  410 ,  420 , and/or  430 . At least one of the internal muffler paths  410 ,  420 , or  430  can make the phase of the portion of the acoustic waves out-of-sync (e.g., at or about ½ of wavelength difference) relative to the portion of the acoustic waves directed through the external muffler path  440   
     It is to be appreciated that the external muffler path  440  can be modified relatively easily, for example, at a working site, because it is readily accessible. The external muffler path  440  can be optimized/modified at the working site by, for example, changing the length of the external muffler path  440  and/or modifying acoustic impedance of the external muffler path  440  by adding or removing other features that can affect acoustic impedance (e.g., any one or more of the configurations as shown in  FIGS. 3A to 3E ). 
       FIG. 5  illustrates that the muffler  500  can have a plurality of external muffler paths  541 ,  542 ,  543 . In the illustrated embodiment, the muffler  500  can include one internal muffler path  511 . The plurality of muffler paths  541 ,  542 , and/or  543  may be optimized for a different range of wavelengths. That is, the external muffler paths  541 ,  542 , and/or  543  can be configured to provide relatively high degrees of phase shift to different ranges of wavelengths. When acoustic waves of a certain range of wavelengths are generated, one or more of the external muffler paths  541 ,  542 , and/or  543  can provide phase shifts to portions of the acoustic waves, so that the phase of the acoustic waves can be out-of-sync with the portion of the acoustic waves directed through the internal muffler path  540 . 
     In operation, when the compressor varies its operation speed, the range of wavelengths of the acoustic waves generated by the compressor can also vary. The acoustic waves can be directed into the muffler. A portion of the acoustic waves may be directed through the internal muffler path  511 . Other portion of the acoustic waves may be directed through the plurality of external muffler paths  541 ,  542 , and/or  543 . At least one of the external muffler paths  541 ,  542 , and/or  543  can make a portion of the acoustic waves out-of-sync (e.g., at or about ¼ of wavelength) relative to the portion of the acoustic waves directed through the internal muffler path  540 . 
     It is to be appreciated that in some embodiments, a muffler can include a plurality of external muffler paths and a plurality of internal muffler paths. 
     Since the external muffler paths  541 ,  542 , and/or  543  can be modified relatively easily, the muffler  500  may be optimized and/or modified relatively easily at a worksite. For example, the acoustic impedance of the external muffler paths  541 ,  541 , and/or  543  can be modified based on the wavelength range of the acoustic waves at the worksite. 
     It is to be appreciated that an existing muffler without an external muffler path may be retrofitted to include one or more external muffler paths, so that acoustic waves with out-of-sync phases can be provided to attenuate the acoustic waves. 
     Aspects 
     Any of aspects 1 to 7 can be combined with aspect 8. 
     Aspect 1. A muffler comprising:
         a first acoustic path; and   a second acoustic path;   wherein the first acoustic path and the second acoustic path are configured to provide different phase shifts to acoustic waves, and the first acoustic path and the second acoustic path are configured to direct acoustic waves to merge after the first and second acoustic paths.       

     Aspect 2. The muffler of aspect 1, wherein the first acoustic path and the second acoustic path have different lengths. 
     Aspect 3. The muffler of aspect 1, wherein the first acoustic path and the second acoustic path have different acoustic impedance. 
     Aspect 4. The muffler of aspect 1, further comprising:
         a muffler housing, wherein the first acoustic path is internal to the muffler housing and the second acoustic path is external to the muffler housing.       

     Aspect 5. The muffler of aspect 1, further comprising:
         a third acoustic path, where in the third acoustic path are configured to provide different phase shifts than the first and second acoustic paths.       

     Aspect 6. The muffler of aspect 5, further comprising:
         a muffler housing, wherein the first and second acoustic paths are internal to the muffler housing and the third acoustic path is external to the muffler housing.       

     Aspect 7. The muffler of aspect 5, further comprising:
         a muffler housing, wherein the first and second acoustic paths are external to the muffler housing and the third acoustic path is internal to the muffler housing.       

     Aspect 8. A method of attenuating acoustic waves, comprising:
         providing a first degree of phase shift to a first portion of the acoustic waves;   providing a second degree of phase shift to a second portion of the acoustic waves; and   merging the first portion of the acoustic waves and the second portion of the acoustic waves;   wherein the first degree of phase shift and the second degree of phase shift are different.       

     The terminology used in this specification is intended to describe particular embodiments and is not intended to be limiting. The terms “a,” “an,” and “the” include the plural forms as well, unless clearly indicated otherwise. The terms “comprises” and/or “comprising,” when used in this Specification, specify the presence of the stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, and/or components. 
     With regard to the preceding description, it is to be understood that changes may be made in detail, especially in matters of the construction materials employed and the shape, size, and arrangement of parts without departing from the scope of the present disclosure. This specification and the embodiments described are exemplary only, with the true scope and spirit of the disclosure being indicated by the claims that follow.