Patent Publication Number: US-10771889-B2

Title: Acoustic filtering

Description:
CLAIM OF PRIORITY 
     This application claims priority under 35 U.S.C. § 119(e) to provisional U.S. Patent Applications 62/257,923, filed on Nov. 20, 2015, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Electronics (e.g., transducers) are often designed with sealed enclosures (e.g., an airtight seal or housing) to protect the electronics from debris, water, and so forth. However, once these electronics are put into use, it is often the case that the airtight housing or seal leaks, because the airtight seals prevent the device&#39;s ability to equalize pressure, e.g., when there is a pressure differential. Pressure differentials are often caused by temperature changes. The temperature change can be internal, external or both. As the external pressure fluctuates (thus causing a pressure differential), the enclosure tries to equalize the internal pressure by drawing in air from the outside. If the housing is completely airtight, pressure builds up inside in the form of a positive or negative buildup. Positive buildup causes the housing to bloat, while negative buildup creates a vacuum. Either type of buildup leads to stress on the seal, which in turn compromises and damages it effectiveness. The compromised seals begin to allow water and contaminants to enter the housing, which can ultimately lead to electronic failure. (See, “The Unknown Problem with Airtight Enclosures,” www.ElectronicsProtectionMagazine.com). 
     To protect against these pressure differentials, more robust seals are often used. However, this solution will remedy the immediate leakage and contamination issues, but is a short-term fix that will ultimately fail because the fundamental problem of pressure differentials has not been addressed. The device is simply more airtight without having a solution for the root cause. (See, “The Unknown Problem with Airtight Enclosures,” www.ElectronicsProtectionMagazine.com). 
     As described in U.S. Pat. No. 7,439,616, a sealing ring is provide on the bottom of a packaged device that will seal the back volume during surface mounting to a user&#39;s board, as show in  FIG. 1  (which is FIG. 3 in U.S. Pat. No. 7,439,616). 
       FIG. 2  is a bird&#39;s eye view of the bottom of package  10  shown in  FIG. 1 . In  FIG. 2 , substrate  14  of package  10  includes cavity  18 . Sealing ring  22  surrounds cavity  18 , making package  10  air-tight, when mounted to a board. 
     SUMMARY 
     In one aspect, a package comprises: a transducer; a substrate comprising an acoustic port, with the transducer attached to a surface of the substrate and over or adjacent to the acoustic port; and a venting mechanism for venting air or sound pressure from a device comprising the package, with the venting mechanism being affixed to the substrate and partially surrounding the acoustic port, and with the venting mechanism being dimensioned to filter out audio frequencies. 
     In this aspect, the venting mechanism includes first and second sidewalls defining a gap between the first and second sidewalls, wherein the gap is dimensioned to have a specific acoustic impedance such that air or sound pressure with an audio frequency range does not enter the venting mechanism and is not sensed by the transducer, and air or sound pressure with a frequency that differs from an audio frequency enters the venting mechanism for venting to an atmosphere. The venting mechanism is an open ring. The venting mechanism forms a vent around the acoustic port. The venting mechanism is fabricated from one or more of solder, a metal, epoxy, plastic and fiberglass. The venting mechanism is associated with a threshold frequency level, wherein the venting mechanism is configured to vent out of the device sound pressure with a frequency that exceeds or is less than the threshold frequency level. The transducer is disposed within a sealed volume of the device. The transducer is a piezoelectric transducer, a silicon microphone, a piezoelectric microphone or a silicon condenser microphone. The transducer is comprised of one or more of AlN, PZT, ScAlN, LiNbO3, LiTaO3, GaN and GaAs. 
     In another aspect, a method include forming a venting mechanism on a package substrate of a package and around an acoustic port of the package substrate, by: forming a first venting sidewall on the package substrate; forming a second venting sidewall on the package substrate that is substantially opposite to the first venting sidewall on the package substrate; wherein a gap between the first and second venting sidewalls is dimensioned to have a specific acoustic impedance to configure the venting mechanism such that air or sound pressure with an audio frequency range that is generated within a device including the package does not enter the venting mechanism and is not sensed by a transducer included in the package, and air or sound pressure that is generated within the device and with a frequency that differs from an audio frequency enters the venting mechanism for venting the air or sound pressure from the device into an atmosphere. In this aspect, forming comprises applying solder around the acoustic port in a shape that forms the venting mechanism. 
     In yet another aspect, a substrate comprises an acoustic port; and a venting mechanism for venting air or sound pressure, with the venting mechanism being affixed to the substrate and partially surrounding the acoustic port, and with the venting mechanism being dimensioned to filter out audio frequencies. 
     In this aspect, the venting mechanism includes first and second sidewalls defining a gap between the first and second sidewalls, wherein the gap is dimensioned to have a specific acoustic impedance such that air or sound pressure with an audio frequency range does not enter the venting mechanism, and air or sound pressure with a frequency that differs from an audio frequency enters the venting mechanism for venting to an atmosphere. The venting mechanism is an open ring. The venting mechanism forms a vent around the acoustic port. The venting mechanism is fabricated from one or more of solder, a metal, epoxy, plastic and fiberglass. The venting mechanism is associated with a threshold frequency level, wherein the venting mechanism is configured to vent out of the acoustic device sound pressure with a frequency that exceeds or is less than the threshold frequency level. The device further includes a piezoelectric transducer, a silicon microphone, a piezoelectric microphone or a silicon condenser microphone. The device further includes a transducer comprised of one or more of AlN, PZT, ScAlN, LiNbO3, LiTaO3, GaN and GaAs. The transducer is disposed within a sealed volume of the other device. 
     All or part of the foregoing may be implemented as an apparatus, method, or electronic system that may include one or more processing devices and memory to store executable instructions to implement the stated functions. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques described herein will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF FIGURES 
         FIGS. 1, 3B, 6A  are each a diagram of a packaged device. 
         FIGS. 2, 4 and 8A-8C  are each a bird&#39;s eye view of a bottom of a package device. 
         FIG. 3A  is a cross-sectional view of a device. 
         FIG. 6B  is a diagram of a device including a packaged device. 
         FIGS. 5, 7  are each a diagram of an equivalent circuit model of a package. 
         FIG. 8D  is a diagram of a vent cross-section. 
         FIG. 9  is an example process for forming a venting mechanism. 
     
    
    
     DETAILED DESCRIPTION 
     Using the techniques described herein, a venting mechanism (e.g., a vent) is built into solder partially surrounding an acoustic port and/or an opening of a device (e.g., a mobile device) including a package with an acoustic transducer. In particular, rather than using a sealing ring to seal a cavity (e.g., an acoustic port) in a package including a transducer, a vent is used around the device opening and/or the acoustic port to vent air pressure from a device, while also preventing unwanted sound from entering the acoustic port. There are various types of acoustic transducers, including, e.g., microphones and other acoustic sensors that detect sound, or mechanical vibration, producing an electrical signal representing the sound or vibration detected. 
     In particular, air pressure generated within a device causes pressure to build up inside the device, which can result in cracking of the internal walls of the device and/or cracking of other internal components in the device. Therefore, the venting or release of this sound/air pressure from the device and into the atmosphere is beneficial. However, if the sound/air pressure being vented has an audio frequency, then the microphone may sense this sound/air pressure, resulting in the user hearing noise or unwanted sound. Generally, an audio frequency is a frequency within the range of 20 Hz-20 kHz. To vent the low frequency sound and/or air pressure to prevent cracking of the internal walls, while preventing the sound/air pressure with audio frequencies from being sensed by a microphone, a microphone package includes a venting mechanism surrounding an acoustic port of the microphone in the device. The venting mechanism is configured to vent low frequency (outside the audio frequency range of 20 Hz-20 kHz) sound/air pressure occurring within the device (e.g., generated inside the device) out to the atmosphere by exhibiting a low acoustic impedance to such sound/air pressure. The venting mechanism exhibits a high acoustic impedance to high frequency sounds (e.g., those sounds within the audio frequency range) generated within the device, thereby preventing these sounds from entering the venting mechanism (and being vented to the atmosphere) and also preventing this sound/air pressure with audio frequencies from being sensed by the microphone. 
     Referring to  FIG. 3A , a cross-sectional view of device  47  is shown. Device  47  includes microphone  48 , which includes transducer  48   a , application-specific integrated circuit (ASIC)  48   b , and a package comprising package lid  48   c  and package substrate  48   d . In this example, package lid  48   c  and package substrate  48   d  collectively form the package for holding transducer  48   a  and ASIC  48   b . Device  47  includes enclosure volume  47   a , e.g., a volume of air inside of device  47 . In this example, microphone  48  is disposed within enclosure volume  47   a.    
     In this example, device  47  includes device opening  43  for entry and release of sound waves and acoustic pressure from device  47 . Microphone  48  includes acoustic port  44  for entry of sound waves into transducer  48   a . In this example, device  47  includes venting mechanism  45  that allows sound to vent from device  47  to the environment surrounding device  47 . Rather than having a sealing ring to seal acoustic port  44  of microphone  48  from enclosure volume  47   a  of device  47 , device  47  includes venting mechanism  45  that is configured to filter out (e.g., prevent entry) audio frequencies (e.g., representing unwanted sound)—thereby acting as an audio filter, while venting out of device  47  other frequencies satisfying various criteria for venting out of device  47  (e.g., the criteria being that the frequencies occur outside the audio frequency range of 20 Hz-20 kHz, are less than or greater than a threshold frequency level, and so forth). 
     Accordingly, at audio frequencies, unwanted sound generated in in enclosure volume  47   a  of device  47  does not reach the acoustic port. Low frequency pressure changes, such as those resulting from atmospheric pressure, are vented to the atmosphere. In this example, venting mechanism  45  is structurally part of microphone  48  (e.g., by being fabricated on top of package substrate  48   d ). 
     In an example, venting mechanism  45  filters sound based on frequency. For sound/air pressure generated inside device  47 , a gap  46  between sidewalls (not shown in this cross section view, see  FIG. 4 ) of venting mechanism  45  is dimensioned to filter out high frequencies (e.g., audio frequencies within a range of 20 Hz-20 kHz). Gap  46  forms a vent (hereinafter referred to as “vent  46 ”) for venting air from inside device  47  to the atmosphere. This sound/air pressure with high frequencies that is generated inside device  47  encounters a high impedance at the gap between the sidewalls of venting mechanism  45 . This high impedance prevents the sound/air pressure at the audio frequencies from entering venting mechanism  45 . As such, this sound/air pressure remains in enclosure volume  47   a  of device  47  and doesn&#39;t enter acoustic port  44 , thereby preventing this high frequency sound/air pressure from being sensed by microphone  48  and preventing this high frequency sound from being heard by a user of device  47 . 
     In this example, sound/air pressure with low frequencies (that are generated inside the device) vent to the atmosphere via vent  46 , e.g., which is a gap between the sidewalls in venting mechanism  45 . Generally, low frequency sound/air pressure is sound/air pressure at a frequency that is inaudible to the human ear and outside the audio frequency range. The gap between the sidewalls of venting mechanism  45  is dimensioned to exhibit a low impedance to these low frequency sounds/air pressure. Due to this low impedance, this low frequency sound/air pressure generated inside device  47  enters the gap between the sidewalls in venting mechanism  45  and vents to the atmosphere. This low frequency sound/air pressure could be sensed by microphone  48 , depending on microphone performance, as the sound/air pressure is venting to the atmosphere. However, because this sound/air pressure is outside the audio frequency range, it does not impact microphone performance or result in audible noise to the user of device  47 . 
     For sound/air pressure coming from the atmosphere, high and low frequencies see low impedance at acoustic port  44  and are sensed by microphone  48 , to enable microphone  48  to pick-up this sound. High frequencies coming from the atmosphere see high impedance to the device body (e.g., enclosure volume  47   a ), e.g., via the gap between the sidewalls of venting mechanism  45 , thereby preventing these high frequencies from entering enclosure volume  47   a  of device  47 . 
     In an example, the acoustic port causes resistance that acts as a natural filter and the acoustic port is electrically connected to the rest of the package. However, the acoustic port is not sealed, because there is a vent. The vent, along with the transducer, also has an inherent resistance that acts as a filter. On some devices, especially wearable devices, the acoustic port of a microphone and the device opening (that allows sound to access and enter the acoustic port) may be the only openings to the environment and the remainder of the device might be sealed. In this case, it is desirable to place a controlled vent to the atmosphere to allow atmospheric pressure changes to equalize inside such a device, e.g., by venting out to the atmosphere. This vent is built into the microphone packaging without impacting microphone performance. 
     Referring to  FIG. 3B , package  30  includes transducer  32  that is affixed to substrate  36 . Cover  38  covers substrate  36 . Acoustic port  34  is formed in substrate  36  to form an acoustic path to transducer  32 . In an example, transducer is a transducer described in U.S. Pat. No. 8,531,088 and/or fabricated using the techniques described in U.S. Pat. No. 8,531,088, the entire contents of which are incorporated herein by reference. Venting mechanism  40  partially surrounds acoustic port  34  to form a vent around acoustic port  34 . 
     Referring to  FIG. 4 , a bird&#39;s eye view of the bottom of package  30  is shown. In this example, venting mechanism  40  is in proximity to port  34 . Venting mechanism  40  is formed from solder that is soldered onto the bottom of substrate  36 . Venting mechanism  40  forms vent  42  to vent air and sound pressure from a device that includes package  30  out to the atmosphere. In this example, venting mechanism  40  has a U shape and is fabricated from solder paste. 
     Venting mechanism  40  includes venting sidewalls (or vent arms)  40   a ,  40   b . In this example, vent  42  is a gap between sidewalls  40   a ,  40   b . When venting sidewalls  40   a ,  40   b  are closer together, venting mechanism  40  has a higher resistance, e.g., relative to a resistance of venting mechanism  40  when venting sidewalls  40   a ,  40   b  are further away from each other. By adjusting the spacing between venting sidewalls  40   a ,  40   b , the resistance of venting mechanism  40  is adjustable. Accordingly, the resistance of venting mechanism  40  can be adjusted to a threshold resistance or frequency to filter high frequencies, such that low frequency sounds (or air pressures) equalize and pass through the vent, while high frequency sounds encounter a high impedance (e.g., at the gap between the venting sidewalls) that prevents these high frequency sounds or air pressure from entering the venting mechanism, thereby ensuring that these high frequency sounds do not enter the acoustic port and ensuring that these high frequency sounds are not audible to a user of the device. As such, these high frequency sounds or air pressure remain in the enclosure volume and are not vented out to the atmosphere. 
     In this example, venting mechanism  40  includes an open ring, rather than a sealed ring. Venting mechanism  40  is attached to a structure, e.g., a board in a mobile device and acts as an acoustic filter that will knock out high frequency roll off. The solder used in forming venting mechanism  40  can be made of various materials, including, e.g., solder, metals, epoxy, fiberglass, plastic and so forth. Additionally, venting mechanism  40  can include various shapes and design structures, e.g., curled, straight, zig zag, U-shaped, and so forth. Additionally, venting mechanism  40  can be structured and fabricated to cover various spec targets, e.g., such as a high frequency response, and so forth. 
     Referring to  FIG. 5 , equivalent circuit model  50  of a package (e.g., package  30 ) is shown. In this example, P represents pressure due to sound coming into the microphone portion of a device (such as a mobile device). R vent  is an acoustic resistance of a vent (e.g., vent  42 ). Lentis an acoustic mass of a vent (e.g., vent  42 ). C dev  is an acoustic compliance of the device and represents a volume of air sealed in the device in accordance with the following equation: 
     
       
         
           
             
               C 
               Dev 
             
             = 
             
               V 
               
                 ρ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   c 
                   2 
                 
               
             
           
         
       
     
     V the volume of air in the device. ρ is the density of air. c is the speed of sound. 
     The vent allows a sealed device to equalize atmospheric pressure. Without the vent, the atmospheric pressure may introduce a pressure differential across C dev  This pressure differential would then stress the walls of the device and could cause cracking or breaking. 
     R AP  is an acoustic resistance of an acoustic port. L AP  is an acoustic mass of an acoustic port. C front  is an acoustic compliance in the front the device. C back  is an acoustic compliance in the back the device. C dia  an acoustic compliance of a diaphragm in the package. L dia  is an acoustic mass of a diaphragm in the package. R Gap  is an acoustic resistance across a gap (e.g., gap  68  in  FIG. 6A ) in the transducer, as described below. In this example, the vent does not impact the acoustical performance of the microphone, because the resistance of the vent allows wanted sound (e.g., sound from the environment) to be received by the microphone, but also filters out unwanted sound (e.g., sound coming from the device) from reaching the microphone. 
     Referring to  FIG. 6A , package  60  (such as a transducer package) includes a transducer  62  (such as a microphone, piezoelectric transducer, a silicon microphone, a piezoelectric microphone, silicon condenser microphone and so forth) with a diaphragm  64  (e.g., the moving portion of the sensor or transducer). The microphone  62 , including the diaphragm  64 , is fabricated using a MEMS process, such as the one described in U.S. Pat. No. 8,531,088, the entire contents of which are incorporated herein by reference. In this example, the transducer  62  comprises of various materials, including, e.g., a piezoelectric material; AlN PZT, ScAlN, LiNbO3, LiTaO3, GaN, GaAs, etc. In one example, the transducer  62  is a piezoelectric microphone that uses piezoelectricity to produce an electrical signal from air pressure variations. 
       FIG. 6A  also illustrates the portions of package  60  that correspond to portions in equivalent circuit model  50  of a package. For example, R AP    61  and L AP    63  of the acoustic port  66  are shown. C front    65  and C back    67  are also shown. In a variation, the transducer comprises silicon and the transducer is a silicon microphone in which the transducer is die bonded to the substrate. The transducer is fabricated on various types of die, including, e.g., a silicon die (of a silicon substrate). In still other variation, the transducer is a silicon condenser transducer or a silicon condenser microphone. Generally, a condenser microphone is a microphone that uses a capacitor to convert the compression and rarefaction of sound waves into electrical energy. Generally, a silicon microphone is a type of acoustic sensor made from silicon or polysilicon. 
     Referring to  FIG. 6B , a variation of  FIG. 6A  is shown in which package  60  is included with device  69  (e.g., a sealed device). In this example, device  69  includes sealed volume  69   a  in which package  60  is located. In this example, transducer  62  is disposed within sealed volume  69   a  of device  69 . 
     Referring to  FIG. 7 , equivalent circuit model  70  of a package (e.g., package  30 ) is shown. In this example, high vent resistance prevents unwanted sound from reaching the microphone port, by filtering out the unwanted sound (e.g., by filtering out audio frequencies, which are likely to be unwanted sound coming from within the device). Pun represents unwanted sound due to pressure (and/or a simulation of unwanted sound via pressure), such as the pressure caused by a user pressing a screen on a mobile device or by pressing buttons on a mobile device. R Rad  is an acoustic radiation (“rad”) resistance. If there was a large amount of resistance caused by external air, there is an increased chance of sound pressure entering the microphone. Lad is radiation (“rad”) mass caused by external air. In this example, portions  72 ,  74 ,  76  and  78  specify portions of model  70  that represent the vent, package, acoustic port and transducer, respectively. R vent  is an acoustic resistance of a vent (e.g., vent  42 ). Lentis an acoustic mass of a vent (e.g., vent  42 ). C dev  is an acoustic compliance of the device. R AP  is an acoustic resistance of an acoustic port. L AP  is an acoustic mass of an acoustic port. C front  is an acoustic compliance in the front the device. C back  is an acoustic compliance in the back the device. Calais an acoustic compliance of a diaphragm in the package. L dia  is an acoustic mass of a diaphragm in the package. R Gap  is an acoustic resistance across a gap in the transducer, as described below. 
     Referring  FIGS. 8A-8C , venting mechanisms  87 ,  94 ,  100  are various shapes and dimensions. In the example of  FIG. 8A , diagram  80  illustrates a bird&#39;s eye view of a bottom of package  82  with supply voltage (V in )  81  and an output voltage  83 . In this example, cavity  86  is a bird&#39;s eye view of an opening into an acoustic port. (In an example, cavity  86  includes the acoustic port). Venting mechanism  87  is fabricated around cavity  86  to vent into a portion of a device that holds package  82 . In this example, “h” represents a longitudinal length of the vent, as shown in  FIG. 8A . “L” represents a cross-sectional length of the vent. “A” represents an area of the vent. 
     Referring to  FIG. 8B , diagram  90  illustrates that a vent surround an opening in an acoustic port can have various shapes and sizes. In this example, package  85  includes opening  92  into an acoustic port (not shown). In an example, opening  92  includes and/or is the acoustic port. In this example, the bottom of package  85  is visually depicted. Venting mechanism  94  at least partially surrounds opening  92  in accordance with the design and shape shown in  FIG. 8B . Venting mechanism  94  forms vent  94   a  to vent or exhaust air/pressure out of the package. In this example, package  85  includes supply voltage (Vin)  91  and an output voltage  93 . Referring to  FIG. 8C , diagram  96  shows a bottom of package  95  with an opening  98  into an acoustic port. In an example, opening  98  includes and/or is the acoustic port. In this example, venting mechanism  100  at least partially surrounds opening  98  and includes a circular shape. Vent  100   a  is formed by an opening in venting mechanism. Package  95  includes supply voltage  102  and output voltage  104 . Referring to  FIG. 8D , a vent cross-section of vent  110  is shown. In this example, “g” ( 112 ) represents the cross-sectional height of the vent  110 . “L” ( 114 ) represents a cross-sectional length of the vent  110 . 
     In the examples described herein, R vent  is determined in accordance with 
                 R   Vent     =       12   ⁢           ⁢   μ   ⁢           ⁢   h         g   3     ⁢   L         ,         
wherein μ is viscosity.
 
     In the examples described herein, acoustic mass of the vent (L vent ) is determined in accordance with: 
                 L   Vent     =         ρ   0       π   ⁢           ⁢     ℛ   2         ⁢     (     L   +     1.7   ⁢           ⁢   ℛ       )         ,         
where ρ the density of air and   is the effective radius of the vent, approximated as the radius of a circle with the same cross-sectional area as the vent.
 
     Referring to  FIG. 9 , process  120  for forming or fabricating a venting mechanism and associated vent includes the following operations. In operation, a system or entity forms ( 122 ) a venting mechanism on a package substrate of a package and around an acoustic port of the package substrate, by: forming ( 124 ) a first venting sidewall on the package substrate; forming ( 126 ) a second venting sidewall on the package substrate that is substantially opposite to the first venting sidewall on the package substrate; dimensions ( 128 ) a gap between the first and second venting sidewalls to have a specific acoustic impedance to configure ( 130 ) the venting mechanism such that air or sound pressure with an audio frequency range that is generated within a device including the package does not enter the venting mechanism and is not sensed by a transducer included in the package, and to configure ( 132 ) the venting mechanism such that air or sound pressure that is generated within the device and with a frequency that differs from an audio frequency enters the venting mechanism for venting the air or sound pressure from the device into an atmosphere. In some examples, the forming comprises applying solder around the acoustic port in a shape that forms the venting mechanism. 
     Embodiments can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations thereof. Apparatus can be implemented in a computer program product tangibly embodied or stored in a machine-readable storage device for execution by a programmable processor; and method actions can be performed by a programmable processor executing a program of instructions to perform functions by operating on input data and generating output. The techniques described herein can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Each computer program can be implemented in a high-level procedural or object oriented programming language, or in assembly or machine language if desired; and in any case, the language can be a compiled or interpreted language. 
     Suitable processors include, by way of example, both general and special purpose microprocessors. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Generally, a computer will include one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD_ROM disks. Any of the foregoing can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits). 
     Other embodiments are within the scope and spirit of the description and the claims. Additionally, due to the nature of software, functions described above can be implemented using software, hardware, firmware, hardwiring, or combinations of any of these. The use of the term “a” herein and throughout the application is not used in a limiting manner and therefore is not meant to exclude a multiple meaning or a “one or more” meaning for the term “a.” Additionally, to the extent priority is claimed to a provisional patent application, it should be understood that the provisional patent application is not limiting but includes examples of how the techniques described herein may be implemented. 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the claims and the examples of the techniques described herein.