Abstract:
An electro-acoustic transducer is provided that comprises a diaphragm and a magnet assembly comprising a magnet and a back plate. The back plate comprises at least one first vent. The diaphragm generates sound during a movement of the diaphragm relative to the back plate. The transducer further comprises a printed circuit board comprising at least one second vent and a cavity between the printed circuit board and the back plate that separates the at least one first vent from the at least one second vent.

Description:
BACKGROUND 
       [0001]    This description relates generally to audio transducers, and more specifically, to an acoustical resistance assembly of a transducer used in an in-ear headphone. 
       BRIEF SUMMARY 
       [0002]    In accordance with one aspect, an electro-acoustic transducer is provided that comprises a diaphragm and a magnet assembly comprising a magnet and a back plate. The back plate comprises at least one first vent. The diaphragm generates sound during a movement of the diaphragm relative to the back plate. The transducer further comprises a printed circuit board comprising at least one second vent and a cavity between the printed circuit board and the back plate that separates the at least one first vent from the at least one second vent. 
         [0003]    Examples may include one or more of the following: 
         [0004]    A first geometry of the at least one second vent relative to the at least one first vent may provide a first frequency response for the transducer. A second geometry of the at least one second vent relative to the at least one first vent may provide a second frequency response different from the first frequency response for the transducer. 
         [0005]    The at least one first vent may include a hole that is offset from an outer diameter of the back plate. 
         [0006]    The at least one first vent may be located at an outer diameter of the back plate. 
         [0007]    The at least one second vent may comprise micro apertures extending through the printed circuit board, or PCB. 
         [0008]    The at least one second vent may range in diameter from 50 μm to 200 μm. 
         [0009]    The at least one second vent may comprise a plurality of air holes extending through the printed circuit board and a scrim material coupled to the printed circuit board and positioned over the air holes. 
         [0010]    The at least one first vent and the at least one second vent may be constructed and arranged to provide an acoustical resistance of air flowing between an external environment and an interior of the transducer, and for shaping a frequency response for the electro-acoustic transducer. 
         [0011]    The at least one first vent of the back plate and the at least one second vent of the printed circuit board may each have a total acoustical impedance that includes a real part and an imaginary part. The real part of the total acoustical impedance of the at least one first vent may be lower than the real part of the total acoustical impedance of the at least one second vent. 
         [0012]    In accordance with another aspect, an electro-acoustic transducer is provided that comprises a diaphragm and a magnet assembly comprising a magnet and a back plate. The back plate comprises at least one first vent hole. The diaphragm generates sound during a movement of the diaphragm relative to the back plate. A printed circuit board comprises at least one second vent hole. A cavity between the printed circuit board and the back plate separates the at least one first vent hole from the at least one second vent hole in the printed circuit board. A scrim material is coupled to a surface of the printed circuit board in the cavity, and is positioned over the at least one air hole. 
         [0013]    Examples may include one or more of the following: 
         [0014]    A first geometry of the at least one first vent hole may provide a first frequency response for the transducer. A second geometry of the at least one vent hole may provide a second frequency response different from the first frequency response for the transducer. 
         [0015]    The at least one first vent hole may be offset from an outer diameter of the back plate. 
         [0016]    The at least one first vent hole may be located at an outer diameter of the back plate. 
         [0017]    The at least one first vent hole and the at least one second vent hole may be constructed and arranged to provide an acoustical resistance of air flowing between an external environment and an interior of the transducer, and for shaping a frequency response for the electro-acoustic transducer. 
         [0018]    The at least one first vent hole of the back plate and the at least one second air hole of the printed circuit board may each include a plurality of vent holes having a total acoustical impedance that includes a real part and an imaginary part. The real part of the total acoustical impedance of the back plate vent holes may be lower than the real part of the total acoustical impedance of the printed circuit board vent holes. 
         [0019]    In accordance with another aspect, an acoustic device is provided comprising a diaphragm and a magnet assembly comprising a magnet and a back plate. The back plate comprises at least one vent hole. The diaphragm generates sound during a movement of the diaphragm relative to the back plate. A printed circuit board comprises at least one micro vent. A cavity between the printed circuit board and the back plate separates the at least one vent hole from the at least one micro vent of the printed circuit board. 
         [0020]    Examples may include one or more of the following: 
         [0021]    A first geometry of the at least one micro vent relative to the at least one vent hole may provide a first frequency response for the transducer. A second geometry of the at least one micro vent relative to the at least one vent hole may provide a second frequency response different from the first frequency response for the transducer. 
         [0022]    The at least one vent hole and the at least one micro vent may each have a total acoustical impedance that includes a real part and an imaginary part, and wherein the real part of the total acoustical impedance of the at least one back plate vent hole may be lower than a real part of a total acoustical impedance of the at least one micro vent. 
     
    
     
       BRIEF DESCRIPTION 
         [0023]    The above and further advantages of examples of the present inventive concepts may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of features and implementations. 
           [0024]      FIG. 1  is an isometric view of a cross-section of a microspeaker with an example of a conventional acoustical resistance assembly. 
           [0025]      FIG. 2  is an isometric view of a cross-section of a microspeaker with another example of a conventional acoustical resistance assembly. 
           [0026]      FIG. 3  is an equivalent circuit diagram for acoustical components of a conventional microspeaker positioned in an earbud. 
           [0027]      FIG. 4  is a graph illustrating frequency response curves corresponding to a conventional microspeaker positioned in an earbud. 
           [0028]      FIG. 5A  is an isometric view of a cross-section of a microspeaker with an acoustical resistance assembly, in accordance with some examples. 
           [0029]      FIG. 5B  is an isometric view of a cross-section of a microspeaker with an acoustical resistance assembly, in accordance with some examples. 
           [0030]      FIG. 6A  is an isometric view of a cross-section of a microspeaker with an acoustical resistance assembly, in accordance with some examples. 
           [0031]      FIG. 6B  is an isometric view of a cross-section of a microspeaker with an acoustical resistance assembly, in accordance with some examples. 
           [0032]      FIG. 7  is an isometric view of a cross-section of a microspeaker with an acoustical resistance assembly, in accordance with some examples. 
           [0033]      FIG. 8  is an isometric view of a cross-section of a microspeaker with an acoustical resistance assembly, in accordance with some examples. 
           [0034]      FIG. 9  is an equivalent circuit diagram for the acoustical components of a microspeaker of  FIGS. 5A-8  positioned in an earbud, in accordance with some examples. 
           [0035]      FIGS. 10A and 10B  are frequency response graphs, in accordance with some examples. 
           [0036]      FIG. 11  is a graph illustrating a relationship between a number of PCB micro vents and hole diameter for a fixed total acoustical resistance of the array of micro vents, in accordance with some examples. 
           [0037]      FIGS. 12A and 12B  illustrate frequency responses corresponding to an acoustical resistance assembly configured with PCB micro vents, in accordance with some examples. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    Modern in-ear headphones, or earbuds, typically include a microspeaker comprising a permanent magnet and a voice coil that is attached to a diaphragm that pushes the air around it, which in turn creates a sound that is output to a user. In doing so, the microspeaker must produce a sufficient sound pressure over the entire frequency range over which the device will be used. 
         [0039]    According to  FIGS. 1 and 2 , each acoustical resistance assembly  10 ,  30  may include a protective cover  12 , a diaphragm  14 , a voice coil  16 , a permanent magnet  18 , a suspension element  17 , a front plate  19 , a back plate  20 , and a printed circuit board (PCB)  22 . The protective cover  12  protects the diaphragm  14  from damage during operation and includes an opening  11  for outputting the sound generated at the diaphragm  14  to an ear canal or the like. 
         [0040]    The diaphragm  14  is coupled to, and driven by, the voice coil  16 . More specifically, as is well-known, the voice coil  16  is positioned in a permanent magnetic field generated by the magnet  18  and will move when an electrical current is applied to the voice coil  16 . The diaphragm  14  can be circular or non-circular in shape, and is coupled to a diaphragm ring  21  or other supporting member via the suspension element  17 , sometimes referred to as a surround. The surround  17  and diaphragm  14  may be constructed as a single component or as separate components. In operation, the surround  17  allows the diaphragm  14  to move in a reciprocating manner in response to an electrical current applied to the voice coil  16 . Movement of the diaphragm causes changes in air pressure, which results in a production of sound. 
         [0041]    The magnet  18  is sandwiched between the front plate  19  and the back plate  20 . The back plate  20  in turn is coupled to the PCB  22 . The back plate  20  can have a pole piece  23  that extends from a base portion of the back plate  20  towards the diaphragm  14 . The voice coil  16  is positioned about the pole piece  23 . 
         [0042]    The assembly  10  shown in  FIG. 1  includes a single vent hole  25  that extends through the back plate  20 . When the diaphragm  14  moves, air is forced through the back plate vent hole  25 . The assembly  30  shown in  FIG. 2  includes a single vent hole  26  that extends through the center of the pole piece  23 . As with the assembly  10  shown in  FIG. 1 , when the diaphragm  14  moves, air is forced through the pole piece vent hole  26 . The vent holes  25 ,  26  can be applied to achieve a range of frequency response shapes, due to the vent holes  25 ,  26  contributing to the acoustical impedance of the respective assemblies  10 ,  30 . 
         [0043]    A covering, or scrim  24 , can be positioned over the back plate vent hole  25  and/or pole piece vent hole  26  to provide an acoustical resistance at the respective vent hole  25 ,  26 . In the example of  FIG. 1 , the PCB  22  is cut short to create space for positioning the scrim material  24  over the vent hole  25 . In the example of  FIG. 2 , the PCB  22  includes an opening  27  to create space for positioning the scrim material  24  over the vent hole  26 . The scrim  24  can be formed of acoustically resistive materials such as a non-woven fabric, woven fabric, wire mesh, or the like. Changes in the acoustical resistance of the air flowing through the scrim  24  may further affect the frequency response of the driver in an earbud or related in-ear headset, the fundamental resonance of the driver, and may also have an impact on the damping of other acoustical resonances in the assembly  10 ,  30 . 
         [0044]      FIG. 3  is a view of an equivalent circuit diagram  40  for acoustical components of a conventional microspeaker for example, including acoustical resistance assembly  10  or  30  described herein. The microspeaker may be inserted in an earbud or related in-ear headset having a sealed back. The various features of the assembly  10  of  FIG. 1  and the assembly  30  of  FIG. 2  are represented by the acoustical impedance circuit  40 . 
         [0045]    An air region between the top surface of the diaphragm  14  and the ear canal (not shown) is represented by an acoustical compliance C AF . The output is the pressure in the front cavity, i.e., at acoustical compliance C AF . The motion of the diaphragm  14  is represented by the volume velocity source U. An air region under the diaphragm  14  in the motor cavity  29  is represented by an acoustical compliance C AM . The active region of the scrim  24  over the vent hole  25 ,  26  is represented by an acoustical resistance R AV . An air region at the back side of the transducer in a sealed earbud enclosure (not shown) is represented by an acoustical compliance C AB . The acoustical system represented by the equivalent circuit  40  permits the frequency response of the assemblies  10 ,  30  to be derived mathematically. In particular, acoustical pressure can be plotted as a dependent variable and input excitation frequencies can be plotted independent variables. The curves  71 - 75  shown in  FIG. 4  correspond to frequency response curves for an acoustical resistance assembly described herein, depending on selected design parameters which as described herein can affect the acoustical resistance R AV  of the assembly  10  or  30 . The acoustical resistance R AV  in turn can have an impact on the sensitivity of the microspeaker. In  FIG. 4 , the horizontal axis represents a frequency range of 10 Hz to 10 KHz, and the vertical axis represents the normalized sound pressure level for varying values of R AV . 
         [0046]    In  FIG. 4 , frequency response curve  71  is generated from an acoustical resistance assembly  10 ,  30  where the vent hole  25 ,  26  is blocked, non-existent, or otherwise prevents air (sound) from passing through the vent hole  25 ,  26 . Here, the acoustical resistance at the vent hole  25 ,  26  is large, e.g., R AV ≈∞. Frequency response curve  72 , on the other hand, is generated from an acoustical resistance assembly  10 ,  30  where the vent hole  25 ,  26  is open so that air passes through the vent hole  25 ,  26  in an uninterrupted manner. Here, the acoustical resistance at the vent hole  25 ,  26  is negligible, e.g., R AV ≈0. Thus, frequency response curves  71  and  72  represent the two extreme cases of no venting and venting with negligible acoustical resistance, respectively. The remaining frequency response curves  73 - 75  illustrate intermediate examples, with varying levels of acoustical resistance. For example, curve  73  indicates that the scrim  24  over the vent hole  25 ,  26  is more porous, and permits more air to pass through the vent hole  25 ,  26  than the scrim  24  corresponding to curve  74 . Similarly, curve  75  indicates that air is more difficult to pass through the vent hole  25 ,  26  due a less porous scrim  24  than the scrim corresponding to curve  74 . 
         [0047]    In order to modify the vents in the back plate  20  to tailor the frequency response, structural changes must be made to the PCB, and possibly the scrim  24 ,  22 , to accommodate for the back plate vent hole modifications, for example, to align the openings in the PCB with the back plate vent holes. Scrim materials are typically available having a discrete set of flow resistances. However, the use of commercially available scrim to modify the characteristics of the microspeaker may require the area of the hole and active area of the scrim  24  to be changed. In configurations having a back plate and a PCB, both the back plate and the PCB may need to be changed to modify the frequency response of the microspeaker in the in-ear headset. 
         [0048]    In brief overview, examples described herein provide a system and method for venting the motor of a microspeaker in a flexible manner, and with reduced design complexity, to achieve a wide range of frequency responses (e.g. those shown in  FIG. 4 ). This is achieved by tailoring the frequency response of the microspeaker in an in-ear headset by modifying the PCB, while maintaining the back plate configuration, for example, without changing the geometry of the back plate vents. Accordingly, a transducer design may be modified for different applications to achieve a desired frequency response. 
         [0049]    Although a microspeaker is shown and described, inventive concepts described herein can equally apply to other small transducers. Referring to  FIGS. 5A and 5B , the microspeaker  100  includes a housing or sleeve  112 , a diaphragm  114 , a coil  116 , a surround  117 , a permanent magnet  118 , a coin  119  or front plate, a back plate  120 , a printed circuit board (PCB)  122 , and a scrim  124 . The sleeve  112  has a hollow interior at which the front plate  119 , back plate  120 , magnet  118 , and coil  116  are positioned. A protective cover  121  can be positioned about the top of the sleeve  112  to protect the diaphragm  114  from damage during operation. 
         [0050]    One or more air holes  125  extend through the PCB  122 . A scrim  124  is positioned on a surface of the PCB  122  facing the back plate  120 , and covers the air holes  125 . The scrim  124  can be attached to the PCB  122  by an adhesive or other coupling mechanism or bonding technique. The scrim  124  and PCB  122  are separated from the back plate  120  by a predetermined distance so that a cavity  127  is formed between the PCB  122  and the back plate  120 . Scrim material may include, but not be limited to, woven monofilament fabric, wire cloth, nonwoven fabric, or related material to further tune the desired level of acoustical resistance, and thus the frequency response of the microspeaker. Accordingly, acoustical resistances of the scrim material can range from 3 to 260 Pa/(m/s), but not limited thereto. Pore sizes can range from 18 um to 285 um, but not limited thereto. 
         [0051]    The air holes  125 , either alone or in combination with the scrim  124  shown in  FIGS. 5A and 5B , form vents (referred to as second vents) which provide a desired level of acoustical resistance for air traveling between an external environment and the cavity  127  through the scrim  124 . The size, shape, location, number, and placement of the vent holes  125  in the PCB  122  can vary, as can the number of vent holes  125 , depending on the desired frequency response for the microspeaker. 
         [0052]    One or more vent holes  132  are located in the back plate  120 . Although vent holes  132  are referred to herein, the term vent hole  132  can also refer to notches or the like that are formed at the periphery of the back plate  120 . In the example of  FIG. 5B , the vent holes  132  are located at the outer diameter of the back plate  120 . Here, notches  132  are formed in the periphery, or outer diameter, of the back plate  120 , where the notch  132  is defined by a portion of the back plate  120 , and where a functional vent is formed when the back plate  120  is inserted into the sleeve  112 . In the example of  FIG. 5A , the vent holes  132  are located inboard from the outer diameter of the back plate  120 . Here, the back plate vent holes  132  can be formed by drilling through-holes in the back plate  120  where the entirety of the hole  132  is surrounded by the back plate  120 . The size, shape, location, number, and placement of the vent holes  132  in the back plate  120  can vary, as can the number of vent holes  132 . 
         [0053]    The back plate vent holes  132  are constructed and arranged to behave principally as an acoustical mass. More specifically, the vent holes  132  each has a cross-sectional area, diameter, or related dimension that is sufficiently large so that the complex acoustical impedance of the vent holes  132  is primarily imaginary or reactive. There will also be a real or resistive component to the complex acoustical impedance of the vent holes  132 . The real part of the total acoustical impedance of all the back plate vent holes combined is significantly lower than the real part of the total acoustical impedance of all the PCB vents combined (including the effect of the scrim  124  if it is present). 
         [0054]    As shown in  FIG. 9 , an air region between the top surface of the diaphragm  114  and the ear drum (not shown) is represented by an acoustical compliance C AF . The output is the pressure in the front cavity, i.e., at acoustical compliance C AF . The motion of the diaphragm  114  is represented by the volume velocity source U. An air region under the diaphragm  114  in the motor cavity  29  is represented by an acoustical compliance C AM . An air region at the back side of the transducer in a sealed earbud enclosure (not shown) is represented by an acoustical compliance C AB . 
         [0055]    In accordance with some examples, the scrim  124  covering the air holes  125  in the PCB  122  is represented by an acoustical resistance R AV  (distinguished from R AV  described with reference to a conventional assembly of  FIGS. 3 and 4 ). In particular, each air hole  125  has an acoustical resistance. The acoustical resistances of all air holes  125  are combined into a single element (R AV ). Air regions in each back plate vent hole  132  are collectively represented by an equivalent mass M AV . In particular, each vent hole  132  has an acoustical mass. The acoustical masses of all vent holes  132  are combined into a single element (M AV ). An air region in the cavity  127  is represented by an acoustical compliance C AG . 
         [0056]    The presence of the back plate vent holes  132  provides additional flexibility with respect to impacting the frequency response of the transducer. As described above, each vent hole  132  acts primarily as an acoustical mass M AV . An acoustical resonance of the system corresponds to the acoustical mass M AV , along with the acoustical compliance of air C AM . The acoustical impedances associated with the back plate vent holes  132 , respectively, can be configured to be parallel to each other. The back plate vent holes  132  can be constructed and arranged to achieve this. In doing so, the total acoustical mass can be reduced, which moves the resonance higher in frequency. This resonance may be dampened due to the acoustical resistance of the PCB  22  (with or without the scrim  24 ), which may be problematic if the acoustical resistance is too low.  FIGS. 10A and 10B  illustrate the impact on pressure sensitivity at the front cavity of the assembly  100  by reducing the number of back plate vent holes  132 , for example, from six vent holes to a single vent hole. 
         [0057]    In some examples, the back plate vent holes  132  are each positioned on an axis that may extend in a direction of diaphragm motion. The PCB air holes  125  can be offset from the back plate vent holes  132 , i.e., positioned on a different axis than the axis along which a neighboring back plate vent hole  132  is positioned. Alignment of the PCB air holes  125  and back plate vent holes  132  is not necessary because the pressure in the cavity  127  is assumed to be uniform at the frequencies of interest. Accordingly, PCB air holes  125  and back plate vent holes  132  can be misaligned with respect to each other, with no penalty with respect to performance. This provides flexibility in the mechanical design of these components so that they can be made easier to fabricate and assemble as compared to conventional approaches. Accordingly, to achieve, for example, to shape, a desired frequency response in a transducer design, only modifications to the PCB air hole geometry are required. 
         [0058]    Turning to  FIGS. 6A and 6B , the acoustical resistance assembly  200  is similar to the assembly  100  of  FIGS. 5A and 5B , except for the absence of a scrim material over the PCB  222 . Instead, the scrim is replaced by a plurality of micro vents  225  or small apertures or holes extending through the PCB  222  to the cavity  127 . The micro vents  225  serve as an “integral vent,” obviating the need for a scrim or the like positioned over a PCB opening to achieve a desired acoustical resistance. The number and/or size of the micro vents  225  can establish the desired damping characteristics, and thus the frequency response, of the assembly  200 . Similar to other examples in  FIGS. 5A and 5B , the acoustical resistance of the assembly  200  can be adjusted by modifying the PCB  222  which in  FIGS. 6A and 6B  includes the addition of micro vents  225 , but without the need to modify the back plate  120 . In addition, the absence of a scrim simplifies the manufacturing process with respect to the assembly  200  due to a reduced part count along with a reduced number of adhesive joints otherwise required to bond the scrim to the PCB. 
         [0059]    The acoustical resistance assembly  200  can be represented by the acoustical impedance circuit  140  illustrated at  FIG. 9  which has been described above. Other equivalent circuits can equally apply. For example, an equivalent circuit can illustrate the back plate vents  132  and PCB micro vents  225  each as a generic acoustical impedance block with both real and imaginary components. 
         [0060]    As described above, the back plate vent holes  132  can behave principally as an acoustical mass. On the other hand, the micro vents  225  are configured to have an area, length, and/or related dimensions to behave principally as an acoustical resistance. A relevant and important feature is for the real part of the total acoustical impedance of all the PCB vents combined (including the effect of scrim if it is present) to be significantly higher than the real part of the total acoustical impedance of all the back plate vent holes. 
         [0061]    The size, shape, location, number, and placement of the micro vents  225  in the PCB  222  can vary, as can the number of micro vents  225 , depending on the desired frequency response for the microspeaker, the mechanical resistance of the microspeaker in a vacuum, manufacturability, and other design considerations. The acoustical resistance provided to the system by each vent hole depends on its length and diameter—in particular, the smaller the diameter, the higher the acoustical resistance (assuming a fixed length), and the longer the hole, the higher the acoustical resistance (assuming a fixed diameter). Additionally, for substantially identical holes, the total acoustical resistance is inversely proportional to the number of holes. Thus, adding holes reduces the total acoustical resistance, while removing holes increases the total acoustical resistance. As an example, for a fixed PCB thickness (and thus vent hole length) of 360 μm, the effect of the acoustical resistance provided by a varying number of holes is shown in  FIGS. 12A and 12B , for holes of diameter 50 μm and 100 μm, respectively. In the example of  FIGS. 12A and 12B , the PCB through which the holes extend has a thickness of about 360 μm. The number of holes in each case range from zero to the maximum number of holes that can fit on a PCB of a given dimension and minimum hole-to-hole spacing. In each case, the approximate number of holes required for a desired frequency response is noted. It can be seen that more vent holes will be required to achieve the desired frequency response when a smaller diameter vent hole is used.  FIG. 11  emphasizes this by depicting the relationship between the approximate number of vent holes required to achieve an example target frequency response for this configuration and the diameter of the vent holes, ranging from 50 μm and 200 μm. The decoupling of the back plate venting and the PCB venting described above allows for greater flexibility in the choice of PCB hole size and number and thus greater control over the frequency response. 
         [0062]    When tuning the damping of the microspeaker, a number of micro vents  225  can be determined. By increasing or decreasing the number of micro vents  225  the frequency response can be changed. The micro vents  225  are offset with respect to a set of back plate vents  132 , and separated from the back plate vents  132  by the cavity  127 , achieving similar benefits as those described with reference to acoustical resistance assembly  100  described in  FIGS. 5A and 5B . 
         [0063]    With reference to  FIG. 7 , the acoustical resistance assembly  300  includes a protective cover  121 , a diaphragm  114 , a voice coil  116 , a suspension element  117 , a front plate  319 , a back plate  320 , and a printed circuit board (PCB)  322 , similar to or the same as those of other embodiments herein. The assembly  300  also includes a magnet  318 , which can be similar to or the same as the magnet  18  of  FIGS. 1 and 2 . In some examples, the magnet  18  is a ring magnet. In other examples, the magnet is a cylindrical magnet. Other magnet types can equally apply. The back plate  322  has a pole piece  123  that extends from a base portion of the back plate  320  towards the diaphragm  114 . The voice coil  116  and magnet  318  are each positioned about the pole piece  23 . 
         [0064]    A cavity  127  is formed by the back plate  320  and a scrim  124  coupled to the PCB  322 . The cavity  127  provides for a volume of air can be represented by an equivalent acoustical compliance C AG  illustrated in the acoustical impedance circuit  140  illustrated at  FIG. 9 . Therefore, the acoustical resistance assembly  300  of  FIG. 7  can be represented by the acoustical impedance circuit  140  illustrated at  FIG. 9 . The presence of the cavity  127  and PCB air holes  325  in  FIG. 7  permits the acoustical resistance to be tuned without modifying the back plate  320 . 
         [0065]    With reference to  FIG. 8 , the acoustical resistance assembly  400  is similar to the assembly  300  of  FIG. 7 , except for presence of micro vents  335  or small apertures extending through the PCB  322  to the cavity  127 . The micro vents  335  serve as an “integral vent,” obviating the need for a scrim or the like positioned over a PCB opening to achieve a desired acoustical resistance, similar to the example illustrated in  FIGS. 6A and 6B . 
         [0066]    A number of implementations have been described. Nevertheless, it will be understood that the foregoing description is intended to illustrate and not to limit the scope of the inventive concepts which are defined by the scope of the claims. Other examples are within the scope of the following claims.