Patent Publication Number: US-11395071-B2

Title: Sound generator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/CN2018/123194, filed on Dec. 24, 2018, which claims priority to Chinese Patent Application No. 201810146756.5, filed on Feb. 12, 2018 and Chinese Patent Application No. 201820043738.X, filed on Jan. 10, 2018, all of which are hereby incorporated by reference in their entireties. 
     TECHNICAL FIELD 
     The present disclosure relates to the technical field of sound generating device. 
     BACKGROUND 
     The sound generating device is an important component in electronic products and is used to convert electrical signals into acoustic signals. The development trend of the electronic products is to get thinner and thinner, and in order to achieve more functions, there are more and more components in the electronic products. Hence, the space reserved for the sound generating device is bound to become smaller and smaller. Furthermore, the electronic products are paying more and more attention to the user&#39;s music experience, so the sound generating device is required to have better sound quality. 
     In order to improve music experience effects, the sound generating device in the prior art installs a sound generator in a box with a volume. The sound generator comprises a housing, and a magnetic circuit system and a vibration system accommodated and fixed in the housing. A rear cavity is formed between the sound generator and the box. The larger the rear cavity, the lower the low-frequency resonance frequency of the product, thereby the low-frequency performance of the product is improved. The sound generating device in the prior art generally has two structures: one structure is similar to the sound box, wherein the box is of a rectangular parallelepiped type, and a sound generator is fixed on a front panel of the box, forming a rear cavity in a thickness direction and rear horizontal direction, which is not conducive to the thinning and miniaturization of the product; another structure is that the box has an accommodating cavity for accommodating a sound generator and a rear cavity located on the side of the sound generator, wherein forming the rear cavity on the side of the sound generator can obtain the largest possible rear cavity volume, but at the same time, it also leads to a larger space occupied by the entire sound generating device in a horizontal direction, which is not conducive to the miniaturization of the product. 
     Furthermore, in the sound generating device in the prior art, the shape of the rear cavity is irregular, and the airflow from the sound generator into the rear cavity is not stable enough to cause problems such as polarization and distortion, and the acoustic effect is not satisfactory. 
     If the volume of the sound generating device of the existing structure is reduced, the volume of the rear cavity of the sound generating device is bound to be reduced. Therefore, it is necessary to provide a new sound generating device, which has a small volume and good performance to meet the development needs of electronic products. 
     SUMMARY 
     The embodiment of the invention provides a sound generator, which can meet the requirements of small volume and have better performance. 
     The invention further provides a sound generator, comprising a housing, a vibration system and a magnetic circuit system; wherein, 
     the vibration system and the magnetic circuit system are sequentially accommodated and fixed at a first end of the housing from top to bottom; 
     the magnetic circuit system is provided with a rear sound hole; 
     the housing comprises a first portion corresponding to the vibration system and the magnetic circuit system, and a second portion integrally extending downward from the first portion beyond a bottom surface of the magnetic circuit system; 
     a second end portion of the housing is integrally provided with a housing bottom wall or separately mounted with a lower cover plate; and a rear cavity which is in communication with the rear sound hole is formed between the second portion of the housing, the bottom surface of the magnetic circuit system and the housing bottom wall or the lower cover plate. 
     Optionally, the housing is a straight cylinder structure with openings at two ends; the vibration system comprises a diaphragm and a voice coil fixed below the diaphragm, the diaphragm being fixed on an end surface of a first end opening of the housing; and the lower cover plate is mounted at a second end opening of the housing. 
     Optionally, the magnetic circuit system comprises a magnetic conductive yoke, and a central magnetic circuit portion mounted on an upper surface of the magnetic conductive yoke and a side magnetic circuit portion; 
     a magnetic gap accommodating the voice coil is formed between the central magnetic circuit portion and the side magnetic circuit portion; and 
     at least one of the central magnetic circuit portion and the side magnetic circuit portion is provided with a permanent magnet. 
     Optionally, an outer side of the side magnetic circuit portion is disposed in close contact with an inner wall of the housing. 
     Optionally, a peripheral side of the magnetic conductive yoke is disposed in close contact with an wall of the housing. 
     Optionally, the magnetic conductive yoke is rectangular, and a corner of the magnetic conductive yoke is provided with a first rear sound hole communicating with the magnetic gap and the rear cavity. 
     Optionally, the central magnetic circuit portion comprises a central magnet and a central magnetic conductive plate provided on a top surface of the central magnet; at the central magnetic circuit portion, the magnetic circuit system is provided with a through hole that sequentially penetrates the magnetic conductive yoke and the central magnet as a part of the rear cavity, and a second rear sound hole communicating with the through hole is provided on the central magnetic conductive plate. 
     Optionally, a ratio of an opening volume of the center magnet to the center magnet volume before opening is less than or equal to 35%. 
     Optionally, a second end of the housing is open, the lower cover plate is mounted at the second end opening of the housing, an inner well of the first end of the housing is provided with a convex edge extending toward a center of the housing, and an upper edge of the magnetic circuit system abuts and is fixed on a lower surface of the convex edge. 
     Optionally, the first end of the housing is open, an inner side of an end surface of the first end opening of the housing has a recessed first step end surface, and the first step end surface has a bottom surface and a side surface for mounting the diaphragm. 
     Optionally, an upper cover plate mounted on the housing is also provided above the diaphragm, and an edge of the upper cover plate is located inside the side surface of the first step end surface. 
     Optionally, a first protrusion is provided outside the bottom surface of the edge of the upper cover plate, and an ultrasound line is provided on the first protrusion; 
     the bottom surface of the first step end surface is provided with a first groove at a position corresponding to the first protrusion, and the first protrusion of the upper cover plate is inserted into the first groove and is ultrasonically welded to a bottom surface of the first groove 
     Optionally, the first end of the housing is open, a second groove is provided outside an end surface of the first end opening of the housing, and the diaphragm is fixed inside the end surface of the first end opening; 
     an upper cover plate mounted on the housing is also provided above the diaphragm, and a second protrusion is provided outside a bottom surface of an edge of the upper cover plate, and the second protrusion extends into the second groove and is fixed by bonding. 
     Optionally, the housing is of a rectangular structure. 
     Optionally, the rear sound hole is provided with a breathable spacer, and the rear cavity is filled with a sound absorbing material. 
     Optionally, the housing bottom wall or the lower cover plate is provided with a filling hole for filling the sound absorbing material, and a cover sheet is encapsulated on the filling hole. 
     Optionally, the cover sheet is provided with air-permeable micro-holes that allow air to pass and do not allow the sound absorbing material to pass; or, 
     the cover sheet is provided with a leak hole, and the leak hole is covered with a damping mesh that allows air to pass and does not allow the sound absorbing material to pass. 
     Optionally, a second end of the housing is open, the lower cover plate is mounted at the second end opening of the housing, and the lower cover plate is made of metal. 
     Optionally, the lower cover plate is of a flat plate shape; or the lower cover plate is of a bowl-shaped structure provided with a bottom wall and a side wall. 
     Optionally, a second end of the housing is open, the lower cover plate is mounted at the second end opening of the housing, and the lower cover plate is adhered to an end surface of the second end opening of the housing by a strand layer; or 
     an inner side of the end surface of the second end opening of the housing is provide with a recessed second step end surface, the second step end surface is provided with a top surface and side surfaces for mounting the lower cover plate; the lower cover plate is of a flat plate shape, an edge of the lower cover plate is provided with a recessed portion recessed toward the rear cavity, the recessed portion abuts on the top surface of the second step end surface and forms a first holding strand groove between the side surfaces of the second step end surface, and the first holding strand groove is coated with strands to fix the lower cover plate on the housing; or, the lower cover plate is of a bowl-shaped structure provided with a bottom wall and a side wall, an end of the side wall of the lower cover plate is bent outward to provide a mounting edge, the mounting edge abuts on the top surface of the second step end surface and forms a second holding strand groove between the side surfaces of the second step end surface, and the second holding strand groove is coated with strands to fix the lower cover plate on the housing; or 
     a plastic edge is injection-molded on a periphery of the lower cover plate, and the plastic edge is ultrasonically welded to the second end opening of the housing. 
     Optionally, the first end of the housing is open, and the second end portion of the housing is integrally provided with a housing bottom wall, the housing bottom wall being made of a plastic material; or, the housing bottom wall comprises an integrally molded metal sheet. 
     In the technical solution provided in the embodiment of the invention, the housing comprises a first portion corresponding to the vibration system and the magnetic circuit system, and a second portion integrally extending downward from the first portion beyond a bottom surface of the magnetic circuit system; and a second end portion of the housing is integrally provided with a housing bottom wall or separately mounted with a lower cover plate, and a rear cavity being formed between the second portion of the housing, the magnetic circuit system, and the housing bottom wall or the lower cover plate. Compared with the prior art, the invention directly forms a sufficiently large rear cavity space from the lower end portion of the housing of the sound generator. Firstly, there is no need to additionally configure the box structure forming the rear cavity, thus it will not increase the occupied space in the horizontal direction, and the peripheral area of the housing of the sound generator determines the size of the space occupied by the entire sound generating device in the electronic product, which helps to achieve miniaturization of the product, and on the basis of miniaturization, it can take into account the volume of the magnetic circuit system and the volume of the rear cavity, thereby ensuring acoustic performance. Secondly, a rear cavity is arranged directly below the vibration system and the magnetic circuit system, and the rear cavity has a regular shape and is close to the rear acoustic hole. Compared with the prior art, the same large rear cavity volume can achieve a better acoustic effect. In addition, the technical solution provided by the embodiments of the invention is only to extend the design of the housing of the sound generator, the structure is simple, and there is no need to perform the assemble between the sound generator and the box or the box structure, which can simplify the manufacturing process and mounting process and increase the production efficiency. In addition, the embodiment of the invention also enlarges the volume of the rear cavity to improve the acoustic performance of the device by providing rear sound holes at the bottom edge and the central position of the magnetic circuit system, and effectively solves the problem that the acoustic resistance of the vibration becomes larger to make the stability of the vibration system becoming worse since the distance between the vibration system and the magnetic circuit of the miniaturized device is small. 
     Other features and advantages of the invention will become clear from the following detailed description of exemplary embodiments of the invention with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings that form a part of the description describe embodiments of the invention and together with the description serve to explain the principles of the invention. 
         FIG. 1  is an exploded schematic view of a sound generator provided by an embodiment of the invention; 
         FIG. 2  is a schematic cross-sectional view of a sound generator provided by an embodiment of the invention; 
         FIG. 3  is a partially enlarged schematic view of  FIG. 2 ; 
         FIG. 4  is a schematic cross-sectional view of a sound generator provided by an embodiment of the invention; 
         FIG. 5  is a schematic structural view of a specific implementation of a magnetic conductive yoke in a sound generator provided by an embodiment of the invention; 
         FIG. 6  is a schematic view of a top surface angle of a sound generator provided by an embodiment of the invention; 
         FIG. 7  is a schematic view of a bottom surface angle of a sound generator provided by an embodiment of the invention; 
         FIG. 8  is a connection structure schematic view of an upper cover plate and a housing in a sound generator provided by an embodiment of the invention; 
         FIG. 9  is a connection structure schematic view of a lower cover plate and a housing in a sound generator provided by an embodiment of the invention; 
         FIG. 10  is an another connection structure schematic view of a lower cover plate and a housing in a sound generator provided by an embodiment of the invention; 
         FIG. 11  is a further connection structure schematic view of a lower cover plate and a housing in a sound generator provided by an embodiment of the invention; 
         FIG. 12  is a partially enlarged schematic view of  FIG. 11 ; 
         FIG. 13  is a further connection structure schematic view of a lower cover plate and a housing in a sound generator provided by an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Various exemplary embodiments of the invention will now be described in detail with reference to the drawings. It should be noted that: unless specifically stated otherwise, the relative arrangement of components and steps, numerical expressions, and numerical values set forth in these embodiments do not limit the scope of the invention. The following description of at least one exemplary embodiment is actually merely illustrative, and in no way serves as any limitation on the invention and its application or use. 
     Techniques and devices known to those of ordinary skill in the related art may not be discussed in detail, but where appropriate, the techniques and devices should be considered as part of the description. In all examples shown and discussed herein, any specific values should be interpreted as exemplary only and not as limitations. Therefore, other examples of the exemplary embodiment may have different values. It should be noted that: Similar reference numerals and letters indicate similar items in the following drawings. Therefore, once an item is defined in one drawing, there is no need to discuss it further in subsequent drawings. 
       FIGS. 1-5  show a schematic structural view of a sound generator provided by an embodiment of the invention. As shown in  FIGS. 1, 2 and 4 , a housing  10 , a vibration system  20  and a magnetic circuit system  30  are included. The vibration system  20  and the magnetic circuit system  30  are sequentially accommodated and fixed at a first end of the housing  10  from top to bottom. Combined with  FIGS. 4 and 5 , the magnetic circuit system  30  is located below the vibration system  20  and fixed in the housing  10 , and the magnetic circuit system  30  is provided with a rear sound bole  40 . As shown in  FIG. 4 , the housing  10  comprises a first portion  1001  corresponding to the vibration system  20  and the magnetic circuit system  30 , and a second portion  1002  integrally extending downward from the first portion  1001  beyond a bottom surface of the magnetic circuit system  30 . A second end portion of the housing  10  is integrally provided with a housing bottom wall (not shown in the figures) or separately mounted with a lower cover plate  50 ; and a rear cavity  60  which is in communication with the rear sound hole  40  is formed between the second portion  1002  of the housing, the bottom surface of the magnetic circuit system  30 , and the housing bottom wall or the lower cover plate  50 . 
     Wherein, according to the actual situation, the above-mentioned housing  10  may be selected to have one end opening or two end opening structures. One end opening structure may be an upper end opening or a lower end opening, and the other end is a closed end. When the closed end is an upper end corresponding to the vibration system, it is allowed to open a small sound hole on the closed end, and after assembling the vibration system and the magnetic circuit system from the opening end, close the open end with a cover plate. 
     In a specific implementation structure, the housing  10  is a straight cylinder structure with two ends opening; as shown in  FIGS. 1 and 2 , a vibration system  20  is installed at a first end opening of the housing  10 ; the vibration system  20  comprises a diaphragm  21  and a voice coil  22  fixed below the diaphragm  21 , the diaphragm  21  being fixed on an end surface of a first end opening of the housing  10 ; and the lower cover plate  50  is mounted at the second end opening of the housing  10 . 
     Compared with the prior art, the technical solution provided by the embodiments of the invention directly forms a sufficiently large rear cavity space from the lower end portion of the housing of the sound generator. There is no need to additionally configure the box structure forming the rear cavity, thus it will not increase the occupied space in the horizontal direction, and the peripheral area of the housing of the sound generator determines the size of the space occupied by the entire sound generating device in the electronic product, which helps to achieve miniaturization of the product, and on the basis of miniaturization, it can take into account the volume of the magnetic circuit system and the volume of the rear cavity, thereby ensuring acoustic performance. Secondly, a rear cavity is arranged directly below the vibration system and the magnetic circuit system, and the rear cavity has a regular shape and is close to the rear acoustic hole. Compared with the prior art, the same large rear cavity volume can achieve a better acoustic effect. In addition, the technical solution provided by the embodiments of the invention is only to extend the design of the housing of the sound generator, the structure is simple, and there is no need to perform the assemble between the sound generator and the box or the box structure, which can simplify the manufacturing process and mounting process and increase the production efficiency. In addition, the embodiment of the invention also enlarges the volume of the rear cavity to improve the acoustic performance of the device by providing rear sound holes at the bottom edge and the central position of the magnetic circuit system, and also effectively solves the problem that because the distance between the vibration system of the miniaturized device and the magnetic circuit is small, the acoustic resistance of the vibration becomes larger and the stability of the vibration system becomes worse. 
     In a specific implementation structure, as shown in  FIGS. 1 and 2 , the magnetic circuit system  30  comprises a magnetic conductive yoke  31 , and a central magnetic circuit portion  301  mounted on an upper surface of the magnetic conductive yoke  31  and a side magnetic circuit portion  302 ; a magnetic gap housing the voice coil  22  is formed between the central magnetic circuit portion  301  and the side magnetic circuit portion  302 ; and at least one of the central magnetic circuit portion  301  and the side magnetic circuit portion  302  is provided with a permanent magnet. In particular, the central magnetic circuit portion  301  comprises a central magnet  32  and a central magnetic conductive plate  33 . The side magnetic circuit portion  302  comprises a side magnetic conductive plate  35  and a side magnet  34 . In order to reduce the volume of the sound generator and maximize the magnetic circuit system, as shown in  FIG. 2 , the outer side of the side magnetic circuit portion  302  is disposed in close contact with the inner wall of the housing  10 . Further, the peripheral side of the magnetic conductive yoke  31  and the inner wall of the housing  10  are also disposed in close contact with each other. 
     Further, the magnetic conductive yoke  31  may be rectangular, and a corner of the corresponding magnetic conductive yoke  31  may be provided with a first rear sound hole  401  communicating with the magnetic gap and the rear cavity  60 . More specifically, as shown in  FIG. 1 , the magnetic conductive yoke  31  is a polygonal structure with four corners provided with notches; at the corner positions of the magnetic conductive yoke  31 , that is, the positions near the edges of the notches, first rear sound holes  401  communicating with the magnetic gap and the rear cavity  60  is provided. 
     Furthermore, as shown in  FIG. 2 ; the central magnetic circuit portion  301  of the magnetic circuit system  30  includes a central magnet  32  and a central magnetic conductive plate  32  provided on the top surface of the central magnet  32 . At the central magnetic circuit portion  301 , the magnetic circuit system  30  is provided with a through hole that sequentially penetrates the magnetic conductive yoke  31  and the central magnet  32  as a part of the rear cavity  60 , and a second rear sound hole  402  communicating with the through hole is provided on the central magnetic conductive plate  33 . 
     The four first rear acoustic holes  401  at the four corners of the magnetic conductive yoke  31  cannot achieve the best air circulation effect with the rear cavity  60 , thus in this embodiment, a second rear sound hole  402  communicating with the through holes in the magnetic conductive yoke  31  and the central magnet  32  is provided on the central magnetic conductive plate  33  as a No. 5 rear sound hole. The four first rear sound holes  401  and the second rear sound hole  402  together constitute the rear sound hole  40  provided on the magnetic circuit system. The No. 5 rear sound hole can not only play the role of expanding the capacity of the acoustic cavity  60 , but also solve the problem that the acoustic resistance of the vibration becomes larger to make the stability of the vibration system becoming worse since the distance between the vibration system and the magnetic circuit of the miniaturized device is small. 
     What needs to be added here is that the central area of the central magnet  32  contributes to the BL of the sound generator (a parameter which measures the strength of the driving system in the sound generator) less than the boundary area. Therefore, when the volume of the rear cavity  60  is limited, the center area of the central magnet  32  is hollowed-out to increase the volume of the rear cavity, which helps to improve the performance of the product. Although the hollowed-out area of the central magnet  32  has little influence on the BL value of the magnetic circuit system  30 , it still has some influence. If the hollowed-out area of the central magnet  32  is too large, its influence on the BL value of the magnetic circuit system  30  cannot be ignored. If the hollowed-out area is too large, the BL value of the magnetic circuit system  30  will be smaller, and the performance of the product will be lower. Therefore, it is necessary to find a balance range such that the increase of the volume of the rear cavity  60  since the center magnet  32  is hollowed-out improves the product performance more than the reduction in the BL value of the magnetic circuit system reduces the product performance, thereby optimizing the product performance. Through simulation, it is known that when the hollowed-out volume of the center magnet  32  accounts for less than 35% of the original volume of the center magnet, the product performance is improved. When the hollowed-out volume of the center magnet  32  exceeds this range, the BL value of the magnetic circuit system  30  sharply decreases. At this time, the increase in the space of the rear cavity  60  has a lower performance improvement effect than the product performance reduction effect caused by the decrease of the BL value of the magnetic circuit system, and the overall performance is the reduction of product performance. Therefore, in the above technical solution provided by the invention, the opening volume of the center magnet should satisfy: the ratio of the opening volume of the center magnet  32  to the volume of the center magnet  32  before opening is less than or equal to 35%, and can be further controlled to 5%-30%. 
     Further, when the second end of the housing  10  is open and the lower cover plate  70  is installed at the second end opening of the housing, as shown in  FIG. 3 , the inner wall of the first end of the housing  10  is provided a convex edge  1003  extending toward a center direction of the housing  10 , and an upper edge of the magnetic circuit system  30  abuts and is fixed on a lower surface of the convex edge  1003 . 
     When the first end of the housing  10  is open, continuing to refer to  FIG. 3 , an inner side of an end surface of the first end opening of the housing  10  has a recessed first step end surface  11 , and the first step end surface  11  has a bottom surface  111  and a side surface  112  for mounting the diaphragm  21 . Referring to  FIGS. 1 and 5 , an upper cover plate  70  mounted on the housing  10  is also provided above the diaphragm  21 , and an edge of the upper cover plate  70  is located inside the side  112  of the first step end surface  11 . 
     More specifically, the sound generator provided in the embodiment of the invention may further include: a centering support  200  provided between the diaphragm  21  and the voice coil  22 , and a reinforcement pan  23  provided on the side of the diaphragm  21  away from the magnetic circuit system  30 ; the reinforcement part  23  is fixed to the diaphragm  21 , as shown in  FIG. 1 . 
       FIGS. 6 and 7  show outer contour schematic diagrams of an implementation form of a sound generator provided by an embodiment of the invention. As shown in  FIGS. 6 and 7 , the housing  10  of the sound generator provided in this embodiment may be a rectangular structure. For example, adopting the means that the sound generator of the technical solution provided by the embodiment of the invention can be prepared to have a plane size of (6-30) mm*(8-30) mm, and then by providing a rear sound hole with a capacity expansion effect on the magnetic circuit system, the purpose of reducing the height dimension of the sound generator is achieved. 
     Further, as shown in  FIG. 8 , a first protrusion  701  is provided outside the bottom surface of the edge of the upper cover plate  70 , and an ultrasound line is provided on the first protrusion  701 ; the bottom surface of the first step end surface  11  is provided with a first groove at a position corresponding to the first protrusion  701 , and the first protrusion  701  of the upper cover plate  70  is inserted into the first groove and is ultrasonically welded to a bottom surface of the first groove. Alternatively, a second protrusion is provided on bottom surface of the first step end surface, and an ultrasound line is provided on the second protrusion; a third groove is provided at a position corresponding to the ultrasonic line on the bottom surface of the edge of the upper cover plate, the second protrusion of the first step end surface is inserted into the third groove and fixed by ultrasonic welding; the realization structure is not provided in the drawings. 
     Further, when the first end of the housing  10  is open, as shown in  FIG. 9 , a second groove  1005  is provided outside an end surface of the first end opening of the housing  10 , and the diaphragm  21  is fixed inside the end surface of the first end opening; an upper cover plate  70  mounted on the housing  10  is also provided above the diaphragm  21 , and a second protrusion  702  is provided outside a bottom surface of an edge of the upper cover plate  70 , and the second protrusion  702  extends into the second groove  1005  and is fixed by bonding. 
     Further, as shown in  FIGS. 1 and 2 , the rear sound hole is provided with a breathable spacer  80 , and the rear cavity  60  is filled with sound absorbing material. The sound absorbing material may be zeolite material, activated carbon material, or other materials with capacity expansion effect, which is not limited in this patent, wherein, the breathable spacer  80  is a mesh cloth that allows air to pass and does not allow sound absorbing material to pass, and is used to isolate the sound absorbing material and prevent it from entering the magnetic circuit system. Filling the rear cavity with sound absorbing material can further increase the volume of the rear cavity, which helps to improve the performance of the sound generator. The way of providing the breathable spacer  80  directly on the rear sound hole  40  can use all the space of the rear cavity to fill the sound absorbing material, thus increasing the filling amount of the sound absorbing material, and achieving a better capacity expansion effect. And, according to the embodiment, “the magnetic circuit system  30  is provided with a through hole that sequentially penetrates the magnetic conductive yoke  31  and the central magnet  32  as a part of the rear cavity  60 , and a second rear sound hole  402  communicating with the through hole is provided on the central magnetic conductive plate  33 ”, In the case that the through hole penetrating through the magnetic conductive yoke  31  and the central magnet  32  increases the rear cavity and is filled with sound absorbing material for the capacity expansion, the second rear sound hole  402  is located at the center of the magnetic circuit system, and the contact rate between the sound absorbing material at the position of the through hole and the air can be increased to achieve the best capacity expansion effect. 
     Further, as shown in  FIGS. 1, 3 and 7 , the housing bottom wall (not shown in the figures) or the lower cover plate  50  is provided with a filling hole  51  for filling the sound absorbing material, and a cover sheet  51  is encapsulated on the filling hole  50 . The cover sheet  52  may be directly a hard sheet that is not air-permeable, and only serves to block the sound absorbing material. As an another embodiment, the cover sheet  52  is also provided with air-permeable micro-holes that allow air to pass and do not allow the sound absorbing material to pass; or, the cover sheet  52  is provided with a teak hole  521 , and the leak hole  521  is covered with a damping  53  that allows air to pass and does not allow the sound absorbing material to pass. The above-mentioned specific embodiment makes the filling hole  51  serve as a leakage hole of the rear cavity, and can be used to balance the air pressure inside and outside the sound generator. Further, the acoustic resistance can be adjusted by adjusting the size of the air-permeable micro-holes or the mesh size of the damping net. 
     In an actual implementation, a lower cover plate  50  is installed at the second end opening of the housing  10 . The lower cover plate  50  in this embodiment may be made of a metal material, which may be made thinner and occupy less space. The lower cover plate  50  is of a flat plate shape as shown in  FIGS. 10, 11 and 13 ); or, the lower cover plate  50  is of a bowl-shaped structure provided with a bottom  501  and a side wall  502  (as shown in  FIG. 9 ). In the embodiment in which the lower cover plate  50  is made of metal and is of a bowl-shaped structure, the metal lower cover plate  50  of the bowl-shaped structure has high strength and takes up little space, and the presence of the side wall  502  forms a part of the rear cavity space. Therefore, the height of the housing  10  can be reduced, thereby avoiding the problem that the excessively high plastic housing needs to increase the wall thickness to ensure the overall structural strength, which will increase the occupied space, and is more conducive to miniaturization of the product. 
     Referring to  FIGS. 9-13 , in the sound generator provided in this embodiment, the lower cover plate  50  may be connected to the second end opening of the housing  10  in the following three ways. Of course, the embodiments of the present invention are not limited to the following connection methods. 
     In the first way, as shown in  FIG. 10 , the lower cover plate  50  is bonded to the end surface of the second end opening of the housing  10  through the strand layer  90 . Specifically, as shown in  FIG. 10 , the edge of the lower cover plate  50  extends to be flush with the outer side wall of the housing  10 , and the board surface of the lower cover plate  50  opposite to the end surface of the second end opening of the housing  10  has a back strand. The lower cover plate  50  is bonded to the end surface of the second end opening of the housing  10  through its own back strand to seal the rear cavity. 
     In the second way, as shown in  FIGS. 9, 11 and 12 , an inner side of the end surface of the second end opening of the housing  10  is provide with a recessed second step end surface  12 , the second step end surface  12  is provided with a top surface  121  and side surfaces  122  for mounting the lower cover plate  50 ; as shown in  FIGS. 11 and 12 , the lower cover plate  50  is of a flat plate shape, an edge of the lower cover plate  50  is provided with a recessed portion  54  recessed toward the rear cavity  60 , the recessed portion  54  abuts on the top surface  121  of the second step end surface and forms a first bolding strand groove  55  between the side surfaces  122  of the second step end surface  12 , and the first holding strand groove  55  is coated with strands to fix the lower cover plate  50  on the housing  10 . Alternatively, as shown in  FIG. 9 , the lower cover plate  50  is of a bowl-shaped structure provided with a bottom wall  501  and a side wall  502 , an end of the side wall  502  of the lower cover plate  50  is bent outward to provide a mounting edge  503 , the mounting edge  503  abuts on the top surface  121  of the second step end surface  12  and forms a second bolding strand groove  504  between the side surfaces  122  of the second step end surface  12 , and the second holding strand groove  504  is coated with strands to fix the lower cover plate  50  on the housing  10 . 
     In the third way, as shown in  FIG. 13 , a plastic edge  100  is injection-molded on a periphery of the lower cover plate  50 , and the plastic edge  100  is ultrasonically welded to the second end opening of the housing  10 . 
     In an another specific embodiment, the first end of the housing may be open, and the second end portion of the housing is integrally provided with a housing bottom wall, the housing bottom wall being all made of a plastic material; or, the housing bottom wall comprises an integrally molded metal sheet for increasing the space. 
     Although some specific embodiments of the invention have been demonstrated in detail by way of examples, it should be understood by a person skilled in the art that the above examples are only intended to be illustrative but not to limit the scope of the invention. It should be understood by a person skilled in the art that the above embodiments can be modified without departing from the scope and spirit of the present invention. The scope of the present invention is defined by the attached claims.