Abstract:
A loudspeaker enclosure accommodates at least one speaker. Sound waves emanating from the rear of the speaker exit through a port in the enclosure. The interior of the enclosure contains at least one partition to lengthen the acoustic path from the rear of the speaker to the port and to cause alternating expansion and contraction of the sound waves emanating from the rear of the speaker. The partitions are arranged in relation to the speaker(s) so as to reduce sound wave reflections that would create non-linearities in the frequency response.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional application of co-pending application Ser. No. 13/214,003, filed Aug. 19, 2011. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of loudspeaker systems, and more particularly to a high efficiency, extended bass speaker enclosure. 
     2. Background Art 
     This inventor&#39;s U.S. Pat. No. 5,012,889 describes a speaker enclosure that provided a significant improvement over then state-of-the-art loudspeaker systems. One of the objects of my earlier invention was to provide a speaker system that yielded a lower or deeper response from a given-sized enclosure over conventional methods of tuning. Another object of my earlier invention was the elimination of various unwanted side effects of conventional tuning methods. For example, the loudspeaker enclosure described in my earlier patent avoided the creation of standing waves, which degrade the speaker&#39;s performance, by reducing parallel surfaces inside the speaker enclosure. 
     The interior of the enclosure was divided into two smaller chambers by means of a partition inclined with respect to the front and rear walls. One such chamber was adjacent to the speaker, while the second or rear chamber was separated from the front chamber by the partition. Below the speaker, a vent or port allowed the rear chamber to communicate with the environment outside of the enclosure. 
     Sound waves emanating from the rear of the speaker were reflected by the inclined partition toward the top of the speaker enclosure. The partition did not extend all the way to the top wall of the enclosure, thereby allowing sound waves to reflect off of the top wall and enter the rear chamber. The sound waves then exited the enclosure through the port below the speaker. The rear waves were thus delayed with respect to the front waves from the speaker to achieve the desired tuning of the speaker system. Furthermore, the arrangement of the interior partition caused the rear waves to be compressed at two locations thereby increasing the effective delay and further improving low frequency response. 
     Another embodiment of a speaker enclosure employing these design principles is described in my subsequently issued U.S. Pat. No. 5,111,905. The disclosures of both U.S. Pat. No. 5,012,889 and U.S. Pat. No. 5,111,905 are fully incorporated herein by reference. 
     Although the speaker enclosures of my earlier patents provided improved bass performance for a given-sized enclosure in comparison to other speaker designs of the time, it was found that reflected sound waves would effectively cancel some of the frequencies of sound and create non-linearities in the frequency response. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention comprise a loudspeaker enclosure accommodating at least one speaker. Sound waves emanating from the rear of the speaker exit through a port in the enclosure. The interior of the enclosure contains at least one partition to lengthen the acoustic path from the rear of the speaker to the port and to cause alternating expansion and contraction of the sound waves emanating from the rear of the speaker. The partitions are arranged in relation to the speaker(s) so as to reduce sound wave reflections that would create non-linearities in the frequency response. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment of the invention in this disclosure are not necessarily to the same embodiment, and they mean at least one. 
         FIG. 1  is a perspective view of a prior art loudspeaker system. 
         FIG. 2  is a cross-sectional view through line  2 - 2  of  FIG. 1 . 
         FIG. 3  is a partially cut-away front elevation view of a speaker enclosure according to an embodiment of the present invention. 
         FIG. 4  is a cross-sectional view through line  4 - 4  of  FIG. 3 . 
         FIG. 5  is a cross-sectional view of another speaker enclosure according to an embodiment of the present invention. 
         FIG. 6  is a cross-sectional view of another speaker enclosure according to an embodiment of the present invention. 
         FIG. 7  is a cross-sectional view of another speaker enclosure according to an embodiment of the present invention. 
         FIG. 8  is a cross-sectional view of another speaker enclosure according to an embodiment of the present invention. 
         FIG. 9  is a cross-sectional view of another speaker enclosure according to an embodiment of the present invention. 
         FIG. 10  is a cross-sectional view of another speaker enclosure according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, for purposes of explanation and not limitation, specific numbers, dimensions, materials, etc. are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known speaker components are omitted so as not to obscure the description of the present invention with unnecessary detail. 
     Throughout this description, reference is made to relative directions, such as “top”, “bottom”, “front”, “back”, “side” and similar terms. It will be understood that these terms refer to directions as the speaker enclosures are illustrated in the drawings; however, this does not dictate the orientations in which the speaker enclosures may be utilized. For example, it may be convenient to place a speaker enclosure on its side so that the “side” of the enclosure as described herein is actually the “bottom” of the enclosure as used. Therefore, the use of such terms is not to be interpreted as limiting the invention in any regard. 
     Referring to  FIGS. 1 and 2 , the loudspeaker enclosure  10  of my earlier patents is shown. Enclosure  10  is a generally box-like structure having a front wall  11 , a top wall  12 , a bottom wall  13 , a rear wall  14 , and opposing side walls  15  and  16 . 
     Loudspeakers  17  and  18  are mounted in respective apertures in front wall  11 . As is conventional in the construction of multi-speaker systems, loudspeaker  17 , also referred to as a woofer, is designed to reproduce bass frequencies, whereas loudspeaker  18  is designed to reproduce frequencies in the mid-range and above. A crossover network (not shown) separates the frequency components radiated by loudspeakers  17  and  18 . Additional loudspeakers or other sound radiating devices may be incorporated in enclosure  10 , either co-located on front wall  11  with loudspeakers  17  and  18  or disposed on one of the other walls of the enclosure. In particular, multiple woofers may be employed in lieu of a single speaker as illustrated. A grill of fabric, foam or other suitable material may be secured to the outer surface of front wall  11  in order to provide a pleasing and decorative appearance. 
     Referring particularly to  FIG. 2 , enclosure  10  includes a partition  20  disposed therewithin. Partition  20  includes a shelf portion  21  that is generally horizontal and that abuts front wall  11  immediately below woofer  17 . Inclined portion  22  is contiguous with shelf portion  21  and extends upwardly and rearwardly therefrom behind woofer  17 , terminating at top edge  22   a . Both shelf portion  21  and inclined portion  22  extend the entire width of enclosure  10  between side walls  15  and  16 . Shelf portion  21  and inclined portion  22  are secured to side walls  15  and  16  so that the interior volume of enclosure  10  is acoustically separated into a first chamber  23  adjacent to woofer  17  and a second chamber  24  between inclined portion  22  and rear wall  14 . Shelf portion  21  is separated from bottom wall  13  by a narrow passage  25 , which communicates with port  19 . Port  19  comprises an aperture formed within front wall  11 , or may be conveniently formed by terminating the lower portion of front wall  11  at shelf portion  21  so that a horizontal slot is formed adjacent to bottom wall  13  extending the entire width of enclosure  10  between side walls  15  and  16 . 
     The above-described speaker system provided improved bass performance for a given-sized enclosure in comparison to contemporaneous speaker designs. However, a problem with this earlier design was that, in a smaller enclosure, the woofer  17  was necessarily placed near the bottom of the inclined portion  22 . This meant that the inclined portion  22  was in close proximity to the back of the woofer. It was found that some of the sound waves would travel to the partition and reflect directly back towards the woofer. These reflected sound waves would effectively cancel some of the frequencies of sound and create non-linearities in the frequency response. Most often, this could be discerned in the midrange frequencies. 
     The cancellation frequency can be determined by measuring the distance from the rear of the woofer cone to the board and doubling it for the round trip. Then, the frequency for a half wavelength of this distance may be determined. At the distance of a half wavelength, the reflected sound from the board will reach the back of the woofer cone 180 degrees out of phase, with the result being partial cancellation. For example, at a distance of 2½ inches from the board, the frequency of cancellation will be approximately 1,350 HZ, which is clearly in the audible range. As the board is slanted, there will be additional frequencies that will be subject to this cancellation. 
     It should be noted that, for simplification purposes, reference is made to the sound wave that travels off the back of the woofer cone and into the enclosure. However, in reality for any given frequency, the sound travels in one direction for the first part or 180 degrees of the cycle and in the opposite direction for the second half or 180 degrees of the cycle. So, depending on the time and frequency, the sound wave may be traveling in either direction inside the enclosure. 
     The design of a speaker enclosure is driven by many considerations. The external dimensions, of course, must be large enough to accommodate the size(s) of the selected speaker(s), but it is generally desired to make the enclosure as compact as reasonably possible. The locations, angles and dimensions of the interior partitions, as well as the location and size of the vent, are then “tuned” to the characteristics of the speaker(s). A particular enclosure designed to deliver superior performance with a particular speaker may not perform well at all with another speaker of equal size, but with different characteristics. Thus, it should be understood that any specific dimensions referred to in the following descriptions of certain embodiments apply to specific speaker applications and would likely need to be adjusted for different speakers. 
       FIG. 3  is a partially cut-away front elevation view and  FIG. 4  is a vertical cross-sectional view of an improved speaker enclosure  100  in accordance with an embodiment of the present invention. Enclosure  100  is similar in many respects to enclosure  10 . Notably different, however, is the placement of speakers  117  and  118 . Woofer  117  is mounted away from shelf portion  121  to minimize cancellation as discussed above. Tweeter  118  is mounted below woofer  117 . By placing the woofer higher within the enclosure and mostly higher than the top of slanted board  122 , the distance from the rear of the speaker to the nearest reflecting surface (the rear of the enclosure) is significantly increased in comparison to my earlier designs. This significantly lowers the frequency of cancellation. Acoustical filling may be inserted in the increased volume behind the woofer to effectively eliminate that frequency inside the enclosure. The result is that the listener receives that frequency directly from the front of the woofer. 
     Shelf portion  121  is generally horizontal and abuts front wall  111  immediately below speaker  118 . Unlike the shelf portion in enclosure  10  described above, shelf portion  121  of enclosure  100  extends further rearwardly toward back wall  114 , leaving gap  123 . Shelf portion  121  is supported above bottom wall  113  by a pair of strips  124  that are spaced apart to establish the width of port  119 . Slanted portion  122  extends upwardly from shelf portion  121  and rearwardly therefrom behind speaker  118 . Both shelf portion  121  and slanted portion  122  extend the entire width of enclosure  100  between side walls  115  and  116 . 
     Sound waves leaving the rear of woofer  117  are compressed as they enter the space between the top of slanted portion  122  and back wall  114 . As the sound waves travel down, there is decompression as the volume increases between slanted portion  122  and back wall  114 . The sound waves are again compressed as they enter gap  123  on the way to port  119 . Reference to “compression” of the sound waves is not meant to imply that the sound waves are compressed in the sense of decreasing their wavelength, and hence increasing their frequency. Rather, it is the reciprocating air masses in which the sound waves are propagated that are cyclically compressed in the regions of decreasing cross sectional area and then decompressed in the regions of increasing cross sectional area. However, it is convenient to refer to this effect as compression/decompression of the sound waves. 
     The physical construction of speaker enclosure  100  is similar to that described above for enclosure  10 . The walls and partitions of enclosure  100  may be constructed from medium density fiberboard (MDF), particle board, plywood or any other suitable material having acoustical properties appropriate for use in a loudspeaker enclosure generally. The individual panels are joined to one another by fasteners and/or glue, taking care that the joints are tight and will not vibrate. 
     In a particular embodiment, speaker enclosure  100  illustrated in  FIGS. 3 and 4  may be adapted for a bookshelf speaker using an approximately 4-inch woofer. In such an embodiment, the enclosure has internal dimensions of approximately 10.5 inches high by 5 inches wide by 5 inches deep. The gap  123  is approximately 0.84 inch and the port dimensions are approximately 3.5 inches wide by 0.375 inch high. 
       FIG. 5  is a horizontal cross-sectional view of a speaker enclosure  200  in accordance with another embodiment of the present invention. Enclosure  200  is adapted for a center channel speaker with a pair of woofers  217   a ,  217   b  and a single tweeter  218 . Partitions  220  each include a first portion  221  that is generally parallel to and spaced apart from the respective side wall  212 ,  213  and that abuts front wall  211  outboard of the respective woofer. Each of partitions  220  further includes a second portion  224  that is generally parallel to and spaced apart from back wall  214 . Third portions  222  of partitions  220  extend forwardly from second portions  224  at respective angles pointing generally toward tweeter  218 . All of the partition portions  221 ,  224  and  222  extend the entire height of enclosure  200  between the top and bottom walls. The distance behind the woofers to the respective partition portions  224  is maximized in this design. The angled portions are mostly off to the sides of the respective woofers and provide compression into the main chamber  226 , shared by both woofers, where decompression occurs. There are two bass-reflex ports  219 , one at the top and one at the bottom. Sound waves are compressed a final time upon entering the space between portions  224  and back wall  214  on the way to the port openings. 
     In a particular embodiment, speaker enclosure  200  has internal dimensions of approximately 15 inches wide by 5 inches high by 5 inches deep. Partitions are spaced apart from the side and rear walls by approximately 0.31 inch, which also defines the width of the ports. 
       FIG. 6  is a vertical cross-sectional view of a speaker enclosure  300  in accordance with another embodiment of the present invention. Enclosure  300  is adapted for a single speaker subwoofer. A shelf  321  is spaced apart from bottom wall  313  and abuts front wall  311  to define a port opening  319 . Slanted partition  322  extends upwardly from shelf  321  and rearwardly therefrom behind speaker  317 . Shelf  321  and slanted partition  322  extend the entire width of enclosure  300  between the side walls. 
       FIG. 7  is a vertical cross-sectional view of a speaker enclosure  400  in accordance with another embodiment of the present invention. Enclosure  400  is adapted for a single speaker subwoofer and is similar to enclosure  300  described above. A first shelf  421  is spaced apart from bottom wall  413  and abuts front wall  411  to define a port opening  419 . Here, however, shelf  421  extends further back toward back wall  414  than does shelf  321  of the previously described embodiment, terminating at gap  423 . Slanted partition  422  extends upwardly from shelf  421  and rearwardly therefrom behind speaker  417 . A second shelf  425  is spaced apart from shelf  421  and is disposed behind partition  422 . Shelves  421  and  425  and slanted partition  422  extend the entire width of enclosure  400  between the side walls. 
     In a particular embodiment adapted for use with an approximately 10-inch woofer, speaker enclosure  300  has internal dimensions of approximately 15 inches high by 15 inches wide by 14 inches deep. The shelves are spaced apart from the bottom wall and from each other by approximately 0.5 inch, which is the vertical dimension of port  319 . 
       FIG. 8  is a vertical cross-sectional view of a speaker enclosure  500  in accordance with another embodiment of the present invention. Enclosure  500  is adapted for a two-speaker side-firing assisted subwoofer with a speaker  517  on each side of the enclosure. A first shelf  521  is generally horizontal and abuts front wall  511  below speakers  517 . A second shelf  525  also abuts front wall  511  and is spaced apart from and below shelf  521 . Slanted partitions  522   a  and  522   b  extend upwardly and downwardly, respectively, from the end of shelf  521 . Shelves  521  and  525  and slanted partitions  522   a  and  522   b  extend the entire width of enclosure  500  between the side walls. 
     In a particular embodiment adapted for use with a pair of approximately 10-inch woofers, speaker enclosure  500  has internal dimensions of approximately 30.5 inches high by 12 inches wide by 20 inches deep. Shelf  521  extends back from front wall  511  approximately 8.5 inches and shelf  525  extends back from front wall  511  approximately 6.5 inches. Partitions  522   a  and  522   b  are each approximately 6 inches in length. Shelves  521  and  525  are spaced apart by approximately 0.625 inch, which is the vertical dimension of port  519 . 
       FIG. 9  is a vertical cross-sectional view of a speaker enclosure  600  in accordance with another embodiment of the present invention. Enclosure  600  is adapted for a two-speaker assisted subwoofer. Subwoofers  617   a  and  617   b  are mounted in side wall  615 , which also includes port  619 . A first shelf  621  is generally horizontal and abuts front wall  611  below speaker  617   b . A second shelf  625  also abuts front wall  611  and is spaced apart from and below shelf  621  to form a rectangular duct leading to port  619 . Slanted partitions  622   a  and  622   b  extend upwardly and downwardly, respectively, from shelf  621 . Shelf  621  and slanted partitions  622   a  and  622   b  extend the entire width of enclosure  600  between the side walls. Shelf  625  abuts side wall  615 , but is spaced apart from the opposite side wall to provide an air path to the rectangular duct leading to port  619 . 
     In a particular embodiment adapted for use with a pair of approximately 10-inch woofers, speaker enclosure  600  has internal dimensions of approximately 39 inches high by 8.5 inches wide by 22 inches deep. Shelf  621  extends back from front wall  611  approximately 10 inches to partitions  622   a  and  622   b  and approximately 2.75 inches beyond, leaving a gap of approximately 9.25 inches to rear wall  614 . Shelf  625  extends back from front wall  611  approximately 9 inches. Partition  622   a  is approximately 11.5 inches in length and partition  622   b  is approximately 10 inches in length. Shelves  621  and  625  are spaced apart by approximately 0.825 inch, which is the vertical dimension of port  619 . 
       FIG. 10  is a cross-sectional view through a speaker enclosure  700  in accordance with another embodiment of the present invention. Enclosure  700  is adapted for use with an in-wall speaker system, which, by its nature, is limited in depth. Woofer  717  and tweeter  718  are mounted within enclosure  700  facing outwardly to project sound waves into the room through a front grill. Due to the limited depth of the enclosure, it is not practical to provide an effective serpentine path behind the woofer as in the previously described embodiments. A partition  720  surrounds a substantial portion of woofer  717 . A vent  719  is located below partition  720  so that sound waves emanating from the rear of the woofer must travel around the partition before exiting through the vent. 
     It will be recognized that the above-described invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the disclosure. Thus, it is understood that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.