Abstract:
The invention features an acoustic waveguide and system for transmitting pressure wave energy produced by an electroacoustical transducer in a medium that propagates pressure wave energy. The acoustic waveguide and system includes a tube defining a spiral-shaped channel with a length of L. The tube has a first end and a second end with the first end closed and the second end open to the medium. The tube has a transducer opening for accommodating an electroacoustical transducer located between the first and second end of the tube. The system includes an electroacoustical transducer mounted to the acoustic waveguide.

Description:
This is a continuation-in-part of application Ser. No. 10/072,784 filed Feb. 8, 2002. 
    
    
     TECHNICAL FIELD 
     This invention relates to acoustic waveguide electroacoustical transducing systems. 
     BACKGROUND OF THE INVENTION 
     For background, reference is made to Bose U.S. Pat. No. 4,628,528 and Bose U.S. Pat. No. 6,278,789 B1, both of which are incorporated by reference herein. 
     It is an important object of the invention to provide an improved acoustic waveguide and electroacoustical transducing system which has a long waveguide channel within a relatively compact structure. 
     SUMMARY OF THE INVENTION 
     In an aspect, the invention features an acoustic waveguide for transmitting pressure wave energy produced by an electroacoustical transducer in a medium that propagates pressure wave energy. The acoustic waveguide has a tube defining a spiral-shaped channel with a length of L. The tube has a first end and a second end with a transducer opening for accommodating an electroacoustical transducer located adjacent to the first end of the tube. The second end of the tube is open the medium. 
     Embodiments may include one or more of the following features. The spiral-shaped channel may have a smoothly changing curvature with radius. Additionally, the inner walls of the waveguide may be contiguous. The effective length of channel L may be approximately one quarter of the wavelength of the lowest frequency pressure wave energy to be transmitted by the waveguide. The lowest frequency to be transmitted corresponds substantially to the frequency below which the output level commences falling substantially continuously with frequency. The tube may define a spiral shaped channel that has a rectangular cross section. The tube may define a spiral shaped channel that has a rectangular cross section. The tube may define a spiral shaped channel that is coiled in a single plane, forming a flat spiral, or coiled in a plurality of planes, forming a helical spiral. 
     In another aspect of the invention, an acoustic waveguide for transmitting pressure wave energy produced by an electroacoustical transducer in a medium that propagates pressure wave energy, the waveguide has a tube having a first end and a second end and formed in a spiral configuration. The first end of the tube is closed and the second end of the tube is open to the medium and a transducer opening for accommodating an electroacoustical transducer is located on the tube between the first end and second end of the tube. The tube defines a first spiral-shaped channel located between the transducer opening of the tube and the first end of the tube and a contiguous second spiral-shaped channel located between the transducer opening of the tube and the second end of the tube. 
     Embodiments may include one or more of the following features. The first spiral-shaped channel defined by the tube may have a length of ⅓L while the second spiral-shaped channel may have a length of ⅔L. The length of the first spiral-shaped channel, ⅓L, plus the length of the second spiral-shaped channel, ⅔L, plus end effect may be approximately equal one quarter of the wavelength of the lowest frequency pressure wave energy to be transmitted by the waveguide. The first and second spiral-shaped channels may each have a smoothly changing curvature with radius. The inner walls of the tube may be contiguous. The first spiral-shaped channel may have substantially the same cross-section as the second spiral-shaped channel. The cross section of the first and second spiral-shaped channels may be rectangular. The tube may be composed of PVC. The tube defining the spiral-shaped channel may be coiled in a single plane, forming a flat spiral. The tube may be coiled in a plurality of planes, forming a helical spiral. A transducer housing may be attached to the tube and the tube may have a second transducer opening located between the tube and the transducer housing. The tube may be of two-piece construction which may be assembled with screws, bolts, clips, adhesive, glue and the like. 
     In another aspect of the invention, a system for transmitting pressure wave energy in a medium that propagates pressure wave energy in a medium, the system includes an electroacoustical transducer having a vibratile surface and a spiral waveguide. 
     Embodiments of the invention may have one or more of the following advantages. 
     A spiral waveguide permits a long waveguide channel within a relatively compact structure. A long waveguide channel improves the bass response of a loudspeaker system, while a compact structure can be particularly convenient in a loudspeaker system where physical space is limited, such as in an automobile or portable stereo. Additionally, a spiral waveguide does not have any abrupt 90 or 180 degree bends in the channel, which minimizes unwanted turbulence in the waveguide channel. A spiral waveguide can also be configured to have an open and a closed end with a transducer positioned at a specific distance between the open and closed end in order to reduce the first peak in frequency response of the acoustic energy transmitted by the waveguide. 
     Other features, objects and advantages will become apparent from the following detailed description when read in connection with the accompanying drawing in which: 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1A is a top view of a top spiral acoustic waveguide member comprising an electroacoustical transducing system having an open end and a closed end. 
     FIG. 1B is a bottom view of the top spiral waveguide member of FIG. 1A; 
     FIG. 1C is a top view of a bottom spiral waveguide member having an open end and a closed end; 
     FIG. 1D is a bottom view of the bottom spiral waveguide member of FIG. 1C; 
     FIG. 2A is a graphical representation of acoustic power output as a function of frequency (i) at the end of a single-ended waveguide and (ii) at the end of the open-ended channel of a two channel waveguide having a 2:1 channel length ratio; 
     FIG. 2B is a graphical representation of the acoustic power output as a function of the frequency at the transducer of (i) a single-ended waveguide and a two-channel waveguide having a 2:1 channel length ratio; 
     FIG. 3A is a top view of a top spiral waveguide member having an open end and a closed end and having a transducer housing; 
     FIG. 3B is a bottom view of the top spiral waveguide member of FIG. 3A; 
     FIG. 3C is a top view of a bottom spiral waveguide member having an open end and a closed end and having a transducer housing; 
     FIG. 3D is a bottom view of the bottom spiral waveguide member of FIG. 3C; and 
     FIG. 3E is a side view of the top spiral waveguide member shown in FIGS. 3A-B attached to the bottom waveguide member shown in FIGS.  3 C-D. 
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     With reference now to the drawings, FIGS. 1A and 1B show the top and bottom view, respectively, of a top waveguide member  10 , while FIGS. 1C and 1D show the top and bottom view, respectively, of a matching bottom waveguide member  11 . A spiral waveguide is formed by attaching a top waveguide member  10  with a bottom waveguide member  11 , thus forming a waveguide channel  20  (with a length L) having an open end  30  and a closed end  31 . In this particular embodiment, the two waveguide members,  10  and  11 , are attached by four screws through the four holes,  41 ,  42 ,  43  and  44 . However, the two waveguide members may be attached by screws, bolts, nails, clips, tabs and slots, tongues and grooves, pins, glue, adhesive, cement and the like. 
     Referring again to FIGS. 1A and 1B, the top waveguide member  10  has a transducer opening  50 , where an electroacoustical transducer such as a loudspeaker transducer (not shown) may be mounted. In this particular embodiment, the bottom waveguide member  11  provides for two holes  61 ,  62  which provide a passage for wire connecting the transducer to an electrical signal source. The transducer opening  50  is located along the waveguide channel  20  such that it divides the waveguide channel  20  into two contiguous channels, an open-ended channel  21  (having a length L 1 ) and a closed-ended channel  22  (having a length L 2 ). Both of the contiguous channels  21 ,  22  have a smoothly changing curvature with radius, substantially identical rectangular cross sections, and are centered about the same spiral axis. 
     The length of waveguide channel  20  plus any end effect is approximately one quarter of the wavelength of the lowest frequency pressure wave energy to be transmitted by the waveguide. For example, if the lowest frequency pressure wave energy to be transmitted by the waveguide is 60 Hz in air at room temperature, the length of the waveguide channel  20  (plus any end effect) is approximately 1.4 meters. 
     The walls of the waveguide channel  20  are hard. PVC, ABS, Lexan, other hard plastic, metal, or wood materials or the like provide suitable material to construct the walls of the waveguide. 
     The transducer may be mounted at any location along the waveguide channel  20  depending on the design of the system. In the embodiment illustrated in FIGS. 1A-1D, the transducer opening  50  is configured to mount an electroacoustical transducer such that path length of the open-ended channel  21  is approximately twice as long as the closed-ended channel  22 . This positioning of the transducer is useful for greatly reducing the first resonance peak that would be present in the frequency response of the acoustic energy transmitted by a single-ended waveguide. 
     FIGS. 2A and 2B show a graphical representation of the acoustic power output as a function of frequency at the open end of a waveguide channel (FIG. 2A) and at the transducer opening (FIG. 2B) (i) with the transducer located adjacent to the closed end of the waveguide channel of length L and (ii) with the transducer located between the open end and the closed end such that the distance between the open end and the transducer is approximately twice as long (⅔L) as the distance between the closed end and the transducer (⅓L). In this particular illustration, the waveguide channel is approximately 1.34 meters in length, has a circular cross section with a cross-sectional diameter of 7.23 cm, and is approximately 56% of the cross-sectional area of the transducer. In this example, a volume located behind the transducer and between the transducer and waveguide channel and is approximately 500 cubic centimeters. A volume located behind the transducer and between the transducer and waveguide channel is not necessary and is preferably as small as practical (ideally zero) if the mechanical dimensions of the transducer, the cross-sectional area of the waveguide and other design restrictions permit it. Removing or reducing the volume between the transducer and waveguide channel in this example would still leave the beneficial results described of a reduction in the first resonance peak. 
     As shown in FIG. 2A, the first resonance peak, which occurs at approximately 200 Hz in this example, is greatly reduced by positioning the transducer at a location that divides the waveguide channel into a closed end channel of length ⅓L and an open ended channel of length ⅔L (i.e., a 2:1 ratio). Similarly, FIG. 2B shows that the transducer output does not experience a corresponding null (i.e, reduced displacement) at approximately 200 Hz. 
     FIGS. 3A-3E show another embodiment of a spiral waveguide electroacoustical transducing system. FIGS. 3A and 3B show the top and bottom view, respectively, of a top waveguide member  10 , while FIGS. 3C and 3D show the top and bottom view, respectively, of a matching bottom waveguide member  11 . FIG. 3E shows a side view of the assembled spiral waveguide electroacoustical transducing system. 
     The waveguide shown in FIGS. 3A-3E is similar in structure to the waveguide shown in FIGS. 1A-1D, having a spiral-shaped waveguide channel  20  with an open end  30  and closed end  31 . A transducer opening  50  is provided in the top waveguide member  10  and divides the waveguide channel  20  into an open-ended channel  21  and a contiguous closed-ended channel  22 . The transducer opening is located along the waveguide channel  20  such that the open-ended channel  21  is approximately twice as long as the closed-ended channel  22 . In this embodiment, the dimension between the top and bottom surfaces of the assembled waveguide is reduced to make a more compact structure by allowing the rear of the transducer to protrude beyond said bottom surface. Said rear of the transducer is covered by back housing  70  which may be formed as an integral part of the bottom waveguide member  11  or it may be formed as a separate structure to be affixed to the rear of the bottom waveguide member  11 . The front side of the transducer faces out of the transducer opening  50 . In the embodiment shown in FIGS. 3A-3E, a volume located behind the transducer and between the transducer and waveguide is created. While from an acoustical performance standpoint, it is normally preferable to have a minimal volume behind the transducer and between the transducer and waveguide, other design considerations such as limitations in the amount of physical space available for the waveguide may necessitate a volume behind the transducer and between the transducer and the waveguide. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the embodiments shown in FIGS. 1A-D and  3 A-E illustrate a spiral waveguide assembly having two contiguous spiral channels,  21 ,  22 , which radiate out from the transducer opening  50 . However, another embodiment of the spiral waveguide may have a single spiral channel radiating out from an inner end to an outer end, with the electroacoustical transducer mounted adjacent to the inner end (thus forming a single-ended waveguide). The transducer may be mounted such that the vibratile surface of the transducer is parallel to the plane of the spiral waveguide channels as shown in FIGS. 1A-D and  3 A-E, or it may be mounted such that the vibratile surface is at the end of a channel, perpendicular to the plane of the spiral waveguide channels. The spiral waveguide may also be formed as a flat spiral (as illustrated in FIGS. 1A-D and  3 A-E) where the waveguide channel is coiled in a single plane, or the waveguide may be formed as a helical spiral (i.e., a helix) where the waveguide channel is coiled in a constantly changing plane. The cross section of the waveguide channel may be rectangular, circular, oval or the like. The length and cross section of the waveguide channels may be modified according to the lowest desired frequency of transmission, medium of transmission, and surface area of the vibratile surface of the transducer. The transducer does not have to be partially or fully enclosed by the waveguide structure with the front of said transducer facing out of the waveguide through hole  50 , but may, for example, be mounted external to said waveguide structure such that the front of the transducer faces into the waveguide through hole  50 . The spiral acoustic waveguide as shown in FIGS. 1A-D and  3 A-E show a two-piece construction of the waveguide channel, however, the two piece construction may consist of a single top or bottom member comprising the waveguide walls and a corresponding bottom or top member which is substantially flat and which, when assembled with the top or bottom member, forms the fourth wall of the waveguide or construction of the waveguide channel may be of a single piece of construction or may be formed from multiple pieces attached together. Additional embodiments may include damping material, such as polyester, disposed within one or more of the waveguide channels. 
     It is evident that those skilled in the art may make numerous modifications of the departures from the specific apparatus and techniques disclosed herein without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques disclosed herein and limited solely by the spirit and scope of the appended claims.