Patent Publication Number: US-8989419-B2

Title: Phase plug with axially twisted radial channels

Description:
BACKGROUND 
     1. Technical Field 
     The field relates to acoustic phase plugs. 
     2. Description of the Problem 
     Horn loading a diaphragm type loudspeaker has long been done in order to control the direction of radiation of sound produced. A horn also increases loudspeaker efficiency in air as it operates as a transformer to improve impedance matching between the loudspeaker and the transmission medium. In effect, a horn makes the air adjacent the loudspeaker diaphragm “stiffer.” It was also early recognized that horns progressively cut-off the throughput of middle and high frequency sound with increasing frequency and, consequentially, operate to distort voices and music. Wente in U.S. Pat. No. 2,037,187 noted that the cutoff characteristic was traceable in a large measure to the fact that the sound waves emanating from the various portions of the diaphragm of the loudspeaker traverse paths of unequal length before reaching the throat of the horn. As a result sound waves propagated from different areas of the diaphragm reach the throat out of phase. For low frequency waves the disparity in phase is not particularly detrimental but at progressively higher frequencies the phase difference can increase to a sufficient degree to cause a marked neutralization (or cancellation) of the sound waves with high frequency speech and music being noticeably diminished in intensity. Horn performance was improved by making the sound front introduced to the throat of the horn increasingly “planar.” 
     Wente proposed a “sound translating device,” or what today would be called a phase plug, to control path distance between sections of a loudspeaker diaphragm and a horn throat. The phase plug was constructed using a tapered cone and a plurality of progressively larger, hollow tapered members with the tapered cone nested in the smallest hollow tapered member and the smallest hollow tapered member nested in the next smallest member to produce a series of annular air ducts/sound channels intermediate a dome shaped portion of the diaphragm and the horn throat. 
     Henricksen in U.S. Pat. No. 4,050,541 proposed a phase plug having radial rather than annular sound channels. This was done in part to simplify manufacture of the phase plug. Avera in U.S. Pat. No. 6,064,745 taught a phase plug with radial channels where the phase plug had a frusto-conical body with a spherical entrance face which conformed to a loudspeaker diaphragm and a planar outlet face. The channels through the plug exhibited equal or slowly increasing aperture area from entrance face to outlet face to minimize diffraction. 
     Whether employed with a horn/waveguide or not, generation of a sound field characterized by planar wavefronts from a diaphragm type loudspeaker reduces distortion at a point of reception, particularly at high frequencies. A wavefront is usually defined as a surface of constant phase. The further characterization of a wavefront as being “planar” means that the sound field exhibits constant phase in a flat surface where the surface is perpendicular to the direction of sound propagation. A sound field can consist of successive planar wavefronts. 
     SUMMARY 
     A loudspeaker assembly comprises a casing with a diaphragm loudspeaker mounted in the casing. A phase plug is mounted in the casing adjacent the diaphragm loudspeaker. The phase plug has a central cone with a longitudinal axis extending from an input end adjacent the diaphragm loudspeaker to an output end with its base at the input end tapering to an apex at the output end. A plurality of vanes extend radially outwardly from the central cone with the plurality of vanes being thickest at the input end and progressively thinning toward the output end. Radially outward edges of the plurality of vanes are twisted relative to the longitudinal axis with the degree of twist being progressively greater with increasing proximity to the output end. The plurality of vanes tapers in width along their longitudinal extent from the input end to the output end. The casing, plurality of vanes, central cone and interior walls define a plurality of radially twisted phase leveling channels having input ports on the input end and outlet ports on the output end of the phase plug. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Understanding of the following description may be enhanced by reference to the accompanying drawings, wherein: 
         FIG. 1  is a bottom or input end perspective view of a phase plug assembly; 
         FIG. 2  is an output end perspective view of the phase plug assembly of  FIG. 1 ; 
         FIG. 3  is a plan view of the phase plug from the phase plug assembly of  FIGS. 1 and 2 ; 
         FIG. 4  is a perspective view of the phase plug; 
         FIG. 5  is a cutaway view of the phase plug; 
         FIG. 6  is a perspective cutaway view of a loudspeaker assembly incorporating a phase plug; 
         FIG. 7  is a side elevation of the phase plug; 
         FIG. 8  is a sectioned view of the phase plug assembly taken perpendicular to its longitudinal axis along section lines  8 - 8  in  FIG. 7 ; 
         FIG. 9  is a cross sectional view of the phase plug assembly taken through the longitudinal axis of the phase plug assembly; 
         FIG. 10  is a bottom or input end perspective view of an alternative phase plug assembly; 
         FIG. 11  is an output end perspective view of the phase plug assembly of  FIG. 10 ; 
         FIG. 12  is a sectioned view of the alternative phase plug assembly taken perpendicular to its longitudinal axis; 
         FIG. 13  is a cross sectional view of the phase plug assembly taken through the longitudinal axis of the phase plug assembly; 
         FIG. 14  is a side elevation of the phase plug of the alternative phase plug assembly; 
         FIG. 15  is an input end perspective view of the alternative phase plug; 
         FIG. 16  is a perspective view from the output end of the alternative phase plug; 
         FIG. 17  is a plan view of the input end of the alternative phase plug; 
         FIG. 18  is a bottom or input end perspective view of a second alternative phase plug assembly; 
         FIG. 19  is an output end perspective view of the phase plug assembly of  FIG. 18 ; 
         FIG. 20  is a sectioned view of the second alternative phase plug assembly taken perpendicular to its longitudinal axis; 
         FIG. 21  is an end view of the second alternative phase plug; 
         FIG. 22  is a side elevation of the second alternative phase plug; 
         FIG. 23  is a cross section taken along section lines  23 - 23  of  FIG. 21 ; 
         FIG. 24  is a cross section taken along section lines  24 - 24  of  FIG. 22 ; 
         FIG. 25  is a perspective view of the output end of the second alternative phase plug; and 
         FIG. 26  is a reverse angle perspective view to that of  FIG. 25 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1-9  a loudspeaker assembly  10  comprises an acoustic transducer assembly  22 , a phase plug sub-assembly  62  and an assembly ring  52 . The acoustic transducer assembly  22  and the phase plug sub-assembly  60  are attached to opposite sides of the assembly ring  52 . Loudspeaker assembly  10  generates acoustic planar wavefronts from an assembly output face  38 . 
     The phase plug sub-assembly  60  includes a carrier  62  which is a single piece molded element. Carrier  62  has a substantially cylindrical outer wall  20  attached to a circumferential assembly ring  52  at one end of the cylinder. Carrier  62  also has a frusto-conical inner casing  50  and a plurality of radial segmented divider ribs  30  formed between the cylindrical outer wall  20  and the frusto-conical inner casing. The frusto-conical inner casing  50  is hollow and defines a frusto-conical void in which the perimeter face  40  of a downstream or output section  14  of a phase plug  18  snugly fits to form a phase plug sub-assembly  60 . 
     Frusto-conical inner casing  50  is open at both ends along the longitudinal axis C of the casing or the phase plug  18 . One end opens onto output face  38  to provide planar output ports  54  which extend radially outwardly from a central longitudinal axis “C”. The other end  37 , surrounding an input surface  36  of the phase plug  18 , is mated to assembly ring  52  and closed by the acoustic transducer assembly  22 . A phase plug  18  input section  12  extends from the enclosed open end of the frusto-conical casing  50  where it is encircled by assembly ring  52 . Planar output ports  54  may feed into the throat of a horn (not shown) or left exposed depending upon the application. 
     Phase plug  18  has a central cone  44  which is symmetric about central longitudinal axis C. Central cone  44  tapers to an apex on the output face  38 . A plurality of radial vanes  46  extend radially outwardly from the central cone  44 . Each radial vane  46  is thickest along an acoustically upstream or input surface  36  (see  FIGS. 2 and 3 ), extends from the input surface  36  to the planar output face  38  and progressively thins from the input surface  36  to an edge along planar output face  38 . The plurality of radial vanes  46  are twisted relative to the central longitudinal axis C with the degree of twist greatest toward the outside perimeter  40  of the output section  14 . The degree of twist grows progressively larger with increasing proximity to the planar output face  38 . 
     The inner casing  50 , the plurality of radial vanes  46  and central cone  44  define a plurality of radially twisted, phase leveling channels  48  having input ports on the input end and outlet ports on the output end of the phase plug  18 . The plurality of radially twisted, phase leveling channels  48  exhibit the same progressive radial twist exhibited by the vanes  46  toward the output face  38  and away from the central cone  44 . The progressive thinning of the plurality of vanes  46  results in increasing latitudinal cross-sectional area of the phase leveling channels  48  at the outlet ports  54  along the planar output face  38  as compared to the inlet ports  56  on the acoustically upstream input surface  36 . The distance or radius of the channels  48  progressively decreases moving downstream so that the cross-sectional area of the channels can be kept constant or allowed to slowly increase. 
     Phase leveling channels  48  exhibit greater minimum distances from the radially outward portions of the cone diaphragm  24  to the planar output face  38  than they exhibit relative to portions of the cone diaphragm closer to the central longitudinal axis C. The degree of twist applied to radial vanes  46  is chosen so that the minimum distance through the phase leveling channels  48  to the planar output face  38  is the same across the device. 
     The base phase plug  18  comprises the input surface  36  and a central base cap  42 . Base cap  42  closes an anti-resonant cavity  16  within central cone  44 . Taken together, input surface  36  and base cap  42  provide a surface which closely conforms to the shape of a cone diaphragm  24  which it faces and which it is proximate to. In this context the term proximate may be taken distances relative to the wavelengths of sound of interest. 
     Cone diaphragm  24  is a vibratile surface generating sound energy in response to electrical signals applied to a voice coil  26 . The central portion of the cone diaphragm  24  is reinforced using an inverted spider support  32 . Voice coil  26  is wound on a central pole mount  34  supporting the cone diaphragm  24 . Changing electrical current in voice coil  26  generates force for moving the cone diaphragm  24  by interacting with the magnetic field from a permanent magnet  28  mounted outside of the voice coil on the inside wall of the steel magnetic shield bucket  58 . 
     Referring to  FIGS. 10-17  an alternative phase plug assembly  60 A is illustrated. Phase plug assembly  60 A is substantially identical to phase plug assembly  60  except that the phase plug  19  incorporates an annular dividing ring  66  which divides the twisted radial channels  48  into radially inner and outer divisions  48 A and  48 B, respectively. The annular dividing ring  66  has a constant width and parallels the taper of the outer surface of the central cone  44 . 
     Referring to  FIGS. 18-26  still another phase plug assembly  60 B is illustrated which is structurally the same as phase plug assembly  60  except for substitution of a alternative phase plug  21 . Phase plug  21  displays a largely closed input surface  36  broken by concentric ring constellations of a plurality of input ports  72 . Input ports  72  are connected by radially twisted channels  75  which run through the body of phase plug  21  to connect with outlet ports  74  on the output surface  38  of the phase plug assembly  60 B. 
     For any given cross section taken perpendicular to the central longitudinal axis of phase plug  21 , the channels  75  are arranged in concentric circular constellations around the axis. The radius of each constellation grows progressively smaller moving acoustically downstream through the phase plug  21 . Channels  75  may increase in cross sectional area from the input port  72  toward the output port  74  for each channel so that the surface area of output surface  38  is largely made up of output ports  74 .