Abstract:
An electro-acoustic transducer has an electro-magnetically driven moving dome and a phase plug having a body and a dome-interface surface, with a compression cavity formed between the dome and the dome-interface surface. The phase plug includes at least first and second annular slots beginning at the dome-interface surface and extending a first depth into the body of the phase plug. The first and second slots are separated by a bridge element at the dome-interface surface and joined by a first bridge passage at the first depth beneath the dome-interface surface. The phase plug also includes an exit slot coupling the bridge passage to a throat at a second depth in the body of the phase plug.

Description:
BACKGROUND 
     This disclosure relates to electroacoustic transducing with a bridged phase plug. 
     A compression driver is a type of electroacoustic transducer in which air is compressed in a compression cavity between a moving diaphragm and a fixed phase plug. Passages in the phase plug, referred to as slots, conduct air from the compression cavity to a listening environment, generally through a throat and a horn. The horn provides impedance matching between the air in the throat and air in the free space of the listening environment and controls the directivity of the radiated sound. 
     Several terms are defined with reference to  FIGS. 1 and 2 . For reference, directions such as “top” and “bottom” or “above” and “below” refer to the drawing itself with the top and bottom margins of the drawing sheet defining up and down. As installed, a phase plug could face in any direction. In a compression driver, the primary moving element is referred to as the dome  10 . In some examples, the dome is a simple spherical section. In some examples, the dome has a complex curvature. The ends of the dome are formed into or joined to a cylindrical section called the skirt  12 . The skirt is joined to a voice coil former or bobbin  14  and a surround  16 , which is in turn fixed to the external structure  18 . In some examples, the surround is formed from an extension of the dome, not a separate part. A voice coil  20  is wound around the bobbin and reacts to a magnet  22  and pole piece  24  to move the bobbin and dome when a current or voltage is applied to the voice coil. Above the dome is a rear cavity  26  bounded by a rear cavity wall  28 . Below the dome is a front or compression cavity  30  bounded by a dome-interface surface  32  of a phase plug  34 . Movement of the dome compresses air in the compression cavity. In the example of  FIGS. 1 and 2 , the dome, skirt, bobbin, surround, external structure, voice coil, magnet, and pole piece are shown abstractly and are not meant to represent any particular design or technology. 
     In a typical phase plug, exemplified in  FIGS. 1 and 2 , one or more slots  36   a ,  36   b ,  36   c  begin at the dome-interface surface of the phase plug and join at the throat  38 , communicating the pressurized air from the compression cavity  30  to the throat  38 . The throat is defined as beginning at the point where the multiple slots are completely joined in a single passage. While we refer to these passages as slots, due to their appearance in a two-dimensional section (e.g.,  FIG. 1 ), they are actually cone-shaped voids in the three-dimensional phase plug, bounded on top and bottom by cones of slightly different radius (if the slots taper in width, as they do in this example) and/or vertical position. In  FIG. 2 , each of  36   a ,  36   b , and  36   c  is seen twice. Given the shape of the slots, the phase plug  34  is composed of several concentric cone-shaped solids  34   a - 34   c  and an outer cylindrical solid  34   d , all joined and held in relative position by supports (not shown) within the slots. The slots  36   a - 36   c  couple the compression cavity  30  to the throat  38 , which in turn couples to a horn (see  FIG. 7 ). 
     SUMMARY 
     In general, in some aspects, an electro-acoustic transducer has an electro-magnetically driven moving dome and a phase plug having a body and a dome-interface surface, with a compression cavity formed between the dome and the dome-interface surface. The phase plug includes at least first and second annular slots beginning at the dome-interface surface and extending a first depth into the body of the phase plug. The first and second slots are separated by a bridge element at the dome-interface surface and joined by a first bridge passage at the first depth beneath the dome-interface surface. The phase plug also includes an exit slot coupling the bridge passage to a throat at a second depth in the body of the phase plug. 
     Implementations may include one or more of the following features. The first and second slots may have approximately equal cross-sectional areas. The exit slot may have a cross-sectional area at the first bridge passage approximately equal to the sum of the cross sectional areas of the first and second slots. The exit slot may begin at the first bridge passage and have a cross-sectional area that increases exponentially with the length of the exit slot from the bridge passage to the throat. The first and second slots may be located at corresponding first and second radial distances from a central axis of the phase plug, the first and second radial distances corresponding to locations of first and second nulls in a standing wave excited in the compression cavity by motion of the dome. The exit slot may begin at a position along the first bridge passage corresponding to a location of a null in a standing wave in a loop including the first bridge passage, the first and second slots, and the portion of the compression cavity joining the first and second slots. 
     A voice-coil may be coupled to the dome and a compliant surround may couple the dome to a surrounding structure. A housing including a dome-facing surface may forms a back cavity between the dome and the dome-facing surface. A horn may be coupled to the output aperture of the phase plug. The phase plug may also include a third slot, the third slot beginning at the dome-interface surface and extending a third depth into the body of the phase plug, the third slot being separated from the second slot by a second bridge element at the dome-interface surface and joined by a second bridge passage at the third depth to the first bridge passage, and the exit slot beginning at the second bridge passage. The phase plug may also include third and fourth slots beginning at the dome-interface surface and extending a third depth into the body of the phase plug, the third and fourth slots being separated by a second bridge element at the dome-interface surface and joined by a second bridge passage at the third depth beneath the dome-interface surface, the second slot and first bridge passage being separated form the third slot and second bridge passage by a third bridge element at the dome interface surface and joined by a third bridge passage at a fourth depth beneath the third bridge element, the exit slot beginning at the third bridge passage. The first depth and the third depth may be approximately equal. The dome may be concave or convex relative to the phase plug. 
     Advantages include providing a smooth output response at high efficiency levels across the entire operating range of the compression driver. 
     Other features and advantages will be apparent from the description and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a sectional elevation view of a conventional compression driver. 
         FIG. 2  shows a cut-away isometric view of a conventional compression driver. 
         FIGS. 3A and 3B  show sectional elevation views of a compression driver having a bridged phase plug. 
         FIG. 4  shows a cut-away isometric view of a compression driver having a bridged phase plug. 
         FIGS. 5 and 6  show cross-sectional elevation views of alternative embodiments of compression drivers having bridged phase plugs. 
         FIG. 7  shows an assembled compression driver and horn. 
         FIG. 8  shows a sectional elevation view of a compression driver having a bridged phase plug and an inverted dome. 
     
    
    
     DESCRIPTION 
     An improved compression driver  100  having a bridged phase plug  102  is shown in  FIGS. 3A ,  3 B, and  4 .  FIG. 3A  identifies the parts of the driver while  FIG. 3B  includes indicators of dimensions and reference points used in describing the geometry of the parts. Reference numbers are omitted in  FIG. 3B  for parts not referred to in discussion of the other parts&#39; geometry. Elements occurring on both sides of the phase plug are only labeled on one side in  FIG. 3B  for clarity. In the bridged phase plug  102 , two slots  104  and  106  begin at the dome-interface surface  108  and extend a short depth  108   a  into the phase plug, where they join at a bridge passage  110 . The bridge passage is separated from the compression volume  30  and the two slots are separated from each other by a bridge element  112 . An exit slot  114  begins at the bridge passage  110  and continues through the body of the phase plug to the throat  116 . The throat ends at an aperture  118 . To establish points of reference, we consider the beginning of the exit slot  114  to be at the opening  110   a  where the lower wall  110   b  of the bridge volume  110  would continue if the exit slot  114  were absent. The end of the exit slot  114  and beginning of the throat  116  is the section  114   a  at a depth  108   b  below the surface  108  where the two halves (as viewed in cross-section) of the exit slot  114  join. 
     The dome  10 , bobbin  14 , surround  16 , voice coil  20 , and other parts external to the phase plug may not vary from the traditional compression driver design, or may be modified in other ways independent of the bridged phase plug. Modifications to the moving parts and external structure are beyond the scope of this disclosure. 
     Various design parameters may be modified to optimize the bridged phase plug  102  for particular performance targets, based on the acoustic attributes of the back cavity, compression cavity, and the moving parts (dome, skirt, bobbin, surround). In particular, the radii  104   c ,  106   c  of the slots  104  and  106  (measured from the centerline  100   a  of the phase plug to the centerlines  104   a ,  106   a  of the slots), the widths  104   b ,  106   b  of the slots, the radius  114   c  where the exit slot  114  joins the bridge passage  110 , and the curvatures of the slots, can all be varied to obtain desired performance. 
     In some examples, the slots  104  and  106  are centered at radii selected to correspond to nulls in low-order axisymmetric, or radial mode, standing waves in the compression cavity induced by motion of the dome. Locating the slots at such nulls minimizes the pressure caused by cavity modes in the compression cavity. The widths of the slots  104  and  106  are selected to control a relationship between the total cross-sectional areas of the two slots. In some examples, the widths are selected so that the two slots have equal or approximately equal areas, which we refer to as a balanced bridge. The particular relationship between the areas of the two slots can be varied to obtain desired performance. In contrast, in some conventional multi-slot phase plugs, each slot&#39;s width is the same, making each slot&#39;s total area proportional to its radius. The balanced bridge design controls pressure peaking in the compression cavity without changing the slot locations. It also reduces the pressure response at the center of the compression cavity over a wide frequency band around the bridge resonance, explained in more detail below. The thickness of the bridge element  112  is tapered so that the cross-sectional areas of the two slots  104  and  106  remain approximately constant along their respective lengths, from the dome-interface surface  108  to the region where they combine and join the exit slot  114 . As shown in  FIG. 3B , we refer to the cross sectional area of the slots  104  and  106  by reference to the widths  104   b ,  106   b  of the slots in cross section at various positions along their lengths. Widths  104   b ,  106   b  are shown at the beginning and ends of the slots  104 ,  106 . Similarly, the width  114   b  of the exit slot  114  is shown at the begging and end of the exit slot. These lines are revolved around the centerline  100   a  of the phase plug to find areas. The cross-sectional area of the exit slot  114  where it joins the bridge passage  110  (i.e.,  110   a  in  FIG. 3B ) sets the compression ratio of the driver. In some examples, the areas of the slots  104  and  106  at the surface  108  and as they continue into the bridge volume are selected to match, in combination, the area of the exit slot  114  where it joins the bridge passage  110 , such that the total cross sectional area of the slots from the surface  108  to the beginning ( 110   a ) of the exit slot  114  is constant and corresponds to the compression ratio of the driver. 
     The radius  114   c  where the exit slot  114  joins the bridge passage  110  is selected to correspond to a null in a low-order, e.g., first order, standing wave in the bridge passage  110 . Also shown in the example of  FIGS. 3A and 3B , the side-walls of exit slot  114  have a smoothly-varying curvature from the bridge passage  110  to the throat  116 . Also in this example, the cross-sectional area of the exit slot  114  grows exponentially from the bridge to the throat, based on the target cutoff frequency of the driver. An exponential curvature helps decrease the length of any acoustic pathway that will be added to the compression driver before it reaches the diffraction slot of a horn. More generally, the total area of the slots changes smoothly along the length from the compression cavity to the throat and is generally constant or monotonically increasing toward the throat. This combination of locations, proportions, and curvatures results in a smooth frequency response at the throat over a wide range of frequencies, at least in cases where the dome  10  moves as a piston. 
     The balanced bridge phase plug has an additional advantage, as compared to conventional multi-slot phase plugs, of controlling loop resonances. In the conventional phase plug of  FIG. 1 , looping resonant waves may exist between the slots, e.g., a wave may exist in slots  36   a  and  36   b , joined by the short section of the compression cavity  30  between the openings of those two slots. Note that such “loops” and the waves in them are complex three-dimensional shapes, not the simple paths implied by the two-dimensional cross sectional views in which they are discussed. The bridge passage  110  between the slots  104  and  106  greatly shortens the loop between those two slots, raising the resonant frequency of the loop. Raising the resonant frequency of the loops tends to move the frequency to ranges where humans are less sensitive to the peaks and dips that loop resonances cause in the response of the transducer. There also tends to be more incidental damping at higher frequencies, so the loop resonances will not be as strong. In addition to raising the resonant frequency of any loop resonances, the balanced bridge design also decreases pressure imbalances between the slots that excite the loop resonances in the first place. 
     Two alternative bridged phase plug designs  200  and  201  are shown in  FIGS. 5 and 6 , respectively (only the right half of each section is shown). In  FIG. 5 , a first slot  204  and a second slot  206  are defined by a first bridge element  210  and joined by a first bridge passage  214 , which is in turn joined to a third slot  208  through a second bridge passage  216  around a second bridge element  212 . The exit slot  218  is joined to the second bridge passage. Alternatively, the two inner slots  206  and  208  may be joined first. In  FIG. 6 , the first bridged slots  204  and  206  are defined by the bridge element  210  and joined by a first bridge passage  214  as before, while a third slot  220  and a fourth slot  222  are defined by an additional bridge element  224  and joined by a second bridge passage  228 . The two bridge passages  214  and  228  are separated by a third bridge element  226  and joined by a third bridge passage  230 , which is joined to the exit slot  218 . Each of these designs may be advantageous in particular compression driver designs, depending on the number and location of nodes in an axisymmetric standing wave of interest, which tend to be a function of the diameter of the dome and compression cavity. 
     A sectional view of an assembled loudspeaker  300  is shown in  FIG. 7 . The loudspeaker includes a compression driver  100  coupled to an exponential horn  302 . Other horn shapes, such as conical, hyperbolic, and tractric, may also be suitable. The bridged phase slot  102  as described above is located in the compression driver  100 , with the throat of the phase plug communicating with the beginning of the horn. As noted above, the throat has an exponential curvature compatible with the curvature of the horn, based on the targeted cutoff frequency of the completed loudspeaker. 
     Another embodiment  400  is shown in  FIG. 8 . In some examples, the dome and motor structure are inverted, such that the convex surface of the dome  10  faces a concave phase plug  402 . In the example of  FIG. 8 , the entire dome and motor structure is inverted. In other examples, only the dome is inverted and the motor components remain on the phase plug side of the structure. In an inverted-dome design, surface normals of the dome-interface surface  408  diverge, whereas in the conventional phase plug, like that shown in  FIG. 3 , surface normals would have converged at the center of the sphere of which the dome-interface surface is a section. If each slot made a relatively straight path from the surface  408  to the throat  416 , their lengths would increase with increasing slot radius. In a bridged phase plug, as shown, slots  404  and  406  begin at the surface and join at a bridge passage  410 , separated by a bridge element  412 . An exit slot  414  connects the bridged slots to the throat  416 , which ends at an aperture  418 . By bending the slots to form the bridge, the effective lengths of slots closer to the centerline are increased, such that all the slots have similar lengths, independent of their starting radii. This design advantageously allows the slots to match the direction of surface normals where they begin, but still join in a common throat with relatively uniform total lengths. 
     Other implementations are within the scope of the following claims and other claims to which the applicant may be entitled.