Patent Publication Number: US-8121330-B2

Title: Phase plug for compression driver

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage application under 35 U.S.C. 371 of PCT/GB2007/001379 filed Apr. 13, 2007, which claims priority of GB 0607452.0 filed Apr. 13, 2006. Both applications, PCT/GB2007/001379 and GB 0607452.0, which are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to loudspeakers, and particularly relates to compression drivers and to phase plugs for compression drivers. 
     2. Description of Related Art 
     A compression driver is a type of loudspeaker in which an acoustically radiating diaphragm radiates acoustic waves into a small cavity. The cavity is connected by a phase plug (also known as a phase adaptor, a phase transformer, an acoustic transformer, etc.) to an aperture, which normally opens into a horn waveguide. The small cavity and throat area present the diaphragm with a high acoustic load, and because of this, it tends to be highly efficient. However, the cavity in front of the diaphragm can cause acoustic problems at high frequencies. In particular, the cavity can exhibit strong resonances (known as cavity modes) at distinct frequencies that are commonly within the working band of the compression driver. These resonances can undesirably introduce large pressure response variations in the output of the compression driver. Additionally, the high pressure levels in the cavity that occur when the resonances are excited are undesirable for driver linearity. The severity of the resonance problem is determined primarily by the shape of the cavity, the design of the phase plug and, more specifically, the location and size of the pathways (channels) through the phase plug. 
     The Journal of the Acoustical Society of America, Volume 25, No. 2, March 1953 (Bob H Smith of the University of California), discloses an investigation of the air chamber of horn type loudspeakers, which includes a method of calculating the positions and sizes of the entrances to the channels (pathways) in a phase plug with annular channels (slots). The aim of the disclosed method is to avoid the excitation of resonances caused by the motion of the air entering and leaving the channels in the phase plug. According to the mathematical analysis presented in that technical paper, in an ideal phase plug with annular channels, the widths of the channels should be very nearly the same irrespective of their radial position in the phase plug, but with increasing radial position the channel width should normally increase very gradually. 
     Whereas the technical paper by Bob Smith considers only the effect of the motion of the air in the channels, in reality resonances are also excited by the motion of the diaphragm itself. The present inventors have performed a new analysis including the latter effect, and have accordingly devised the present invention. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, a first aspect of the present invention provides a phase plug, comprising a body having an input side for receiving acoustic waves and an output side for transmitting acoustic waves, the body including a plurality of channels extending from the input side to the output side for propagating acoustic waves through the body, wherein the input side comprises an input surface which includes a plurality of openings constituting entrances for the channels, the input surface being substantially part of a sphere or an ellipsoid in shape, and wherein the areas of the openings vary with radial position on the input surface, the radial position being measured in a direction extending perpendicularly from a central axis extending through the input surface, the variation in the areas being a function of the cosine of an angle subtended at the centre of the sphere or a focus of the ellipsoid between the central axis and the radial position. 
     In some preferred embodiments of the invention, as well as the areas of the openings varying with radial position on the input surface, the variation in the areas of the openings may be described by a mathematical relationship which includes the radial position as a function of the relationship. Preferably, the mathematical variation in the areas of the openings is substantially proportional to a function in the range r·cos 1/2 φ to r·cos 2 φ, where r is the radial position and φ is the angle. Most preferably, the variation in the areas of the openings is substantially proportional to r·cosφ, where r is the radial position and φ is the angle. 
     In especially preferred embodiments of the invention, one or more of the openings has the form of one or more slots, each slot having a constant or varying width. (Preferably substantially all of the openings have the form of slots.) For example, in some embodiments, each slot has a substantially constant width, but the widths of the slots vary with radial position on the input surface of the phase plug. Such versions of the invention preferably have a plurality of slots arranged spaced apart from each other in an annular fashion around the central axis of the phase plug. (There will generally be connection parts extending across the annular slots, to join together the parts of the phase plug body that are separated from each other by the slots.) In other embodiments, each slot has a varying width. Such versions of the invention preferably have a plurality of slots arranged in a radial fashion around the central axis of the phase plug. Yet other embodiments of the invention are a combination of these two versions, in which the phase plug includes one or more slots arranged in an annular fashion around the central axis and also includes one or more slots arranged in a radial fashion around the central axis. The annular slot(s) may be situated closer to the central axis than the radial slot(s), or vice versa, and/or the annular slots and radial slots may alternate in a radial direction extending away from the central axis, for example. In all such main types of phase plug according to the invention, the widths of the slots preferably vary with radial position as a function of the cosine of the angle φ. 
     A second aspect of the invention accordingly provides a phase plug, comprising a body having an input side for receiving acoustic waves and an output side for transmitting acoustic waves, the body including a plurality of channels extending from the input side to the output side for propagating acoustic waves through the body, wherein the input side comprises an input surface which includes a plurality of slots constituting entrances for the channels, the input surface being substantially part of a sphere or an ellipsoid in shape, and wherein the widths of the slots vary with radial position on the input surface, the radial position being measured in a direction extending perpendicularly from a central axis extending through the input surface, the variation in the slot widths being a function of the cosine of an angle subtended at the centre of the sphere or a focus of the ellipsoid between the central axis and the radial position. 
     In some embodiments of the invention, the variation in the widths of the slots (with radial position on the input surface) may be described by a mathematical relationship which includes the radial position as a function of the relationship. This is preferably the case for slots that are arranged in a substantially radial orientation on the input surface about the central axis, for example. Thus, for example, the width of each slot may vary substantially in proportion to a function in the range r·cos 1/2 φ to r·cos 2 φ, where r is the radial position and φ is the angle. More preferably, the width of each slot may vary substantially in proportion to r·cosφ, where r is the radial position and φ is the angle. For phase plugs in which one or more of the slots are arranged in a substantially radial orientation on the input surface about the central axis, they preferably are joined to each other via an opening at an axially central region of the input surface. 
     Additionally or alternatively, for some embodiments of phase plug according to the invention, the variation in the widths of the slots (with radial position on the input surface) may be described mathematically by means of a relationship that does not include the radial position as a function of the relationship. This is preferably the case for slots that are substantially annular or substantially part of an annulus, in shape, for example. Thus, for example, the widths of the slots may vary substantially in proportion to a function in the range cos 1/2 φ to cos 2 φ, where φ is the angle. Preferably, the widths of the slots vary substantially in proportion to cosφ, where φ is the angle. As mentioned above, for embodiments of the phase plug having one or more annular slots, each slot preferably is arranged such that the axis of its annulus is substantially coaxial with the central axis of the phase plug, and preferably each slot has a substantially constant width, but the widths of the slots vary with radial position on the input surface of the phase plug. 
     In some embodiments of the invention, the input surface is concave, e.g. for use with a diaphragm having a convex radiating surface. Alternatively, in other embodiments of the invention the input surface is convex, e.g. for use with a diaphragm having a concave radiating surface. 
     A third aspect of the invention provides a compression driver, comprising a phase plug according to the first or second aspect of the invention, and an acoustically radiating diaphragm situated adjacent to the input side of the phase plug. 
     The diaphragm of the compression driver preferably has either a convex or a concave acoustically radiating surface. Preferably, the acoustically radiating surface of the diaphragm is substantially part of a sphere or an ellipsoid in shape. Advantageously, the acoustically radiating surface of the diaphragm may be substantially rigid. 
     The compression driver preferably includes a horn waveguide situated adjacent to the output side of the phase plug. In at least some embodiments of the invention, the horn waveguide is non-circular in cross-section perpendicular to the central axis. For example, the horn may be oval in cross-section, or indeed substantially any shape. However, for many embodiments of the invention, the horn waveguide is substantially circular in cross-section perpendicular to the central axis. 
     The horn waveguide may be substantially frusto-conical (i.e. the horn waveguide may be substantially conical but truncated at the throat of the horn). However, the horn waveguide may be flared, e.g. flared such that it follows a substantially exponential curve, or a substantially parabolic curve, or another flared curve. Other horn waveguide shapes are also possible. 
     The horn waveguide may be a static waveguide, or it may itself be an acoustically radiating diaphragm, e.g a cone diaphragm. Consequently, in some embodiments of the invention, the horn waveguide may comprise a driven acoustically radiating diaphragm. The horn diaphragm may be driven substantially independently of the dome-shaped diaphragm, for example such that the horn diaphragm is arranged to radiate acoustic waves of generally lower frequency than is the dome-shaped diaphragm. Consequently, the loudspeaker may include a drive unit to drive the horn diaphragm. An example of a suitable arrangement (but without a phase plug according to the present invention) in which the horn waveguide itself comprises an acoustically radiating diaphragm, is disclosed in U.S. Pat. No. 5,548,657. 
     A fourth aspect of the invention provides a combination loudspeaker comprising an acoustically radiating horn diaphragm, a driver for the horn diaphragm, and a compression driver according to the third aspect of the invention located in, or adjacent to, a throat of the horn diaphragm. Preferably the compression driver is arranged to radiate high frequency sounds, and the horn diaphragm preferably is arranged to radiate low or mid-range frequency sounds. 
     It is to be understood that any feature of any aspect of the invention may be a feature of any other aspect of the invention. 
     The phase plug preferably is formed from one or more of: a metal or metal alloy material; a composite material; a plastics material; a ceramic material. 
     The diaphragm of the compression driver preferably is formed from a substantially rigid low density material, for example one or more of: a metal or metal alloy material; a composite material; a plastics material; a ceramic material. Some preferred metals for forming a suitable metal or metal alloy material include: titanium; aluminium; and beryllium. The acoustically radiating surface of the diaphragm of the compression driver may be formed from a specialist material, for example diamond (especially chemically deposited diamond). 
     The horn waveguide may be formed from any suitable material, for example one or more of: a metal or metal alloy material; a composite material; a plastics material; a fabric material; a ceramic material. For those embodiments of the invention in which the horn waveguide is an acoustically radiating diaphragm, it preferably is formed from a plastics material or a fabric material, for example. Metal and/or paper may be preferable in some cases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: 
         FIG. 1  is a cross-sectional schematic representation of one embodiment of a compression driver according to the invention; 
         FIG. 2  is a partial cross-sectional schematic representation of a first embodiment of a phase plug according to the invention, together with an acoustically radiating diaphragm; 
         FIG. 3  shows six views (( a ) to ( f )) of a second embodiment of a phase plug according to the invention; 
         FIG. 4  is a schematic diagram indicating the radial position r and the angle φ used to define features of the invention; 
         FIG. 5  is a graphical representation indicating variations in channel entrance opening areas and slot widths of preferred embodiments of phase plugs according to the invention; and 
         FIG. 6  is a schematic cross-sectional representation of a combination loudspeaker according to the invention, comprising a convex radiating diaphragm, a phase plug of the type illustrated in  FIG. 3 , and a radiating horn diaphragm. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a cross-sectional schematic representation of one embodiment of a compression driver according to the invention. The compression driver comprises an acoustically radiating diaphragm  1  having a concave acoustically radiating surface situated adjacent to an input side of a phase plug  3 . On an opposite (output) side of the phase plug  3  is a horn waveguide  5 . The diaphragm  1 , phase plug  3 , and horn waveguide  5  have a central axis X-X extending therethrough. The diaphragm  1 , phase plug  3 , and horn waveguide  5  are arranged such that acoustic waves generated by the diaphragm  1  are propagated through channels  7  extending through the phase plug  3  from the input side to the output side of the phase plug and are then received and propagated by the horn waveguide  5 . The diaphragm  1  is driven by means of a driver assembly comprising a centre pole part  9 , an outer pole part  11 , and a magnet  13 . Specifically, an annular skirt portion of the diaphragm  1 , which projects from the circumference of the acoustically radiating surface, carries an electrically conductive coil, and the coil and skirt portion of the diaphragm are situated in a gap  15  between the centre pole part  9  and the outer pole part  11 , which gap has a magnetic field extending across it. A clamp ring  17  and a rear enclosure part  19  are also shown. 
     Everything described above with reference to schematic  FIG. 1  is conventional and well known to the skilled person. The novelty of the present invention lies primarily in the details of the phase plug, which will now be described. 
       FIG. 2  is a partial cross-sectional schematic representation of a first embodiment of a phase plug  3  according to the invention, together with an acoustically radiating diaphragm  1 , of a compression driver as illustrated schematically in  FIG. 1 . The annular skirt portion  21  of the diaphragm  1  which carries an electrically conductive coil  23 , and which projects from the circumference  25  of the acoustically radiating surface, is shown schematically in  FIG. 2 . The acoustically radiating surface  27  of the diaphragm is concave, and lies adjacent to a correspondingly convex input surface  29  of the phase plug  3 . Both the concave acoustically radiating surface  27  and the convex input surface  29  comprise part of a sphere (or an ellipsoid, but preferably a sphere) in shape, and they are substantially concentric. The phase plug  3  includes a plurality of channels  7  extending from its input side (adjacent to the diaphragm  1 ) to its output side (closer to the horn waveguide  5 ) for propagating acoustic waves through the body of the phase plug. Consequently, the input surface  29  of the phase plug  3  includes a plurality of openings  31  constituting entrances for the channels  7 . More particularly, the phase plug  3  includes three substantially coaxial annular channels  7 , having respective coaxial annular slot entrance openings  31   a ,  31   b  and  31   c . Annular slot  31   a  is the closest to the central axis X-X, annular slot  31   c  is the furthest from the central axis X-X, and annular slot  31   b  is situated between slots  31   a  and  31   c . Each slot  31  has a substantially constant (fixed) width for substantially its entire extent, but the width of each slot is different to the width of each other slot, in a particular defined relationship (described below). 
     The inventors of the present invention have found that if the areas, and the widths, of the slots  31  vary as a function of the cosine of the angle subtended at the centre of the sphere (or a focus of the ellipsoid) defining the input surface  29  of the phase plug between the central axis X-X and the radial position of the slot on the input surface, then the phase plug can significantly reduce, or can even substantially eliminate, the excitation of acoustic resonances (cavity modes) in the region between the diaphragm  1  and the throat of the horn waveguide  5 . The definitions of the angle (which is designated as φ) and the radial position (which is designated as r) are illustrated in  FIG. 4 . The radial position r is measured in a direction extending perpendicularly from the central axis X-X extending through the input surface  29  of the phase plug  3 . (The particular value of the angle φ and the particular value of the distance r shown in  FIG. 4  constitute just one such angle and one such radial distance; each of the slots  31  will have its own particular value of the angle φ and the radial distance r, defined and measured from the central axis X-X as shown in  FIG. 4 .) 
     The inventors have found, in particular, that acoustic resonances can be significantly reduced (or even substantially eliminated) if the variation in the areas of the openings  31  (e.g. slots) is substantially proportional to a function in the range r·cos 1/2 φ to r·cos 2 φ. Thus, for example, for some embodiments of the invention, the variation may be substantially proportional to r·cosφ. 
     The variation in the areas of the slots  31   a ,  31   b  and  31   c  of  FIG. 2  is shown graphically in  FIG. 5 , in which the horizontal axis indicates the angle φ of each slot (in degrees), and the vertical axis indicates the open area of each slot (in arbitrary units). Each of the slots  31   a ,  31   b  and  31   c  is indicated on the graph (as a small labelled oval), together with the functions r·cos 1/2 φ, r·cosφ and r·cos 2 φ. As can be seen, all of the slots fall within the range defined by the limits r·cos 1/2 φ and r·cos 2 φ. 
     Additionally (as mentioned above) the widths W of the annular slots of the phase plug  3  illustrated in  FIG. 2  preferably vary substantially in proportion to a function in the range cos 1/2 φ to cos 2 φ. More preferably, the widths W of the slots vary approximately in proportion to cost. The widths W of the slots are indicated in  FIG. 2 . 
       FIG. 3  shows six views (( a ) to ( f )) of an alternative embodiment of a phase plug  3  according to the invention. The phase plug  3  of  FIG. 3  comprises a body having an input side  33  for receiving acoustic waves and an output side  35  for transmitting acoustic waves. A plurality of channels  7  extends from the input side  33  to the output side  35  for propagating acoustic waves through the body of the phase plug  3 . The input side  33  comprises a concave input surface  29  which includes a plurality of openings  31  in the form of slots, which constitute entrances for the channels  7 . The input surface is substantially part of a sphere (or an ellipsoid, but preferably a sphere) in shape. The slots  31  are arranged in a substantially radial orientation on the input surface  29  about the central axis X-X. In the embodiment illustrated in  FIG. 3 , the phase plug  3  includes seven channels, and thus seven slots, but fewer, or a greater number, of slots could be used instead. Each channel  7  (and thus also each slot  31 , which is an entrance of a channel) is partially defined, and separated from neighbouring channels  7 , by a pair of spaced apart fins  37 . Because there are seven channels there are also seven radially arranged spaced-apart fins  37 . Each fin projects towards the central axis X-X from an outer circumferential part  39  of the phase plug  3 . The circumferential part  39  has a generally frusto-conical shape, with its smallest radius adjacent to the input side  33  and its largest radius adjacent to the output side  35 . 
     The area distributions of the slots  31 , and thus also the widths of the slots, vary with radial position r on the input surface  29  of the phase plug  3  illustrated in  FIG. 3 . More particularly, the area distributions and the widths of the slots  31  vary as a function of the radial position r and the cosine of the angle φ (which are defined in the same way as illustrated in  FIG. 4 ). Specifically, the variation in both the area distributions of the slots  31 , and the widths of the slots  31 , is substantially proportional to a function in the range r·cos 1/2 φ to r·cos 2 φ, for example approximately proportional to r·cosφ. Because such a variation in slot width would mean that the width of each slot reduces to zero at the central axis X-X (where r=0), the phase plug could include an axially central part of the phase plug body where all of the fins  37  are joined together. However, in order to comply with the ideal mathematical variation in slot width, any such axially central part of the phase plug body would ideally need to be vanishingly small in radius (which is difficult or impossible to achieve). Thus, the physical embodiment of the phase plug at the central axis X-X will generally be an approximation to the ideal mathematical variation in slot width, e.g. either comprising a small axially central part of the phase plug body or comprising an axially central opening  38  which joins all of the slots to each other. The latter version is the one illustrated in  FIG. 3 . 
     While the slot openings  31  on the input surface  29  of the phase plug  3  of  FIG. 3  comply with the above-mentioned mathematical relationships, the output sides of the channels  7  do not necessarily comply with those mathematical relationships. In  FIG. 3 , each channel  7  widens in an approximately exponential manner in a direction parallel to the central axis X-X from the input side  33  to the output side  35 . As shown in view ( f ) of  FIG. 3 , the output edge  41  of each fin  37  has a thin substantially constant width. Additionally, the output edge  41  of each fin  37  curves substantially continuously from the circumferential part  39  at the output end  35  of the phase plug  3 , to the radially innermost part of the fin at the input surface  29 . 
       FIG. 6  is a schematic cross-sectional representation of a combination loudspeaker  51  according to the invention, comprising a convex dome-shaped radiating diaphragm  53 , a phase plug  3  of the type illustrated in  FIG. 3 , and a radiating horn diaphragm  55 . The convex radiating diaphragm  53  and the phase plug  3  are located in the throat of the horn diaphragm  55 . The convex radiating diaphragm  53  is arranged to radiate high frequency sounds, and the horn diaphragm  55  is arranged to radiate low or mid-range frequency sounds. The combination loudspeaker  51  includes a “surround”  57  in the throat of the horn diaphragm  55  that supports the convex radiating diaphragm  53  via a flexible annular web  59 , and attached to this surround  57  is a support  61  for the phase plug  3 . An inner cylindrical part  65  of the horn diaphragm  55  carries a conductive coil of a driver for the horn diaphragm, which extends into a magnetic gap of the driver (not shown). The horn diaphragm  55  is supported by a second flexible annular web  67  at its outer periphery, and the outer periphery of the second flexible annular web  67  is attached to an outer support  69 . 
     It will be understood that other embodiments of the invention, and modifications of the described and illustrated embodiments of the invention, are possible within the definitions of the invention provided in the appended claims.