Abstract:
This invention provides a two-stage phasing plug located within a compression driver. The two-stage phasing plug housed within the compression driver may be coupled to a horn. The two-stage phasing plug includes first and second phasing plugs. The advantages of having a two-stage phasing plug is that the first and second phasing plugs may be simpler to manufacture, cost less and the overall dimensional tolerances may be tightly controlled. The higher dimensional tolerances may be obtained because the first phasing plug may be made from a unitary work-piece, and therefore, may be tooled and cut in the same machining set up. This allows the unitary work-piece to be machined and cut very accurately when compared to assembling separate components together during the manufacturing process. Since the most dimensionally critical area is the rear side of the first phasing plug, the tolerances of the second phasing plug may not be as critical. Thus, a more expensive material, such as steel, may be used for the first phasing plug, and less expensive material, such as plastic, may be used to manufacture the second phasing plug.

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 09/921,149, filed Jul. 31, 2001, entitled TWO-STAGE PHASING PLUG SYSTEM IN A COMPRESSION DRIVER, which claims priority to U.S. Provisional Patent Application, Ser. No. 60/221,692 filed Jul. 31, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a compression driver, phasing plug and an assembly of a compression driver phasing plug having a tight dimensional tolerance. 
     2. Related Art 
     A compression driver typically comprises a pole piece made of ferromagnetic material having a magnetic air gap to receive a voice coil. The exit or opening of the compression driver is adaptable for coupling to the throat of a horn. A diaphragm, usually circular with a central dome-shaped portion, is mounted adjacent the rear opening of the bore to allow the diaphragm to freely vibrate. Attached to the edge of the diaphragm&#39;s dome is a cylindrical coil of wire, the voice coil, oriented so that the cylindrical axis of the coil is perpendicular to the diaphragm and coincident with the axis of the pole piece bore. A static magnetic field, usually produced by a permanent magnet, is applied so that an alternating signal current flowing through the voice coil causes it to vibrate along its cylindrical axis. This in turn causes the diaphragm to vibrate along the axis of the bore and generate sound waves corresponding to the signal current. The sound waves are directed through the bore toward its front opening. 
     The front opening of the bore is usually coupled to the throat of a horn, which then radiates the sound waves into the air. In the description that follows, the term “throat” is used to mean either downstream end or exiting end of the pole piece bore or the actual entrance of a horn. Interposed between the diaphragm and the pole piece bore is a perforated structure known as a phasing plug for impedance matching the output of the diaphragm to the horn. Within the phasing plug are one or more air passages or channels for transmission of the sound waves. The surface of the phasing plug adjacent to the diaphragm corresponds spherically and is positioned fairly close to the diaphragm while still leaving an air gap, or compression region, in which the diaphragm can vibrate freely. 
     The phasing plug performs two basic functions. First, because the cross-sectional area of the air channel inlets are smaller than the area of the diaphragm, the air between the diaphragm and the phasing plug (i.e., the compression region) can be compressed to relatively high pressures by motion of the diaphragm. This is what allows a compression driver to output sound at greater pressure levels than conventional loudspeakers where the diaphragm radiates directly into the air. The efficiency of the loudspeaker is thus increased by virtue of the phasing plug being placed in close opposition to the diaphragm to minimize the volume of air between the diaphragm and the phasing plug. Second, as the name “phasing plug” implies, the path lengths of the air channels within the phasing plug may be equalized so as to bring all portions of the transmitted sound wave into phase coherence when they reach the throat. Without such path length equalization, sound waves emanating from different air channels would constructively or destructively interfere with one another at certain frequencies so as to distort the overall frequency response. 
     Manufacturing the compressor driver phasing plug, however, can be a time consuming and expensive process. For example, to make a compression driver and phasing plug, a number of parts need to be assembled either by gluing or press-fitting the parts together, and then the assembly is machined for finishing. Unfortunately, the labor intensive process of assembling the number of parts adds cost to the manufacturing process. Moreover, the tight dimensional tolerances that must be kept are difficult to achieve. That is, because of the inherent variances that exist in casting each part, when they are combined, the size of the air passages or channels may vary, i.e., one air passage may be smaller or larger than the specification requires, so that there is distortion in the frequency response. Therefore, there is still a need to manufacture a compression driver phasing plug that is easy to manufacture yet with tight dimensional tolerances. 
     SUMMARY OF THE INVENTION 
     This invention provides a two-stage compression driver having tight dimensional tolerances. The compression driver may include a two-stage phasing plug having a first phasing plug and a second phasing plug. The first phasing plug is adapted to receive the second phasing plug, and vice versa. When the two phasing plugs are combined, they form the two-stage phasing plug within a compression driver. The first phasing plug may be made of a unitary work-piece that has a rear side and an intermediate side. The rear side of the unitary work-piece may have a dome or convex shape. The thickness between the first side and the intermediate side of the unitary work-piece may be substantially constant so that the intermediate side has a concave shape. 
     To form slots within the first phasing plug, the unitary work-piece is cut so that slots are formed between the rear and the intermediate sides. In other words, slots are cut within the unitary work-piece to form the first phasing plug. The slots are formed in the work-piece to provide air channels or air passages. In particular, the air channels within the first phasing plug may be equalized so as to bring all portions of the transmitted sound wave into phase coherence when they reach the intermediate side of the first phasing plug. The slots may be formed using a variety of methods known to one ordinarily skilled in the art, such as water jet, laser, and machine tools. With regard to material, the first phasing plug may be made of steel. 
     The second phasing plug also has an intermediate side and a front side. The intermediate side of the second phasing plug may be adapted to associate or flush with the intermediate side of the first phasing plug. For example, the intermediate side of the second phasing plug may have a convex or dome shape so that it substantially matches the concave shape of the intermediate side of the first phasing plug. The second phasing plug may be formed from different material, such as plastic, than the first phasing plug. 
     The second phasing plug may be made in a variety of ways. One way is to assemble formed plastic parts that easily “snap” or glue together. The second phasing plug may have slots that form air channels or air passages so that the first and second phasing plugs, when mated, form continuous air channels through the first and second phasing plugs that transmit sound waves into phase coherent or time synchronization when the)y reach the throat of a horn. 
     The first and second phasing plugs may be easy to manufacture, cost less, and the overall dimensional tolerance may be tightly held because the first phasing plug is made from a unitary work-piece. Therefore, the phasing plugs may be tooled and cut in the same machining set up. This allows the unitary work-piece to be machined and cut very accurately when compared to assembling separate components together to manufacture a phasing plug. For the phasing plug to perform properly, the rear side of the first phasing plug (i.e., the side adjacent to the diaphragm), needs to be cut or machined accurately to a tight tolerance. The second phasing plug needs to be cut or machined accurately as well, but it is not necessary to cut or assemble the second phasing plug to the same level of precision as the rear side of the first phasing plug. That is, the performance of the two-stage phasing plug depends more on how well the first phasing plug is cut than the second phasing plug. To minimize the cost of manufacturing the two-stage phasing plug, accurately cut steel may be used to manufacture the first phasing plug, and a less expensive material, such as plastic, may be used to assemble the second phasing plug. By using different materials the material costs of the two-stage phasing plug may be reduced. 
     Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the alt upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood with reference to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is an overview of a compression driver having a two-stage phasing plug adapted to couple to a horn. 
         FIG. 2  is a cross-sectional view of a compression driver with a two-stage phasing plug. 
         FIG. 3  is a cross-sectional view of a first phasing plug. 
         FIG. 4  is an enlarged view of the first phasing plug of FIG.  3 . 
         FIG. 5  is a side view of the first phasing plug illustrated in FIG.  3 . 
         FIG. 6  is a bottom view of the first phasing plug illustrated in FIG.  3 . 
         FIG. 7  is a cross-sectional view of another embodiment of a two-stage phasing plug. 
         FIG. 8  is a cross-sectional view of another embodiment of a two-stage phasing plug. 
         FIG. 9  is a side-view of a second phasing plug. 
         FIG. 10  is a top view of a second phasing plug of the embodiment illustrated in FIG.  8 . 
         FIG. 11  is a bottom view of a second phasing plug illustrated in FIG.  8 . 
         FIG. 12  is a cross-sectional view of a second phasing plug of the embodiment illustrated in FIG.  8 . 
         FIG. 13  is a side view of an inner piece of the second phasing plug illustrated in FIG.  8 . 
         FIG. 14  is a side view of a centerpiece within the second phasing plug illustrated in FIG.  8 . 
         FIG. 15  is a cross-sectional view of the embodiment illustrated in FIG.  14 . 
         FIG. 16  is a side view of an outerpiece within the second phasing plug illustrated in FIG.  8 . 
         FIG. 17  is a cross-sectional view of the outerpiece illustrated in FIG.  16 . 
         FIG. 18  is a cross-sectional view of a housing forming the second phasing plug of the embodiment illustrated in FIG.  8 . 
         FIG. 19  is a cross-sectional view of an alternative two-stage phasing plug. 
         FIG. 20  is a cross-sectional view of another embodiment of the two-stage phasing plug. 
         FIG. 21  is a cross-sectional view of a phasing plug. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Phasing plugs perform two functions. First, the phasing plug provides acoustic load, i.e., acoustic amplification to the throat of the horn. This is done through acoustic impedance matching, and generally depends on the compression ratio and the distance between the diaphragm and the phasing plug. Therefore, to match the impedance, the height of the dome formed in the phasing plug and the width of the slots both need to be accurate because the height of the dome affects the distance between the diaphragm and the phasing plug; and the width of the slots affects the compression ratio. Put differently, because the cross-sectional area of the slots (or air channel inlets) are smaller than the area of the diaphragm, the air between the diaphragm and the phasing plug (i.e., the compression region) can be compressed to relatively high pressures by motion of the diaphragm. This allows a compression driver to output sound at greater pressure levels than conventional loudspeakers where the diaphragm radiates directly into the air. The efficiency of the loudspeaker is thus increased by virtue of the phasing plug being placed in close opposition to the diaphragm to minimize the volume of air between the diaphragm and the phasing plug. 
     Second, the phasing plug provides equalized path length to its orifice so that all of the transmitted sounds are in phase. Without such path length equalization, sound waves emanating from the different air channels or air passages would constructively or destructively interfere with one another at certain frequencies to distort the overall frequency response. To minimize such distortion and to maximize the impedance matching, the two-stage phasing plug needs to be manufactured to a tight dimensional tolerance. In other words, the path length will be eschewed, if the dimensions deviate from the specified dimensions and, therefore, distortion will occur. Moreover, the shape and height of the dome and the width of the slots on the rear side (the side adjacent to the diaphragm) of the first phasing plug that create the acoustic impedance matching need to be accurate for the two-stage phasing plug to perform properly. 
       FIG. 1  illustrates a general overview of a compression driver  100  having a two-stage phasing plug assembly  102  and a diaphragm  104  adapted to couple to a horn  106 . The two-stage phasing plug assembly  102 , comprised of the first phasing plug  108  and the second phasing plug  110 , is adapted to couple to the throat  112  of the horn  106 . The diaphragm  104  may be adapted to be juxtaposed to the first phasing plug  108  to drive air through the two-stage phasing plug assembly and then to the throat  112  of the horn  106 . 
     To manufacture a two-stage phasing plug with tight tolerances in the critical areas, the two-stage phasing plug  102  may be divided into two pieces comprising a first phasing plug  108  and a second phasing plug  110 . The first phasing plug  108  may be made from a unitary work-piece and is machined to shape the dome surface  114  and its height and may be cut to form the slots (see also FIGS.  2 - 6 ). In other words, tolerances can be tightly held because the first phasing plug is machined from a unitary work-piece. With regard to the second phasing plug  110 , the accuracy may not be as critical as the dimensional requirements in the first phasing plug. Therefore, the second phasing plug may be assembled from a number of components made of less expensive material, such as plastic, paper material or any material and allows for materials having lower tolerances. Alternatively, the first phasing plug may be assembled from a number of pieces that are glued or fitted together and adapted to associate with the second phasing plug. Also, the second phasing plug may be made from a unitary work-piece as well. 
       FIG. 2  illustrates a cross-sectional view of the two-stage phasing plug assembled within the compression driver  100 . A cover  202  encloses the entire assembly. The diaphragm  200  may be adjacent or juxtaposed to the first phasing plug  108 . Moreover, the second phasing plug  110  may be flush within the first phasing plug  108  to form the two-stage phasing plug assembly. In this embodiment, a three circular slots  204 ,  206 , and  208  may be formed between the first and second phasing plugs  108 ,  110  to form air passages or channels so that air between the diaphragm  200  and the first phasing plug  108  may be compressed through the three slots. Compressed air then exit through the throat of the horn. 
     As illustrated in  FIG. 3 , the first phasing plug  108  may have a rear side  300  and a first intermediate side  302 . In this embodiment, the rear side  300  may have a convex or dome shape, while the first intermediate side  302  may have a concave shape. On the first intermediate side  302 , the first phasing plug  108  has a cavity  308  adapted to receive the second phasing plug  110 . The cavity  308  may have a cylindrical shape having a diameter “d” and the intermediate side  302  forming a base for the cavity  308 . Moreover, the first phasing plug  108  has a flange  304  adapted to couple to the throat  112  of the horn  106  illustrated in FIG.  1 . To do so, the flange  304  has a threaded opening  306  to receive a bolt to couple to the throat  112  of the horn. 
       FIG. 4  illustrates a plurality of slots, three circular slots  204 ,  206 , and  208  in this embodiment, formed between the rear and first intermediate sides  300  and  302 . Moreover, the three slots  204 ,  206 , and  208  have a substantially similar slot length L between the rear and first intermediate sides  300  and  302 . The slots forming the air channels may expand from the rear side  300  to the first intermediate side  302 . That is, the width of the cut on the rear side  300  may be smaller than the width of the cut on the first intermediate side  302 . Besides the slots, a pair of indentations  400  may be made forming a first bridge  402  between the pair of indentation so that the inner plate  404  is not cut away from the first phasing plug  108  because of the slot  204 . Similar indentations and bridges may be made to hold a center plate  406  and an outer plate  408  in place. 
     The plurality of slots form air passages or channels so that air between the diaphragm and the rear side  300  may be compressed into the plurality of slots. The radial distance δ 1  generally represents the radial diameter of the first slot  204 . The radial distance δ 2  separates the two slots  204  and  206 . The radial distance δ 3  separates the two slots  206  and  208 . The radial distances δ 1 , δ 2 , and δ 3  may be substantially similar to the wavelength of the highest frequency the two stage-phasing plug  100  needs to produce such that any cancellation, if at all, occurs at the highest frequency possible outside of the audio band. That is, as the diaphragm compresses, air pressure waves are formed, and some of the pressure waves takes a longer path to the slots than other pressure waves. For instance, pressure waves at the center of two slots must travel, half of the radial distance, i.e., δ/2, further than pressure waves near the same two slots. If distance δ/2 is equal to one-half of the wavelength, then the pressure waves at δ/2 distance from any of the slots are out of phase with the pressure waves near the slots, thus canceling each other. 
     Put differently, “standing waves” as generally known to one skilled in the art, typically occur in the cavity between the diaphragm and the rear side  300  of the first phasing plug  108 , which can interfere with or cancel the pressure waves passing through the slots in the phasing plug. To minimize the interference from the standing waves, the radial distances δ 1 , δ 2 , and δ 3  may be positioned on the rear side  300  of the first phasing plug  108  based on a methodology developed by Bob Smith in a paper entitled “An Investigation of the Air Chamber of Horn Type Loudspeakers” JASA, Vol. 25, No. 2, published March of 1953, that is incorporated by reference into this application. 
     As stated in Bob Smith&#39;s paper:
         Any one of the modes may be suppressed by making the horn throat an annulus which is located at the node, of this mode. If it is necessary to suppress two modes, two annuluses (slots) are required. These annuluses can be located at the nodes of the second mode and thus do not excite it. Each annulus does excite the first node, but the excitation by the second annulus is out of phase with that of the first annulus. By suitable choice of annulus widths, complete cancellation of the first mode results. Thus, the first two modes are suppressed. The process can be carried out for any number of annuluses, i.e., in the general casae of “m” annuluses the first “m” modes can be suppressed.   The air chamber theory developed here suggests the following design procedure: The diaphragm size is selected by the power requirements of the loudspeaker. One then computes the frequencies of the modes associated with this diaphragm from Eq. (13), decides how many modes have to be suppressed, and chooses this number of annuluses. The radii of these annuluses are determined from Eq. (26) and the relative widths from the set of Eqs. (25).       

     Equation (13) of Bob Smith&#39;s paper states that: 
     The resonant frequencies of the higher modes are
 
 f   n   =p   n   c /2 πa, 
 
and the resonant wavelengths are λ n =2πa/p n ,
 
λ 1 =1.64 a, λ   2 =0.896 a, λ   3 =0.618 a, λ   4 =0.471 a. 
 
     Equations (25) and (26) of Bob Smith&#39;s paper states that: 
     The first a modes can be suppressed by letting “j” take on integral values from 1 to m. This produces a set of simultaneous equations:
 
 A   1   J   o ( k   1   r   1 ) . . .  A   m   J   o ( k   1   r   m )=0
 
 A   1   J   o ( k   m   r   1 ) . . .  A   m   J   o ( k   m   r   m )=0  (25)
 
Any set of annulus areas and radii which satisfy Eq. (25) will suppress the first m modes. One way of doing this is to choose the radii such that
 
 J   o ( K   m   r   i )=0  i =1 , . . . m,   (26)
 
i.e., choose the radii to be at the nodes of the “m”th mode of Jo. This reduces Eq. (25) to “m−1” equations. These equations can be solved simultaneously for the area of each annulus. For the case of one, two, or three annulus the proper radii and widths of annulus are
         for m=1: r 1 =0.628a and ω 1  arbitrary;   for m=2: r 1 =0.334a, r 2 =0.788a, ω 1  arbitrary, and ω 2 =1.004ω 1 ;   for m=3: r 1 =0.238a, r 2 =0.543a, r 3 =0.853a, ω 1  arbitrary, ω 2 =1.025ω 1 , and ω 3 =1.065ω 1 .       

     In general, incorporating more slots in the phasing plug further suppresses the lower frequency standing waves. Alternatively, with enough slots in the phasing plug, the occurrence of the standing waves may be outside of the audio band such that the interference may not be noticeable to a listener at all. As such, the radial distances δ 1 , δ 2 , and δ 3  each may vary depending on the application of the compression driver. In general, the benefit of having more slots is balanced with the increase in cost associated with incorporating more slots into the phasing plug. 
     For example, the first phasing plug  108  according to  FIG. 4  may have the following exemplary dimensions. The slot width for the slot  204  on the rear side  28  may be from about 0.02 inches to about 0.10 inches, and in particular about 0.06 inches; while on the first intermediate side  302 , the width of the slot  204  may be from about 0.02 inches to about 0.15 inches, and in particular about 0.077 inches. The width for slots  206  and  208  may be substantially similar to the width of the slot  204 . The radial distances δ 1 , δ 2 , and δ 3  may be about 0.5 inches to provide a compression ratio to be about 6:1 to about 12:1, and in particular about 10:1. 
     The first phasing plug  108  may be made from a work-piece that has been machined and cut. For example, a work-piece may be initially formed from a cast that is cylindrical in shape. To accurately cut the rear side  300  into a dome surface, the work-piece may be installed in a spindle or lathe and tooled to form the dome shape according to the specification and tolerance. The work-piece may be cut with a tool that is computer controlled so that the rear surface  300  may be cut accurately to form the dome shape in one pass. Other methods known to persons skilled in the art may be used to polish or carve the rear side  300  to satisfy the tolerance requirement. The work-piece may be initially cast or forged with sufficient tolerances that it may not need to be carved or polished to satisfy the specification. 
     Once the rear surface  300  has been machined, the slots  204 ,  206 , and  208  may be partially pierced between the rear and first intermediate sides  300  and  302 . This may be done using a variety of machining tools as known to one skilled in the art. Then, the slots may be cut through the first phasing plug  108  between the rear side  300  and first intermediate sides  302  using a water jet or other suitable cutting mechanism, except for the bridges between the plates  404 ,  406 , and  408 . For example, a water jet may be injected from the rear side  300  until it cuts through the first intermediate side  302 . With regard to the indentations, the water jet does not cut in those areas. One of the advantages with the water jet is that it expands as it cuts so that the water jet naturally makes the slots  204 ,  206 , and  208  that expand from the rear side  300  to the first intermediate side  302 . Therefore, there is no additional machining that needs to be done to expand the slots or air channels from the rear side  300  to the first intermediate side  302 . Alternatively, a laser, cutting tools, or plasma cutting methods or any other methods known to one skilled in the art may be used to cut the slots as well. 
       FIG. 5  illustrates a side view of the first phasing plug  108  that has been machined on the rear side  300  to form a dome shape having a particular dimensional tolerance, and cut to have the slots  204 ,  206 , and  208 . The slot  204  defining the inner plate  404 , the slot  206  defining the center plate  406 , and the slot  208  defining the outer plate  408 . 
       FIG. 6  illustrates the bottom view of the first phasing plug  108  showing the first intermediate side  302 . Although the dimensional tolerance on the first intermediate side  302  may not be as critical as the rear side  300 , the first intermediate side  302  may be machined as well so that the thickness between the rear and first intermediate sides  300 ,  302  is substantially constant. Again the slot  204  defines the inner plate  404 . The center plate  406  is between the two slots  204  and  206 . And the outer plate  408  is between the two slots  206  and  208 . To hold the plates together, an inner bridge  602  is formed between the inner plate  404  and the center plate  406 , a center bridge  604  is formed between the center plate  406  and the outer plate  408 , and an outer bridge  606  is formed between the outer plate  408  and the edge  608  of the first phasing plug  108 . Moreover, a number of threaded openings  608  are formed to receive a bolt to couple to the throat of a horn. 
     The two-stage phasing plug may have a number of slots depending on the application. For instance,  FIG. 7  illustrates a two-stage phasing plug  700  including a first phasing plug  702  and a second phasing plug  704  with four slots  706 ,  708 ,  710 , and  712 . And  FIG. 8  illustrates a two-stage phasing pug  800  including a first phasing plug  802  and a second phasing plug  804  with five slots  806 ,  808 ,  810 ,  812 , and  814 . Note that in this example, the first intermediate side  816  is substantially flat rather than being concave as in the other embodiments. With additional slots in the two-stage phasing plug, the radial distances need to be smaller to accommodate more slots on the rear side  818 . As such, to maintain the compression ratio on the compression driver, which may be generally defined as the overall surface area of the rear side of the first phasing plug in relation to the overall opening area of the slots on the rear side, the width of the slots need to be reduced as well. In general, the compression ratio may be between about 6:1 and about 12:1, and in particular about 10:1. 
     As illustrated in  FIG. 8 , the thickness between the first intermediate side  816  and the rear side  818  need not be constant. For example, the first intermediate side  816  or the base of the cavity may be a substantially flat surface rather than being a curved surface as illustrated in FIG.  3 . 
       FIGS. 9-12  illustrate by way of example the second phasing plug  110  configured to substantially fill the cavity  308  of the first phasing plug  108  illustrated in FIG.  3 .  FIG. 9  illustrates the second phasing plug  110  having a second intermediate side  900  and a front side  902 . The second intermediate side  900  substantially matches the shape of the first, intermediate side  302  so that when the first and second intermediate sides are adjacent they are substantially flush together. In other words, there is little gap, if any, between the first and second intermediate sides  302 ,  900 . 
     As illustrated in  FIG. 10 , the second phasing plug  110  has a plurality of slots  1000 ,  1002 , and  1004  that correspond to the slots  204 ,  206 , and  208 , respectively, in the first phasing plug  108 . Moreover, the slot  1000  generally defines an inner piece  1010 . Between the two slots  1000  and  1002  is a centerpiece  1012 , and between the slots  1002  and  1004  is an outerpiece  1014 . That is, the second intermediate side  900  is comprised of the inner piece  1010 , the centerpiece  1012 , and the outerpiece  1014 , which flush against the inner plate  404 , the center plate  406 , and the outer plate  408  on the first intermediate side  302  of the first phasing plug  108 , respectively. In other words, the second intermediate side  900  substantially matches the first intermediate side  302  so that when the second phasing plug  110  is inserted into the cavity of the first phasing plug  108 , the second intermediate side  900  may be substantially flush against the first intermediate side  302 . To substantially fill the cavity  308 , the second phasing plug  108  may have a cylindrical shape with a diameter “D” that is equal or slightly less than the diameter “d” of the cavity  308  in FIG.  3 . Therefore, the second phasing plug  108  may be press-fitted into the cavity  308 . Alternatively, glue may be used to securely hold the second phasing plug  110  within the cavity  308  of the first phasing plug  108 . 
     In another embodiment, the second phasing plug  110  may be interchangeable so that the compression assembly  100  may be adaptable for a particular application by simply changing the second phasing plug. That is, the second phasing plug may be releaseably held in the cavity of the first phasing plug, so that the second phasing plug may be removed and replaced with a different phasing plug depending on the application. 
       FIG. 11  illustrates the slots  1000 ,  1002 , and  1004  exiting through the front side  902  of the second phasing plug  110 . As illustrated in  FIG. 12 , the slots  1000 ,  1002 , and  1004  expand from the second intermediate side  900  to the front side  902 , i.e., the exit side. Moreover, the width of the slots  1000 ,  1002 , and  1004  in the second intermediate side  900  are substantially similar to the corresponding slots  204 ,  206 , and  208  on the first intermediate side  302 . This way, the slots forming the path lengths or air channels from the first and second phasing plugs transition smoothly and continuously. In this embodiment, the front side  902  is substantially flat such that the second phasing plug may be fully inserted into the cavity  308 , as shown in FIG.  2 . Alternatively, the front side  52  may extend into the throat  112  of the horn  106 . 
     The second phasing plug  110  may be assembled using a variety of methods. One such method is illustrated in  FIGS. 13-18 . As dimensional accuracy in the second phasing plug  110  is not as critical as in the first phasing plug  108 , the second phasing plug may be assembled together, unlike the first phasing plug  108 , which may be made from a unitary work-piece. That is, in this embodiment, an inner piece  1300 , the centerpiece  1400 , the outerpiece  1600 , and a housing  1800  are assembled to make the second phasing plug  110 . 
       FIG. 13  illustrates the inner piece  1300  having a cone shape with a pair of flanges  1302 . The inner piece  1300  has an inner surface  1304  that is a portion of the second intermediate side  900 , which flush against the inner plate  404  along the first intermediate side  302  of the first phasing plug  108 .  FIGS. 14 and 15  illustrate the centerpiece  1400  having a funnel shape with a bore  1402 ; and a center surface  1404  that is a portion of the second intermediate side  900  and fits flush against the center plate  406  of the first phasing plug  108 . Moreover, the centerpiece  1400  has a pair of divots  1406  adapted to receive the pair of flanges  1302 , so that the inner piece  1300  may be press-fitted into the bore  1402  of the centerpiece  1400 . Likewise, the centerpiece  1400  has three flanges  1408  so that the centerpiece may be press-fitted into the outerpiece  1600 . 
       FIGS. 16 and 17  illustrate the outerpiece  1600  having a funnel shape as well. The outerpiece  1600  has an opening  1602 , and three divots  1604  adapted to receive the three flanges  1408  from the centerpiece  1400 . That is, the centerpiece  1400  may be press-fit into the opening  1602  of the outerpiece  1600 . Likewise, the outerpiece  1600  has an outer surface  1606  that fits flush against the outer plate  408  of the first phasing plug  108 . Moreover, the outerpiece  1600  has three flanges  1608 . 
       FIG. 18  illustrates the housing  1800  having a cylindrical shape with a diameter “D” and an opening  1802 . Within the opening  1802  are three divots  1804  which are adapted to receive the three flanges  1608  so that the outerpiece  1600  may be press-fit into the housing  1800 . Accordingly, the second phasing plug  108  as shown previously in  FIGS. 9-12  may be assembled by press-fitting the inner piece  1300  into the center piece  1400 , then press-fitting the center piece  1400  into the outerpiece  1600 , and then press-fitting the outerpiece  1600  into the housing  1800 . 
     With regard to the expansion of the slots through the two-stage phasing plug  102 , the slots may expand gradually in a straight line through the first phasing plug  108  and then to the second phasing plug  110 , as illustrated in FIG.  2 . Alternatively, as illustrated in  FIG. 19 , the first phasing plug  1908  may have slots  1912 ,  1914 ,  1916 , and  1918  expanding gradually in a straight line but in the second phasing plug  1910 , the slots  1912 ,  1914 ,  1916 , and  1918  expand in a curve or in any conic profile, i.e., hyperbolic, parabolic, etc. shape so that the length of the each slots through the two-stage phasing plug  1900  between the rear side  1920  and the front side  1922  are substantially constant. Moreover, the slots  1912 ,  1914 ,  1916 , and  1918  exit through the second phasing plug  1910  substantially parallel with the center axis  1950 . That is, air exits through the slots substantially parallel with the center axis  1950 . 
     Still further, as illustrated in  FIG. 20 , in another embodiment, a two-stage phasing plug  2000  may have slots  2012 ,  2014 ,  2016 , and  2018  through the first phasing plug  2008  that expand in a curve or in any conic profile, i.e., hyperbolic, parabolic, etc. shape as well as in the second phasing plug  2010 . Here, the first phasing plug  2008  may be assembled from a number of pieces rather than being formed from a unitary piece. Also, the slots  2012 ,  2014 ,  2016 , and  2018  exit through the front side  2022  of the second phasing plug  2010  at an acute angle relative to the center axis line  2050 . In other words, as air exit through the slots  54 , air diverges off of the center axis line  2050  at an acute angle φ, such as between about 5° and about 25°. One of the advantages here is that as air exit through the slots  2012 ,  2014 ,  2016 , and  2018  in a divergent direction so that the direction of the air is in alignment with the contour of a horn that flares out as well. In other words, with this embodiment, pressure waves leave the slots in the direction that conforms to the shape of the horn. 
       FIG. 21  illustrates yet another embodiment of the invention, where a phasing plug  2100  may be made of a number of pieces rather than in two stages as discussed above. That is, slots  2112 ,  2114 ,  2116 , and  2118  may be formed through the phasing plug  2100  which are curve comprised of number of pieces assembled together like the second phasing plug  110  assembled together as illustrated in  FIGS. 9 through 12 . 
     The first phasing plug may be made of any ferromagnetic material such as steel. Alternatively, any other materials known to one skilled in the art may be used as well. The second phasing plug, on the other hand, may be made of less expensive and easier to work with material such as plastic or any material known to one skilled in the art. Any method may be used to make the second phasing plug, such as well-known molding processes. Also, machining and cutting processes are well known to one skilled in the art and may be selected based on the tolerance requirements. 
     Although the invention is generally described in terms of the one embodiment above, numerous modifications and/or additions to the above-described embodiment would be readily apparent to one skilled in the art. For example, the slots may be cut in any configuration. U.S. Pat. No. 4,050,541, is incorporated by reference into this application and discloses a radial slot configuration. U.S. Pat. No. 5,117,462, is incorporated by reference into this application discloses a whole array. The first intermediate surface  302  may also have a convex surface rather than a concave surface. 
     Phasing plugs have been made with many designs. Perhaps the most frequently used type is one having annular cross-sections that usually increase in area as the principal radius of each annulus decreases in moving toward the throat of a speaker. This is shown, for example, in U.S. Pat. No. 2,037,187, entitled “Sound Translating Device,” issued to Wente in 1936 and incorporated by reference. Another type is the salt shaker design, so called because holes at the spherical outer surface of the plug that extend through to the throat of the speaker resemble the holes of a salt shaker. Another design that has been used, shown in U.S. Pat. No. 4,050,541, entitled “Acoustical Transformer for Horn-type Loudspeaker,” couples the diaphragm region to the throat by radial slots extending from the axis of cylindrical symmetry of the speaker and is incorporated by reference into this application. 
     While various embodiments of the application have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of this invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.