Abstract:
A loudspeaker assembly, including an acoustic waveguide; an acoustic driver mounted in the waveguide so that a first surface radiates sound waves into the waveguide so that the sound waves are radiated from the waveguide; and an acoustic volume acoustically coupled to the acoustic waveguide for increasing the amplitude of the sound waves radiated from the acoustic waveguide.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of, and claims priority of, U.S. patent application Ser. No. 12/020,978, published as U.S. Published Pat. App. 2009/214066 A1, now U.S. Pat. _______. 
     
    
     BACKGROUND 
       [0002]    This specification describes an improved acoustic waveguide. Acoustic waveguides are described generally in U.S. Pat. 4,628,528. Some specific aspects of acoustic waveguides are described in U.S. Pat. 6,771,787 and in U.S. patent application Ser. No. 09/753,167. 
       SUMMARY 
       [0003]    In one aspect, a loudspeaker assembly, comprises: an acoustic waveguide; an acoustic driver mounted in the waveguide so that a first surface radiates sound waves into the waveguide so that the sound waves are radiated from the waveguide; and an acoustic volume acoustically coupled to the acoustic waveguide for increasing the amplitude of the sound waves radiated from the acoustic waveguide. The acoustic waveguide may be substantially lossless. The acoustic volume may be for increasing the amplitude of sound waves of a wavelength equal to the effective acoustic length of the waveguide. The acoustic waveguide may have curved walls forming walls of the acoustic volume. The acoustic waveguide may have curved walls forming walls of an acoustic volume acoustically coupled to the acoustic waveguide to increase the acoustic radiation from the waveguide. The acoustic volume may be tear drop shaped. The waveguide walls may form walls of another acoustic volume coupled to the acoustic waveguide. The loudspeaker assembly may further comprise electronic components positioned in the acoustic volume. The loudspeaker assembly may further comprise a coupling volume for acoustically coupling the acoustic waveguide to the acoustic volume and the combination of the coupling volume and the acoustic volume may form a Helmholtz resonator may have a Helmholtz resonance frequency that is outside the operating range of the loudspeaker assembly. The acoustic driver may be mounted so that a second surface of the acoustic driver radiates directly to the environment. The waveguide may comprise multiple curved sections substantially defining the acoustic volume. The acoustic waveguide may substantially define another acoustic volume. The acoustic volume may be teardrop shaped. The waveguide may have an effective acoustic length, and the acoustic volume may have acoustic paths each having a length that is less than 10% of the effective acoustic length of the loudspeaker assembly, or the acoustic paths may have a length that is greater than 10% of the effective acoustic length of the loudspeaker assembly and that is within a range of lengths that does not result in a dip in a frequency response. The acoustic volume may comprise a baffle structure causing the length of an acoustic path to be within the range of lengths. The waveguide may have a substantially constant cross-sectional area. A closed end of the waveguide adjacent the acoustic driver may have a larger cross-sectional area than an open end of the waveguide. 
         [0004]    In another aspect, a loudspeaker assembly, comprises: an acoustic driver; an acoustic waveguide with substantially continuous walls acoustically coupled to the acoustic driver so that a first surface of the acoustic driver radiates into the acoustic waveguide and so that the waveguide radiates acoustic radiation from an open end of the waveguide; and the waveguide comprises a structure for increasing the amplitude of the acoustic radiation that is radiated from the open end of the waveguide. The structure for increasing the amplitude may comprise an acoustic volume, acoustically coupled to the acoustic waveguide. The acoustic waveguide may be substantially lossless. The acoustic waveguide may have curved walls forming walls of an acoustic volume acoustically coupled to the acoustic waveguide to increase the acoustic radiation from the waveguide. The acoustic waveguide walls may form walls of a teardrop shaped acoustic volume. The waveguide walls may form walls of another acoustic volume coupled to the acoustic waveguide. The loudspeaker assembly may further include electronic components positioned in the acoustic volume. The loudspeaker assembly may further comprise a coupling volume for acoustically coupling the acoustic waveguide to the acoustic volume; and the combination of the coupling volume and the acoustic volume may form a Helmholtz resonator having a Helmholtz resonance frequency that is outside the operating range of the loudspeaker assembly. The acoustic driver may be mounted so that a second surface of the acoustic driver radiates into the environment. The waveguide may comprise multiple curved sections substantially defining at least one acoustic volume, coupled to the acoustic waveguide. The acoustic waveguide may substantially define another acoustic volume, coupled to the acoustic waveguide. The acoustic volume may be teardrop shaped. The waveguide may have an effective acoustic length; the acoustic volume may have acoustic paths each having a length that is less than 10% of the effective acoustic length of the loudspeaker assembly, or each having a length that is greater than 10% of the effective acoustic length of the loudspeaker assembly and that is within a range of lengths that does not result in a dip in a frequency response. The acoustic volume may comprise a baffle structure causing the length of an acoustic path to be within the range of lengths. The waveguide may have a substantially constant cross-sectional area. The waveguide may have a cross sectional area at a closed end adjacent the acoustic driver than at an open end. 
         [0005]    In another aspect, a loudspeaker apparatus comprises an acoustic waveguide and an acoustic driver having a first radiating surface and a second radiating surface, the acoustic driver mounted to the waveguide so that the first surface radiates acoustic energy into the acoustic waveguide so that the acoustic radiation is radiated from the waveguide. The loudspeaker apparatus may be characterized by a cancellation frequency at which radiation from the second surface is out of phase with the radiation from the waveguide, resulting in destructive interference between the radiation from the waveguide and the radiation from the second surface, resulting in a reduction in acoustic output from the loudspeaker apparatus at the cancellation frequency. The loudspeaker apparatus may have an acoustic volume, acoustically coupled to the waveguide to increase the amplitude of the radiation from the waveguide resulting in less reduction in acoustic output from the loudspeaker apparatus at the cancellation frequency. 
         [0006]    Other features, objects, and advantages will become apparent from the following detailed description, when read in connection with the following drawing, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0007]      FIGS. 1A and 1B  are geometric objects useful in understanding some of the other figures; 
           [0008]      FIG. 2  is a diagrammatic view of a waveguide assembly; 
           [0009]      FIGS. 3A and 3B  are diagrammatic views of waveguide assemblies; 
           [0010]      FIGS. 3C and 3D  are diagrammatic cross-sectional views of waveguide assemblies; 
           [0011]      FIGS. 4A-4G  are diagrammatic views of waveguide assemblies; 
           [0012]      FIGS. 5A and 5B  are diagrammatic views of a waveguide assembly; 
           [0013]      FIGS. 6A and 6B  are diagrammatic views of a portion of a waveguide assembly; 
           [0014]    and 
           [0015]      FIGS. 7A-7D  are drawings of a practical implementation of loudspeaker systems with waveguide assemblies including features shown diagrammatically in other figures. 
           [0016]      FIG. 8  is a diagrammatic view of a portion of a waveguide wall, an opening and an acoustic volume. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIGS. 1A and 1B  show some geometric objects useful in understanding some of the figures that follow.  FIG. 1A  is an isometric view of two waveguides  6  and  7 . Waveguides  6  and  7  are depicted as structures having rectangular cross-sections in the Y-Z plane and an X-dimension longer than both the Y- and Z- dimensions. The area dimension in the Y-Z plane (hereinafter the “area dimension”) of waveguide  6  is A and the linear dimension along the Y-axis is h. In the specification, there are references to changes in the area dimension. In the corresponding figures, changes to the area are depicted by changes in dimension in the Y-direction, holding the dimension in the Z-direction uniform. So for example, a waveguide  7  with an area dimension of  2 A would be depicted in the corresponding figure by a doubling of the linear dimension h along the Y-axis to  2   h .  FIG. 1B  shows the waveguides of  FIG. 1A  as cross sections in the X-Y plane and includes some additional elements. Except where otherwise specified, the waveguides in the following figures are shown as cross-sections in the X-Y plane, with the longest dimension in the X-dimension. Except where otherwise specified, “length” refers to the length of the acoustic path through the waveguide. Since waveguides are frequently bent or curved, the length may be greater than the X-dimension of a device incorporating the waveguide. Acoustic waveguides typically have at least one open end  18  and may have a closed end  11 . An acoustic driver  10  is typically mounted in the closed end  11  as shown, but may be mounted in one of the walls  13  as represented by the dashed line. In the figures that follow, the acoustic driver is shown as mounted in closed end  11 . 
         [0018]      FIG. 2  shows a first waveguide assembly  100 . An acoustic driver  10  is mounted in one end of a waveguide  12 A that is low loss and preferably substantially lossless through the frequency range of operation of the waveguide. The waveguide  12 A has a cross-sectional area A and an effective acoustic length  1 . The waveguide has a tuning frequency which is determined principally by the effective acoustic length of the waveguide, which is the physical length plus end effect corrections. End effect corrections may be determined using estimation techniques or empirically. For simplicity, in the figures the length l will be shown as the physical length and the term “length” will refer to the effective acoustic length. The waveguide  12 A has a volume given by lA. 
         [0019]      FIG. 3A  shows a second waveguide assembly. An acoustic driver  10  is coupled to a waveguide  12 B that is low loss and preferably substantially lossless through the frequency range of operation of the waveguide. Waveguide  12 B has a physical length βl and a cross-sectional area βA, where β is a factor &lt;1. The volume of the waveguide  12 B is, β 2 lA. Acoustically coupled by opening  34  to the waveguide  12 B is an acoustic volume or chamber  22 . The volume of the chamber  22  is lA-β 2 lA , so that the volume of the waveguide  12 B plus the volume of the chamber  22  is the same as the volume of the waveguide  12 A of  FIG. 2 . An effect of the chamber  22  is that the waveguide  12 B has essentially the same tuning frequency as the waveguide  12 A of  FIG. 2  despite having a shorter length. An advantage of the waveguide of  FIG. 3A  is that (except as described below in the discussion of Helmholtz resonators and in the discussion of  FIGS. 6A and 6B ) the chamber  22  can be many shapes so long as the chamber  22  has the correct volume dimension. So, for example, as shown in  FIG. 3B , the walls of chamber  22  can form a gradually curved surface  31  which forms the walls of the waveguide  12 B. A waveguide having a gradual curve causes less turbulence and undesirable noise than waveguides with a more abrupt curve or change in direction and also use space efficiently. As long as the intended volume is maintained, the dimensions of chamber  22  may have a wide range of values, except as discussed below in the discussion of  FIGS. 6A and 6B . 
         [0020]      FIGS. 3C and 3D  show cross-sections of a waveguide assembly in the Y-Z plane, so that the x-dimension (the longest dimension of the waveguide) is perpendicular to the sheet of the drawing. In the waveguide of  FIG. 3C , the chamber  22  has a dimension in the Y direction and the Z direction that is larger than the Y and Z dimension of the waveguide  12 B so that the chamber partially or completely envelops the waveguide. If desired, for example for ease of manufacture, a barrier  46  or a barrier  48  or both may be placed in the waveguide  12 B or the chamber, respectively (so that there are two waveguides  12 B- 1  and  12 B- 2  or two chambers  22 A and  22 B or both), and achieve the same acoustic result as if there were no barriers. Sight lines  52 ,  54 , and  56  will be referenced below. To eliminate high frequency peaks, there may be a small amount of acoustically resistant material in accordance with U.S. Pat. 6,278,789 in the waveguide of  FIG. 3A  and in the waveguides of all subsequent figures. 
         [0021]    The concepts of reducing the cross-sectional area and length of a waveguide and adding a chamber to the waveguide as shown in  FIGS. 3A and 3B  can be applied to portions of waveguides, for example stepped portions of stepped waveguides, as well as whole waveguides, for example stepped waveguides.  FIG. 4A  shows a stepped waveguide  12 C according to U.S. Pat. 6,771,787. An acoustic driver  10  is mounted in one end of the stepped waveguide  12 C. The stepped waveguide  12 C has four sections  24 - 27  along the length of the waveguide, with section  24  adjacent the acoustic driver and section  27  adjacent the open end  18  of the waveguide. The sections are of substantially equal length l. Section  24  has a cross sectional area A 1 , section  25  has a cross sectional area A 2 , which is larger than A 1 ; section  26  has a cross sectional area A 3 , and section  27  has a cross sectional area A 4  which is larger than cross sectional area A 3 . The volume V 1  of section  24  is A 1 l, the volume V 2  of section  25  is A 2 l, the volume V 3  of section  26  is A 3 l and the volume V 4  of section 26 is A 4 l. In conventional waveguides, radiation from a surface of the acoustic driver that faces the environment (hereinafter the exterior surface) is out of phase with radiation from the surface of the acoustic driver that faces into the waveguide. At wavelengths equal to the effective acoustic length of the waveguide, the radiation from the waveguide and the radiation from the exterior surface of the waveguide destructively interfere, reducing the combined radiation of the waveguide and the acoustic driver. In a waveguide system according to  FIG. 4A , the radiation from the waveguide is greater than the radiation from the exterior surface of the acoustic driver, and therefore the dip in the combined radiation from the waveguide and the exterior surface is eliminated. In one embodiment, the waveguide assembly of  FIG. 4A , A 1 =A 3 , A 2 =A 4 , and 
         [0000]    
       
         
           
             
               
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         [0000]    The operation of the waveguide assembly of  FIG. 4A  is described in U.S. Pat. 6,711,787. 
         [0022]      FIG. 4B  illustrates a waveguide system using chambers acoustically coupled to the waveguide so that the waveguide is shorter than a corresponding conventional waveguide. An acoustic driver  10  is mounted in one end of a waveguide  12 D. Waveguide  12 D, and waveguides in the subsequent figures, is low loss and preferably substantially lossless through the frequency range of operation of the waveguide. The waveguide  12 D has a cross sectional area equal to the cross sectional area A 1  of sections  24  and  26  of the waveguide of  FIG. 4A . Sections  25  and  27  of  FIG. 4A  have been replaced by sections  25 ′ and  27 ′, respectively. Sections  25 ′ and  27 ′ have a length of βl and a cross-sectional area A′ 2  equal to β A   2  where β is a number 0&lt;k&lt;1. In this example, 
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         [0000]    so that the waveguide of  FIG. 4B  has a uniform cross-sectional area A throughout the length of the waveguide. Sections  24 ′ and  26 ′ have a cross-sectional area of A and volumes (V 1  and V 3  respectively) of lA. Sections  25 ′ and section  27 ′ have a cross-sectional area of A′ 2  and volumes (V′ 2  and V′ 4  respectively) of β 2 A 2 l . At a distance d 1  (where l&lt;d 1 &lt;l+βl, in one example 
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         [0000]    from the acoustic driver end of the waveguide, a chamber  22  is acoustically coupled to the waveguide through an opening  34 . At a distance d 2  (where l+βl+l&lt;d2&lt;l+βl+βl+l+βl , in one example 
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         [0000]    from the acoustic driver end  11  of the waveguide, a chamber  29  is acoustically coupled to the waveguide through an opening  38 . Chamber  22  has a volume dimension V c  of A 2 l (1-β 2 ) so that V′ 2 +V c =V 2 , and chamber  29  has a volume dimension V D  of A 4 l(1-β 2 ) so that V′ 4 +V c =V 4 , so that the total volume occupied by the assembly of  FIG. 4B  and the total volume occupied by the assembly of  FIG. 4A  are substantially equal. As stated above, so long as the chambers have the correct volume, the volume can have any shape, orientation, or linear dimensions of the chambers, except as shown below in  FIGS. 6A and 6B  and discussed in the corresponding portion of the specification. 
         [0023]    The opening  34  or  38  may have an area such that it may form, with the chamber  22  or  29 , respectively, a Helmholtz resonator which could have adverse acoustic effects on the operation of the waveguide system. Helmholtz resonators are described in, for example, http://www.phys.unsw.edu.au/jw/Helmholtz.html, a copy of which is attached as an appendix. However, the dimensions of the opening  34  and of the chamber  22  can be selected so that the Helmholtz resonance frequency is at a frequency that does not adversely affect the operation of the waveguide system or that is outside the operating frequency range of the waveguide. Selecting dimensions so that the Helmholtz resonance frequency is outside the operating frequency of the waveguide can be done by making the width of openings  34  and  38  to the chambers  22  and  29  respectively, close to (for example &gt;50% of) the width of the chambers. 
         [0024]    The tuning of the waveguide  12 D of  FIG. 4B  is essentially the same as the tuning of the waveguide  12 C of  FIG. 4A . Sections  24 ′ and  26 ′ of  FIG. 4B  have the same effect on the tuning of the waveguide as sections  24  and  26  of  FIG. 4A . Sections  25 ′ and  27 ′ of  FIG. 4B  have the same effect on the tuning of the waveguide as sections  25  and  27  of  FIG. 4A , even though the physical length of sections  25 ′ and  27 ′ of  FIG. 4B  is βl which (since β&lt;1) is shorter than the physical length l of sections  25  and  27  of  FIG. 1 . 
         [0025]    The figures disclosed above are merely illustrative and not exhaustive and many variations are possible. For example, the waveguide may have more than four sections; sections such as sections  25 ′ and  27 ′ may have different lengths; the volume dimensions of sections such as  25 ′ and  27 ′ may have different volume dimensions; the combined volume dimensions such as V 3  and V 4  may not be equal to V 2 ; and as will be seen below, different configurations of the chambers are possible (for example, there may be different numbers of chambers, and the chambers may have different volume dimensions, shapes, and placements along the waveguide as will be described below). 
         [0026]    In addition to providing the same tuning frequency with a waveguide of shorter length, the waveguide system of  FIG. 4B  has the same advantage of  FIG. 4A  with regard to eliminating the dip in the combined output of the acoustic driver and the waveguide at frequencies at which the corresponding wavelength equals the effective length of the waveguide. At these frequencies, the acoustic output of the waveguide is greater than the acoustic output radiated directly to the environment by acoustic driver, so the combined radiation from the waveguide and the acoustic driver is greater than the combined output from a conventional waveguide system. The waveguide assembly of  FIG. 4B  is also less prone than the waveguide assembly of  FIG. 4A  to wind noises that can occur at abrupt area discontinuities. 
         [0027]      FIG. 4C  shows a variation of the waveguide assembly of  FIG. 4B . In the waveguide assembly of  FIG. 4C , the chamber  22  of  FIG. 4B  is replaced by chambers  22 A and  22 B with a total volume equal to the volume of chamber  22 . The entrance to chamber  22 A is placed at distance d 1  such that 
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         [0000]    from the acoustic driver, in one example 
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         [0000]    and the entrance  34 B to chamber  22 B is placed at distance d 2  such that 
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         [0000]    Chamber  29  of  FIG. 4B  is replaced by chambers  29 A and  29 B with a total volume equal to the volume of chamber  29 . The entrance  38 A to chamber  29 A is placed at distance d 3  such that 
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         [0000]    and the entrance  38 B to chamber  29 B is placed at distance d 4  such that 
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         [0000]    from the acoustic driver, in one example 
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         [0000]    The effect of the tuning of the waveguide assembly of chambers  22 A and  22 B is substantially the same as the effect of chamber  22  of  FIG. 4B , and the effect of on the tuning of the waveguide assembly of chambers  29 A and  29 B substantially is the same as the effect of chamber  26  of  FIG. 4B  and have the same beneficial effect of alleviating the dip in the output of the waveguide assembly at the frequency at which the wavelength equals the effective length of the waveguide. Generally, using multiple chambers permits the tuning frequency to more closely match the tuning frequency of the equivalent stepped waveguide such as the waveguide of  FIG. 4A . 
         [0028]    Aspects of  FIGS. 4A ,  4 B, and  4 C can be combined. For example, the waveguide assembly of  FIG. 4D  has a chamber  32  coupled to the waveguide  12 E in the first section at distance d 1 , where l&lt;d 1 &lt;l+βl and a stepped section  27  beginning at distance d 2 =l+βl+l. The waveguide assembly of  FIG. 4E  has a waveguide  12 F with a stepped section  25  beginning at distance d 1 =l and a chamber  29  at a distance d 2 &gt;l+l+l. Aspects of  FIGS. 4A ,  4 B, and  4 C can also be implemented in a tapered waveguide if the type shown in  FIG. 1  of U.S. Pat. 6,771,787, as shown in  FIG. 4F . For use in a tapered waveguide, the size of the chambers and the location of the openings from the waveguide to the chambers may be determined by modeling. A waveguide such as the waveguide with substantially continuous walls such as the waveguide of  FIG. 4F  may be less subject to wind noises that may occur at abrupt area discontinuities. The waveguide assembly of  FIG. 4G  is a diagrammatic view of a practical waveguide assembly incorporating elements of  FIGS. 4A-4E . The implementation of  FIG. 4G  has six 2.25 inch acoustic drivers  10 A- 10 F and dimensions as shown. 
         [0029]      FIG. 5A  shows an implementation of the waveguide assembly shown schematically in  FIG. 4B  illustrating walls of chambers  22  and  29  forming multiple curved surfaces  31 A and  31 B which also forms walls of the waveguide resulting in less turbulence than would occur with a more abrupt curve, while using space efficiently. The reference numbers in  FIG. 5A  indicate similarly numbered elements in the corresponding waveguide system of  FIG. 4B .  FIG. 5B  shows an implementation of the waveguide shown schematically in  FIG. 4E  illustrating walls of chamber  29  and stepped section  25 . The reference numbers in  FIG. 5B  indicate similarly numbered elements in the corresponding waveguide system of  FIG. 4E . 
         [0030]      FIGS. 6A and 6B  illustrate another feature of a waveguide assembly. In  FIG. 6A , waveguide  12 B is acoustically coupled to a chamber  22  through an opening  34 . Acoustic waves enter the opening  34  and propagate into the chamber  22  along a number of acoustic paths, for example path  66 A until the acoustic waves encounter an acoustic boundary. There may be many acoustic paths along which the acoustic waves propagate; for simplicity only one is shown. 
         [0031]    Generally, it is desirable to configure the chamber so that the lengths of all acoustic paths are significantly shorter than one-fourth of the effective acoustic length of the waveguide  12 B. If the length of one of the acoustic paths is not significantly shorter than one fourth (for example, not shorter than 10%) of the effective acoustic length of the waveguide, output dips may occur at certain frequencies. In one example, a waveguide assembly similar to waveguide assembly of  FIG. 4B  is tuned to 44 Hz, so that it has an effective acoustic length of 1.96 m. (6.43 feet). A chamber  22  with a volume of 1851.1 cc (114 cubic inches) is coupled to waveguide  12 B at a position 39.6 cm (15.6 inches) from the closed end  11 . Chamber  22  has an acoustic path  66 A (see  FIG. 6A ) that has a length of 40.6 cm (16 inches), that is 
         [0000]    
       
         
           
             
               
                 
                   40.6 
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                   cm 
                 
                 
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         [0000]    of the effective acoustic length of the waveguide assembly. An undesirable dip in the frequency response may occur at about 200 Hz. Depending on factors such as the distance of the chamber  22  from the closed end  11 , the dip in the frequency response may occur when the length of acoustic path  66 A is as short as 25.4 cm (10 inches), which is 
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         [0000]    of the effective acoustic length of waveguide  12 B. 
         [0032]    One way of eliminating the frequency response dip is to reconfigure chamber  22  so that acoustic path  66 A has a length shorter than 10% (in this case 19.6 cm) of the effective acoustic length of the waveguide system. However in a practical waveguide, it may be difficult to reconfigure the chamber so that acoustic path  66 A has a length of less than 10% of the effective acoustic length of the waveguide system. 
         [0033]    Another way of eliminating the frequency response dip is to add structure to the chamber  22  that changes the length of an acoustic path such as  66 A to a length that does not cause a frequency response dip.  FIG. 6B  shows the waveguide system of  FIG. 6A  with baffles  42  inserted into the chamber so that the length of acoustic path  66 B is 50.8±1.3 cm (20±0.5 inches). The waveguide system of  FIG. 6B  does not have the frequency response dip of the waveguide system of  FIG. 6A . The path length dimensions at which dips may occur and the range of path lengths at which dips do not occur, and the variance of the path length with regard to the placement of the chamber opening relative to the ends of the waveguide can be determined by modeling or experimentation. If the situation shown in  FIGS. 6A and 6B  occurs, it is generally desirable to shorten the path length because the tolerance (the range of path lengths that result in no dip) is wider. In the example above, any length shorter than 25.4 cm is suitable, but the tolerance of the longer acoustic path is only ±1.3 cm. 
         [0034]      FIGS. 7A and 7B  show a practical implementation of an audio reproduction device incorporating a waveguide assembly having features shown diagrammatically in previous figures. The elements in  FIGS. 7A and 7B  correspond to similarly numbered elements in the previous figures. The dashed lines in  FIGS. 7A and 7B  illustrate the boundaries of the chambers  22  and  29 .  FIG. 7A  is a cross section in the X-Z plane of the audio reproduction device. The waveguide assembly  12 B has the form of the waveguide assembly of  FIG. 3C  and the cross section is taken along a sight line corresponding to sight line  52  or  54  of  FIG. 3C ; the cross sections taken along sight lines corresponding to sight lines  52  and  54  are substantially identical. There is a barrier  46  (of  FIG. 3C , not shown in this view) resulting in the waveguide assembly having two waveguides.  FIG. 7B  is a cross section in the X-Z plane, taken along a sight line corresponding to sight line  56  of  FIG. 3C . The acoustic driver  10  (of previous figures), not shown in this view is coupled to the waveguide  12 B. Compartments  58  and  60  are for high frequency acoustic drivers (not shown), which are not germane to the waveguide assembly. In the implementation of  FIGS. 7A and 7B , volume V 1  of chamber  22  is about 1861 cm 3  (114 cubic inches); the volume V 2  of chamber 29 is about 836 cm 3  (51 cubic inches); the physical length of the waveguide is about 132.1 cm (52 inches); the center of opening  34  to chamber  22  is located about 39.6 cm (15.6 inches) from closed end  11  and the width of opening  34  is about 3.8 cm (1.5 inches); the center of opening  38  to chamber  29  is about 11.7 cm (4.6 inches) from the open end 18 of the waveguide and the width of opening  38  is about 3.8 cm (1.5 inches); and the waveguide is tuned to about 44 Hz. 
         [0035]    The waveguide assembly of  FIG. 7C  has two low frequency acoustic drivers  10 A and  10 B. The elements in  FIG. 7C  correspond to similarly reference numbered elements in the previous figures. The second section of the waveguide  12  has coupled to it two chambers  22 A and  22 B by openings  34 A and  34 B, respectively. The fourth section of the waveguide  12  has coupled to it a single chamber  26  by opening  38 . The walls of the waveguide  12  form walls (which for the purposes of this application includes following substantially the same outline as the walls) of chambers  22 A and  22 B and substantially enclose chambers  22 A and  22 B. Chambers  22 A and  22 B are “teardrop” shaped to provide large turning radii for the waveguide, providing a lessening of turbulence than would occur with smaller turning radii or with sharp bends. Chamber  26  provides a large chamber with low air velocity that provides a convenient location for electronics components  36 . The low velocity air causes less turbulence when it encounters the electronics  36 . The irregular, multiply curved shape of chamber  26  permits the assembly to be fit efficiently into a small device enclosure  34 . High frequency acoustic drivers do not radiate into the waveguide  12 . 
         [0036]    The waveguide assembly of  FIG. 7D  is a practical implementation of the waveguide illustrated schematically in  FIG. 4F . The elements of  FIG. 7D  correspond to similarly reference numbers in  FIG. 4F . 
         [0037]      FIG. 8  shows an enlarged view of an implementation of the opening  34  and the chamber  22 . The size of the opening  34  is intentionally greatly exaggerated for purposes of explanation. The opening  34  is formed by a portion of the walls bent inwardly toward the volume. Similar to the implementations of  FIGS. 7A ,  7 B, and  7 D, the opening  34  is configured so that the wall  13  and the opening  34  form a continuous surface; that is, there are no discontinuities such as a right angle between the opening and the wall. An opening configured as in  FIGS. 7A ,  7 B,  7 D, and  8  is advantageous because the continuous, smooth configuration of the opening causes less turbulence than an opening that is, for example, a right angle relative to the opening. 
         [0038]    Other embodiments are in the claims.