Patent Publication Number: US-11026017-B2

Title: Directive multiway loudspeaker with a waveguide

Description:
FIELD OF THE INVENTION 
     The present invention relates to loudspeakers. In particular, the present invention relates to loudspeakers equipped with a waveguide. 
     To be exact, the present invention relates to the preamble portion of claim  1 . 
     PRIOR ART 
     In the prior art especially loudspeakers with two or more drivers (multiway loudspeakers) have exhibited problems with sound diffractions created by discontinuities on the front baffle surface (Face) of the loudspeaker. In practice the high frequency driver (tweeter) has been the most critical part in this sense. The applicant of the present application has created solutions where the surroundings of the tweeter have been formed as a continuous waveguide for high and midrange frequency audio signals either merely for a tweeter and/or midrange driver or alternatively for a coaxial midrange-tweeter driver. 
     In this application, these kinds of sound sources are referred to as waveguide drivers and they include any drivers located in the centre of this three dimensional waveguide structure. By these solutions good sound quality and accurate directing of the sound energy may be achieved. However, the frequency range and effectiveness of the waveguide for controlling the directivity of radiation depends on the size of the waveguide, determined to a great extent by the surface area covered by the waveguide, and therefore the size of the front baffle (Face) of the loudspeaker. Small waveguide area limits directivity control to high frequencies, such as the tweeter range only. A large waveguide area enables extending the frequency range of directivity control towards lower frequencies, such as the midrange driver frequency range. 
     When a smaller size loudspeaker is designed, all the drivers usually cannot be placed in the center of the waveguide (such as the low frequency radiator, the woofer) the surface area taken by these other drivers and the drivers themselves will either limit the baffle area available for the waveguide or additionally create harmful diffractions of audio energy, causing deterioration of the quality of the audio signal audible to the listener. 
     In the prior art there have been attempts to create a loudspeaker with one or more waveguides on the front side of the loudspeaker. The applicant of the present application has earlier created various solutions like this, e.g. in EP-application 14168925.7 and application PCT/FI2014/050757. In these applications were presented solutions where non-coaxial drivers were positioned such that they are not disturbing the waveguide form created on the front surface (Face) of the enclosure and if positioned on the same surface (the front side (Face) of the enclosure) they are covered with a material that functions advantageously as a solid surface in selected frequencies and restricts penetration of the frequencies emitted by the sound source(s) for which the waveguide has been designed and on the other hand being permeable to other frequencies, more specifically the frequencies radiated by the non-coaxial driver(s), typically woofer(s), emit. 
     Covering the low frequency driver may cause some problems with the dynamic performance of the driver because the volume displacement of air by the driver requires sufficient openings to allow flow of air. In addition the sub volume in front of the woofer may cause unwanted resonances. 
     AIM OF THE INVENTION 
     In accordance with the invention at least some of the problems described above are solved by acoustically connecting either resistive or reactive resonators to the sub volume of the woofer such that the total volume of the loudspeaker stays as small as possible. Advantageously these resonators are located at least partially around the coaxial element. In addition, the aim of the invention is to improve the dynamical performance of the woofer(s). 
     More specifically, loudspeaker according to the invention is characterized by what is stated in characterizing portion of claim  1 . 
     According to one embodiment of the invention, the loudspeaker includes at least one resonator acoustically connected to the sub volume, the resonator being tuned to at least one of unwanted resonances of the sub volume. 
     Advantages Gained with the Invention 
     Considerable advantages are gained with the aid of the present invention. 
     With help of one embodiment of the invention the low frequency driver may be covered and yet problems with the resonances caused by the sub volume of the woofer may be suppressed. In some embodiments the suppression may take place in multiple frequencies by multiple resonators tuned to different frequencies. 
     With help of the invention the entire front surface (Face) of the loudspeaker can be formed as a continuous waveguide for mid- and high frequencies without any disturbing resonances on form the sub volume of the bass driver, yet keeping the total volume of the loudspeaker as small as possible. By this measure the whole audio range from 18-20000 Hz may be directed precisely to one “sweet spot” and in addition the rest of the sound energy is divided to the listening room due to the full waveguide form of the loudspeaker such that the loudspeaker enclosure itself does not essentially affect to the frequency response in other directions than the main direction. 
     In other words, in the traditional loudspeakers where the complete baffle plate is either planar or only partly curved as a waveguide, the signal formed into other directions than the “sweet spot” will be reflected from the walls of the listening room in a non controlled manner. The invention however provides an enclosure where the sound pressure is optimally distributed to all directions, whereby also the wall reflections sound natural to human ear. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the following, certain preferred embodiments of the invention are described with reference to the accompanying drawings, in which: 
         FIG. 1  presents a front view of a loudspeaker according to one preferred embodiment of the invention. 
         FIG. 2  presents a cross section of a loudspeaker according to  FIG. 1 . 
         FIG. 3  presents a detailed cross section of a loudspeaker according to  FIG. 1 . 
         FIG. 4  presents a graph of frequency responses of a woofer cavity and corresponding resonators in accordance with the invention. 
         FIG. 5  presents a cross section of a woofer sub volume in accordance with the invention. 
         FIG. 6  presents a cross section of a second woofer sub volume in accordance with the invention. 
         FIG. 7  presents a cross section of a third sub volume in accordance with the invention. 
         FIG. 8 a    presents a front view a woofer in accordance with the invention. 
         FIG. 8 b    presents a cross section A-A of a woofer of  FIG. 7   a.    
         FIG. 9  presents a cross section of a third woofer sub volume in accordance with the invention. 
         FIG. 10  presents a front view of a loudspeaker according to one alternative embodiment of the invention, 
         FIG. 11  presents a cross section of a loudspeaker according to  FIG. 9 . 
         FIG. 12  presents a front view of a loudspeaker according to another preferred embodiment of the invention. 
         FIG. 13  presents a view of a loudspeaker system according to one preferred embodiment of the invention. 
         FIG. 14  presents a cross sectioned view of a loudspeaker according to one preferred embodiment of the invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     List of Used Terms 
     
         
           1  loudspeaker 
           2  enclosure 
           3  waveguide driver, also coaxial drive or tweeter only 
           4  woofer, low frequency driver, additional driver 
           5  front port (opening) for the woofer, low frequency driver having an outer rim on the surface of the enclosure  2  the rim defining a plane of the rim of the front port 
           6  acoustically selectively transparent layer 
           7  support structure for the acoustically transparent layer 
           8  three dimensional waveguide surface, also a front surface (Face) of the enclosure  2  radiating the main acoustic power having a smooth continuous surface with axially symmetrical features around the centre of the waveguide driver  3   
           9  sweet spot for multiple loudspeakers 
           10  first acoustic axis 
           11  second acoustic axis 
           12  tweeter 
           13  midrange driver 
           15  front portion (wall) of the enclosure, (may also be a waveguide surface  8 ), a frontal baffle portion, the front portion radiating the main acoustic power and including the waveguide surface  8  and having a plane  28  perpendicular to the first acoustic axis  10   
         B 1  frequency band of the waveguide driver  3   
         B 2  frequency band of non-coaxial driver  4   
         C cross over frequency band between bands B 1  and B 2   
         d cavity depth of the panel resonator 
           20  first port, also side opening having an outer rim defining a first port plane on the enclosure surface. 
           21  side portion (wall) of the enclosure 
           22  sub volume, also front space of woofer, low frequency driver, part of the inner volume  27   
         W width of sub volume 
         L length of sub volume 
           23  side wall of the sub volume (front space) forming a spacer between the driver  4  and the enclosure  2 , the tangent in the middle of the side wall  23  having an angle different than zero to the plane  28  of the front portion  15 , typically an angle around 90 degrees. 
           25  back portion of the enclosure, having a plane defined by a tangent formed in the middle of the back portion  25  being typically parallel with the plane of the front portion  15 . The plane of the back portion  25  may have various different angles in accordance with the invention. 
           26  ambient volume 
           27  inner volume of the enclosure  2   
           28  plane of the front portion 
           29  plane of the side portion  21 , determined by the tangent of the center of this portion 
           30  plane of the back portion, determined by the tangent of the center of this portion plane of the front port  5   
           31  plane of the first port  20 , the a plane  31  of the front port  5  and a plane  32  of 
           32  any of the first ports  20  has an angle α greater than 0 degrees, preferably more than 45 degrees when the first port  20  is not located on the back portion  25   
           33  spacer, a part between the woofer and the front portion  15 , either integral part of the enclosure  2  or a separate element 
           34  reflex port 
         α angle between the plane  31  of the front port  5  and the plane  32  of the first port  20   
           40  resonator 
           40 ′ sub resonator 
           41  suppressive material of the resonator 
           43  frequency response of the sub volume 
           44  frequency response of the resonator 
         f 0  resonance frequency 
           45  neck of the Helmholtz resonator 
           46  cavity of the Helmholtz resonator 
           47  woofer cover 
           48  cover tubes 
           50  panel of the panel resonator 
       
    
     In accordance with  FIG. 1  one embodiment of the invention the loudspeaker  1  includes a coaxial waveguide driver  3  comprising a tweeter  12  and a midrange driver  13  around it. The coaxial driver  3  is positioned in the centre of the three dimensional waveguide surface  8 , also a front surface (Face) of the enclosure  2 . The enclosure is typically made of cast metal, advantageously aluminium. Also other castable or moldable materials, such as λtic combination may be used as a material of the enclosure. 
     The waveguide surface  8  radiates the main acoustic power of the driver  3 . The waveguide  8  has a smooth continuous surface with axially symmetrical features around the centre of the waveguide driver  3 . Two woofer drivers  4  are positioned symmetrically on both sides of the waveguide driver  3  inside the enclosure  2  and narrow ports (openings)  20 , first ports are formed just behind the waveguide surface for the woofers  4  in order to let the acoustic energy out from the enclosure  2 . These first ports  20  are in this embodiment in the narrow front ends of the enclosure  2  and these ports are partially visible from the listening direction. In other words the first port  20  is a U-form slot. 
     With dashed line are presented the woofers  4  and outlines of the woofer sub volumes  22  and resonators  40  connected to the woofer sub volume  22 . The function of the resonators  40  is to suppress resonations of the woofer sub volume  22 . These resonators  40  are positioned partially behind the coaxial driver  3  and each sub volume  22  has two resonators on both sides of the coaxial driver  3 . The sub volume  22  has width W and height H such that the ratio W/H is around 1.8 and typically in the range of 1.0-5. The resonators  40  are typically an integral part of the enclosure. 
     The resonators are dimensioned such that the longest dimension, in this time length is λ/4 or alternatively λ/2 of the wavelength to be suppressed. In other words if the sub volume  22  has an unwanted resonance at wavelength λ, the resonator should be λ/4 long. In frequency domain this means that at resonance f 0 , λ=v/f 0 , where v is the velocity of sound. Advantageously the resonator  40  is filled with a suppressive material  41  like PES wool, open-cell foam material, fibre glass, mineral wool, felt, or other fiberous or open cell or porous materials, or alternatively of any solid material that is manufactured in the place of the volume such that the material an open cell or fiberous structure where the cell size or the fiber size as in the dimensional area of 1 um (micrometer) to 1 mm (millimeter). 
     With reference to  FIG. 2 , the resonators  40  may be also are located at least partially behind the coaxial driver  3 . 
     Referring to  FIGS. 2 and 3  the two woofers  4  positioned symmetrically around the coaxial driver form an equivalent large woofer radiating essentially along the same acoustic axis  10  through ports  20  as the waveguide driver  3  even though the woofers have their own acoustic axis  11 . 
     In other words the loudspeaker  1  includes a first driver  3 , which is configured to produce a first frequency band B 1  and a corresponding first acoustic axis  10 , and a second driver  4 , which is configured to produce a second frequency band B 2 , which is different from the first frequency band B 1  but may overlap in a cross-over region, and which second frequency band B 2  has a second acoustic axis  11 . The enclosure  2  encloses said drivers  3 ,  4  and comprises a three dimensional waveguide  8  positioned on a front surface of the enclosure  2  and around the first driver  3 . 
     As described above the second acoustic axis  11  of individual woofer drivers are non-coaxial with the first acoustic axis  10 , however the resultant axis of the multiple symmetrical woofers working together (equivalent woofer driver) has the same acoustic axis as the coaxial driver, waveguide driver  3 . This symmetry is however not required in all embodiments of the invention. The axes  10  and  11  may be parallel or non-parallel. 
     Referring to  FIGS. 2 and 3  the woofer  4  is positioned inside the enclosure  2  such that a sub volume  22  is formed in front of the woofer  4  and limited by the woofer  4  itself and side walls  23 . The resonator  40  is acoustically connected to the sub volume  22 . A suitable suppressing material  41  may be used inside the resonator  40  in order to further attenuate the unwanted frequencies. 
     The side walls  33  of the sub volume (front space)  22  form a spacer between the driver  4  and the enclosure  2  sealing the sub volume  22  from the rest of the inner volume  27  of the enclosure  2 . In more detail the inner volume  27  is limited by the enclosure  2  walls, namely front portion  15 , side portions  21  and back portion  25 . 
     Typically the first ports  20  are directed substantially orthogonally in relation to first  10  and second  11  axes, most preferably in the range of 60-120 degrees in relation to these axes. However when the first ports  20  are conducted to the back portion  25  of the enclosure  2 , e.g. by channels, the difference between the direction of the first ports  20  and the axes  10  and  11  may be even 180 degrees. 
     The total area of the first ports  20  is the critical feature, therefore the first ports  20  may be only one single first port  20  for each woofer  4  as presented in the figures or may be formed of multiple first ports  20  like a grid with an area corresponding one single port. 
     The first ports  20  should not disturb the three dimensional waveguide surface  8 , and therefore they are advantageously positioned on the side portions  21  of the enclosure  2 . Of course these first ports  20  may be conducted to the back portion  25  of the enclosure  2  by suitable tubes or channels (not shown). In other words the first ports  20  form air passages to areas outside the three dimensional waveguide  8  of the front portion  15  of the enclosure  2 . 
     The graph of  FIG. 4  shows frequency response of the sub volume  22  of the woofer  4  (solid line) with one resonance at f 0  and corresponding frequency response of a resonator  40  acoustically connected to the sub volume  22  (dashed line), while the resonator  40  compensates for the unwanted resonance of the sub volume  22 . 
       FIG. 5  shows an alternative embodiment with two resistive resonators ( 40 ) with different lengths for two unwanted frequencies of the sub-volume. Also one or two resistive broad band resonator may be used, advantageously filled with suppressive material. In this case the mechanical dimensions (length, width and depth) of the resonator cavity define the tuning frequency or frequencies of the resonator. 
       FIG. 6  shows an alternative embodiment with one reactive Helmholtz resonator  40 . In general reactive resonators have high quality factor and they are very effective narrow band resonators. Also these type of resonators can be installed several in one sub volume  22  if there are several sharp unwanted resonances. This type of resonator is also tuned to the unwanted frequency or frequencies f 0 . The dimensioning of the Helmholtz resonator is explained in the following: 
     The resonance arises from the effect of the acoustic air mass neck of the resonator  40  and the series resonance circuit created by the acoustic compliance of the air volume of the chamber of the resonator. Close to the resonance frequency, the Helmholtz resonator attenuates the unwanted resonance of sub volume  22 . The neck-cavity system of the resonator  40 , can be derived from the air volume of the cavity of the resonator and the diameter of the neck and its length. 
               f   0     =       c     2   ⁢   π       ⁢       A   LV               
in which f 0  is the resonance frequency, c is the speed of sound, A is the cross-sectional area of the neck, L is the length of the neck, and V is the volume of the chamber.
 
       FIG. 7  shows an alternative embodiment with one reactive panel resonator as a resonator. This embodiment is dimensioned in the following way based on the panel  50  mass per unit and cavity depth d: 
     Panel resonator/membrane absorber resonant frequency f is defined in the following way:
 
 f= 60√{square root over ( md )}
 
where m
 
m=acoustic mass per unit area of panel  50  (kg/m 2 )
 
d=cavity depth
 
     Stiffness of the membrane fixing is assumed to be negligible 
       FIG. 8 a    shows as a top view a woofer  4  having a planar cover  47  and short tubes  48  forming as well a Helmholtz resonator where the tubes are the necks and the volume between the cover and the woofer cone forms the volume of the resonator. In  FIG. 8 b    this solution is presented as a A-A cross section. The tuning principle is the same as in  FIGS. 5 and 6 . 
       FIG. 9  shows another alternative solution, where the resonator  40  is formed between the frontal baffle portion and  15  and the sub volume  22  of the woofer. The resonator may be either resistive type without any neck portion or reactive type if the opening to the sub volume  22  is made as a tube. The tuning principle is the same as in previous figures. 
     Typically the loudspeaker in accordance with the invention functions in accordance with well-known bass reflex principle, where the low frequency driver  4  is tuned in resonance with help of the compliance of the air volume contained inside the enclosure  27  and the air volume contained inside the reflex port  34  of  FIG. 2 . 
     One embodiment of the invention ( FIGS. 10-11 ) can be also described in the following way: 
     The loudspeaker  1  comprises an enclosure  2  defining an inner volume  27  and including a frontal baffle portion  15  (front portion), which has a front port  5  for providing a fluid passageway between the inner volume  27  and the ambient volume  26  of the enclosure  2  and a side portion  21  extending rearward from the periphery of the baffle portion  15 . The side portion  21  forms side walls or the enclosure  2 . The enclosure further includes a back portion  25 , which is typically essentially parallel with the frontal baffle portion  15  and forming the back side of the enclosure  2 . The loudspeaker  1  further comprises a driver  4  attached to the enclosure  2 , such that the driver  4  is arranged at a distance from the baffle portion  15 , forming a sub volume  22  inside the enclosure  2  such that a sub volume  22  is formed between the driver  4  and the baffle portion  15  by a spacer  33 , wherein said front port  5  acts as a front port between the sub volume  22  and the ambient volume  28  of the enclosure  2 . In accordance with this embodiment a first port  20  is formed to the enclosure  2  either in the side portion  21  or back portion  25  in order to connect the sub volume  22  and the ambient volume  26  with each other. 
     In accordance with  FIG. 10  one embodiment of the invention two woofer drivers  4  are positioned on both sides of the waveguide driver  3  inside the enclosure  2  and suitable ports (openings)  5  are formed for the woofers  4  in order to let the acoustic energy out from the enclosure  2 . 
     With reference to  FIG. 11 , the openings  5  are covered with an acoustically transparent layer  6  forming part of the waveguide surface  8 . If needed the acoustically transparent layer  6  may be supported from below with support bars  7 . The woofer driver  4  is typically spaced from the acoustically transparent layer  6 . 
     Referring to  FIG. 10  the two woofers  4  form an equivalent large woofer radiating essentially along the same acoustic axis  10  as the waveguide driver  3  even though the woofers have their own acoustic axis  11 . 
     In other words the loudspeaker  1  includes a first driver  3 , which is configured to produce a first frequency band B 1  and a corresponding first acoustic axis  10 , and a second driver  4 , which is configured to produce a second frequency band B 2 , which is different from the first frequency band B 1  but may overlap in a cross-over region, and which second frequency band B 2  has a second acoustic axis  11 . The enclosure  2  encloses said drivers  3 ,  4  and comprises a three dimensional waveguide  8  positioned on a front surface of the enclosure  2  and around the first driver  3 . The three dimensional waveguide  8  comprises an acoustically selectively transparent portion  6  which is acoustically essentially reflecting to sound waves of the first frequency band B 1  propagating in a direction angled to the first acoustic axis  10 , the waveguide portion  6  is essentially transparent to sound waves of the second frequency band B 2  propagating in the direction of the second acoustic axis through the waveguide portion  6 , and the second driver  4  is positioned inside the enclosure  2  behind the acoustically selectively transparent portion  6 . 
     As described above the second acoustic axis  11  of individual woofer drivers are non-coaxial with the first acoustic axis  10 , however the resultant axis of the multiple woofers working together (equivalent woofer driver) has the same acoustic axis as the coaxial driver, waveguide driver  3 . This symmetry is however not required in all embodiments of the invention. The axes  10  and  11  may be parallel or non-parallel. 
     Referring to  FIGS. 10 and 11  the woofer  4  is positioned inside the enclosure  2  such that a sub volume  22  is formed in front of the woofer  4  and limited by the woofer  4  itself, side walls  23  and the acoustically selectively transparent layer  6 . To the sub volume  22  is connected a resonator  40 , which is tuned to unwanted frequencies created by the sub volume  22 . The resonator  40  may be either resistive or reactive. With resistive resonator the suppressive characteristics are of broad band type. In other words the notch around the center frequency f 0  created by resistive resonator is not so sharp like in the reactive resonators. The side walls  33  of the sub volume (front space)  22  form a spacer between the driver  4  and the enclosure  2  sealing the sub volume  22  from the rest of the inner volume  27  of the enclosure  2 . In more detail the inner volume  27  is limited by the enclosure  2  walls, namely front portion  15 , side portions  21  and back portion  25 . 
     In some embodiments of the invention the acoustically selectively transparent layer  6  may be replaced by a mechanically protective grid, the grid limiting in this case the sub volume, as well as the inner volume  27 . Advantageously the first ports  20  are formed in the side walls  23  of the sub volume  22  and to the side portions  21  of the enclosure  2  in order to optimize the operation of the woofer  4 . Without these first ports  20  the performance of the woofer  4  may be compromised. The first ports  20  may be positioned on any of the side portions  21 , e.g. on the short side portions  21  as shown in the figures or alternatively to the long side portions  21 . 
     Typically the first ports  20  are directed substantially orthogonally in relation to first  10  and second  11  axes, most preferably in the range of 60-120 degrees in relation to these axes. However when the first ports  20  are conducted to the back portion  25  of the enclosure  2 , e.g. by channels, the difference between the direction of the first ports  20  and the axes  10  and  11  may be even 180 degrees. 
     The area of these first ports  20  is typically 5-50% of the area of the openings  5  for the woofer  4 , most advantageously in the range of 10-20% of the area of the openings  5  for the woofer  4 . The total area of the first ports  20  is the critical feature, therefore the first ports  20  may be only one single first port  20  for each woofer  4  as presented in the figures or may be formed of multiple first ports  20  like a grid with an area corresponding one single port. 
     The first ports  20  should not disturb the three dimensional waveguide surface  8 , and therefore they are advantageously positioned on the side portions  21  of the enclosure  2 . Of course these first ports  20  may be conducted to the back portion  25  of the enclosure  2  by suitable tubes or channels (not shown). In other words the first ports  20  form air passages to areas outside the three dimensional waveguide  8  of the front portion  15  of the enclosure  2 . 
     Typically the second driver  4  is positioned inside the enclosure  2  behind the acoustically selectively transparent portion  6  and spaced from it, such that a sub volume  22  is formed inside the enclosure  2  and separated from the inner volume  27  by the driver  4  and side walls  23  formed as a spacer between the driver  4  and the front portion  15  of the enclosure  2 . 
     In connection with the acoustically selectively transparent layer  6  essentially reflecting means reflection or absorption of at least 50-100% of the acoustic energy, preferably in the range of 80-100%. 
     In the same way essentially transparent means transparency of at least 50-100% of the acoustic energy preferably in the range of 80-100%. 
     In the following additional advantageous properties of the acoustically selectively transparent layer  6  are presented: 
     The thickness of the layer  6  is advantageously:
         felt, about 1 . . . 5 mm thick   open cell plastic foam, about 1-20 mm thick, pore diameter less than 1 mm   thin fabrics as such or as a part of the layer  6         

     The layer  6  should attenuate the acoustical radiation of the waveguide driver  3 , meaning typically in frequencies above 600 Hz. 
     In other words the layer  6  should have an acoustical impedance (or absorption) as a function of frequency therefore functioning as an acoustical filter in the following way:
         lowpass when the sound from woofer driver  4  is going through   attenuation (e.g. caused by turbulence or absorption with high losses) for high frequencies from waveguide driver  3  causing strong reflection of the acoustic waves at mid and high frequencies   high reflectance for high frequencies of the driver  3         

     Advantageously the layer  6  is formed of holes or pores or their combination in the following way:
         if single layer  6  is used holes should have smaller diameter than 1 mm   if multiple layers  6  are used holes with diameter smaller than 1 mm, may work   also, if multiple layers  6  are used holes with diameter larger than 1 mm, may work (not tested yet)   microstructure like felt and open celled plastic work       

     The properties for the ideal material for layer  6  are the following:
         gas permeable (=porous)   low acoustical losses up to the crossover frequency C (woofer  4 )   high acoustical reflectance slightly above the crossover frequency c   known materials fulfilling the above criteria:
           felt, about 1 . . . 5 mm thick   open cell_plastic foam, about 1-20 mm thick, pore diameter less than 1 mm   
               

     The layer  6  may cover the loudspeaker front (tweeter  12  excluded) or only the holes  5 . 
     The layer  6  may be also formed as a metal structure, like mesh or grid with on one or several layers in accordance with the above requirements for porosity and frequency properties. This kind of structure could be formed e.g. by a stack of perforated metal sheets or plates of thickness around 0.2-2 mm. The properties of this kind of stack could be adjusted by placement (distribution) of the holes or pores, percentage (openness) of the holes or pores, and the spacing of the plates from each other. The hole or aperture diameter may vary typically around 0.3-3 mm. The spacing between the sheets or plates is typically around 0.2-2 mm. 
     A metal structure described above is advantageous, because its propertied can be adjusted freely and the external properties like colour can be as well selected without limitations. 
     The crossover frequency C is typically the following:
         low frequency f&lt;600 Hz (woofer output range)   high frequency f&gt;600 Hz (midrange and/or tweeter output range)       

     In accordance with the invention in combination with the large waveguide  8 :
         woofer  4  is placed behind the waveguide surface  8     two or more (e.g.  4 ) woofers  4  can be used in order to obtain directivity, woofers may be positioned symmetrically in relation to the coaxial driver       

     Also an embodiment with only one woofer is possible, however directivity for low frequencies will not be obtained beyond what is provided by the size of the air displacing surface of the woofer in combination with the size of the front baffle of the loudspeaker enclosure. 
     In alternative embodiments of the invention the selectively transparent portion  6  may be replaced by a mechanically protective grid not having complete properties of selective transparency. 
     In accordance with  FIG. 12  the resonator may be divided into multiple independent sub resonators  40 ′, each having its own resonance frequency. 
       FIG. 13  shows the typical positioning of the loudspeakers  1  in accordance with the invention, where the loudspeakers are directed to the listening position, sweet spot  9 . Due to the fact that the complete front surface of the enclosure  2  is formed as a waveguide  8 , a very good directivity is achieved. Additionally the waveguide form  8  causes a uniform distribution of all frequencies to all directions in the listening room and therefore the reflections from the walls, ceiling and floor cause no coloration of the sound.  FIG. 13  indicates also the front portion  15 , side portions  21  and back portion  25  of the loudspeaker  1  enclosure  2 . 
     In  FIG. 14  is presented a loudspeaker in which suppressive material  41  is positioned in the resonator cavity  40 . Only the upper cavities  40  in the figure are filled with the material but in reality both upper and lower cavities  40  will be filled with suppressive material.