Patent Publication Number: US-4322578-A

Title: Method and devices for the omnidirectional radiation of sound waves

Description:
BACKGROUND ART 
     One of the major drawbacks of conventional high fidelity sound-diffusers, which consist of several frontally oriented loudspeakers, is that high frequencies are radiated in a directional way, i.e. with a reduced angle of emission. This is because sound radiation remains spherical, and therefore omnidirectional, only as long as the radiated wave-length is much greater than the diameter of the sound source, which is generally only true for low frequencies. For example, the wave length of a typical low frequency sound wave is 3.44 m, which is significantly greater than a typical low frequency loudspeaker diameter of 30 cm. Consequently, low frequency sound exhibits a wide emission angle. 
     However, for increasingly higher frequency sound waves, the wave length decreases, and eventually becomes smaller than the diameter of the source. At this point, nearly the entire wave energy is radiated directionally along the speaker axis. Consequently, the source emits plane waves exhibiting a narrow emission angle. 
     Consequently, conventional diffusers of this kind, though provided with high-quality parts, have many drawbacks. For example, the response of high frequencies varies considerably throughout the room, and may be too high along the axis giving rise to a squeaky and tiring sound, and considerably attenuated in lateral positions, e.g. 30 degrees off axis. Consequently, the timbres of instruments tend to be distorted, since it is well known that timbres are distinguished by the harmonics of the highest frequency. Another drawback is that the positioning of the diffuser in a common domestic room becomes extremely difficult. Thus, if a highly reflective wall or other object is in the axial path of the diffuser, chain reflections and undesirable echoes occur. If, on the other hand, an absorbing surface is in the axial path of the diffuser, high frequencies are completely absorbed, and the resulting sound tends to be too deep. Another drawback occurs in stereophonic listening, where, in addition to the difficulty of accurately positioning two diffusers, the listening area in which the two stereophonic messages can be heard in their full range of frequencies is rather limited. Out of this narrow area, resulting from the superpositioning of two narrow angles of high frequency radiation, the timbre range of the stereophonic messages is distorted, in that certain positions exhibit an exaggerated stressing of the high-pitched tones of one channel and a damping of the tones of the other channel. Another drawback results from the nature of high fidelity sound reproduction. Thus, reproducing sound in high fidelity means, as far as possible, recreating the atmosphere of the concert hall, where only a small part of the sound reaches the listener directly, the majority being reflected sound. This fact explains and confirms psychoacoustic research which indicates that the human ear tolerates very high levels of reflected sound pressure, and finds lower sound levels unpleasant when sound hits the ear directly. 
     DISCLOSURE OF THE INVENTION 
     One object of this invention is to provide a method for radiating acoustic waves, free of the drawbacks of conventional methods, especially those mentioned above, and which results in a truly uniform omnidirectional radiation. Another object of the invention is to provide a system which, by a proper choice of dimensions and the use of means completely extraneous to the conventional technology, provides an increased selectivity in the omnidirectional radiation of high frequencies, and increased separation of frequencies in the medium-high frequency range. A further object of this invention is the provision of simple, effective and reliable devices for implementing the method of the invention. 
     The method of the present invention for obtaining uniform radiation of acoustic waves in the frequency band from 20 to 28,000 Hz as emitted by conventional speakers, e.g. woofer, mid-range and/or tweeter, and particularly of those at high frequencies, and for increasing selectivity and separation of medium to high frequency waves, is characterized by the fact that: 
     (i) the acoustic waves in the lower part of the acoustic band, for instance those emitted by a woofer, undergo omnidirectional reflection only; 
     (ii) a very small part (15% average) of waves having frequencies immediately above the upper limit of the aforementioned lower band, particularly waves emitted, for instance, by a mid-range and/or a tweeter, also undergo only omnidirectional reflection; and 
     (iii) the remainder of the aforementioned acoustic waves having frequencies above the upper limit of the woofer, i.e. those emitted by a mid-range and/or a tweeter, undergo a series of diffractions and reflections, the diffractions occurring as the waves pass through successively smaller transparent zones in a series of reflectors. In one preferred embodiment of the invention, the sound sources emitting frequencies above the upper limit of the woofer are arranged on a surface to emit sound in a direction orthogonal to the surface, towards a reflection-diffraction system. 
     The preferred reflection-diffraction system comprises a set of reflectors having surfaces parallel to each other and to the surface on which the sound sources are arranged. Of the n reflectors, n-1 have zones transparent to predetermined bands of the aforementioned sound waves, the zones being dimensioned to act as punctiform sources of spheric sound waves. The nth reflector is without a transparent zone. 
     It is recommended to construct the reflection-diffraction system with a set of superimposed panels of highly waterproof reflecting material. The transparent zones are preferably defined by holes in the panels in line with the axis of the sound sources, or lying within the emission cone of these sources, the diameters of the zones being equal to preselected wave lengths corresponding to a particular frequency in the incident sound waves. According to the invention, the distances among panels and the diameters of the holes decrease as the distance from the sound sources increases. 
     In an alternative preferred embodiment, the sound waves in the lowest frequency band, i.e. those emitted by a woofer, undergo omnidirectional reflection via a pyramid-shaped reflection system having its axis aligned with the axis of the woofer, and having its apex penetrating the cone of the woofer. Preferably, the woofer emits waves in a direction opposite to that of the higher frequency sound sources. For example, the medium to high frequency sources may be arranged on one wall of a housing, with the low frequency sources arranged on the opposite wall. Generally, the pyramid-shaped reflector and the panels comprising the reflector-diffraction system, as well as the housing for the sound sources, are made of polymers, or copolymers of acrylic monomers, particularly of alkylic, or of compositions mainly based on the above-mentioned acrylic homocopolymers. 
     The various features and advantages of the invention will be more clearly understood from the following detailed description and annexed drawings of the preferred forms thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a partial vertical section illustrating one embodiment of a device in accordance with the present invention; 
     FIG. 2 is a vertical section of an alternative embodiment of a device in accordance with the present invention; 
     FIG. 3 is a vertical section of a further alternative embodiment in accordance with the present invention; 
     FIG. 4 is a vertical section of a still further embodiment in accordance with the present invention; 
     FIG. 5 is a vertical section of yet another embodiment in accordance with the present invention; and 
     FIG. 6 is a top plan view of the reflectors and upper surface of the housing of the embodiment illustrated in FIG. 5. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     To simplify the explanation of the principles of this invention, the medium to high frequency section is treated separately from the low frequency section. 
     Medium/High Frequency Section 
     In FIG. 1, A is a housing having an upper surface A&#39;, in the center of which is a source S of medium to high-frequency sound waves. S may comprise, for example, a tweeter having a cupola S&#39;, which is fed, as by an electric filter, with signals having a frequency of 4000 to 20,000 Hz, and therefore emits sound waves ranging from 4000 to 20,000 Hz. According to the invention, a reflection-diffraction system SRD is placed over the cupola S&#39;. The reflection-diffraction system SRD comprises a plurality of reflectors R, of which n-1 reflectors, i.e. the reflectors from R to R n-1 , have on a part of their respective surfaces, one or more concentrated zones ZCT which are transparent to sound waves emitted by the source S. The surface of the nth reflector, i.e. the reflector R n , does not have a transparent zone. 
     As used herein, the term &#34;reflector&#34; comprises any means of suitable shape, dimensions and material such that one surface thereof is impervious to and substantially completely reflects incident waves, except for losses due to friction. As used herein, the term &#34;concentrated transparent zones&#34; means small parts of the above-mentioned reflectors from R 1  to R n-1 , where sound waves are transmitted and not reflected. 
     In FIG. 1, the reflection-diffraction system SRD according to the invention is made up of a set of 4 reflectors (n=4) R B , R C , R D  and R E , the first three of which (n-1=3), that is to say R B , R C  and R D , have concentrated transparent zones, ZCT 1 , ZCT 2  and ZCT 3 , while the last reflector R E  is totally reflecting, i.e. has no zones of transparency. According to the invention, the above-mentioned concentrated transparent zones ZCT 1 , ZCT 2  and ZCT 3  are arranged along the axis X of source S and comprise circular holes, T B , T C  and T D  respectively, having diameters corresponding to the wave length of at least one of the frequencies of the incident waves from the source S. With this arrangement, the result is the following: 
     (a) A smaller part of the sound waves emitted by S and designated in FIG. 1 by the reference Oa is directly reflected by R B  and spread in all directions. For instance, if in FIG. 1 S emits sound in the wave band from 4 to 20 KHz, and the hole T B  of the first reflector R B  has a diameter of 4.3 cm., equal to a wave length λ=4.3 and therefore to a frequency of 8000 Hz, the sound waves having frequencies from 4000 to 8000 Hz will be substantially reflected by R B  and ultimately pass out of the cavity between A&#39; and R B , which is open laterally. 
     (b) In the zone ZCT 1  the portion of the wave having a frequency of 8000 Hz, as well as those portions having a frequency near 8000 Hz, give rise to diffractions, and the center of the hole ZCT 1  becomes a punctiform source SP of a spheric sound wave having the same frequency as the incident wave. This latter new wave undergoes reflections between the reflectors R B  and R C  and is ultimately propagated in all directions towards the outside, as shown by O b , spreading at 360 degrees through the open slit of the cavity between the panels R B  and R C . 
     (c) Sound waves with a frequency higher than 8000 Hz carry on their way through the first hole T B , reaching the second hole T C  representing the concentrated transparent zone ZCT 2 . If the diameter of the hole T C  is 3.2 cm., corresponding to a frequency of 10,700 Hz, frequencies from 8000 Hz to about 10,700 Hz are reflected by R C  and spread at 360 degrees by the cavity between the first and second panels R B  and R C  which, as noted, is open on all sides. 
     (d) The same is true for the transparent zone ZCT 3 , the hole of which has a diameter equal to 2.5 cm., corresponding to 13,750 Hz. Consequently, ZCT 3  becomes a punctiform source and all waves from 13,750 to 20,000 Hz are reflected and omnidirectionally propagated by the open-sided cavity between the reflector-diffractor R D  and the reflector R E  which, being the last of the set, has no transparent zones. 
     By employing a reflection-diffraction system, arranged, according to the invention, in such a way that the transparent zones of the individual panels are on the axis of emission of the source S, the surprising result occurs that medium and high frequencies are propagated omnidirectionally, and the musical message propagated to the outside, after passing through the reflection-diffraction system, is free from distortion due to intermodulation, owing to the effect of the diffractions occurring in the concentrated transparent zones. Furthermore, the resultant separation of the instruments produces an effect of musical presence and realness which is not likely to occur with conventional diffusers. 
     Low-Frequency Section 
     The low-frequency section is illustrated with reference to FIG. 2. FIG. 2 shows a complete device 5 for carrying out the method invented, including both a medium-high frequency section according to the principles described above, and a low-frequency section. The latter is represented by a woofer, whose cone W&#39;, having, for example, a diameter equal to 186 mm, opens into the hollow space 7--7&#39; in the lower wall A&#34; of the housing A. As shown, wall A&#34; is parallel and opposite to the wall A&#39; where the sources of medium to high frequency sound are arranged. The waves emitted by the woofer are incident on a pyramid-shaped reflector RP whose axis is aligned with the axis of the woofer. The apex V of the reflector RP extends into the front W&#34; of the woofer, while the base BB of reflector RP rests on a reflector R&#39;. The reflector R&#39; is without zones of transparency, and is therefore similar to the reflector R E  in FIG. 1. 
     The sound waves emitted by the woofer are reflected and omnidirectionally propagated by the reflector RP, passing out of the open-sided cavity between the confronting surfaces of A&#34; and R&#39;. The standards or uprights 9 which support the box A, the reflector-diffractors R B  and R C , and the last reflector R E , do not take up much room, and thus do not significantly interfere with the desired omnidirectional radiation. 
     In FIG. 2, the medium to high-frequency section is made up of two sources, i.e. a tweeter TW and a midrange MD, both arranged on the wall A&#39; of housing A. The reflector-diffractors R B  and R C  have two concentrated zones of transparency each, represented by the holes T B  and T&#39; B  in R B  and by T C  and T&#39; C  in R C . The holes T B  and T C  are aligned with the cupola S&#39; of the tweeter TW, while the holes T&#39; B  and T&#39; C  are aligned with the axis of the cone C&#39; of the midrange MD. 
     In a preferred form of the diffuser illustrated in FIG. 2, the reflectors R&#39; and R E , the reflector-diffractors R B  and R C , and the walls A&#39; and A&#34; of the housing A are all square shaped, being 330 mm on a side. The tweeter TW has a cupola S&#39; having a diameter of 80 mm, T B  and T C  have diameters of 43 mm and 25 mm, respectively, the midrange MD has a diameter of 90 mm, and the diameters of T&#39; B  and T&#39; C  are 60 mm and 43 mm, respectively. The distance between R&#39; and A&#34;, which corresponds to the height of the lower uprights 9 and roughly to the height of the pyramid RP, is 90 mm, while the height of the housing A is 200 mm, so the volume of A is 330×330×200 mm. One of the delicate factors according to the invention is the distance among the reflectors. By trial and error, optimal values were found. They are: a distance of 60 mm between A&#39; and R B , 50 mm between R B  and R C , and 32 mm between R C  and R E . 
     In FIG. 2, very satisfactory results were also obtained by employing square shaped panels R&#39;, R B , R C  and R E , and square shaped walls A&#34; and A&#39;, each being 380×380 mm; a cone-shaped woofer having a diameter of 230 mm; a distance between R&#39; and A&#34; of 90 mm; a housing A 220 mm high; a trumpet-shaped tweeter about 75 mm high with an opening, in a hole in A&#39;, of 50×100 mm; a hole T B   having a diameter of 32 mm and a hole T C  having a diameter of 20 mm; a midrange MD with cupola having a diameter of 125 mm; and a hole T&#39; B  having a diameter of 90 mm and a hole T&#39; C  having a diameter of 60 mm. 
     The distances between A&#39; and R B , between R B  and R C , between R C  and R E  were kept unchanged, i.e. equal to 60 mm, 50 mm and 32 mm respectively. The thickness of the various reflectors and reflector-diffractors is very important too. For example, in the last-mentioned FIG. 2 embodiment, the best results were achieved with the following thicknesses: R&#39;=8 mm; R B  and R C  =3 mm and R E  =5 mm. 
     Good results were also obtained with a device consisting of two loudspeakers only, a tweeter with a cupola having a diameter of 80 mm in A&#39;, and a common cone-shaped woofer having a diameter of 186 mm in A&#34;. In the two-speaker embodiment, R&#39;, A&#34;, A&#39;, R B , R C  and R E  are all square shaped, being 300×300 mm; the height of the box A is 180 mm; the distance between R&#39; and A&#34; is 90 mm; the pyramid RP has a base BB of 120 mm, and a height from 8 to V of 120 mm as well; and the diameters T B  and T C  are 43 mm and 25 mm, respectively. The distances between A&#39;, R B , R C  and R E  remain unchanged, that is to say 60 mm, 50 mm and 32 mm, the thicknesses of R&#39; and R E  are 8 mm and 5 mm, respectively, and the thicknesses of R B  and R C  remain at 3 mm. 
     In FIG. 3, a particularly interesting embodiment is shown. The FIG. 3 embodiment has two housings A and A 1 , and two woofers W and W 1  with coaxial tweeters TW and TW 1 . As shown, W is disposed in the lower wall A&#34; of the housing A, and W 1  is disposed in the upper wall A&#39; 1  of the housing A 1 . A reflector R&#39; is equidistant between A&#34; and A&#39; 1 , and the bases of two pyramidal reflectors R P  and R&#39; P&#39;  are secured to the reflector R&#39; with their respective vertical axes coaxial with W and W 1 . The following are suitable dimensions for the diffuser illustrated in FIG. 3, which I refer to as a boxer type diffuser: walls A&#39;, A&#34;, A&#39; 1 , A&#34; 1 , the reflector R&#39; as well as the lower panel R, which preferably rests on four small wheels 15, are all square shaped, being 480×480 mm; the height of housings A and A 1  is 250 mm; the diameters of woofers W and W 1  are 300 mm; the base as well as the sides of pyramids RP and RP&#39; is 140 mm; the distance between A&#34; and R&#39; and between R&#39; and A&#39; 1  is 110 mm; the distance between A&#34; 1  and R is 50 mm; and the thickness of R&#39; and R is 8 mm. 
     In FIG. 4, the boxer-type diffuser of FIG. 3 is coupled to a high-frequency section of the type illustrated in the upper part of FIG. 2. Thus, in FIG. 4 the high frequency section comprises a tweeter TW 2  and a midrange MD 1  in the wall A&#39; of housing A, and the three reflectors R B , R C  and R E . R B  and R C  have two holes each, T B  and T&#39; B , and T C  and T&#39; C , respectively. The holes T B  and T C  are on the same line as the opening of TW 2 , and the holes T&#39; B  and T&#39; C  are aligned with the cupola of MD 1 . In FIG. 4, the dimensions of the housing A differ from those of A&#39;. Likewise, the dimensions of W also differ from those of W&#39;. In a practical form of realization of this diffuser, all the panels are 480×480 mm (as in FIG. 3); the height of housing A is 200 mm and that of A&#39; is 250 mm; and the diameter of W is 230 mm and that of W&#39; 354 mm. Since the vertices V and V&#39; of RP and RP&#39;, respectively, slightly penetrate the cones of W and W&#39;, the distances between R&#39; and A&#34; and between R&#39; and A&#39; 1  are 90 mm. 
     In FIG. 5 a more sophisticated embodiment of the invention, including, for the high-frequency section, a cupola-shaped midrange MD, disposed between a tweeter TW which receives signals coming from the electric filter FE ranging from 4000 to 20,000 Hz, and a tweeter STW, which receives signals from FE ranging from 8000 to 25,000 Hz. The reflector-diffusers R B , R C , R D  have three holes each, T B , T&#39; B  and T&#34; B  in R B  ; T C , T&#39; C  and T&#34; C  in R C , and so on. The holes T B , T C  and T C  are aligned with the cupola of TW; the holes T&#39; B , T&#39; C  and T&#39; D  are aligned with the cupola of the speaker MD; and the holes T&#34; B , T&#34; C  and T&#34; D  are aligned with the cone of STW. As far as the low-frequency section is concerned, the woofer W receives signals ranging from 20 to 800 Hz from FE and transforms them into sound waves which are directed towards the pyramidal reflector R P . The housing A is supported by the uprights 9 and 10 which are secured to the panel R&#34;, which in turn is supported by four rubber wheels 15. The four upper standards 9&#39; and 10&#39; are secured to the wall A&#39; of the housing A, and support the reflector-diffractors R B , R C  and R D , as well as reflector R E . Preferably, an iron plate 20 secures R E  about a threaded bolt 21 which is secured at its lower end by a screw nut 22. The lower uprights 9 and 10 are preferably held in place by a threaded rod 23 and nut 23&#39;. The upper standard 10&#39; may be coupled, as by a screw 24, with a block 25 made of the same material as the housing A and reflectors R B , R C , R D , R E  and R&#34;. 
     In a preferred form of the diffuser of FIG. 5, the end of speaker STW seated in A&#39; has a rectangular section 50×100 mm and is aligned with holes T&#34; B , T&#34; C  and T&#34; D  having diameters of 32 mm, 25 mm and 20 mm, respectively. MD has a cupola with a diameter of 125 mm and TW has a cupola with a diameter of 80 mm. MD and TW are respectively aligned with holes having the following diameters: T&#39; B  =90 mm, T&#39; C  =60 mm, T&#39; D  =43 mm; T B  =43 mm, T C  =32 mm and T D  =25 mm. All the reflectors, as well as the housing A, are square with sides of 480 mm. The height of the box A is 250 mm. The woofer W has a diameter of 354 mm, while the pyramidal reflector RP consists of an equilateral triangle with sides of 170 mm. The height of the lower uprights 9 and 10 is 110 mm. The distances among the various reflectors are: Between A&#39; and R B  =60 mm; between R B  and R C  =50 mm; between R C  and R D  =50 mm; and between R D  and R E  =32 mm. 
     The iron plate 20 is square with dimensions of 35×35 mm. The thickness of the housing A and lower panel R&#34; is 8 mm; the thickness of R E  is 5 mm; and that of the three reflector-diffractors R B , R C , R D  is 3 mm. Preferably, the housing A is made by joining the horizontal panels A&#39; and A&#34; with the vertical panels A&#34;&#39; by miter joints as shown at 30. 
     In all the embodiments discussed hereinabove, the most suitable material for the planar reflectors, pyramidal reflectors and reflector-diffractors is a polymeric composition based on acrylic homo-copolymers, especially of methyl-methacrylate, plain or in compound with other monomers such as styrene, vinyl chloride, etc. Also, compositions based on the above-mentioned methacrylic polymers are suitable. 
     Among the above-mentioned materials, products marketed under the trademarks &#34;PERSPEX&#34; of I.C.I., &#34;VERDRIL&#34; of MONTEDISON, and &#34;PLEXIGLAS&#34; of ROHM &amp; HAAS, have given excellent results. It will be apparent, however, that other similar materials can be used instead. The physical characteristics of a polymer of methylmethacrylate, 500,000 molecular weight, especially of a &#34;PERSPEX&#34; obtained by total polymerization through melting, are, according to ASTM: 
     Specific gravity: (D792) 1.17-1.20 
     Tensile strength kg/sq.cm.: (D638) 560-770 
     Compressive strength kg/sq.cm.: (D695) 770-1330 
     Impact strength: (D256) 2.1-2.7 
     Rockwell hardness: (D785) M80-M100 
     Thermal conductivity: (C177) 0.5-0.7 
     Dielectric strength: (D149) 450-550 
     Dielectric constant: (D150) 3-3.5 (100 cycles) 
     Dielectric factor: (D150) 0.04-0.06 (100 cycles) 
     It will be apparent that other synthetic crystalline materials, having rigidity (i.e. absence of its own resonances), indexes or reflection and absorption, dielectric constant, etc., comparable to those of polymethylmethacrylate, may also be used. The supporting uprights are preferably comprised of aluminum. A further advantage of the diffusers according to the invention is that they are suited for use with conventional loudspeakers having common transducers and exponential cone, cup or funnel shapes. 
     The panels are preferably parallelepipeds with square or rectangular surfaces ranging from 10 cm to 100 cm., preferably from 14 cm to 80 cm, and with a thickness of from 1.5 mm to 15 mm, and preferably from 2 mm to 9 mm. The diameters of the holes in the reflector-diffractors range from 5 mm to 125 mm and preferably from 10 mm to 100 mm. The distances among panels, and between the surface of the first housing and the first panel, may vary from 90 mm to 15 mm, and preferably from 80 mm to 20 mm, with the distances between panels decreasing as the distance from the sound sources increases. 
     The five top plan views of FIG. 6 illustrate the arrangement of the speakers STW, MD and TW on A&#34;, the disposition of holes T&#34; B , T&#39; B  and T B  in R B , of holes T&#34; C , T&#39; C , T C  in R C  ; of holes T&#34; D , T&#39; D  and T D  in R D  ; and of the plate 20 on R E . It was surprisingly found that the best results are obtained when the centers of the loudspeakers on A&#39; and the centers of the holes in the panels R B , R C  and R D  are aligned along a diagonal. It was found, furthermore, that it is advantageous and more desirable to arrange the midrange in a position behind at least one tweeter relative to the listener. 
     The sound performance of the devices described hereinabove, though depending partly on the quality of the components (e.g. loudspeakers) used, have shown: an excellent response in the frequency range from 30 Hz to 20,000 KHz, linear, within 3 db; a very high efficiency (sound pressure) with 1 W input: 92-94 db, 1 m. off.; very low harmonic and intermodulation distortion (remarkably lower than that obtained by placing the loudspeakers in the conventional arrangement); very high dynamics (capability to pass instantaneously from low to very high levels of sound pressure); capacity to reproduce sound without distortions and without listening trouble for very high sound levels (112-115 db at 4 m. off with 100 W applied power.); quick and prompt response to transient impulses due to short rise times; exceptional selectivity and a strong presence for large orchestras and choirs; excellent response to impulsive basses (i.e. percussions) due to the immediate damping of the oscillations of the woofer by the coupled pyramid, which besides radiating sound at 360 degrees, avoids the return of the emitted waves which could give rise to interferences; and very pleasant aesthetics due to the very advanced design and to the materials used for the first time in the field of high fidelity, thereby allowing appropriate placement in any domestic room, with classic or modern furniture. This &#34;optimum optimorum&#34; of features is present in all devices according to the invention with input power in the range from 10 to 250 W. 
     A further advantage is achieved by using the embodiment, shown in FIGS. 3 and 4, wherein two housings A and A 1  are employed, and wherein a speaker is fitted in the bottom of the upper housing and another in the top of the lower housing, the two speakers being in confronting relation. The confronting loudspeakers can be of various types (e.g. woofer, midrange or tweeter or combinations thereof as in FIG. 3). In these embodiments, by virtue of the opposition of the two loudspeakers, the sound waves radiated in phase are compressed among themselves and, owing to this reciprocal compression, gain a higher efficiency in db (sound pressure level), a stronger presence, and clearer audibility both for singing and speaking pieces.