the loudspeaker unit shown in fig1 and 2 comprises a cabinet box 2 having a front plate 4 , on which , near the top thereof , is mounted a tweeter 6 . just underneath the tweeter 6 is mounted a woofer 8 , which is a combined driver for the bass and the middle tone frequences , while well below the woofer 8 the front plate 4 is provided with an acoustic port 10 . the rear wall of the cabinet is designated 12 , and in front of this wall is mounted an intermediate partition plate 14 , which , together with the rear wall 12 , confines a closed rear chamber 16 of the cabinet . on the partition plate 14 and just behind the woofer 8 is mounted another , corresponding woofer 18 , which in the example shown is mounted rearwardly radiating and thus forwardly projecting into a chamber 20 common to both drivers 8 and 18 and located between the front plate 4 and the partition plate 14 . the two woofers are mutually rigidly connected through a connector tube 22 , which is arranged between the respective magnet systems 23 of the woofers and is stiff against both axial compression and axial pulling forces . the connection 22 should not necessarily consist of a closed tube , which will occupy a certain volume of the common chamber 20 ; by way of example , one or more thin rods or a cross profiled connector element may be used instead of a closed tube . the woofers 8 and 18 are coupled , geometrically , in counter phase , such that their respective diaphragms will move concurrently towards and away from the common chamber 20 . the front woofer 8 will radiate the sound direct from the front plate 4 and will at the same time , together with the rear woofer 18 , transmit the same sound to the common chamber 20 , though here in counter phase relative the directly radiated sound . however , since the acoustic port 10 is located substantially spaced , e . g . some 50 cm , from the front woofer 8 , the sound as transmitted through this port will be substantially in phase with the directly radiated sound , and test have shown that the total result is a good sound quality or sound picture seeming to originate from a single sound source , despite the said distance between the woofer and the acoustic port . if desired one or more addtional pairs of woofers may be mounted in the cabinet , whereby the loudspeaker unit may operate with very high effects from frequences even up to some 2 - 4 khz and with a very good bass reproduction without any expensive large diameter bass loudspeaker having to be used . advantageously an acoustic damping material may be laid into the rear chamber 16 as well as into one or more partial areas of the chamber 20 , but it is deemed unnecessary to describe this in more detail here . the driver 18 may optionally , be mounted in an inverted position on the partition plate 14 , whereby it will produce an increased amount of sound energy to the chamber 20 . however , it is then correspondingly more difficult to provide the rigid connection 22 between the two drivers . the rear chamber 16 should not necessarily be a regular box space extending over all the height and width of the cabinet , but it will be appreciated that with the illustrated regular design of the chamber 16 the space or volume of an overall simply designed cabinet will be advantageously utilized . the sub unit consisting of the two loudspeakers 8 and 18 and the rigid connection 22 therebetween will be mountable in the cabinet by insertion into the respective mounting holes in the plates 4 and 14 , and the outer rim portions of the chasis of the loudspeakers will hereby be holdable in the holes by intermediary of respective resilient ring members 24 serving to seal the mechanical connection and additionally to absorb any resulting mechanical vibrations of the said sub assembly 8 , 22 , 18 . in practice it has been found that with the system shown and described it is perfectly possible to make use of loudspeakers 8 and 18 adapted for reproduction of both the lowermost and the middle range frequences , whereby it is possible to avoid the use of correspondingly specialized loudspeakers as well as an associated electrical cross - over unit . the rearmost chamber 16 should not necessarily be entirely closed , inasfar as it may have an opening such as a rearwardly directed acoustical port . obviously , the said qualification of the closed chamber will then be reduced , but , some other qualified combination effect of the two chambers may still be possible . in general , experiments and calculations have shown that the double chamber design with the two antiphase loudspeakers offers many advantageous possibilities based on modifications of the several parameters as constituted by the volumes of the two chambers , the type or types of the two loudspeakers , and the location and size of the acoustic port of at least the front plate . an important finding is that the closed rear chamber as accounting for a remarkably good bass reproduction may show this effect for a volume , which is considerably smaller than the volume correspondingly required for a pure or single chamber closed type system . it should be mentioned that it has been found advantageous to use a rear loudspeaker 18 , in which the resilient suspension of the movable system is softer than in the front loudspeaker 8 . principally , in order to obtain the advantages of the double chamber system as far as the sound reproduction is concerned , the two or at least two loudspeakers 8 and 18 should not necessarily be located on a common center axis ; thus , the loudspeaker 18 could be located on the top plate of the chamber 20 cooperating with an upper closed chamber . it will be appreciated , however , that the requirement of the two loudspeakers working together for producing the sound in the chamber 20 automatically results in a mechanical antiphase operation when the loudspeakers are arranged on a common axis , whereby the additional and very important advantage of an overall non - vibrating loudspeaker cabinet is achieved . in fig3 is shown a loudspeaker cabinet 30 , the front plate of which carries a tweeter 32 , an intermediate loudspeaker 34 , and a woofer unit 36 comprising a front radiating woofer 38 and another corresponding woofer 40 , which is arranged back to back with the woofer 38 , with the magnetic systems 42 engaging and secured to each other . when the magnetic systems are uniformly polarized in the joined woofers the polarizations will be opposite adjacent the meeting plane between the woofers , and as already mentioned this will induce an increase of the magnetic flux in the annular air gaps for the voice coils of both of the woofers . the two woofers 38 and 40 are electrically coupled in phase , whereby the back to back arrangement will involve that the respective voice coils and diaphragms will operate in counterphase , such that the unit will not transfer noticeable oscillations to the cabinet . fig4 shows the two woofers arranged acoustically in series . the woofers are special in that the central pole pieces of their magnetic systems are shaped as a tube 44 , such that a rather wide internal air channel 46 is provided between the diaphragm areas of the woofers . the diaphragms 47 of the front woofer is entirely closed , the front end of the associated voice coil cylinder being closed by a central diaphragm portion 48 , while the voice coil cylinder inside the diaphragm is provided with an annular row of holes 50 open towards an outer space 52 behind the diaphragm 47 . the woofer chassis , designated 54 , constitutes a closed cone , such that the space 52 is a closed space . the rear woofer 40 is provided with a rigid , rearmost wall member 56 , which is circumferentially sealed to the chassis 58 of this woofer , such that behind the diaphragm 60 thereof a space or chamber 62 is confined . in this woofer the voice coil cylinder is not covered by any diaphragm portion , i . e . the otherwise closed chamber 62 is in open connection with the air channel 46 . the outer diameter of the diaphragm 60 is slightly larger than the outer diameter of the diaphragm of the front woofer 38 , whereby the lacking diaphragm portion across the voice coil cylinder is compensated for . the rear chassis 58 is perforated through openings 59 in such a manner that the front side of the diaphragm 60 is in open connection with the air inside the cabinet 30 . in operation the opposed diaphragms 47 and 60 will be moved towards and away from each other , and arrows a in fig4 illustrate the associated air flow when the diaphragms are moved towards each other . hereby the air in the foremost chamber 54 as well as behind the central diaphragm portion 48 will be compressed , while at the same time the rear chamber 62 will expand , whereby the air will flow rearwardly in the system , through the central channel 46 . when the diaphragms move inversely a corresponding air flow will be created in the opposite direction , and during the operation the air may thus pulsate from one side to the other without meeting any substantial resistance . it will be appreciated that this corresponds to the operative characteristics of the said known compound systems , in which the foremost diaphragm is relieved from the air spring action on the rear side thereof as well as from the distorting , direct contact with the air inside the cabinet ; by the invention this contact is established to the front side of the rear diaphragm 60 . still , the decisive advantage of the system will be that it does not create considerable external oscillations , and that the tight juxtaposition of the two magnetic systems will account for an increased flux intensity in the annular operation gaps of the voice coils . it is highly advantageous , but not really necessary that the flow connection between the chambers 52 and 62 be established centrally through the magnetic systems of the loudspeakers ; thus , as examplified further below it will be within the scope of the invention to make use of one or more external connections between these chambers . fig5 illustrates a widely similar combination of the two woofers , though here with the woofers interconnected in parallel , acoustically . the central portion 48 ( fig4 ) of the front woofer is here removed , such that the central channel 46 is forwardly open . the chassis 54 of the front woofer is perforated behind its diaphragm 47 , as shown by holes 55 , such that the rear side of this diaphragm is in open connection with the air in the cabinet 30 . the rear woofer is designed principally as shown in fig4 though here with the rigid rear wall shaped as a planar member , without the cone shaped depression as shown in fig4 ; with this shape of the wall in fig4 it is intended to adapt the volume of the chamber 62 to the volume of the front chamber 52 , which is irrelevant in connection with fig5 . when the foremost diaphragm 47 in fig5 is moved forwardly so as to compress the front air the rear diaphragm will be moved rearwardly , which will result in a compression of the air in the chamber 62 . hereby the air will flow out forwardly through the passage 46 so as to be front radiated in phase with the air as pushed forwardly by the foremost diaphragm 47 . correspondingly , air will be drawn rearwardly through the passage 46 when the rear diaphragm is moved forwardly , while also the front diaphragm 47 will create a vacuum on the outer air in response to being moved rearwardly . the central passage 46 may well be considered &# 34 ; narrow &# 34 ; compared with the outer diameters of the woofer diaphragms , i . e . the associated sound hole , designated 64 , is relatively small , but since the space or chamber 62 behind the sound hole is of a large diameter the air pulsations in and out through the sound hole will be considerably stronger than corresponding to the pulsations in front of a diaphragm portion of the size of the sound hole , i . e . a remarkable increase of the sound pressure of the front woofer will be achieved . since both of the diaphragms 47 and 60 are in open connection with the interior of the cabinet the volume of the cabinet should be correspondingly large , but the woofer unit according to the invention still presents the advantage that the double woofer on the front side of the cabinet will occupy the mounting space of a single woofer only , just as the unit will additionally be advantageous with respect to suppressed cabinet oscillations and improved efficiency of the magnet systems of the woofers . also the embodiment of fig5 may be modified by renouncing the central air passage 46 and submitting therefor some corresponding external channel connection . thus , the chamber 62 may be connected to a separate acoustic port on the front side of the cabinet 30 through one or more internal channels , and it is hereby even possible to avoid the inevitable resistance effect of the narrow central channel 6 , just as it will be possible to increase the effective area of the front diaphragm 47 by the central portion 48 as in the embodiment according to fig4 . in practice the double woofer unit should not necessarily be constituted by two separate , built - together woofers , as the unit may be designed as a single integrated construction . in such an integrated unit as suggested already in fig5 it may be considered to renounce the said advantage of a close juxtaposition of two separate magnetic systems and instead make use of a single magnetic system having the required two associated annular voice coil gaps , inasfar as the simplicity of such a system might well justify the use of a magnetic system of a strength sufficiently high to create strong flux fields in the annular gaps without the use of any external magnetic influence . in fig6 is shown a double woofer unit of the disclosed type comprising two woofers joined back to back and surrounded by a cylindrical casing 64 having a front mounting flange 66 . the casing 64 is open at both ends and is provided with opposed side openings 68 , through which the space between the interfacing sides of the front and rear woofer diaphragms is connectable with outer space or channel means . this unit is mountable in a cabinet as shown in fig7 and 8 , seen from respective orthogonal angles , such that the lateral openings 68 are open towards the internal of the cabinet , while the space behind the rear diaphragm is flow connected , inside the casing 64 , with the space behind the rear side of the front diaphragm , corresponding to the embodiment of fig4 though now with external connecting channel means in stead of the central passage 46 . of course , the unit shown in fig6 could well be designed in accordance with fig5 so as to present a central sound hole in the front diaphragm 47 . alternatively , the rear side of same unit will be connectable throuth external channel means to an acoustic port of the cabinet . the mounting possibilities of the double woofer unit are further illustrated in fig9 - 18 , which are believed to be self - explanatory . these figures show different constellations of the woofer unit as associated with various wall portions of the loudspeaker cabinet , with the latter having various internal designs , some of which comprises one ore more tubular acoustic ports . in fig1 - 18 the schematically indicated loudspeaker unit may be of a type as shown in either of the fig2 and 3 . in the additional embodiments , the first and second loudspeakers are designed for effective reproduction of both bass and middle tone frequencies , such that the unit constitutes a so - called two - way unit including only a single crossover - network .