GB1589998A

GB1589998A - Multiple driver loudspeaker system

Info

Publication number: GB1589998A
Application number: GB32004/77A
Authority: GB
Original assignee: Bose Corp
Current assignee: Bose Corp
Priority date: 1976-09-02
Filing date: 1977-07-29
Publication date: 1981-05-28
Also published as: AU512097B2; FR2363955A1; SE7708202L; JPS617799B2; JPS5330317A; IT1083817B; DE2760480C2; DE2739523C2; AU2787477A; DE2739523A1; SE415624B; NL7709527A

Description

(54) MULTIPLE DRIVER LOUDSPEAKER SYSTEM (71) We, BOSE CORPORATION of 100 The Mountain Road, Framingham, Massachusetts 01701, United States of America, a Corporation organised under the laws of the State of Massachusetts, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates in general to loudspeaker systems and more particularly concerns a novel loudspeaker system characterized by unusually realistic reproduction of sound that is compact and relatively easy and inexpensive to manufacture in large -quantities while maintaining good quality control and producing relatively high sound levels in response to relatively low input electrical power levels. The present invention achieves the performance level of the internationally famous BOSE 901 DIRECT/REFLECT ING loudspeaker system described in British Patent Specifications Nos. 1253044 and 1253045.

Both that system and a preferred embodiment of this system include eight speakers on a pair of rear panels or baffles that each form an angle of about 30 with the wall upon which the rear speakers direct their sound and one speaker on the front panel or baffle that faces the normal listening area. This arrangement radiates the desired ratio of about 8:1 reflected sound to direct sound while projecting the image of a musical performance located on a stage that is about a foot behind the wall when the enclosure is about a foot in front of the wall so that it is possible to hear the full stereo spread from a wide range of listening positions including directly in front of one enclosure. That system and a preferred embodiment of the present invention also both include an active equalizer for establishing essentially uniform acoustic power radiation as a function of frequency over substantially the entire audio frequency range.

The BOSE 901 loudspeaker system has received an unprecedented series of rave reviews in the United States and many other countries. While that system performs well when driven with power amplifiers of moderate capacity, higher power amplifiers are required to produce high acoustic levels at the lower audio frequencies.

It is known in the prior art to use ported enclosures to obtain higher acoustic power levels at lower frequencies with a given electrical input power. And a simple port works satisfactorily in a conventional woofertweeter loudspeaker system and is used, for example, in the BOSE Model 301 DIRECT /REFLECTING loudspeaker system. However, it was discovered that simply porting the loudspeaker cabinet in the system described in British Patent Specifications Nos. 1253044 and 1253045 did not provide satisfactory performance. It was discovered that in the vicinity of port resonance all the small loudspeakers did not operate in phase with the result that the excursions of the drivers working together increased to compensate for the excursions of the out-ofphase drivers, causing the drivers to enter the nonlinear region of operation at relatively modest sound levels.

Accordingly, it is an important object of the invention to provide an improved loudspeaker system. The invention consists in a loudspeaker system comprising an enclosure having cavity defining means providing a plurality of cavities, with a driver opening for each cavity, the driver openings each accommodating respective like loudspeaker drivers, the cavities being characterised by acoustic compliance, said loudspeaker drivers each being seated in a respective one of said driver openings and connected in phase, the enclosure having an opening accommodating a port tube providing acoustic mass that resonates with said acoustic compliance at a predetermined frequency in the low range of audio frequencies, said cavity defining means being constructed and arranged to provide air channelling means from said loudspeaker drivers into said port tube whereby said port tube is common to all of said cavities, means being provided for preventing the cones of said loudspeaker drivers from exhibiting out-of-phase movement when energized with an electrical signal having spectral components in the low range of audio frequencies embracing and near said resonant frequency, said preventing means comprising the separation of the drivers into individual cavities.

Advantageously, said means for preventing the cones of said loudspeaker drivers from exhibiting out-of-phase movement comprises means for reducing the Q of the resonant system formed by said cavity defining means and the said acoustic mass.

In order to make the invention clearly understood, reference will now be made to the accompanying drawings which are given by way of example and in which: Fig. 1 is a perspective view of a loudspeaker assembly according to the invention showing the three rear-venting port tubes; Fig. 2 is a front view of a loudspeaker assembly according to the invention with the front grille cloth removed to show the mounting of the front driver and the base of the bullets that are cantilevered from the front baffle into the port tubes for the rear drivers; Fig. 3 is a perspective view of an enclosure as seen from the right rear without the top and sides; Fig. 4 is a top view of the assembly with portions cut away and some portions in section to illustrate the internal structure; Fig. 5 is a rear view of a loudspeaker assembly according to the invention with the grille cloth removed; Fig. 6 is a combined block-schematic circuit diagram of one channel of a system according to the invention with a preferred form of active equalizer.

Fig. 7 is a graphical representation of typical responses of the equalizer of Fig. 6 plotted to a common frequency scale; Fig. 8 is a fragmentary view of a baffle portion illustrating structural details of means for accommodating twist-and-lock driver assemblies; and Fig. 9 is a diagrammatic representation which does not form part of the invention but which is given for explaining the operation of means for preventing the cones of the loudspeaker drivers from exhibiting out of phase movement.

With reference now to the drawing and more particularly Fig. 1 thereof, there is shown a perspective view of a loudspeaker assembly according to the invention as seen from the left rear. The assembly includes top, bottom, left and right panels 11, 12, 13 and 14, respectively. Left and right grilles 15 and 16, respectively cover respective angled rear baffles 52 and 33 that each carry four closely-spaced full-range drivers connected in series (Fig. 5) through which left and right port tubes 17 and 18, respectively protrude for venting four cavities behind each respective baffle as described below. Left and right port tubes 17 and 18 surround respective concentric tapered left and right bullets 21 and 22 that comprise means for establishing laminar air flow in the annular region of slightly tapered radial width between the bullet and the surrounding tube.

A central port tube 23 protrudes through a collar 28 defining a port opening in a junction between the rear baffles for venting the cavity behind the driver on the front baffle (FIG. 2).

Referring to Fig. 2, there is shown a front view of a loudspeaker assembly according to the invention with the front grille cloth removed to illustrate certain structural features. The same reference symbols identify corresponding elements throughout the drawing. The front baffle 24 is preferably made of high impact plastics, such as styrene, and formed with a central opening 25 for accommodating front loudspeaker driver 26' and left and right openings 26 and 27, respectively, surrounded by annular rims (not visible in Fig. 2) to which the bases 31 and 32 of left and right bullets 21 and 22 are secured to cantilever bullets 21 and 22 from front baffle 24. Recess 19 is for accommodating the fastener that secures a symbol or logo to the front of the assembly.

Referring to Fig. 3, there is shown a perspective view of an assembly according to the invention without the top and side panels as seen from the right rear illustrating structure defining the respective cavities for the rear drivers. Right rear baffle 33 is formed with four openings 34, 35, 36 and 37, for accommodating respective drivers, each at one end of a respective cavity closed at the other end by front baffle 24. A horizontal partition 41 and portions of port tube 18 separate the cavity behind opening 36 from the cavity behind opening 37, and apertures 42 and 43, respectively, couple these cavities into the entrance of port tube 18 adjacent front baffle 24 over lip portions 44' and 45', respectively, that help establish a smooth flow of air from each cavity into the associated port tube. The area of each of apertures 42 and 43 is large enough to avoid audible noises when reproducing passages in the low bass region while being small enough to prevent acoustical coupling, between drivers. It will be appreciated that the apertures 42 and 43 provide an air channel leading to the port tube 18 and common to the four drivers of baffle 33, a similar arrangement existing for baffle 52.

A suitable cross sectional area for each aperture is 2.25 square inches. The aperture areas were established by pushing the tubes as close to the front baffle as practical without producing undesired audible noises when the drivers were energized with a low frequency signal. Studs 38 are for receiving staples to secure the grille cloth assembly.

Vertical partition 44 and inwardly sloping partition 45 isolate the cavity before opening 36 from the cavity before opening 35. Similarly vertical partition 46 and downwardly and inwardly sloping partition 47 isolate the cavity before opening 37 from the cavity before opening 34. However, the four cavities, although individual to the four drivers, are shared in common by the port tube 18.

It is desired that the cavity behind each driver be of substantially the same volume.

By making panels 45 and 47 slope inwardly, cavity volume is added to the cavities adjacent the sides to compensate for the shorter span between front panel 24 and rear panel 33 for the centre.

Referring to Fig. 4, there is shown a top view of the loudspeaker assembly according to the invention partially in section and with portions at different depths cut away to illustrate features of the invention. The cavity behind front driver 26' is defined by the generally cylindrical member 51 connected at the rear to separate port tube 23 that vents through collar 28 at the junction between the two angled rear panels 33 and 52. A vertical partition 53 which extends above and below cylindrical member 51 and port tube 23 separates the cavities associated with the inner pairs of rear drivers.

The volume of generally cylindrical member 51 is substantially equal to the volume of each of the other eight cavities in the enclosure and coacts with port tube 23 to establish a cavity-port tube resonance of substantially 40 Hz. Each of the other cavities coacts with the associated port tube to establish a cavity-port tube or mass-compliance resonance of substantially 40 Hz.

While the invention may be practiced by fabricating the various partitions and other members as separate pieces, in a preferred form of the invention, the front baffle, the partition 53, the port tube 23 and the generally cylindrical member 51 is a unitary structure formed by injection molding, each rear baffle, the associated port tube and associated partitions is a unitary piece formed by injection molding, and the bullets are unitary pieces formed by injection molding. The preferred material is plastics. A feature of the invention is that only three molds are required, one for the front baffle assembly, a second for the rear baffles and a third for the bullets because bullets 21 and 22 are identical and rear baffles 33 and 52 are identical, collar 28 being formed of two semicircular portions that mate together. The result of this arrangement is high reproducibility at relatively low cost while establishing good acoustic properties.

A feature of the invention resides in having all the cavities vented toward the rear. It has been discovered that venting to the rear where the loudspeaker assembly is closer to the wall results in improved bass response as compared with the conventional approach of venting to the front. There is no problem with the wall obstructing the flow of air from the vents because the preferred position of the loudspeaker assembly is about a foot from an adjacent wall.

Referring to Fig. 5, there is shown a rear view of the loudspeaker assembly with the rear grille cloth removed.

Referring to Fig. 6, there is shown a combined block-schematic circuit diagram of an exemplary embodiment of one channel of an active equalizer connected to a receiver for energizing nine drivers in series according to the invention. For stereo there are two of these channels. Representative parameter values are set forth.

Referring to Fig. 7, there is shown a graphical respresentation as a function of frequency of the response of the active equalizer shown in Fig. 6, for the extreme settings of the mid-bass and treble controls, the middle curve being the normal setting and the effect of moving the below 40 switch to the decreased position. The circuitry includes a number of features. There is a 3-pole sharp cutoff network that effectively sharply cuts off the response below a frequency (e.g.

32Hz) which is at or slightly below the cavity-port tube resonance of about 40 Hz.

Another feature is the compensation for driver rim resonance in the region between 1 and 2 kHz. Still another feature is the provision of the mid-bass control which affects the response between 100 and 300 Hz to accommodate for various listening environments and the treble control which affects the response only above 2 kHz.

The active equalizer couples signal spectral components of substantially the full range of audio frequencies in the inphase connected drivers.

The equalizer should establish a substantially uniform radiated power response of the loudspeaker system as a function of frequency at least in the range of frequencies between the cavity-port tube resonant frequency and the fundamental resonance of the loudspeaker system, and preferably as a function of frequency for substantially the full range of audio frequencies.

Referring to Fig. 8, there is shown a portion of a baffle illustrating structural details preferably molded therein for accommodating twist-and-lock drivers. A baffle includes for each driver an opening 62 inside a depressed annular surface for accommodating a mating rear annular surface on a driver when the driver is mounted in opening 62. The diameter of opening 62 is just large enough to accommodate the portion of the driver basket rearward of the mating rear annular surface. Three equiangularly spaced recesses for accommodating mating tabs of a driver are defined by structures 64 open at the counterclockwise edges for receiving the driver tabs and are formed with notches 65 for mating engagement with corresponding protrusions on the driver to lock the driver in place when twisted fully clockwise. The span of the slit in a direction perpendicular to the baffle is preferably slightly less than the driver tab thickness so that rotating a driver clockwise until a driver protrusion mates with a notch 65 results in each tab being firmly engaged while the outside surface of a lip on the driver basket parallel to the driver axis snugly engages the wall portions 66 extending perpendicularly from the baffle to establish a substantially fluid-tight seal with a driver without gaskets, other soft material such as Mortite or screws to significantly reduce assembly costs while improving reliability.

Having described the physical arrangement of the invention and some important features, it is appropriate to consider certain principles of operation. One aspect of the invention is concerned with reducing cone excursion at a given sound level. An important function of the present invention is to improve the linear motion of the cones and significantly increase the dynamic range over which the loudspeaker system accurately reproduces the bass notes of musical instruments. To this end there are 14 principal regions inside the enclosure; the nine cavities behind the drivers, the three port tubes 17, 18 and 23 venting at the rear and the two mixing regions at the front of port tubes 17 and 18. At times the air in port tubes 17 and 18 moves faster than 60 miles an hour and would produce undesired audible noise in the absence of specific features of the invention that aerodynamically establish laminar flow in the mixing regions and inside tubes 17 and 18. To this end the front ends of tubes 17 and 18 present a curved surface established by the folded-over front lip portion, and bullets 21 and 22 are formed as shown to have outer surfaces approaching the inside surfaces of tubes 17 and 18 at the rear ends, the front portion curved outwardly as shown to coact with the curved lip at tubes 17 and 18 to provide a smooth transition region into the port tubes and a gradually tapered tip at the rear outside tubes 17 and 18. Gradually reducing the cross sectional area of the port tubes helps keep the air flow laminar and causes the port tubes to function as a low pass filter which helps confine high frequency noise inside the enclosure. There-after, the air stream diverges. It is preferable to can tilever the bullets 21 and 22 as shown without introducing supports in the region between bullet and tube because the supports might tend to distort the laminar flow and thereby introduce undesired audible effects.

The air confined in each port tube 18 may be regarded as an acoustic mass in series with the air in each cavity which may be regarded as an acoustic compliance in parallel with three other acoustic compliances each resonated by four times the effective acoustic mass of the associated port tube to establish a cavity-port tube resonance at substantially 40 Hz, the same frequency at which the cavity defined by cylindrical member 51 and port tube 23 are resonant. At relatively few Hertz below the fundamental resonance, typically below 32 Hz, the active equalizer sharply curtails the electrical power to the drivers because applying increasing levels of electrical power to the drivers at these frequencies would produce additional deflection of the cone that might well extend into the nonlinear region without providing appreciable audible acoustic power. Accordingly, a feature of the invention is to arrange the active equalizer so that there is a sharp decrease in response as a function of frequency below about 32 Hz, typically at at least 18 db per octave. It may be advantageous to cut off sharply below the fundamental cavity-port tube resonance of 40 Hz to maximize dynamic range without significant loss of reproduced spectral components present in most music. The choice of 32 Hz still provides adequate dynamic range while facilitating reproduction of very low bass components present in some music.

It is also desirable to sharply reduce the response of the equalizer above the highest audible frequency of 15 kHz to prevent spurious inaudible signals from overloading the amplifiers or speakers and thereby effectively increase the dynamic range of the reproducing system for audible frequencies.

The active equalizer also is arranged to help obtain optimum performance in the presence of varying room acoustics and speaker placement in different listening rooms. The treble frequency contour control adjusts the high frequencies to compensate for materials that might affect the high frequency absorption of the room or for curtains or other lightly absorbing materials that might be located on the wall behind the speakers where complete reflection at these frequencies is preferred. The mid-bass control adjusts for those frequencies most affected by placing the speaker in different locations in the room and for different amounts of mid-bass absorbing materials in the room.

In a multiple-driver loudspeaker system where the drivers operate in the same frequency range and share a common port tube, there is a potential instability associated with variations in the characteristics between drivers. The problem may occur essentially in the frequency range between resonance of the cavity acoustic compliance with the port tube acoustic mass, typically about 40 Hz, and the fundamental resonance of the loudspeaker system, determined by enclosure volume and driver electromechanical characteristics, typically about 150 Hz. Consider the theoretical two-driver case represented in Fig. 9 with drivers 71 and 72 connected in phase and electrical forces Fx and Fy applied to drivers 71 and 72, respectively, driving the cones inward. If driver 72 is stronger so that the force Fy is greater than the force Fx, the pressure inside cavity 73 may cause the cone of driver 71 to move outward in phase opposition to the movement of the cone of driver 72. In extreme cases the voice coil on driver 71 may be driven completely outside the air gap, and this excessive motion will cause undesirable distortion, reduction in maximum bass output and potential early failure of the driver. It will be appreciated that this problem is more severe when the drivers operate over the low audio frequency range where the port is effective in increasing the pressure on the cones. The in-phaseconnected drivers in the system of the invention are prevented from having one or more cones move in phase opposition to the others, when energized with an electrical signal having spectral components in the low range of audio frequencies embracing and near the resonant frequency of the system, for example a range from just below the resonant frequency of the cavity acoustic compliance and port tube acoustic mass to the fundamental resonance of the system.

This is at least partly due to the division of the enclosure into separate cavities, to reduce coupling as described above.

An additional advantageous approach involves controlling the Q of the resonant system comprising the acoustic compliance of the cavity and the acoustic mass of the port tubes. The differences between drivers (Fy.

Fx) /Fx which can be tolerated depend firectly upon spider stiffness, cavity volume and inversely upon the number of drivers in excess of one, atmospheric pressure, the square of the area of the cones and Q + 1, where Q is related to the sharpness of the port tube-cavity resonanace. If the enclosure compliance has a pair of complex zeros associated with the port tube, which can be determined by solving an equation of the form s2 + WoS/Q + Wo Q may be defined as shown in the equation.

The Q is strictly a function of the enclosure and port tube parameters, and does not depend significantly upon driver characteristics. The Q may be lowered by restricting the flow of air in the port tube, for example, by inserting a piece of open-cell foam in the port tube or fuzzing the inside of the port tube 74 with a flocking material 75. Alternatively, the inside of the cavity may be arranged to dissipate energy or combinations of increased dissipation in the cavity and port tube. This damping increases the stability of the system and increases the tolerable driver variations without having undesired out-ofphase driver cone movement.

A function of port tube 75 is to lower the excursion of the drivers to reduce distortion in a frequency range around the cavity-port tube resonance. The Q may be lowered from values typically as high as 15 or 20 down to one or two to increase stability while retaining the advantage of the port tube in reducing distortion.

The equation relating the tolerable force differences is given by: VoKs Fx (N-1 (Q+1)P0A2 where VO is the enclosure volume, Ks is the driver spider stiffness, N is the number of drivers, Q is as defined above, PO is atmospheric pressure, and A is the area of the driver cones.

It is preferred that the means for damping be inserted in the port tube where velocity is relatively high and relatively easy to resist for producing the desired Q-reducing dissipation. However, damping may also be introduced in the cavity, preferably by means responsive to pressure because pressure is relatively high, such as movable sides supported in dash pots or by other suitable dissipative means.

In an exemplary embodiment of the invention the plastics ports are preferably made of impact polystyrene such as Monsanto 4200, the width of the assembly is substantially 21 inches wide, 12 3/8 inches high and substantially 13 inches deep. The volume of each cavity is substantially 177 cubic inches. Port tubes 17 and 18 are substantially 9.5 inches long, have an inside diameter of 1.62" and outside diameter of 1.82" at the rear end, an inside diameter of substantially 2.42" at the front end with the outside diameter of the folded over lips being substantially 3.62" and the folded over portion being substantially 1.00". Center port tube 23 typically has an inside diameter of 0.65" and outside diameter of 0.85" and is substantially 9" long, substantially half of that length extending inside cylindrical member 51 whose inside diameter is substantially 6" and length to the portion that tapers inwardly at an angle of substantially 30' being 6 3/16". Front baffle 24 is preferably curved along a radius of 35.5".

The loudspeaker drivers are 4 1/2" and may be of the type used in the BOSE 901 - loudspeaker each having a voice coil impe- - dance of substantially 8 ohms connected in series-parallel with three drivers in each - bank to provide a nominal impendance of substantially 8 ohms; however, the drivers are preferably drivers each having a nominal voice coil impedance of 0.9 ohm established by a single-layer edge-wound rectangular aluminum wire voice coil, the voice coils of all of the drivers being connected in series.

WHAT WE CLAIM IS : 1. A loudspeaker system comprising an enclosure having cavity defining means providing a plurality of cavities, with a driver opening for each cavity, the driver openings each accommodating respective like loudspeaker drivers, the cavities being characterised by acoustic compliance, said loudspeaker drivers each being seated in a respective one of said driver openings and connected in phase, the enclosure having an opening accommodating a port tube providing acoustic mass that resonates with said acoustic compliance at a predetermined frequency in the low range of audio frequencies, said cavity defining means being constructed and arranged to provide air channelling means from said loudspeaker drivers into said port tube whereby said port tube is common to all of said cavities, means being provided for preventing the cones of said loudspeaker drivers from exhibiting out-ofphase movement when energized with an, electrical signal having spectral components in the low range of audio frequencies embracing and near said resonant frequency, said preventing means comprising the separation of the drivers into individual cavities.

2. A loudspeaker system in accordance with claim 1, wherein the embracing frequency range is from just below said predetermined frequency to the fundamental resonance of said loudspeaker system related to the volume of said cavity defining means and the electromechanical parameters of said loudspeaker drivers.

3. A loudspeaker system in accordance with claim 1,wherein said air channelling means defines a coupling aperture between each cavity and said common port tube of cross sectional area small enough to provide substantial acoustic isolation between adjacent cavities at low bass frequencies and large enough to transmit air therethrough at said low bass frequencies for coupling each cavity to said common port tube.

4. A loudspeaker system in accordance with claim 3, wherein the volume of each said cavities is substantially the same.

5. A loudspeaker system in accordance with claim 3, wherein said driver openings are on the rear of said enclosure, the system further comprising, at least another port tube, said cavity defining means defining at least another cavity adjacent to a driver opening at the front of said enclosure and connected to said another port tube.

6. A loudspeaker system in accordance with claim 5, wherein the volume of each said cavities is substantially the same.

7. A loudspeaker system in accordance with claim 5, wherein all said port tubes vent to the rear.

8. A loudspeaker system in accordance with any one of claims 3 to 7, wherein said common port tube includes means defining a tapered annular hollow region for establishing substantially laminar airflow therein.

9. A loudspeaker system in accordance with any one of claims 1 to 8, and further comprising active equalizing means including means for coupling signal spectral components of substantially the full range of audio frequencies to the in-phase-connected drivers, means for establishing a substantially uniform radiated power response of said system

Claims

**WARNING** start of CLMS field may overlap end of DESC **. inside diameter of 0.65" and outside diameter of 0.85" and is substantially 9" long, substantially half of that length extending inside cylindrical member 51 whose inside diameter is substantially 6" and length to the portion that tapers inwardly at an angle of substantially 30' being 6 3/16". Front baffle 24 is preferably curved along a radius of 35.5". The loudspeaker drivers are 4 1/2" and may be of the type used in the BOSE 901 - loudspeaker each having a voice coil impe- - dance of substantially 8 ohms connected in series-parallel with three drivers in each - bank to provide a nominal impendance of substantially 8 ohms; however, the drivers are preferably drivers each having a nominal voice coil impedance of 0.9 ohm established by a single-layer edge-wound rectangular aluminum wire voice coil, the voice coils of all of the drivers being connected in series. WHAT WE CLAIM IS :

1. A loudspeaker system comprising an enclosure having cavity defining means providing a plurality of cavities, with a driver opening for each cavity, the driver openings each accommodating respective like loudspeaker drivers, the cavities being characterised by acoustic compliance, said loudspeaker drivers each being seated in a respective one of said driver openings and connected in phase, the enclosure having an opening accommodating a port tube providing acoustic mass that resonates with said acoustic compliance at a predetermined frequency in the low range of audio frequencies, said cavity defining means being constructed and arranged to provide air channelling means from said loudspeaker drivers into said port tube whereby said port tube is common to all of said cavities, means being provided for preventing the cones of said loudspeaker drivers from exhibiting out-ofphase movement when energized with an, electrical signal having spectral components in the low range of audio frequencies embracing and near said resonant frequency, said preventing means comprising the separation of the drivers into individual cavities.

9. A loudspeaker system in accordance with any one of claims 1 to 8, and further comprising active equalizing means including means for coupling signal spectral components of substantially the full range of audio frequencies to the in-phase-connected drivers, means for establishing a substantially uniform radiated power response of said system as a function of frequency at least in the range of frequencies between said predetermined resonant frequency and the fundamental resonance of said system, and means for sharply attenuating signal spectral components below a predetermined frequency at or slightly below said predetermined resonant frequency.

10. A loudspeaker system in accordance with claim 9, wherein said means for establishing, serve for establishing a substantially uniform radiated power response as a function of frequency of said system for substantially the full range of audio frequencies.

11. A loudspeaker system in accordance with claim 9, wherein the attentuation imparted to signal spectral components by said means for sharply attenuating below said predetermined frequency is at least 18 db per octave.

12. A loudspeaker system in accordance with claim 8, wherein said means defining an annular region comprises a bullet cantilevered from a member forward of said port tube.

13. A loudspeaker system in accordance with claim 12, wherein said bullet extends through the entire length of said common port tube.

14. A loudspeaker system in accordance with claim 13, wherein said common port tube is formed with a folded over lip adjacent the air channelling means for establishing smooth air flow at the entrance of said com

mon port tube.

15. A loudspeaker system in accordance with claim 14, wherein there are four of said cavities surrounding said common port tube and each of said coupling apertures is located between said lip and said forward member and extends over an angle of substantially 90 degrees about the axis of said common port tube.

16. A loudspeaker system in accordance with claim 15, wherein the arrangement of four cavities, four drivers and a port tube common to the four cavities, is duplicated to provide a loudspeaker system having eight drivers.

17. A loudspeaker system in accordance with claim 16, wherein the two sets of four cavities extend between a front member of the enclosure and respective rear angled baffles forming an obtuse angle with each other.

18. A loudspeaker system in accordance with claim 17, and further comprising a third port tube, said cavity defining means defining a ninth cavity adjacent to a driver opening in said front member, a driver being seated in the last said opening and being connected in phase with all of the other drivers, said third port tube being for venting said ninth cavity to the outside of said enclosure and extending from said ninth cavity through an opening embracing the junction between said rear angled panels.

19. A loudspeaker system in accordance with claim 18, and further comprising active equalizing means including means for coupling signal spectral components of substantially the full range of audio frequencies to the in-phase-connected drivers, means for establishing a substantially uniform radiated power response of said system as a function of frequency at least in the range of frequencies between said predetermined resonant frequency and the fundamental resonance of said system, and means for sharply attenuating signal spectral components below a predetermined frequency at or slightly below said predetermined resonant frequency.

20. A loudspeaker system in accordance with claim 19, wherein said means for establishing includes means for establishing a substantially uniform radiated power response as a function of frequency of said system for substantially the full range of audio frequencies.

21. A loudspeaker system in accordance with claim 19 or 20, wherein the attentuation imparted to signal spectral components by said means for sharply attenuating below said predetermined frequency is at least 18 db per octave.

22. A loudspeaker system in accordance with claim 1, wherein said means for preventing the cones of said loudspeaker drivers from exhibiting out-of-phase movement comprises means for reducing the Q of the resonant system formed by said cavity defining means and the said acoustic mass.

23. A loudspeaker system in accordance with claim 22, wherein said means for reducing comprises damping means carried by said port tube.

24. A loudspeaker system in accordance with claim 22, wherein said means for reducing comprises damping means carried by said cavity defining means.

25. A loudspeaker system in accordance with claim 23, wherein said damping means comprises means for resisting the flow of air through siad port tube.

26. A loudspeaker system in accordance with claim 25, wherein said means for resisting comprises open-cell foam material in the passageway defined by said port tube.

27. A loudspeaker system in accordance with claim 25, wherein said means for resisting comprises resistive material on the inside of said port tube.

28. A loudspeaker system in accordance with claim 27, wherein said resistive material comprises flocked material.

29. A loudspeaker system in accordance with claim 22, wherein said means for reducing is effective in reducing a Q in the range of 15 to 20 to a Q in the range of 1 to 2.

30. A loudspeaker system in accordance with claim 22, wherein said means for reducing reduces the Q to a value sufficiently high so that the port tube, providing acoustic mass, coacts with said cavity defining means to increase the pressure inside said cavity defining means in a frequency range near that of said predetermined resonant frequency to significantly reduce the excursions of the cones of said loudspeaker drivers.

31. A loudspeaker system in accordance with claim 30, wherein said frequency range extends between said predetermined resonant frequency and the fundamental resonance of said loudspeaker system determined by the electromechanical characteristics of said loudspeaker drivers and the effective volume of said enclosure.

32. A loudspeaker system substantially as hereindescribed with reference to and as shown in Figures 1 to 8 of the accompanying drawings.