Abstract:
A loudspeaker system is provided for installation in a space between a front panel and an enclosed area behind the front panel of a partition such as a wall, ceiling or floor fronting a listening area. Electroacoustical transducers are provided which have a two sided vibratory diaphragm driven by an electrical signal. An enclosure mounts the electroacoustical transducers such that one side of the vibratory diaphragm is in contact with air outside the enclosing between the front and rear panels of the partition, with the enclosure being configured to substantially enclose and define a specific volume of air within the enclosure having a predefined acoustic compliance and which is in contact with the other side of the vibratory diaphragm of the electroacoustical transducers. The enclosure is mounted to the structural partition such that the enclosure extends into the space behind the front panel of the partition so that the one side of the vibratory diaphragm contacts a volume of air outside the enclosure within the space behind the front panel of the partition. A passive radiator such as a port which has a specific acoustic mass is provided for coupling the specific volume of air enclosed by the enclosure to the air outside the enclosure in the listening area. A compression plate is provided spaced between the transducer diaphragm and the rear panel to isolate the rear panel from intense direct sound pressure from the transducer.

Description:
CROSS - REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 07/294,150, filed Jan. 5, 1989. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a sub-woofer loudspeaker system and method for compact, efficient installation in structural partitions, such as walls, ceilings, floors, or automobile panels. 
     The generation of people now entering mid-career and raising families of their own are also the first generation to have grown up with the easy availability of reasonably priced high-fidelity sound reproduction equipment and an ever expanding selection of popular music. As a result of the demographic changes that are occurring in this group, they are spending increasing amounts of time at home. However, high quality reproduction of recorded music continues to be an important part of their lives. Along with maturity and adult responsibilities, however, appearance of their homes has also become important. 
     While it is not difficult to design small and inconspicuous loudspeaker systems for reproducing the higher frequency ranges of recorded music, the requirements for reproducing the lower range of frequencies traditionally result in large, obtrusive speaker systems. Such large speaker systems can detract from the appearance of a room, not to mention leading to problems in furniture placement, etc. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a speaker system of high quality and extended low frequency range which can be inconspicuously installed into the typical structural partitions, such as walls, ceilings or floors of a home or business, in the relatively small volume between the front and rear panels of the partitions. The principles of this invention are also applicable to installing such a speaker system in panels of an automobile interior. 
     It is another object of this invention to provide such a speaker system in which system performance is relatively independent of the specific conditions found in the structural partitions at the time of installation. 
     It is a further object of this invention to provide such a speaker system which is reasonably efficient over a frequency range broad enough to allow it to be used with small, independently mounted speaker systems specifically designed to reproduce the middle and higher frequency ranges. 
     It is a still further object of this invention to provide such a speaker system which is flexible enough to permit mounting in virtually any of the myriad combinations of materials and construction methods which may constitute the partitions of a given building, whether being newly constructed or existing, and to provide certain isolation of the speaker system from a rear panel of the partition in which the speaker system is installed. 
     Briefly, in accordance with one embodiment of the invention, a loudspeaker system is provided for installation in a space defined by a front panel and an enclosed area behind the front panel of a structural partition. For example, the structural partition is a wall, ceiling or floor having a front panel fronting a listening area and having a rear panel. Electroacoustical transducing means is provided which has a two sided vibratory diaphragm with means provided for coupling an electrical signal to the electroacoustical transducing means for driving it. Enclosure means is provided for mounting the electroacoustical transducing means within the partition such that one side of the vibratory diaphragm is in contact with air outside the enclosure means, with the enclosure means being configured to substantially enclose and define a specific volume of air within the enclosure having a predefined acoustic compliance and which is in contact with the other side of the vibratory diaphragm of the electroacoustical transducing means. Means are provided for mounting the enclosure means to the structural partition such that the enclosure means extends into the space behind the front panel of the partition so that the one side of the vibratory diaphragm contacts a volume of air outside the enclosure means within the space behind the front panel of the partition. A passive radiating means characterized by having a specific acoustic mass is provided for coupling the specific volume of air enclosed by the enclosure means to the air outside the enclosure means in the listening area. With such an arrangement, the electroacoustical transducer itself and the enclosure are concealed within the structural partition, while the volume of air outside the enclosure means within the space behind the front panel of the partition is substantially acoustically isolated over the approximate frequency range of operation of the electroacoustical transducing means from the volume of air outside the enclosure means within the listening area. A compression plate is provided in spaced relationship to and facing the one side of the vibratory diaphram in contact with air outside the enclosure to provide isolation of the rear panel of the partition from the vibratory diaphram. 
     Other objects and advantages of the present invention will appear from the accompanying drawings considered in conjunction with the detailed description of a preferred embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical equivalent circuit diagram of a prior art arrangement disclosed in a 1979 paper by Laurie Fincham. 
     FIG. 2 is a graph of the frequency response of the circuit of FIG. 1. 
     FIG. 3A is a schematic diagram of a speaker system in accordance with the present invention, illustrating the manner of installation in a structural partition. 
     FIG. 3B is a schematic diagram of an alternate embodiment of a speaker system in accordance with the present invention using a drone cone as a passive radiator into the listening area. 
     FIG. 4 is a front elevation of the speaker system of the present invention shown installed in a structural partition. 
     FIG. 5 is a cross-sectional side view of the speaker system of FIG. 4. 
     FIG. 6 is an electrical equivalent circuit diagram of the speaker system of FIGS. 3-5. 
     FIG. 7A is a graph of the frequency response of the speaker system of FIGS. 3-6 for a volume of air contained within the structural partition in which the system is mounted, of a relative volume value of 10. 
     FIG. 7B is a graph of the frequency response of the speaker system of FIGS. 3-6 for a volume of air contained within the structural partition in which the system is mounted, of a relative volume value of 100, ten times that of FIG. 7A. 
     FIG. 8 is a schematic diagram of a speaker system as in FIG. 3 but including an acoustic trap for removing unwanted frequencies in the system output to the listening area. 
     FIG. 9 is a front elevation of the speaker system of FIG. 8. 
     FIG. 10 is a cross-sectional side view of the speaker system of FIG. 9. 
     FIG. 11 is a schematic diagram of a speaker system as in FIG. 8 but further including an acoustic mass and an acoustic compliance (Helmholtz resonator) coupled to the port tube for, removing specific unwanted frequencies. 
     FIG. 12 is a cross-sectional side view of a speaker system installed in a partition in accordance with an embodiment of the invention wherein a compression plate is used to isolate the speakers from a rear panel of the partition. 
     FIG. 13 is a pictorial view partially broken away showing the speaker system of FIG. 12 installed in a partition. 
    
    
     DETAILED DESCRIPTION 
     Perhaps the most vexing problem of installing a high quality sub-woofer system in a typical structural partition such as a wall, is the thickness of the wall itself. A typical single-family residential wall is constructed with sheet-rock fastened to two by fours. However, a two-by-four is now only 1.5&#39; by 3.5&#39;. Sheet-rock may be as little as 0.5&#39; thick. This means that there is, at most, four inches to work with from the outside face of the wall or front panel to the inside face of the sheet-rock opposite, i.e. the rear panel. Sixteen inches between wall studs is considered standard, leaving 14.5 inches in width to work with. An adequate conventional cabinet size for obtaining deep bass response from an eight inch driver with moderate efficiency might be 1.5  cubic feet, at a minimum. Enclosure wall thicknesses of 1/16 inch would have to be considered a minimum. This would indicate that a cabinet over 50 inches high would be required to achieve the required volume for a single eight inch driver, assuming the driver itself was shallow enough to fit. 
     One possibility is that a speaker design might rely on the volume of air enclosed by the wall itself to substitute for an enclosure. However, the variety of construction techniques and materials used make it impossible to consider any volume of enclosed air as standard, let alone questions of leakage or wall stiffness. 
     The solution to this problem, in accordance with the present invention, is to provide a system that builds on a novel variation of a woofer type known as a &#34;band-pass&#34; sub-woofer. This design concept was first explained in detail in a paper entitled &#34;A Bandpass Loudspeaker Enclosure&#34;, presented to the Audio Engineering Society in May of 1979 by Laurie Fincham of KEF Electronics Limited, U.K. The concept was treated in somewhat greater theoretical detail again in a paper entitled &#34;Bandpass Loudspeaker Enclosures&#34; presented to the Audio Engineering Society in November, 1886 by Earl Geddes of Ford Motor Company. Moreover, in October of 1985 U.S. Pat. No. 4,549,631 was granted to Dr. Amar Bose for an extension of this design concept. 
     In both the Fincham and Geddes papers a double cavity design is disclosed wherein the two cavities are separated by a baffle on which is mounted one or more transducers. The first cavity is sealed while the second cavity is &#34;ported.&#34; That is, the cavity is ported by being provided with an opening of a specific cross-sectional area and length which contains a specific acoustic mass of air. The mass and compliance of the transducer forms a driven resonant system with the compliance of the air in the first sealed cavity. The acoustic mass of air in the port forms a second resonant system with the compliance of the air in the second cavity. The combination of the two is represented by the equivalent electrical circuit shown in FIG. 1. 
     In FIG. 1, the various elements shown will be immediately recognized by anyone skilled in the art. Values are calculated from measurable system parameters and correspond as follows: 
     Eg--voltage output of a constant voltage generator 
     Rg--output impedance of the generator 
     Re--voice coil DC resistance of transducer 
     Le--voice coil inductance of transducer 
     Res--mechanical loss of transducer 
     Cmes--acoustic mass of transducer 
     Lces--acoustic compliance of transducer suspension 
     Lceb1--acoustic compliance of sealed cavity 
     Rleb1--leakage loss of sealed cavity 
     Lceb2--acoustic compliance of ported cavity 
     Rleb2--leakage loss of ported cavity 
     Cmep--acoustic mass of air in port 
     Analysis of the equivalent circuit of FIG. 1 shows that the frequency response output of the system of FIG. 1 using the two cavities is a band-pass characteristic, as shown in FIG. 2. 
     As disclosed by both Geddes and Bose, the frequency range of the band-pass may be extended by using a port in the sealed cavity also. This second port is tuned to a different frequency such that the phase of the acoustic outputs of the two ports adds where they overlap to create a smooth overall response. 
     The present invention departs from the systems of the prior art described above in that it dispenses with the first sealed cavity altogether. Referring to FIG. 3A, there is shown a diagrammatic cross-sectional view illustrating the principles of the present invention. A structural partition 11, such as a wall, floor or ceiling, has a front panel 12 and a rear panel 13 separated by a space 14 enclosed therebetween. An enclosure 16 has an electroacoustical transducer mounted therein. Specifically, in FIG. 3A two separate transducers 17 and 18 are mounted in a wall of the enclosure 16. The transducers 17 and 18 have a two-sided vibratory diaphragm, one side of which faces into the air space 14 of the structural partition 11 and the other side of which faces into an air volume 19 defined by and substantially enclosed by the configuration of the enclosure 16. Terminals 21 and 22 in FIG. 3A diagrammatically illustrate provision for coupling electrical signals to the transducers 17 and 18 for driving them. As shown in FIG. 3A, a passive radiator is used for coupling the specific volume of air 19 defined within the enclosure 16 to the air outside the front panel 12 constituting the listening area. In the specific embodiment of FIG. 3A, this passive radiator comprises a port opening 23 from the interior of the enclosure 16 to the outside listening area. 
     FIG. 3B is similar to FIG. 3A, and like elements in FIG. 3B have been given identical reference numerals to corresponding elements in FIG. 3A. The alternate embodiment of the invention shown in FIG. 3B is one in which the passive radiator means for coupling the specific air volume 19 within enclosure 16 to the outside listening area is a drone cone 24 instead of a port. 
     FIG. 4 is a front elevation of the speaker system of FIG. 3A in accordance with this invention shown installed in a structural partition such as a wall, and FIG. 5 is a cross-sectional view of the speaker system of FIG. 4. Elements in FIGS. 4 and 5 have been given the same reference numerals as corresponding elements shown diagrammatically in FIG. 3A. As shown in FIG. 5, the front and back panels 12 and 13 of the structural partition such as a wall are typically spaced by two-by-fours 26. 
     As shown in FIGS. 3A, 4 and 5, the loudspeaker system in accordance with the present invention comprises an enclosure with a baffle for the mounting of one or more transducers on one or more sides and a port opening on another side. The entire system is mounted into a wall or other partition such that the transducers are inside the wall and the port opening is exposed to the listening area, i.e., inside a room. The enclosure or volume of air 14 formed by the front and back panels and other structural components of the partition 11 serves mainly to prevent the acoustic radiation from the other side of the transducers facing the air volume 14 from interfering destructively with the desirable acoustic radiation from the port 23. 
     It has previously been assumed, quite naturally, that the variability in the characteristics of the enclosure formed by the panels of the partition or wall (e.g., volume, leakage loss, vibration loss, internal loss, etc.) would preclude the choice of any one set of design parameters which would be suitable for all mounting situations one might encounter. However, experiments have shown that the volume of air enclosed inside wall or structural partitions of quite disparate construction materials and techniques invariably appears, acoustically, to be quite large with substantial leakage and internal losses. These losses are of such a magnitude as to substantially minimize the effect on tuning of the system of changes of up to a factor of ten in the apparent volume of the enclosed air. In addition design parameters for the rest of the system can be chosen such that the performance will be substantially unchanged for the vast majority of mounting situations. 
     Referring now to FIG. 6, there is shown an electrical equivalent circuit diagram of the speaker system of FIGS. 3-5. The elements shown in FIG. 6 follow the same convention as the circuit of FIG. 1, with the addition of some new elements which correspond as follows: 
     Rleb1--leakage losses for wall cavity 
     Rieb1--internal and vibrational losses of wall cavity 
     Rleb2--leakage losses for ported cavity 
     Rieb2--internal losses of ported cavity 
     Riep--internal losses of port 
     Leakage and vibrational losses are usually negligible for commercially constructed loudspeaker enclosures but have been shown, by experiment, to be significant for most wall mounting situations. In addition, size and space limitations prohibit the use of a port arrangement optimized for minimum internal loss. Therefore, port internal losses play an important role in the ultimate performance of the system. Leakage loss for the ported cavity should be negligibly small while internal losses will be a controllable design parameter. The equivalent electrical circuit element values for a preferred embodiment of the invention as shown in the drawings are as follows: 
     Eg--1.00 Volt 
     Rg--0.01 Ohm 
     Le--0.20 mH 
     Re--2.20 Ohm 
     Lces--8.50 mH 
     Res--12.00 Ohm 
     Cmes--962.00 uf 
     Rleb1--8.00 Ohm 
     Lceb1--50.00 mH 
     Rieb1--5.00 Ohm 
     Rleb2--0.02 Ohm 
     Lceb2--2.70 mH 
     Rieb2--30.00 Ohm 
     Cmep--1950.00 uf 
     Riep--6.00 Ohm 
     An analysis of this circuit of FIG. 6 shows that appropriate choices for the transducer and ported cavity parameters makes the system performance substantially independent of the characteristics of the wall cavity. Specifically as shown by FIGS. 7A and 7B, the calculated frequency response for two values of the volume of air enclosed within the wall but differing by a factor of ten (Vol.=10 in FIG. 7A, Vol.=100 in FIG. 7B) is virtually nil. Experiments have confirmed the predictions made by this model. 
     In accordance with one preferred embodiment of the invention, the two transducers 17 and 18 are 6.5 inch drivers. The entire enclosure 16 has approximate dimensions of 12 inches wide, 18 inches high and 3 inches deep. These dimensions allow the system to be mounted in the depth of a standard two-by-four stud wall or partition without impairing performance. The circuit element values used above are calculated from easily realizable system parameters. In addition, as particularly shown in FIGS. 4 and 5, the system may be mounted essentially flush into the wall or other partition and &#34;painted out&#34; leaving only a roughly 6 square inch port opening 23 as the only evidence of its presence. An additional advantage of the present invention is that its band-pass characteristics substantially reduce the cost and complexity of the electrical crossover network required to blend its performance with the higher frequency units. 
     As previously mentioned in connection with FIG. 3B, one variation on the system of the present invention is to use a drone cone 24 as the passive radiator output of the system. An advantage to this approach is that a drone cone radiator may be constructed with much less loss than the practical realization of the port version of the system in the preferred embodiment discussed above. This would contribute to improved efficiency at the lower frequencies reproduced by the present invention. An obvious disadvantage to such an arrangement, however, is that a drone cone passive radiator for this application, say on the order of 8 inches in diameter, would have a much larger surface area than that of the port opening and would be much more visually obtrusive. 
     It should be clear that the present invention is not limited to loudspeaker systems for mounting only in wall, floor or ceiling structural partitions. The same principles are applicable to mounting in structural partitions in the interior of automobiles, where many of the same conditions (mainly of uncertainty) apply to situations where a consistent level of performance is required in a variety of different thru-panel mounting situations. Thus, the schematic drawing of FIGS. 3A and 3B apply where the partition 11 is a partition in an automobile with front panel 12 being an interior panel of the automobile. 
     It should also be noted that the preferred embodiment of the present invention, which uses at least two transducers 17 and 18 mounted in the enclosure, offers an additional advantage. Specifically, one of the transducers can be electrically driven by one of the two stereo output channels and the other transducer driven by the other of the two stereo output channels. Such an arrangement creates a center channel sub-woofer without the need for electrically combining the two channels. 
     One difficulty or potential problem should be addressed at this point. Specifically, the port opening 23 (FIGS. 3A, 4, 5) will act as a transmission line at frequencies where the port length is an odd multiple of one-half wavelength. At these frequencies, energy will be transmitted from the interior of the ported cavity to the listening area with very little attenuation. Usually the frequencies at which this occurs will be far enough above the desired operating range that they can be easily attenuated with a simple low-pass network at the input to the transducers. However, when the length of the port is relatively long, the lowest transmission line frequency may be too close to the operating range to permit attenuation using a simple network. The solution to this problem, in accordance with the present invention and as shown in FIGS. 8, 9 and 10, is to provide an acoustic trap 27 to eliminate the undesirable frequencies. This trap may be a tube sealed at one end and opening into the side of the port at its other end, with its length being one-quarter of the wavelength of the lowest undesirable frequency. As an alternative, and as shown schematically in FIG. 11, the trap may consist of a Helmholtz resonator 28 opening into the side of the port. A Helmholtz resonator, as known to those skilled in the art, consists of an acoustic mass and an acoustic compliance tuned to resonate at the undesirable frequency. In this case, the resonator would consist of a small sealed cavity of appropriate volume connected to the side of the port by a tube containing the desired acoustic mass, as shown in FIG. 11. 
     In accordance with the one preferred embodiment of the present invention as discussed above, the port dimensions created an unwanted transmission line frequency at approximately 500 Hz, which was removed by the use of a quarter wave trap (FIGS. 8, 9 and 10) approximately 6.3 inches in length and 1.4 inches in diameter. 
     One problem which can result from the installation of speaker systems in partitions such as the walls of modern buildings is that, unless the back of the speaker system is fully enclosed by a rigid cabinet, significant amounts of sound are transmitted through the opposite face of the wall immediately behind the loudspeaker, i.e. the rear panel, and hence into whatever space of room adjoins the room where the installation is being made. 
     Where good frequency performance is desired from a speaker system, the rear of the loudspeaker diaphragm must radiate into a sufficiently large volume of enclosed air. While the volume of air enclosed between the two faces or panels of a typical wall partition is usually large enough, lack of adequate rigidity in typical wall construction leads to the undesirable transmission of sound through the back or rear panel of the wall as discussed above. This problem is exacerbated when high sound pressure levels of low and mid frequencies are produced within the wall and when the spacing between the back of the sound radiating elements or electroacoustical transducers and the rear wall face behind the loudspeaker is small or restricted. Although a rigid rear enclosure or &#34;back box&#34; would prevent this, space restrictions encountered when making in-wall loudspeaker installations frequently make the use of back boxes of sufficient size extremely difficult or impossible. 
     In accordance with one aspect of the present invention, and as shown for example in FIGS. 4, 5, 9 and 10, the sound radiating elements or electroacoustical transducers 17, 18 are spaced less than one inch from the rear panel of the partition behind the system in a typical installation. Experiments have shown that, when installed in a wall of typical wood stud and wallboard construction, significant sound was transmitted through the opposite wall face above 200 Hz. 
     Turning now to FIGS. 12 and 13, there is shown an embodiment of the invention which addresses the problem of sound transmission through the opposite wall of a partition in which a loudspeaker system is installed. FIG. 12 is a cross-sectional side view of a speaker system installed in a partition in accordance with an embodiment of the present invention wherein a compression plate is used to isolate the speakers from a rear panel of the partition, and FIG. 13 is a pictorial view, partially broken away of the system of FIG. 12. Like reference numerals are used in FIGS. 12 and 13 as in FIGS. 1-11 to refer to the same elements. 
     A structural partition is formed of front panel 12 and rear panel 13 spaced by studs 26. Enclosure 16 has electroacoustical transducers or sound radiating elements 17 and 18 mounted in its wall. The transducers 17 and 18 have two-sided vibratory diaphragms, one side of which faces into the air space 14 of the partition or wall, and the other side of which faces into an air volume 19 defined by and substantially enclosed by the configuration of the enclosure 16. A passive radiating means, such as port 23 couples the specific air volume 19 within enclosure 16 to the outside listening area fronted by front panel 12. FIGS. 12 and 13 also show use of a plinth member 31 useful for mounting the enclosure 16 to the front panel 12 of the wall. 
     FIGS. 12 and 13 show a compression plate 32 mounted to the back wall of the enclosure 16 by two side members 33 and 34 all of which are suitably fastened together as by adhesives or fasteners. In one embodiment of the invention the compression plate was spaced approximately three quarters of an inch from the sound radiating elements 17 and 18 by the side members, but this distance can obviously be increased or decreased. The compression plate 32 is a rigid plate formed of any suitable material and forms, with the side members 33 and 34, an enclosure in back of the sound radiating elements which is substantially sealed on the back and sides but open on the top and bottom. This forms in effect a partial enclosure. The function of this partial enclosure is to isolate the portion of the rear panel 13 immediately behind the sound radiating elements, while permitting the system to continue to &#34;see&#34; the entire volume of air 14 within the partition or wall. Above and below the loudspeaker system this partial enclosure is entirely open to the air within the wall. In these areas the volume velocity of sound is spread over a substantially larger cross-sectional area and results in much lower sound pressure, which in turn serves to minimize excitation of the rear panel or wall surface behind the system. The partial enclosure, due to its narrow depth dimension, does add acoustic mass to the sound radiating elements requiring that adjustments be made to the tuning of the system to maintain optimum performance. Suitable tuning adjustments, such as the volume of the enclosure 16, etc. are well within the level of those skilled in the art. 
     Experiments have shown that the sound transmitted through the rear panel behind the system is reduced by an average of nearly 10 db above 200 Hz to 500 Hz, and that acceleration of the wall surface behind the system is reduced by more than 2 db above 110 Hz. 
     The compression plate technique illustrated in FIGS. 12 and 13 can be used with virtually any in-wall loudspeaker system wherein the sound radiating elements are open to the rear partition panel, to provide isolation of that rear panel from the intense sound pressure produced in the small space behind the sound radiating elements. Thus, this aspect of the invention is not limited to a system and method constituting a &#34;bandpass&#34; sub-woofer, but is applicable to other systems and methods for in-wall loudspeaker installations as well. Moreover, the compression plate need not be flat as shown in FIGS. 12 and 13, but may conform in shape to accommodate specific requirements of any system. Experiments have shown that the total area open to the air volume within the partition or wall may be as little as one third the total area of the sound radiating elements to be partially enclosed by the compression plate and its supports. Furthermore, the volume and dimensions of the partial enclosure are important only in that they affect the acoustic mass of the system and hence the tuning of the system. It has also been shown by experiment that the partial enclosure may be open or partially open on the sides and that the compression plate itself may be partially open. Care, however, must be taken to avoid a geometry which creates a mass of air operating like a port where the primary openings of the partial enclosure join the air volume within the wall. 
     Although the present invention has been described and illustrated in connection with specific presently preferred embodiments, it should be understood that many variations are possible without departing from the true spirit and scope of the present invention, which is to be measured by the following claims.