Abstract:
A speaker assembly and its method of operation are described wherein a plurality of drivers are spatially adjustable relative to each other and to a selected critical listening point in order to establish maximum sound coherency for the system. Means for adjustment and calibration of the relative positions of the drivers are provided to facilitate their location upon a mounting sub-assembly, interchange of components within the total speaker assembly, and synchronization of the drivers for one or more desired listening positions within a listening environment. The speaker system preferably includes a low frequency range sub-assembly having one or more drivers supported in fixed relation within the total speaker system, a housing for the low frequency range sub-assembly including selective band-pass energy filtration means for isolating each driver from its enclosure and an internal structure providing increased structural rigidity and reducing non-linear turbulence within the enclosure.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to improvements in loudspeaker accuracy in the field of high fidelity sound reproduction and more particularly to a method and apparatus for achieving such improvements. 
     The prior art has long been concerned with apparatus and methods of operation for speaker systems in order to accurately reproduce various sounds from an electronic input. 
     It has been thought by many that the ideal speaker would be a point-source from which all the frequencies being reproduced would emanate. Limitations of technology have prevented the construction of such a driver. In attempting to achieve sufficient acoustical output across the entire audible frequency band, the prior art has generally resorted to the use of a plurality of drivers of differing construction, each type reproducing a specific portion of the frequencies within that band. Moreover, it has been generally recognized that linear reproduction of the audible frequency range is important to achieve realism in the reproduced sound. 
     More recently, the importance of polarity and phase alignment of the drivers within the system, relative to each other, has been recognized. U.S. Pat. No. 3,824,343 issued July 16, 1974 to Dahlquist identifies and deals with the problem of &#34;time delay distortion&#34; as a barrier to &#34;coherent sound&#34; in multiple driver systems. Dalquist observes that, in a multiple driver system where all drivers are mounted in the same mechanical plane, the leading edge of a single pulse (an electrical test signal which most meaningfully simulates a short duration musical transient) applied to the system input will be reproduced acoustically by the system as a series of pulses that will appear to the listener as a distorted form of the original pulse. This &#34;time delay distortion&#34; has a number of causes. Initially, a lower mass driver-diaphragm, such as a tweeter dome, can be expected to reach more quickly to the pulse than the more massive woofer cone, so its portion of the pulse will arrive at the listener&#39;s ear before that of the woofer. In addition, the various elements in a crossover network may effect a group delay on portions of the input signal&#39;s spectrum. Thus, while the total energy output of a time delay distorted system may be identical with that of the applied signal, the waveform generated by the system may be substantially different. 
     In order to compensate for the differing &#34;rise time&#34; of each type of driver, defined as the inherent time lag between impression of a voltage and driver response, Dahlquist contemplated a fixed relative arrangement of the drivers so that the pulse wave form front contribution from each driver would be theoretically synchronized and would reach the ear of the listener at the same time. For this purpose, the higher range speakers were located somewhat further away from the listener than the lower range speakers. 
     However, because significant distances existed between the drivers of the above system while the location of the drivers relative to each other within the system were fixed, the proper pulse-arrival time alignment of the above system has been found in the present invention to be not only dependent upon freedom of arrangement for the speaker system within a selected listening environment, but more particularly, upon the location of the listening point relative to the speaker system. Speaker systems of the type contemplated by the present invention may be used in many different types of environments including, for example, studios or homes. In many such environments, there exist certain constraints which limit freedom of location for the speaker system. At the same time, a listener, particularly one with a discerning ear, desires to establish the speaker system so that maximum sound coherence is achieved at a predetermined critical listening point. Location of the critical listening point may or may not be determined by choice. 
     Accordingly, the predetermined relation of components within a speaker system of the type referred to above may not be adaptable to a given listening environment. The result may be a need for compromise between arrangement of the speaker system within the listening environment and the quality of sound reproduced by the system. 
     There has thus been found to remain a need for a method and apparatus in a speaker system for achieving maximum sound coherency for a plurality of drivers of a speaker system relative to a critical listening point of a selected listening environment while allowing the location of the speaker system and the critical listening point to be dictated at least in part by considerations imposed, for example, by the environment itself and/or by the listener. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a speaker assembly which may be adapted to a listening environment in order to produce highly coherent sound at a selected critical listening point in the environment. It is also an object of the invention to provide a method for adjusting such a speaker assembly in order to achieve this same purpose. 
     Generally, the present invention contemplates a speaker assembly including a plurality of sub-assemblies, each including single or multiple drivers, the various sub-assemblies covering the same or different frequency ranges, and being adapted to produce highly coherent sound at a critical listening point in a selected listening environment. Within the speaker assembly of the invention, selected drivers within the system are adjustable relative to the critical listening point. The adjusting means include calibration means for determining the relative positions of the drivers, for example on a mounting sub-assembly, when adjusted for pulse-arrival time alignment at a critical listening point. 
     Similarly, the invention contemplates a method for mounting or adjusting a plurality of drivers in a speaker assembly, the speaker assembly being arranged in a predetermined locale of a listening environment which also has a selected critical listening point. According to the method of the present invention, the drivers are independently supported with at least a portion of the drivers being adjustable relative to the critical point. A microphone and/or listener is then placed at the critical listening point as a monitor in order to adjust the respective drivers for maximum sound coherency of the system at that point. 
     The method and apparatus of the present invention also permit a number of additional features. For example, drivers adapted for interchangeability within the system will have calibrated positions which permit their interchange while maintaining substantial sound coherency at the critical listening point according to a calibration &#34;correction factor&#34;. 
     In other words, the calibration correction factor may be engineered to compensate for the time delay differences between the interchanged drivers and allow the system to correctly maintain sound coherency at the critical listening point. 
     Furthermore, the invention contemplates construction of a housing, particularly for one or more low frequency range drivers, the housing including sound absorptive means for isolating the driver or drivers from the main part of the housing and an internal structure with sound wells behind the driver(s) in order to substantially increase the rigidity of the housing and to reduce turbulence, or non-linear movement of air, within the housing. 
     Additional objects and advantages of the invention are made apparent in the following description, having reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a side view in elevation of a speaker assembly constructed in accordance with the present invention. 
     FIG. 2 illustrates a simplified schematic representation of a speaker system with a coplanar arrangement of its drivers in a manner common in the prior art as described above, where sound wavefront information from the different drivers tend to reach the ear of the listener at different times and cause blurring or incoherence of the sound as heard by the listener. 
     FIG. 3 is a view similar to FIG. 2 with the drivers of the speaker system being illustrated in alignment for achieving maximum sound coherency at a critical listening point occupied by a listener. 
     FIG. 4 is yet another view similar to that of FIG. 3, illustrating inability of a system of the prior art to accomodate a different critical listening point relative to the speaker system. 
     FIG. 5 is a fragmentary view, with parts in section, from the speaker system of FIG. 1 illustrating adjustment means for positioning a selected driver relative to the critical listening point and means for locking the driver in that position while providing a calibrated reading of the driver position. 
     FIG. 6 is another fragmentary view from the speaker system of FIG. 1 illustrating adjustment and calibration means for a selected driver, or driver assembly, relative to the other drivers within the system while said driver(s) may remain fixed in relation to the critical listening point or to the listening environment. This may be accomplished by adjusting the position of a mounting sub-assembly which supports the other drivers relative to the &#34;fixed&#34; component. 
     FIG. 7 is a fragmentary view, with parts in section, illustrating a sound isolating arrangement for mounting the woofer driver(s) to the low frequency housing. 
     FIG. 8 is a side view with parts in section of the housing for the woofer(s) referred to above in FIG. 7. 
     FIG. 9 is a view taken along section line IX--IX of FIG. 8. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A speaker system constructed in accordance with the present invention is generally indicated at 11 in FIG. 1. Preferably, the invention contemplates a speaker assembly including a relatively large number of drivers, or multiple driver sub-components, covering different frequency ranges in order to provide various desired characteristics such as accurate tonal balance, transient response, and power handling. However, for purposes of simplicity, it is believed that the present invention may be clearly understood from the simplified representation for the speaker assembly 11 which is illustrated in FIG. 1 as including a single low frequency range driver, or woofer, unit 13, two midrange units indicated at 15 and 19, and a single tweeter unit 17. 
     The drivers are illustrated as being supported by two mounting sub-assemblies generally indicated at 12 and 21. 
     The woofer unit 13 is preferably mounted upon, and in fixed relation to thesupport base 12. The upper mounting subassembly 21 rests upon an adjustablesupport 23, and extends upward from the woofer unit 13 to provide support for the other driver units 15, 19, and 17. 
     Since the construction of the drivers is generally conventional, with the exception of the woofer housing 14, it is not believed necessary to show components of the driver units 15, 19, and 17 in greater detail. 
     The speaker assembly 11 of the present invention is particularly adapted for adjusting the various drivers generally fore and aft relative to a critical listening point in a listening environment in order to establish maximum sound coherency for the system at that point. 
     In this connection, reference is made to FIGS. 2-4 which illustrate variousselected listening points and environments. The locale for the speaker assembly is generally indicated at 32 in each of FIGS. 2-4. A first critical listening point is indicated in each of the three figures at 30 while a second selected critical listening point is indicated at 34 in FIG. 4. 
     The manner of adjustment for the drivers in the speaker assembly to achievemaximum sound coherency is described in greater detail below in connection with FIGS. 2-4. It is initially noted, however, that while at least some of the drivers in the speaker assembly of FIG. 1 are adjustable fore and aft relative to the critical listening point, the present invention contemplates adjustment of the drivers along any axis relative either to each other or to a critical listening point in order to achieve a desired time or phase alignment. It is also noted that while in the speaker assembly of FIG. 1, and as described in the preferred embodiment of the invention, the adjustment apparatus for the drivers is calibrated in orderto provide benefits such as those described above, the present invention contemplates any adjustment means for the drivers whether or not such means are calibrated. Referring to FIG. 1, it is noted that where rods areused to support and adjust a driver, such as the tweeter 17, one rod may becalibrated, as rod 22, while the second, as rod 24, need not be calibrated. 
     Referring to FIG. 5, it may be seen that the driver housing is rigidly affixed to an elongated rod 22 via a mounting such as 47. The rod is slidably mounted through a hole in a mounting block 44, which is in turn rigidly mounted as part of the mounting sub-assembly 21. The calibration markings are generally indicated at 46, and may be indexed, for example, to the face of the mounting block 44. Means for locking the rod in position are provided by a set screw, indicated at 45. 
     Referring to FIG. 6, the position of the woofer 13 relative to the other drivers in the system is adjusted by sliding the mounting sub-assembly 21 along a calibrated track 23 which is in turn rigidly affixed to the wooferhousing 14. The calibration markings are generally indicated at 49, and maybe indexed, for example, to the front edge of the mounting sub-assembly 21.Locking bolts, indicated at 48, are provided to lock the assembly 21 in place. 
     Referring now to FIGS. 2-4, and particularly to FIG. 2, which is representative of the prior art, the low frequency range unit 13 and the high frequency range unit 17 are arranged in a co-planar relationship. An electrical sound signal in the form of a square wave as generally indicated at 27 is applied to the drivers 13 and 17 from a suitable source26 through a crossover network 25. Without further consideration of factorspresent in the drivers and in the crossover network, it may be assumed for purposes of example that reproduced sound components 28 and 29 from the respective drivers 13 and 17 are slightly out of phase as they reach the listener, as represented by the delay component generally indicated at 33.Thus, the listener at critical listening point 30 would hear the reproducedsound components in a distorted blur, without the time domain coherence contemplated by the present invention. 
     Referring now to FIG. 3, a similar situation to that of FIG. 2 is represented. However, in this case the drivers 13 and 17 are positioned with a tweeter 17 somewhat behind the woofer 13, as is generally contemplated in the above cited Dahlquist patent. It may be seen that the delay between the sound components 28 and 29 as heard by listener at critical listening point 30 has been eliminated, rendering a coherent sound. It should be noted that the speaker system 32 of FIG. 3 is capable of producing coherent sound for only one critical listening point, and that said point will be at a fixed distance from the speaker system. 
     Referring now to FIG. 4, the same speaker system from FIG. 3 is indicated at 32. In this case it may be seen that a second listener at critical listening point 34 would hear a delay, or incoherence, between reproduced sound components 28 and 29. Further adjustment of the speaker system wouldbe necessary in order to provide sound coherency at the critical listening point 34. This illustrates the need for apparatus and means to provide maximum sound coherence at a variety of given critical listening points. 
     Reference is now made to FIGS. 7-9 in order to disclose preferred construction details for the woofer housing 14. Generally, the woofer(s) 13 are mounted on a front panel of a woofer housing as illustrated for example at 40 in FIGS. 7 and 8. 
     Referring particularly to FIG. 7, the woofer unit fragmentarily representedat 13 is secured to the front panel 40 by one or more mounting screws such as indicated at 36. Sound absorbing material is arranged between the frontpanel 40 and the woofer unit 13 as well as between the front panel 40 and the remainder of the woofer housing 14 in order to isolate the woofer housing 14 from mechanical vibrations induced by the frame of the woofer unit 13, thus significantly reducing unwanted acoustical radiation from the housing 14 itself. The front panel 40 may, for example, be formed froma rigid material such as plywood. The sound absorbing means contemplated bythe present invention comprises a sandwich panel arranged on the forward surface of the front panel 40 adjacent to the housing 14 as indicated at 41. Both of the sandwich panels 39 and 41 are of known construction which is not shown in greater detail herein. However, it is noted that each of the panels is of a laminate construction employing special sound absorbingplastics, and may include layers of metal plate. 
     The effect of this use of the sound absorbing layers as illustrated in FIG.7, is to prevent the mechanical transmission of vibrations from the woofer frame 13 to the front panel 40, as well as from the front panel 40 to the woofer housing 14. Thus, there may be a significant reduction of structural vibration within the housing and a significant reduction of their accompanying re-radiation to the air, while at the same time maintaining the woofer unit(s) in rigid relation to the other drivers within the system in order to maintain maximum sound coherency. 
     It should be noted at this point that, in the prior art, woofer units have some times been isolated from their housings by, for example, soft rubber bushings which, although perhaps effectively achieving mechanical isolation, may not be able to maintain the woofer in rigid relation to theother drivers of the system and thus not be able to maintain maximum sound coherency as contemplated by the present invention. 
     In any event, the front panel 40 is secured to the woofer housing 14 by oneor more screws 38 passing through the front panel and the sandwich plate 41. The screws 36 are similarly passed through the woofer frame 13, the sandwich plate 39, the front panel 40, and are engaged by the nuts such asthat indicated at 37. It may be seen that the nuts 37 are also isolated from both the woofer frame 13 and the sandwich plate 41 in order to further prevent sound transmission from the woofer 13 to the housing 14. 
     Referring also to FIGS. 8 and 9, the woofer housing 14 is preferably constructed to achieve enhanced rigidity of the housing as well as dampingof sound waves and reduction of non-linear air flow or turbulance within the housing. Since the low frequency range units of many speaker systems may generate considerably more mechanical energy and movement of air than other drivers in the system, the proper damping, or control, of these components is important if accurate, coherent sound is to be reproduced bythe system. 
     For this purpose, a plurality of angled dividers 51 is arranged in spaced apart relation within the housing 14 to form sound wells extending rearwardly from the woofer driver(s) 13 in parallel relation with its axis. The dividers 51 are mechanically interconnected with each other and the inside surfaces of the housing 14, for example, by glue and &#34;tongue-in-groove&#34; construction, so that the sides of the housing are rigidly braced and the sound wells are separated from each other. The sound wells 42 all open forwardly toward the woofer(s) 13 and are individually filled with sound damping material such as, for example, fiber batting sold for example under the trademark Dacron. Thus, as sound waves propagate rearwardly from the driver(s), they pass into the separatesound wells 42 which prevent lateral turbulence effects from developing within the woofer housing 14. A brace element 43 also extends from the rear surface 50 to a central position on the front panel 40 in order to further stabilize the front panel and rigidify the entire housing 14. 
     Referring momentarily to FIG. 8, it may be seen that the rear surface 50 for the sound wells 42 could also form the back of the housing 14. However, the surface 50 is preferably spaced apart from the back of the woofer housing in order to form a space that may be used for further damping of sound waves or perhaps, for example, for storage or other purposes. 
     DESCRIPTION OF THE METHOD OF OPERATION 
     While it is believed that the method of operation contemplated by the present invention, for the adjustment of drivers relative to a critical listening point in order to establish maximum sound coherency at that point, is apparent from the above description, that method is described below in order to facilitate a more complete understanding of the invention. 
     In general, it may be desirable for reasons such as apparent image size to mount the drivers in a relatively spaced-apart configuration. It then becomes particularly desirable to establish a proper relation between the drivers and the critical listening point in order to achieve coherent sound reproduction at that point. Thus, while the present invention contemplates in general any adjustment of drivers along any axis within a speaker system, the invention particularly contemplates adjustment fore and aft of selected drivers within the speaker system illustrated in FIG. 1, assuming that the spaced-apart relation of the drivers has been predetermined in order to achieve various desired sonic characteristics. It should be noted that the method described below would serve equally well for adjustment of drivers along any axis in order to obtain maximum coherency of sound at a given desired point. 
     The pulse or phase response characteristics for any of a variety of driversmay be calculated or experimentally determined in a known manner. However, the present invention contemplates a method for adjusting the positions ofdrivers within a system relative to a critical listening point in order to compensate for the rise time characteristics of the drivers themselves, and also for effects that may be caused by associated equipment in the entire sound reproduction system. 
     With the speaker system 11 established at a predetermined locale within a selected listening environment, one or more critical listening points are also determined within the environment. In order to accurately monitor theemanations of the various drivers, a monitor such as a microphone or the like is mounted at one of the critical listening points, and coupled to a time domain measurement device such as an oscilloscope (not shown). 
     The woofer unit 13, being fixed in relation to the critical listening point, is used to establish a reference time delay component between the speaker system and the critical listening point. A sharp pulse is applied to the woofer unit via all, or a significant portion of, the sound reproducing system. The time elapsed from the generation of the electricalpulse and the arrival of its leading edge is then noted. The same pulse is then applied in turn to each of the remaining driver assemblies 15, 17, and 19 in turn. The position of each driver is then adjusted so that the leading edge of its reproduced pulse arrives at the critical listening point at the same time as the previously determined reference pulse. Thus,all the drivers may be adjusted so that their reproduced sound components will come into synchronization for a listener at that critical listening point. 
     The position of each driver assembly may now be noted using the calibrationsystems illustrated in FIGS. 5 and 6. The relation between the drivers may now be recorded in connection with the first critical listening point. This process of adjustment, calibration, and recording may be repeated foreach of any additional critical listening points. 
     It is apparent that all or part of the speaker system 11 could be disassembled and reassembled with the proper relation between the drivers being reproduced without repeating the monitoring steps described above. At the same time, the speaker system could easily and repeatedly be adjusted to produce maximum sound coherency at any of the previously determined critical listening points, by simply referring to the calibrations recorded for those points as described above. 
     It is also apparent that interchangeable driver components for a speaker system could be designed with precalibrated positions, in order to facilitate their interchange into a previously adjusted speaker system while substantially maintaining maximum sound coherence as described above. 
     Various other modification and changes will be readily apparent within the scope of the present invention in accordance with the method and apparatusdescribed above. Many variations upon the mounting sub-assembly are possible beyond that illustrated in FIG. 1. For example, components could be ceiling mounted, or mounted with a motor drive to facilitate automatic adjustment of their relative positions. While the preferred embodiment of the present invention includes calibration means for reasons described above, any means for adjustment of drivers, with or without calibrations, according to the method described above would fall within the scope of theinvention. Accordingly, the scope of the present invention is defined only by the following claims.