Abstract:
A stereo loudspeaker system with a right channel and a left channel respectively feeding R and L signals to four speakers (including a right main speaker, a right sub-speaker, a left main speaker and a left sub-speaker) includes a transformer that, depending on the frequency, magnetically couples or isolates the two channels at the right and left sub-speakers. At low frequencies, the transformer isolates the right channel from the left one so that the R signal goes primarily only to the two right speakers, and the L signal goes primarily only to the two left ones. At high frequencies, both the right and left main speakers still receive their respective R and L signals; however, the transformer&#39;s magnet coupling conveys a differential R-L signal to the right sub-speaker and a differential L-R signal to the left sub-speaker, thereby producing an expanded acoustic image and realistic ambient field.

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to loudspeaker systems and more specifically to stereo systems with two channels each having main and sub speakers. 
     BACKGROUND 
     Some stereo loudspeaker systems have right and left channels driving four speakers, wherein the four speakers include a pair of speakers on the right and another pair on the left. Each pair comprises a main speaker and an associated sub-speaker, so the system includes a right main speaker, a right sub-speaker, a left main speaker and a left sub-speaker. Examples of such systems are disclosed in U.S. Pat. Nos. 4,489,432 and 4,638,505, both of which are specifically incorporated herein by reference. 
     In some cases, the right and left channels feed R and L signals to the right and left main speakers, respectively. In addition, at lower frequencies, R and L signals are applied respectively to the right and left sub-speakers as well. At certain higher frequencies, however, a differential R-L signal is sent to the right sub-speaker, and a differential L-R signal is sent to the left sub-speaker. Although the proposed benefits of this are well known to those of ordinary skill in the art, actually achieving the desired results has been an elusive goal due to an assortment of problems pertaining to some frequencies, various signal conditions and/or certain amplifiers. 
     Various attempts to avoid some problems seem to create others. The &#39;505 patent, for instance, discloses using a combination of capacitors and inductors for providing a desired response at both high and low frequencies; however, resonance and distortion seem to occur at a transitional point between high and low frequencies. In what perhaps is an attempt to avoid resonance created by a combination of capacitors and inductors, some known systems omit the capacitors of the &#39;505 patent, add a crossover wire between the two sub-speakers, and rearrange the circuit such that two inductors, without the capacitors of the &#39;505 patent, provide desired responses at both high and low frequencies. A prior art Polk Audio electrical schematic SDA 2B/CRS+ Schematic NC refers to such a crossover wire as an “IC wire from the crossover.” A crossover wire connecting two sub-speakers, however, can provide a low DC impedance connection between the right and left minus input terminals of the amplifier, which can create problems with certain amplifiers, particularly amplifiers of the bridge type output. 
     Other known prior art systems insert a capacitor along the shunt to break the shunt&#39;s otherwise low DC impedance path. Adding the capacitor, unfortunately, reintroduces the resonance issue as an undesirable artifact. Over certain frequencies (e.g., 50-200 Hz) and/or under certain conditions (e.g., when L=−R, or when L exists while R=0), the capacitor resonates with the series inductors, resulting in sudden shifts in phase and level changes in the sub-speakers. In some cases, the resonance of the added capacitor with the series inductors produces poor transient response in the 50-200 Hz range. The sudden shift in phase, level and poor transient response produce undesirable audible effects, such as objectionable coloration in the upper bass and lower midrange frequencies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an electrical schematic of an example loudspeaker system in accordance with the teachings disclosed herein. 
         FIG. 2  is an electrical schematic similar to  FIG. 1  but showing the addition of an example bridge style amplifier system. 
         FIG. 3  is an electrical schematic similar to  FIG. 1  but showing the addition of an example non-bridge style amplifier system. 
         FIG. 4  is an electrical schematic similar to  FIG. 1  but showing the system operating in a high frequency mode. 
         FIG. 5  is an electrical schematic similar to  FIG. 4  but showing the system operating in a low frequency mode. 
         FIG. 6  is a schematic diagram showing the loudspeaker system of  FIG. 1  installed within an example enclosure. 
         FIG. 7  is a schematic diagram similar to  FIG. 6  but showing the loudspeaker system installed within an alternate enclosure arrangement. 
         FIG. 8  is a schematic diagram similar to  FIGS. 6 and 7  but showing yet another example enclosure arrangement. 
         FIG. 9  is a graph showing a comparison of SPICE simulation results of anticipated amplitude responses of sub-speaker signals when signal R=L. 
         FIG. 10  is a graph showing a comparison of SPICE simulation results of anticipated phase responses of sub-speakers when signal R=L. 
         FIG. 11  is a graph showing a comparison of SPICE simulation results of anticipated amplitude responses of sub-speaker signals when only one channel R or L is active. 
         FIG. 12  is a graph showing a comparison of SPICE simulation results of anticipated phase responses of sub-speakers when only one channel R or L is active. 
         FIG. 13  is a graph showing a comparison of SPICE simulation results of anticipated amplitude responses of sub-speaker signals when signals R and L are opposite each other (e.g., R=1 while L=−1). 
         FIG. 14  is a graph showing a comparison of SPICE simulation results of anticipated phase responses of sub-speakers when signals R and L are opposite each other (e.g., R=1 while L=−1). 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-8  illustrate an example stereo loudspeaker system  10  having a right channel  12  and a left channel  14  that deliver an audio signal R  16  and an audio signal L  18  to right and left sets of speakers. In some examples, the right set of speakers includes a right main speaker  20  and a right sub-speaker  22 , and the left set of speakers includes a left main speaker  24  and a left sub-speaker  26 .  FIG. 1  shows a basic electrical schematic of one example of system  10 ,  FIG. 2  shows system  10  including an example bridge style amplifier system  28 ,  FIG. 3  shows system  10  including an example non-bridge style amplifier system  29 ,  FIGS. 4 and 5  show example audio signal flow patterns at different frequencies, and  FIGS. 6 ,  7  and  8  show examples where multiple speakers of system  10  share the same acoustic volume within various enclosures. 
     To achieve the benefits and overcome the limitations of the loudspeaker systems disclosed in U.S. Pat. Nos. 4,489,432 and 4,638,505, both of which are specifically incorporated herein by reference, loudspeaker system  10  includes a transformer  30  having a certain relationship with sub-speakers  22  and  26 . Transformer  30  eliminates the need for a conventional crossover wire between the two sub-speakers  22  and  26 , thus making system  10  compatible with a variety of amplifiers, including those with a bridge type output. 
     In the example shown in  FIG. 2 , amplifier system  10  has a right plus amp  32  connected to both the right main speaker  20  and right sub-speaker  22 , a right minus amp  34  connected to both the right main speaker  20  and a first winding  36  of transformer  30 , a left plus amp  38  connected to both the left main speaker  24  and left sub-speaker  26 , and a left minus amp  40  connected to both the left main speaker  24  and a second winding  42  of transformer  30 . To prevent a small random DC voltage from driving an unopposed and surprisingly high DC error current through a crossover wire between the left and right minus amps  34  and  40 , the crossover wire is eliminated and instead a magnetic coupling  44  of transformer  30  provides a DC current break  46  that is effectively between the left and right minus amps  34  and  40 . 
     Depending on the amplifier&#39;s output frequency of signal R  16  and/or signal L  18 , the magnetic coupling  44  between the transformer&#39;s windings  36  and  42  either isolates or combines signal R  16  and signal L  18 , as applied to right and left sub-speakers  22  and  26 . In some examples, at low frequency (e.g., a predetermined range of frequencies below 200 Hz), right channel  12  primarily feeds audio signal R  16  to right main speaker  20  and to right sub-speaker  22 , and left channel  14  primarily feeds audio signal L  18  to left main speaker  24  and to left sub-speaker  26 . At high frequency (e.g., a predetermined range of frequencies above 200 Hz), right channel  12  and left channel  14  still feed signal R  16  and signal L  18  to their respective right and left main speakers  20  and  24 ; however, transformer  30  operating at higher frequency conveys signal R  16  and signal L  18  across magnetic coupling  44  between windings  36  and  42 . 
     Consequently, at relatively high frequency, magnetic coupling  44  transmits signal L  18  from second winding  42  to first winding  36 . First winding  36 , in turn, applies signal L  18  (e.g., an attenuated portion thereof) to a right sub negative terminal  48  of right sub speaker  22  while signal R  16  is applied to a right sub positive terminal  50 , thereby feeding right sub-speaker  22  a right differential signal  52  (i.e., signal R  16  minus signal L  18 , also denoted as an R-L signal). Likewise, magnetic coupling  44  transmits signal R  16  (e.g., an attenuated portion thereof) from first winding  36  to second winding  42 . Second winding  42  then applies signal R  16  to a left sub negative terminal  54  of left sub speaker  26  while signal L  18  is applied to a left sub positive terminal  56 , thereby feeding left sub-speaker  26  a left differential signal  58  (i.e., signal L  18  minus signal R  16 , also denoted as an L-R signal). The term, “terminal” refers to an electrically conductive point and not necessarily a connector, plug, socket or screw. 
     In addition to terminals  48 ,  50 ,  54  and  56 ; other electrical points of loudspeaker system  10  include right main speaker  20  having a right main positive terminal  60  and a right main negative terminal  62  connected to right channel  12 , left main speaker  24  having a left main positive terminal  64  and a left main negative terminal  66  connected to left channel  14 , a right plus input terminal  68  and a right minus input terminal  70  on right channel  12 , a left plus input terminal  72  and a left minus input terminal  74  on left channel  14 , and transformer  30  having a plurality of terminals. The transformer&#39;s plurality of terminals include first winding  36  having a first point  76  and a second point  78 , and second winding  42  having a third point  80  and a fourth point  82 . 
     In the example shown in  FIG. 3 , amps  34  and  40  are omitted for the non-bridge style amplifier  29 . For signal stability, a ground  84  connects to right minus input terminal  70 , right main negative terminal  62 , second point  78 , fourth point  82 , left minus input terminal  74  and left main negative terminal  66  in a certain strategic arrangement. Such an arrangement creates a first electrical path  86  from right main negative terminal  62  to ground  84 , a second electrical path  88  (via first winding  36 ) from right sub negative terminal  48  to ground  84 , a third electrical path  90  from left main negative terminal  66  to ground  84 , and a fourth electrical path  92  (via second winding  42 ) from left sub negative terminal  54  to the ground  84 . This arrangement is believed to work particularly well when (a) first electrical path  86  is of lower impedance than second electrical path  88  at frequencies greater than 200 Hz, (b) third electrical path  90  is of lower impedance than fourth electrical path  92  at frequencies greater than 200 Hz, (c) second electrical path  88  passes through the transformer&#39;s first winding  36 , and (d) fourth electrical path  92  passes through the transformer&#39;s second winding  42 . 
     In some examples, a frequency of 200 Hz defines the loudspeaker system&#39;s transition between its high-frequency mode ( FIG. 4 ) and its low-frequency mode ( FIG. 5 ). During the low frequency mode, as shown in  FIG. 5 , signal R  16  passes through both the right sub-speaker  22  and the transformer&#39;s first winding  36 , and signal L  18  passes through both the left sub-speaker  26  and the transformer second winding  42 . During the high frequency mode, as shown in  FIG. 4 , signal L  18  is applied to left sub-speaker  26 , magnetically transferred from the transformer&#39;s second winding  42  to first winding  36 , and is applied to right sub-speaker  22 . At the same time, signal R  16  is applied to right sub-speaker  22 , magnetically transferred from the transformer&#39;s first winding  36  to second winding  42 , and is applied to left sub-speaker  26 ; whereby left differential signal  58  (L-R) is applied to left sub-speaker  26  and right differential signal  52  (R-L) is applied to right sub-speaker  22 . 
     Although 200 Hz, in some examples, defines the loudspeaker system&#39;s transition between its high-frequency mode and its low-frequency mode, the transition point is approximate and does not necessarily occur abruptly nor precisely at 200 Hz. In some examples, as the frequency decreases toward 200 Hz, transformer  30  attenuates the −L portion of right differential signal  52  (R-L), as applied to right sub-speaker  22 , and attenuates the −R portion of left differential signal  58  (L-R), as applied to left sub-speaker  26 . At some predetermined frequency at or below 200 Hz, right differential signal  52  (R-L) becomes predominantly signal R  16 , and left differential signal  58  (L-R) becomes predominantly signal L  18 . 
     While some examples of system  10  have 200 Hz as the chosen defining transition frequency between the system&#39;s high and low frequency modes, other examples of system  10  have transition frequencies higher and lower than 200 Hz. In some examples, however, 200 Hz is chosen because only the frequency range between about 200 Hz and 1,000 Hz is used by a listener&#39;s directional hearing mechanism to determine the direction of a sound on the basis of inter-aural time delays, as taught in U.S. Pat. No. 4,638,505. 
     The relationship between the sub-speakers&#39; impedance and the transformer winding inductance helps determine or establish the transition frequency of system  10 . In some examples, for a transition frequency of about 200 Hz, each sub-speaker  22  and  26  has a nominal impedance of 8 ohms while each winding  36  and  42  of transformer  30  has a nominal impedance of about half that or about 4 ohms at 200 Hz. Other examples of system  10  vary from those values. For instance, in some examples, right sub-speaker  22  has a sub-speaker impedance at 200 Hz, first winding  36  of transformer  30  has a winding impedance at 200 Hz, and a ratio of the sub-speaker impedance to the winding impedance at 200 Hz is between 0.5 and 8. 
     Although it is possible to establish a 200 Hz transition frequency by means other than by using transformer  30 , such alternate means may create various problems, such as resonance, distortion, audible artifacts, excess sub-speaker stress, abrupt phase shifts and incompatibility with certain amplifiers (particularly bridged type output amplifiers). Some of these problems may be more noticeable during certain operating condition, such as, for example, when R=−L, or when L is present while R=0, or when R is present while L=0. System  10  with transformer  30 , however, mostly avoids these problems, as illustrated in  FIGS. 9-14 . 
       FIGS. 9-14  provide comparisons of a SPICE simulation circuit based on an example of system  10  (solid lines  94   a - 94   f ) with transformer  30  and a SPICE simulation circuit based on a conventional system without transformer  30  (dashed lines  96   a - 96   f ). The term, “SPICE,” as known to those of ordinary skill in the art, is a Simulation Program with Integrated Circuit Emphases, which is a known general-purpose, open source electrical circuit simulator. 
     In  FIGS. 9 ,  11  and  13 ; the vertical y-axis, or ordinate, shows voltage amplitude expressed in decibels. In  FIGS. 10 ,  12  and  14 ; the vertical y-axis, or ordinate, shows phase shift expressed in degrees. In  FIGS. 9-14 , the horizontal x-axis, or abscissa, is a frequency scale in units of Hertz. 
       FIGS. 9 and 10  illustrate sub-speaker response when signal R  16  equals signal L  18  (i.e., R=L, mono). In  FIG. 9 , solid line  94   a  represents the amplitude response of left sub-speaker  26  of system  10  over a range of frequencies, and dashed line  96   a  represents the amplitude response of a conventional system (e.g., a system based on U.S. Pat. No. 4,638,505). In  FIG. 10 , solid line  94   b  represents the phase response of left sub-speaker  26  of system  10  over a range of frequencies, and dashed line  96   b  represents the phase response of a conventional system. 
       FIGS. 11 and 12  illustrate sub-speaker response when the loudspeaker system receives a signal from only the right or left channel. In the example of  FIGS. 11 and 12 , the loudspeaker system receives only signal L  18  while right channel  12  is basically at ground. In  FIG. 11 , solid line  94   c  represents the amplitude response of left sub-speaker  26  of system  10  over a range of frequencies, and dashed line  96   c  represents the amplitude response of a conventional system. In  FIG. 12 , solid line  94   d  represents the phase response of left sub-speaker  26  of system  10  over a range of frequencies, and dashed line  96   d  represents the phase response of a conventional system. 
       FIGS. 13 and 14  illustrate sub-speaker response when signal R  16  is opposite in amplitude to signal L  18  (i.e., R=−L). In  FIG. 13 , solid line  94   e  represents the amplitude response of left sub-speaker  26  of system  10  over a range of frequencies, and dashed line  96   e  represents the amplitude response of a conventional system. In  FIG. 14 , solid line  94   f  represents the phase response of left sub-speaker  26  of system  10  over a range of frequencies, and dashed line  96   f  represents the phase response of a conventional system. 
     At midrange frequencies, the loading presented by an acoustic volume usually does not significantly influence the response of an enclosed speaker. At low frequencies, however, the response of a speaker is controlled to a great extent by the ratio of the speaker&#39;s total diaphragm area to acoustic volume. In cases where two or more speakers operate within the same acoustic volume, the ratio of the speakers&#39; total diaphragm area to acoustic volume will change accordingly (e.g., change by a factor of two where two identical speakers share a common enclosure). An excessively high diaphragm/volume ratio can create adverse loading, depending on the relationship of the right and left channel signals. So, under certain conditions, it is difficult or impossible for a frequency tailoring circuit to tune some stereo loudspeaker systems for optimum low frequency response. 
     With transformer  30 , however, sub-speakers  22  and  26  can operate in a consistent manner at low frequencies regardless of the relationship of channel signal R  16  and signal L  18 . Some examples of system  10  can thus be optimally tuned by way of a known frequency tailoring circuit  98  ( FIGS. 2 and 3 ) without the need for compromise necessitated by a main speaker and its associated sub-speaker operating and interacting within the same acoustic volume. Consequently, transformer  30  makes loudspeaker system  10  particularly suited for various enclosure configurations including, but not limited to, the examples shown in  FIGS. 6 ,  7  and  8 . The example of  FIG. 6  shows speakers  20 ,  22 ,  24  and  26  plus transformer  30  installed within a single enclosure  100 , whereby all four speakers  20 ,  22 ,  24  and  26  share a common acoustic volume  102  defined by enclosure  100 . The example of  FIG. 7  shows speakers  20  and  22  in one enclosure  104  and speakers  24  and  26  in another enclosure  106 . So, right main speaker  20  and right sub-speaker  22  share a common acoustic volume  108  defined by enclosure  104 , and left main speaker  24  and left sub-speaker  26  share a common acoustic volume  110  defined by enclosure  106 .  FIG. 8  basically shows an example where enclosures  104  and  106  share a common dividing wall  112 . Although transformer  30  can be mounted inside or outside of any speaker enclosure, transformer  30  sharing an enclosure with multiple speakers  20 ,  22 ,  24  and/or  26  simplifies the construction of system  10 , as shown in  FIGS. 6 ,  7  and  8 . In some examples, speakers  20 ,  22 ,  24  and  26  are contained separately within four individual enclosures. 
     The following provides additional clarification. The terms, “right” and “left” are nondescript labels used merely for distinguishing one item from another in the same way the labels, “first” and “second” would do, and so the terms, “right” and “left” do not necessarily determine an item&#39;s relative location to a listener or to another item. Thus, for example, saying that a right main speaker and a right sub-speaker share the same acoustic volume is equivalent to saying that a left main speaker and left sub-speaker share the same acoustic volume. The terms, “speaker” and “driver” are equivalent and used interchangeably. The terms, “amp” and “amplifier” are equivalent and used interchangeably. The term, “negative” as it relates to identifying various terminals is simply a distinguishing label and thus does not mean that the signal on such terminals are necessarily negative. A DC current interruption or break refers to a substantially non-electrically conductive path. The term, “frequency tailoring circuit” refers to any known circuit of capacitors, inductors, resistors and/or other electrical components for enhancing or correcting signals conveyed to a speaker. The term, “high frequency mode” simply means that the audio signal is at a predetermined high frequency range above a predetermined transitional frequency (e.g., above 200 Hz), and the term, “low frequency mode” simply means that the audio signal is at a predetermined low frequency range below the predetermined transitional frequency. The term, “effectively between” as it pertains to the magnet coupling relative to the windings of the transformer means that the magnetic coupling is situated to transmit a magnetic field from one winding to the other. The term, “effectively between” as it pertains a DC current break or interruption being effectively between two amps means that the DC current break or interruption is generally void of an electrically conductive DC path connecting one amp to the other. The term, “magnetic coupling” refers to structure being of a material and being at a position to transmit an appreciable magnetic field from one winding to another. Examples of materials for a magnet coupling include, but are not limited to, iron and alloys of iron. 
     Although certain example methods, apparatus and articles of manufacture have been described herein, the scope of the coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.