RELATED APPLICATION 
     This application claims priority from U.S. application Ser. No. 11/075,469 filed Mar. 8, 2005, now U.S. Pat. No. 7,451,708, which claims priority from U.S. Provisional Application No. 60/551,652 filed Mar. 8, 2004 both of which are herein incorporated by reference. 
    
    
     COPYRIGHT NOTICE 
     © 2004-2005 QSIndustries, Inc. A portion of the disclosure of this patent document, including but not limited to the drawings, contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR §1.71(d). 
     TECHNICAL FIELD 
     This invention pertains to on-board generation of sound effects in motorized toys, and particularly in diesel, electric, and steam locomotives and other model types. 
     BACKGROUND OF THE INVENTION 
     On-board sound became very popular in 1994 when QSI introduced a high-quality, low-cost sound system for three-rail O&#39;Gauge locomotives and when the Lionel Corporation introduced a competitive digital system about the same time. In only about three years, sound was offered in almost all top-of-the-line three-rail O&#39;Gauge locomotives and was starting to appear in the low-end model three-rail locomotives as well. 
     The advent of inexpensive on-board digital sound for three-rail O&#39;Gauge trains only became possible and popular when small affordable digital microprocessors became available that would allow sufficient sound processing within the limited space of model train locomotives. Sufficient space within the locomotive was not only needed for the electronics but was also required for acoustic sound quality and speaker placement. As the three-rail O&#39;Gauge manufacturers began to install sound in smaller O&#39;Gauge engines, they were forced to use smaller speakers, which often sacrificed sound quality. 
     A typical sound installation is shown in  FIG. 1  for an O&#39;Gauge three-rail diesel in this partial cut-away view of cab and chassis. Most three-rail O&#39;Gauge engines have two vertically mounted DC “Can” permanent magnet motors  100  and  101  mounted directly over the diesel trucks  102  and  103  with worm and worm gears to drive one of the metal axles directly in each truck. The other axles in the trucks are linked to the driven axle with spur gears so all or most of the locomotive&#39;s wheels have power. Flywheels,  104  and  105 , are often added to the motor shafts to provide coasting action should power be lost. It is also believed by most customers that the flywheel improves low speed performance. The motors are powered and controlled by the Sound and Motor Drive Electronics  106 . 
     The chassis, motors and electronics are enclosed by the diesel body,  109  (also called a diesel cab). There are three popular diesel cab styles, wide-bodied cabs,  1700 , as shown in  FIG. 17 , narrow-bodied cabs with high hoods,  1800 , as shown in  FIG. 18  and narrow-bodied cabs with low hoods,  2700  as show in  FIG. 27 . All three diesel types have about the same chassis width as measured at the engineers cab area,  1701 ,  1801 , and  2701  even though the hood or body width, W- 1 , W- 2 , W- 3 , varies. The narrow-bodied types of engines have outside walkways along the sides of the hooded area that make up the extra width. The wide-bodied diesels have the advantage of more interior space for electronics, speakers, and motors. The narrow-bodies, however, are more popular and account for larger variety and larger number of prototypes and models manufactured and sold. 
     Model diesel cabs are usually made of injection molded plastic although brass cabs are also popular albeit expensive and a few die-cast cabs have been produced as well. The diesel cabs may also have openings that can affect the sound quality such as the fans,  1703  and  1803 , and ventilation vents or grills,  2702 ,  2703  and  1702 ,  1802 , and their counterparts on the other side of the body,  1704  and  1804 , not visible in the views shown in  FIGS. 17 and 18 . Other possible openings are the windows, portholes on the side, smoke stacks, etc. Engine models, like electric locomotives (GG-1, EP-5, E-33, etc.) have similar drive methods and sound issues as diesels. Cabs for electric locomotives also include wide body and narrow body types. 
     The vertical motor layout shown allows sufficient room between the motors  100  and  101  to mount a full-featured sound and motor control electronics board,  106 . A single speaker  107 , is usually mounted facing down or facing up in the fuel tank area below the chassis  156 . It is either sealed face up to a vent hole in the chassis,  110 , or mounted face down and sealed to the bottom of the fuel tank,  108 , and covering venting holes  111 . In either case, the speaker&#39;s front wave and back wave are separated. When mounted facing up, the front wave is vented into the sealed diesel cab area and the back wave is vented to the outside through the holes in the bottom of the fuel tank. When mounted facing down in the fuel tank, the front wave exits through the series of holes,  111 , and the back wave enters the sealed diesel cab area through vent hole  110 . The diesel cab area or “interior volume” defined by the chassis,  156 , and diesel body  109  acts as a resonate cavity and improves both the base response and the audio volume. There appears to be little difference in sound quality between mounting face-up or face-down. However, for narrow-bodied diesels, the vent hole,  110 , must have a diameter less then or equal to the interior width of the diesel cab which will partially block the speaker&#39;s front wave if the speaker is mounted face-up. Since the fuel tanks have about the same width, a full width speaker can be mounted face-down in most diesel types. 
     To avoid confusion between what we mean by back wave and front wave, we will restrict our examples to the method where the speaker is mounted face-down with front wave exiting to the outside through the holes,  111  and back wave entering the cab area through hole  110 . 
     One advantage of using vertical motors mounted directly over the trucks is that it provides an unencumbered interior space above the fuel tank to allow sound to enter the cab area and sufficient room to mount the sound circuit board,  106 , to the chassis surface, and to allow heat sinking the circuit board to the metal chassis as required. The circuit board width and mounting are generally designed to allow sound to pass around the circuit board and/or around the mounting studs,  175 , to utilize as much of the diesel cab volume as possible. 
     Sound installation in smaller gauge engines: It becomes more difficult to use the same motor and flywheel design for smaller gauge engines like HO and N scale, which, in turn, requires different designs for sound systems, acoustics and speaker installation. There is little difficulty in achieving high quality acoustic sound design for most steam engines since the sound can be installed in the tender in the same way it is done for O gauge steam locomotives as shown in  FIG. 25 . (In  FIG. 25 , speakers,  2531  are mounted in the tender, vented out the chassis vent holes  2532 . An audio electronics/ motor control circuit board  2506  connects to a motor  2500  in the locomotive  2501  which, in turn, drives gear tower  2512  to drive the wheels.) However, installation in HO diesels and electrics, and in particularly, in narrow body, low hood types is problematic. This can be better understood by examining how most HO drive systems operate. 
     The most common type of motor and drive train for HO and N type engines is shown in  FIG. 2 . Instead of two vertical motors, each with flywheels, only one horizontal motor,  200 , is used with two flywheels,  204  and  205 , mounted on the front and rear motor shafts. Power is transmitted through drive shafts,  216  and  217 , through telescoping universal joints  214  and  215 , to gear-towers  212  and  213 , that in turn power the trucks  202  and  203 , respectively. The telescoping universal joints  214  and  215  allow the trucks sufficient rotation and vertical motion to navigate through curved and uneven track. The drive shafts,  216  and  217 , are connected to the metal flywheels at mounting assembly points  219  and  220  to ensure that the drive shafts are locked to rotate with the flywheels. 
     The motor,  200 , is usually mounted directly to the chassis  256 . In many designs, the motor is mounted low into the fuel tank area,  208 , to provide a lower profile, lower center of gravity and more headroom for circuit board,  206 . The fuel tank area is often filled in with metal to increase the engine weight with a decorative outer plastic shell to provide model detail. By definition for this patent, the term “drive-train”, as distinct from the motor and trucks, constitutes the components of motor shaft, and driveline, and depending on the design may include one or more of flywheel(s), universal joint(s), gear-tower(s), gear tower(s) driveline support(s), pulley(s) and belt(s). In other words, the drive-train generally can be said to connect the motor to the truck(s). 
     Because the fuel tank is usually solid and because the motor sits directly over the fuel tank, there is little or no room in the tank area to mount speakers or to vent the back wave into the cab interior volume, to improve the sound quality. In addition, the fuel tank is close to the track in HO and N scale locomotives, which will cause too much sound energy to reflect back and degrade the sound quality and volume. To mitigate these problems, speakers are often mounted on the inside of the cab, directly interior of decorative grills or fan openings, such as  1703 ,  1702  in  FIG. 17 and 1802  and  1803  in  FIG. 18 , and  2703  and  2702  in  FIG. 27 , to vent the sound directly to the outside. Vent and fans are often limited in surface area, however, or are shaped in a way that does not allow the maximum speaker area possible to vent sound to the outside and the resultant sound quality and volume are poor. It is desirable to use the largest speakers possible, which is not always consistent with the available grill and vent area for particular locomotive models. 
     In addition, because the motor is not directly mounted over the trucks, there is usually more opportunity for the back wave to escape through large openings needed for the gear tower or other truck drive mechanisms to rotate as the truck turns to negotiate through curved track. These gear tower holes are shown by open areas  221  and  222  in  FIG. 2  and in  FIG. 4 , which shows a top view of the chassis with cab removed. Any sound escaping through these holes can easily be propagated to the outside through the open truck assemblies,  202  and  203 . Mixing of back wave and front wave from these open areas,  221  and  222 , over the trucks further degrades the sound quality. 
     Another restriction in the acoustic design is the space consumed from added metal weights. These weights are used to increase the overall tractive force of the engine and are sometimes used to hold or support lamps, lighting boards, electronic circuit boards (such as DCC decoders), etc. within the engine&#39;s interior. Any acoustic design must not compromise the pulling power or other features of the locomotive too severely or it will decrease the desirability of the engine. 
     SUMMARY OF THE INVENTION 
     Sound System Guidelines: Some of the principle aspects of the present invention are the following: 
     1. Separate back wave from front wave: It is important for sound quality that the interior cab volume be sealed as much as possible to prevent the back wave from escaping, for example through grill or fan openings or other openings in the diesel cab, or through the openings in the chassis where the motors connect to the trucks. To the extent there is leakage of the back wave to the outside, it will mix with the speaker front wave and cause destructive interference for some base tones and potentially constructive interference for some of the higher frequency tones. The respective path lengths for the front wave and the escaping back wave and the position of the listener will determine which frequency components are degraded or changed. Usually, since the acoustic chamber and path lengths are short, any back wave escape causes degradation of the sound quality. If the back wave is allowed to escape close to the front wave, the degradation is more severe. As the scale of the model decreases, this becomes more of a problem since the distances between front and back waves becomes smaller. 
     2. Engine cab and chassis materials affect sound quality: In addition to above concerns, the back wave can also effectively escape by exciting or vibrating the locomotive cab, which, in turn, re-radiates the sound. This can help or degrade the sound quality, depending on the cab material. Die cast cabs generally will re-radiate very little sound and produce excellent sound quality although the maximum sound volume is usually reduced. Plastic cabs are the most transparent to sound and produce tinny sounds but with higher volume. Brass cabs produce the best quality and at high volume levels. However, because there are so many unknown or uncontrolled variables in the acoustic modeling for re-radiating cabs, it is difficult to determine an optimal sound design that applies to all engine types. 
     3. Use the biggest speaker possible: No matter how clever we have been in our acoustic design using small speakers, the best sound always comes from using the biggest speaker possible. The biggest practical speaker usually has a diameter about the same as the width of the engine chassis. We call this a “full width speaker”. 
     4. Use the biggest resonant cavity possible: The volume for the sealed back wave can have a big affect on the sound system base response. Whenever possible, always maximize the cavity volume. 
     5. Vent the sound under the engine: Propagating sound upward into the open air seems to produce lower quality sound unless the listener is directly over the speaker. The sound has no opportunity to reflect against different parts of the layout such as buildings, mountains, etc. that add both volume and presence. Our experience is that a preferred design choice is to propagate the sound under the locomotive or out the sides of the locomotive through vents and grills. If sound is vented under the locomotive, one should be mindful of the affect of trucks and other obstacles and other factors that can either improve or degrade sound quality. Do not vent sound straight down too close to the track where it can be reflected back and decrease volume and sound quality. 
     6. Impedance Matching: One other design concern is impedance matching of the sound produced by the speaker or speakers to the outside. Although there is seldom enough room to make this the highest priority, due consideration should be given to impedance matching whenever there is an opportunity to do so. 
     To further describe and illustrate the invention, on-board model railroad speaker enclosure designs are presented that allow maximum sized speakers, improve impedance matching of sound to the outside of the locomotive, separate back and front speaker waves while maintaining the standard horizontal drive-train in model train locomotives. Illustrative embodiments are presented for diesels or electric locomotives: one for wide bodied diesels where the speakers lie flat within the locomotive, another for narrow bodied, high hood diesels using an A-frame enclosure and a third for narrow bodies diesels with low hoods, using a slanted or lean-to type of enclosure. A fourth speaker placement design is also included that uses a large flat speaker but requires a change in the standard drive train. Two additional speaker placement designs show installation in steam locomotives and steam locomotive tenders. The various embodiments disclosed herein are provided by way of explanation and not limitation. 
     Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  (prior art) shows the interior of a typical three-rail O&#39;Gauge sound equipped locomotive using vertical motors with top mounted flywheels, electronic circuit board mounted on studs and speaker mounted in fuel tank area. 
         FIG. 2  (prior art) shows the interior of a typical two-rail non-sound equipped engine with a single horizontal motor with two flywheels, drivelines, and universal couplings connected to gear towers at both ends of locomotive. 
         FIG. 3  illustrates one embodiment for implementing sound in an engine with horizontal drive trains by placing speakers flat over speaker enclosures located over holes in the chassis floor. 
         FIG. 4  (prior art) is a top view of the locomotives interior in  FIG. 2  showing the drive train and openings at each end to accommodate the gear tower rotational movement. 
         FIG. 5  is a top view of the locomotive&#39;s interior in  FIG. 3 , showing how the speakers can be located in a wide-bodied diesel or electric type locomotive model. 
         FIG. 6  shows how the enclosure in  FIG. 3  can be extended to cover the gear tower holes to improve the sound quality by minimizing back and front speaker wave mixing. 
         FIG. 7  is a top view of  FIG. 6  showing the extended speaker enclosure that covers the gear tower area. 
         FIG. 8  shows an end cross-sectional view of an A-frame speaker enclosure that allows large speakers to be used in narrow-bodied diesels and still maintain the common horizontal drive train. 
         FIG. 9  is a top view of the locomotive of  FIG. 8  indicating how the A-frame straddles parts of the drive train. 
         FIG. 10  shows a detail of one type of A-frame speaker enclosure with drive line passing through the A-frame interior under the speakers. 
         FIG. 11  is an end view of an A-frame design showing how impedance matching can be improved by flaring the ends of the A-frame interior where it connects to the chassis openings under the locomotive. 
         FIG. 12  shows another embodiment of the A-frame design where sound is vented out through grill or vent openings in the sides or roof of the locomotive rather than through openings at the bottom of the A-frame under the engine. 
         FIG. 13  shows the magnetic field lines from a typical Mylar speaker with most of the magnetic flux from the back contained by the speaker ferromagnetic magnet casing support structure. 
         FIG. 14  shows two similar speakers facing each other with their opposing magnetic fields repelling each other. 
         FIG. 15  shows two similar speakers facing each other with the magnet in one of the speakers reversed so the magnetic fields aid each other and attracting the two speakers. 
         FIG. 16  shows two such attracting speakers mounted in a ferromagnetic A-frame speaker enclosure with enhanced field lines connecting the two speaker backs through the A-frame material and enhanced field strength in the center of the speakers and voice coil area. 
         FIG. 17  (prior art) is a wide-bodied diesel cab. 
         FIG. 18  (prior art) is a narrow-bodied diesel cab. 
         FIG. 19  shows an A-frame speaker enclosure where the base has been flared out in all directions to improve impedance matching to the outside. 
         FIG. 20  shows a speaker larger than the interior width of a narrow body diesel mounted on an elliptical speaker tube open at the bottom where the drive line passes through the tube. 
         FIG. 21  shows the cross section of a narrow body diesel with speaker mounted at a tilt on the speaker tube with chassis hole that allows sound to emanate over the truck area under the locomotive. 
         FIG. 22  shows a top view of the entire narrow body locomotive with two speakers, mounted at a slant with speaker enclosure extensions that cover the gear tower areas. 
         FIG. 23  shows an extended A-frame to allow additional speakers for improved acoustics. 
         FIG. 24  shows a lean-to arrangement for narrow-bodied engines with a high hood where a full width speaker sits diagonally across the available cross section of the engine body with enclosure underneath. 
         FIG. 25  (prior art) shows cross section of steam engine and tender model where full width speakers are mounted flat down on tender chassis and where sound is vented through vent holes to the area under the tender. 
         FIG. 26  shows a mid-range speaker mounted in engine boiler and larger bass speaker mounted in tender. 
         FIG. 27  (prior art) is the body of an NW-1 diesel switcher, which is a good example of a narrow body, low hood body type. 
         FIG. 28  shows an extended enclosure to cover all parts of the drive train, motor and openings in the chassis to provide a completely sealed chamber inside the engine body for the front wave to minimize mixing of back wave and front wave. 
         FIG. 29  shows an alternative horizontal drive train method where only one truck is powered from a horizontal motor and gear tower. Power is coupled to the second truck though a second driveline that passes through end holes in the fuel tank. 
         FIG. 30  shows the addition of an electronic soundboard and full-width speaker mounted flat in the area normally used for the second gear tower. 
         FIG. 31  shows a method to mount a full width speaker mounted flat in a narrow-bodied diesel cab area. 
         FIGS. 32A-32C  illustrate an alternative embodiment of the A-Frame structure with vertically mounted speaker or speakers, called a Cube Enclosure. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Wide-bodied Diesels:  FIG. 3  shows one method to overcome these difficulties for wide-bodied diesels. Speakers for wide bodied diesels can have a diameter equal to the inside width dimension of the diesel cab. The speakers  330  and  331  are each supported by a generally hollow speaker mounting enclosure or “speaker tube”  334  and  335 . The chassis has additional openings,  332  and  333 , to allow the front wave from the speaker to propagate under the locomotive and pass to the outside through the open trucks and other open areas under the engine. These openings to vent the speaker front wave through the chassis can be configured as one large opening, as shown, or as a plurality of smaller openings as convenient. The drivelines,  216  and  217  and the universal joints,  214  and  215 , pass through custom holes at the front and back of the speaker tubes. These driveline holes are only large enough to allow the drivelines or other components of the drive-train to pass through and operate without mechanical interference. This minimizes the front wave from the speaker mixing with the back wave and causing destructive interference of the sound.  FIG. 5  shows a top view of the chassis with the speakers,  330  and  331 , shown mounted to the speaker tubes  335  and  334  respectively. 
     One problem with this design is that the back wave and front wave speaker sounds can still mix through gear tower holes,  221  and  222 . These holes can be covered and sealed as shown in  FIG. 6 , where enclosures  634  and  635  cover the gear tower holes,  221  and  222 . The speaker tube enclosure is maintained by the interior walls  636  and  637 , which directs the sound to speaker openings  332  and  333 . If space allows, the speaker tubes can also be designed with flared ends to provide an open horn structure or exponential curve to improve impedance matching of the sound to the outside. Examples of flared transitions are shown in  FIGS. 11 and 19 .  FIG. 7  shows a top view where enclosures  635  and  634  cover the chassis speaker tube holes  332  and  333  as well as gear tower holes  221  and  222 . The enclosures also provide additional weight to compensate for openings in the chassis and provide additional traction. If there is headroom available over the motor and flywheels, the two separate enclosures can be extended to cover the motor flywheel area to provide a single enclosure over the entire chassis. This allows additional freedom to design the shape of the speaker resonator tubes to provide better impedance matching to the outside. 
     In relatively short, wide bodied locomotives where there is not enough room to mount the speakers between the flywheels and the gear tower, the speakers can often be mounted directly over the gear towers. In this case, the speaker tubes each enclose a gear tower and isolate the open area in the chassis where the gear tower passes though the chassis to the trucks. This method, like the method described above, will separate and isolate the speaker back wave from the front wave. 
     This method for short wide bodied locomotives may require a lower gear tower design to allow the speaker cone to move up and down without interference from the top of the gear tower. Also, because the speaker tube is filled with more of the drive train components, there will be more interference to the sound passing to the outside of the locomotive. 
     Narrow-bodied Diesels: Narrow bodied diesels provide an additional problem. Although the chassis width stays the same, the diesel cab hood region is only about half as wide over most of the length of the locomotive. The drive mechanism consisting of motor and drive train remains essentially the same. The narrower body and drive mechanism prevents mounting speakers flat that span the width of the chassis. Smaller speakers with a diameter that can fit from side to side inside the narrower body are usually too small to produce good volume and sound quality. 
     Instead of laying the speakers flat as in the previous examples, we propose mounting the speakers at an angle.  FIG. 8  shows the end view of a narrow bodied model locomotive where two large speakers are placed in an A-frame structure,  850 , over a hole in the chassis  821  to allow the speaker front waves to pass to the bottom of the locomotive out through openings in the diesel truck and other open areas under the engine. The diesel cab detail is not shown except for its outline,  809 . This novel method of mounting the speakers over the drivelines and/or universal joint or other drive train components, allows large speakers to be mounted in narrow bodied engines without altering the essential horizontal-motor and drive-train design that is so common in O, HO and N scale model diesel locomotives. 
       FIG. 9  shows a top view of the chassis in  FIG. 8 . Here the angled speakers,  830  and  831 , look oval shaped setting in the A-frame structure,  850 . In order to prevent the back wave from mixing with the front wave and to provide better impedance matching to the A-frame chassis hole, both the front and rear ends of the A-frame structure should be sealed off as much as possible. In this case, where the A-frame only spans the driveline, small holes,  951  and  952 , can be added to the ends of the sealed A-frame structure to allow the driveline,  216 , and/or other parts of the drive-train to pass through with minimum mixing of front and back speaker waves. If there is room and it is desirable to have a longer A-frame for acoustic reasons, the A-frame can be extended. An extended A-frame can also allow additional speakers as shown in  FIG. 23  where four speakers are indicated. 
       FIG. 10  shows the detail of the A-frame structure,  850 . Both ends of the A-frame structure have small holes,  951  and  952  to allow driveline,  216 , to pass through. The holes  951  and  952  are just large enough to allow unencumbered movement of the driveline and/or related drive train parts, including lateral motion due to the rotation of the gear tower,  212 , (see  FIG. 9 ) and any vertical motion generated by the diesel truck moving up and down during travel. The speaker  830  is shown on one face while the other speaker,  831 , cannot be seen from this view. Speaker  830  is shown facing in towards the interior of the A-frame with speaker magnet,  1053  and back-wave speaker vent holes  1054  facing out. The second speaker,  831 , is also facing towards the interior of the A-frame structure. When operating, speakers will be phased to both move inwards to the A-frame interior or outwards from the A frame interior in response to the same applied speaker forcing function. The A-frame design could be used with only one speaker but the amount of acoustic energy would be reduced. 
     The A-frame structure also improves impedance coupling to the outside through a tapered tube that opens up at the bottom. The interior of this enclosure can also be designed to have a shape that is optimized for impedance matching such as a exponential curve.  FIG. 11 , shows interior surface of the speaker enclosure with speakers  831  and  830  and with curved or exponential shaped surfaces,  1155 , and continuing through the chassis  256  to improve impedance matching. Both the ends and sides of the enclosure,  850 , can be tapered with a curved surfaces,  1901  and  1902 , as shown in  FIG. 19 . The interior surface is also tapered out in the same proportion whose curves continue through the chassis opening under the enclosure. 
       FIGS. 32A-32C  show an A-Frame structure where the speakers are mounted vertically instead of at a slant (as shown in  FIG. 10  and  FIG. 11 ). We call this alternative embodiment of the A-frame enclosure a “Cube” enclosure.  FIG. 32A  shows front, side, top and bottom view of the Cube. The bottom view shows an opening for front wave sound to propagate out of the Cube interior.  FIG. 32A  shows a Cube with two speakers, mounted on opposite sides. If only one speaker is used, the opening on the other side is sealed to prevent front wave sound from escaping.  FIG. 32B  shows a perspective drawing of the Cube enclosure.  FIG. 32C  shows how the Cube is utilized in much the same manner as an A-Frame in the model locomotive, with the driveline or other drive train components passing through the interior of the Cube as necessary. This structure is preferred if the speaker magnets or speaker thickness is too large or the drivetrain components are too large to fit in the more restricted area of the slanted speaker structure in  FIG. 10 . Just like a slanted A-Frame structure, the Cube can be designed to allow the front wave to propagate out through the bottom opening to the exterior of the locomotive. The backwave will propagate out speaker vent holes to the interior of the locomotive where this back wave sound will be contained or at least controlled to minimize it from mixing with the front wave sound. 
     To prevent problems with the front wave escaping through the holes at the ends of the A-frame needed for drive train components, the speaker enclosure could be extended to cover all parts of the drive train, motor and openings in the chassis as shown in  FIG. 28 . In this embodiment, the drive train components and motor are shown outlined in dotted lines. This enclosure would provide a substantially sealed chamber inside the engine body for the front wave to prevent any mixing of back wave and front wave. Here, the “wings” shown in  FIG. 12  have been removed. 
       FIG. 24  shows another arrangement for narrow-bodied engines with a high hood. Here fitting the speaker diagonally across the available cross section of the diesel cab and supporting it with a speaker enclosure that is like one-half an A-frame achieves extra room for large diameter speakers. This lean-to speaker enclosure can also provide more room for a larger speaker magnet,  2453 , to provide better sound quality. Just like the A-frame structure, components of the drive train will run through the lean-to enclosure under the speaker. 
     If the engine has a low body, it may not be suitable for a lean-to enclosure that uses the full cross sectional hood area for a speaker since there could be interference with some of the drive train components. In this case the speaker can be mounted at a lesser slant and elevated.  FIG. 20  shows the end section of a narrow-bodied diesel with a low hood. Here the speaker,  2031 , is too large to fit flat in the width of the locomotive but will fit if mounted at a slant on top of tubular speaker enclosure  1233 .  FIG. 21  shows an end view of the same locomotive, showing the speaker,  2031 , mounted at a slant. The speaker enclosure is designed to be high enough on the low end to clear the gear tower and high enough at the high end to allow the largest possible speaker. A hole in the chassis,  2121 , is provided to allow the sound to exit under the locomotive as shown in the partial cut-away end-view of the cab and chassis. 
     In  FIG. 20 , the driveline,  217 , is the drive train component that runs through the enclosure but it could be other components as well such as universal joint,  215 , or gear tower,  213 . Like the wide-bodied locomotive, the enclosure can be placed or extended over the gear tower to prevent the back wave from escaping through the gear tower hole,  222  and mixing with the front wave.  FIG. 22  shows a top view of the entire locomotive with two speakers,  2031  and  2030  mounted at a slant as indicated by their elliptical shape in the drawing and speaker enclosure extensions  2180  and  2181 . To balance the sound, one speaker might be slanted high on the left side while the other speaker is slanted high of the right side of the engine although this will probably not make much difference. If there is room and it is desirable to have a longer lean-to for acoustic reasons, the lean-to can be extended. An extended lean-to can also allow additional speakers in the same manner as the extended A-frame that is shown in  FIG. 23 . 
     A modification of the A-frame structure is shown in  FIG. 12  where only a segment of the engine&#39;s body and chassis is drawn. Here the A-frame,  1250  bottom has been sealed up, and the closest end shown has been opened up completely to allow the driveline,  216  and/or other parts of the drive train to pass through unencumbered. The far end of the A-frame should be sealed up as much as possible to prevent the front wave from the two speakers from escaping through the chassis opening,  222 , for the gear-tower,  213 . 
     In the embodiment of  FIG. 12 , the A-frame mounting assembly is sealed below. One end of the structure is opened up to vent the “front wave” into the cab. Back wave is vented outside through the air vent screens on the side of the cab. Alternatively, the front wave can be vented straight down through chassis. The “wings”  1263  on one end of the A-frame seal against the cab sidewall; so the “backwave” vents throughout interior volume of the cab, although it is generally sealed as explained below. 
     The back of speaker  830  can be seen as before but now the front of the second speaker,  831 , can be seen through the A-shaped hole. Also shown is a partial outline of the narrow-bodied locomotive cab,  1800 . Decorative vent or grill in the diesel cab,  1804  is shown drawn on one side of the locomotive; the second grill,  1802 , is not shown in this figure since it would be part of the removed cutaway cab area. In any case, the vent holes are open to air to allow back wave sound from speakers  830  and  831  to pass directly out through the grills to the outside. The sides of the A-frame ends are extended with wings,  1260 ,  1261 ,  1262  and  1263  to the edges of the diesel cab interior which prevents back wave sound from either speaker passing into other parts of the cab area. The front waves from speakers,  830  and  831  can pass to the interior of the cab through the open A-frame end but can no longer vent directly through the bottom of the A-frame to underneath the locomotive. These back waves are prevented from mixing with the speaker&#39;s front waves within the cab area by the same wings,  1260 ,  1261 ,  1262 , and  1263 . 
     in this drawing  FIG. 12 , the diesel cab and chassis are shown terminated at the closest end  256  in the Figure. However, the chassis continues to the end of the engine and is sealed from allowing the front waves from leaving the enclosed diesel cab and chassis area. This structure allows the front wave to resonate within the closed diesel cab area while preventing the front wave from mixing with the back wave. This structure has some drawbacks compared to the method described in  FIG. 10  since to prevent back and front waves from mixing, the wings,  1260 ,  1261 ,  1262  and  1263  as well as the top of the A-frame, must seal well to the sides and top of the diesel cab interior. In addition, it can be difficult to find the best location for this A-frame structure in some locomotive models where the decorative grill or vent openings may be inconveniently located. It is also more difficult to manufacture diesel cabs where the grills are open. 
     Note that in both structures,  FIG. 10  and  FIG. 12 , we speak of the speaker back wave and front wave where the front wave is considered emanating from the open cone area of the speaker while the back wave is usually thought of emanating from air vent holes near the speaker magnet casing. In both structures,  FIG. 10  and  FIG. 12 , either of the speakers could have been mounted facing in or facing out, provided the speaker housing, magnet casings, etc. would not interfere with the drivelines, flywheels, and diesel cab. The advantage with facing speakers in is that most speakers have electrical connections on the back that make it easier to connect to the electronics. If the speakers face out, the electrical wire connections will likely be on the inside the A-frame, which might interfere with the driveline or flywheel motion. 
     There is also an issue of the speaker stationary magnetic flux from the permanent magnets surrounding the voice coil. Generally, the magnetic field,  1300 , runs axially though the center of the speaker cone as shown in  FIG. 13 . The magnetic field,  1371 , from the back pole of the magnet is contained or routed by the low reluctance of the speaker&#39;s ferromagnetic voice coil encasement to run along the speaker&#39;s structure,  1330  to the edge of the speaker where it joins with the magnetic flux emanating from the center of the speaker cone to form a continuous loop. This improves the magnetic field strength in the voice coil and hence improves the speaker efficiency. The magnetic field direction is irrelevant for the efficiency of an individual speaker but is shown by North-South indicator  1372  for comparative reasons. By convention, magnetic lines of force (B field) emanate from the North Pole and enter the South Pole of a magnet. If two such speakers,  1430  and  1431 , are placed face to face, the speaker magnets will repel each other as shown by the magnetic lines of force in  FIG. 14 . Here indicators  1472  and  1473  show the North-South polarities for speakers  1430  and  1431  respectively. This will decrease the magnetic field in each voice coil and hence will somewhat reduce the speakers&#39; effectiveness. On the other hand, if the magnet in one speaker is reversed, then the fields will attract each other as shown in  FIG. 15 . Here indicators  1572  and  1573  show the North-South polarities for speakers  1530  and  1531  respectively. This results in higher concentration of magnetic flux through the centers of the speaker&#39;s voice coils with return paths for the flux at the edges of the speaker&#39;s metal structure and support ring. Note that when the magnet is reversed in one of the speakers, the electrical connections to one of the speaker&#39;s voice coils must be reversed to phase both speakers to work together. 
     In addition, if the A-frame structures shown in  FIGS. 8 ,  10 ,  11 , and  12  are made of ferromagnetic material, and one of the speaker magnets is reversed in polarity as described above, then the magnetic reluctance is reduced for the magnetic path connecting the backs of the two speakers.  FIG. 16  is the same as  FIG. 11  except that the magnet in  1631  is reversed from the magnet in  1630  as shown by the indicators  1672  and  1673 . The field from the back of both speakers now takes the low reluctance path through the ferromagnetic material used for the A-frame structure as shown by the concentrated field  1674  in the upper portion of the A. Actually, magnetic fields will be concentrated anywhere the reluctance is low such as the closed end structure of the A-frame structure shown in  FIG. 10 . In any case, the overall field strength for the flux,  1675 , through the voice coils of speakers  1630  and  1631 , are increased for an A-frame made of ferromagnetic material since the high reluctance region is restricted to the air gap between speakers. 
     Note that in the above A-frame structures, the speakers can be mounted facing out as well as facing in or one speaker can be mounted facing out and the other one facing in. It does not make that much difference in the design of the structures but the wiring to the speakers may need to be changed to ensure proper phasing. However, if the A-frame is constructed of ferromagnetic material, the most efficient design is for both speakers to face each other. It should be noted that the present invention can be applied to all model scales. 
     Special Installation Methods for Difficult Locomotives: In order to provide more room for large speakers and electronics in model locomotives, we can use an alternative drive train method. In this case, only one truck is powered from a vertical motor, gear tower or belt drive. Power is coupled to the second truck though a second driveline that passes through end holes in the fuel tank. 
       FIG. 29  shows this method being used with a horizontal drive train. Here the motor,  2900 , is connected to two flywheels,  2905  and  2904 . A single driveline,  2916  connects to universal joint,  2914 , which connects to gear tower,  2912  which powers truck  2990 . The other truck receives power from driveline  2992  that connects to truck  2990  through universal joint,  2991 . The driveline,  2992  connects to telescoping coupling,  2993 , that allows the trucks to rotate freely as they pass through curves but continue to pass torque though to universal joint  2995  and to truck  2996 . The fuel tank,  2997 , has an internal channel that allows the telescoping coupling,  2993 , and two drivelines,  2992  and  2994  to pass through without interference. 
     Whether the method to power the first truck is a vertical motor or a horizontal motor, we will call this the “coupled-truck drive train” when a horizontal driveline is connected between the trucks under the locomotive to transfer power.  FIG. 30  shows the coupled-truck drive train method used for a wide bodied diesel. The area normally used for the second gear tower is now free for the installation of a full width speaker,  3031 , flat on the chassis. The large chassis opening,  3098  allows sound to pass under the locomotive to the open areas in truck,  2996 . The truck is mounted to pivot,  3099  that is attached to a bar that is connected to the chassis at the edges. 
     The coupled-truck drive train also has utility for installing full width speakers (slightly smaller diameter than the chassis width) in narrow-body, low-hood locomotives.  FIG. 31  shows the front portion of a narrow bodied diesel with the speaker,  3131 , being mounted flat in the chassis area in such as way that the speaker magnet housing,  3153  is covered by the engineers cab and the flatter areas of the speaker are partly covered by the chassis at the edges,  3101  and  3102 . The cab area is usually not large enough to cover the entire speaker. Mounting the speaker in this way allows the cab to cover the thicker center of the speaker, leave the speaker back wave vents open to the interior of the engine, allow the chassis to cover the edges of the speaker so they cannot be seen from the outside of the engine, and still have a full width speaker to vent sound under the locomotive. In  FIG. 31 , the speaker is shown mounted from under the chassis and held in place by speaker retainers  3171  and  3172 . The truck pivots on support  3199  that connects to the chassis. 
     Steam Locomotives: Steam engines usually present fewer problems for sound installation because of the extra room in the tenders as shown by the cross section in  FIG. 25 . However, having the train sounds come from the tender is not prototypical and with larger engines (e.g., O&#39;Gauge and G&#39;Gauge), it is quite obvious. Even with large HO engines, it is apparent that the sounds are not coming from the correct region of the engine. With some steam engines it is possible to place the speaker in an acoustically sealed steam boiler.  FIG. 26  shows a speaker,  2633 , placed towards the front of the boiler. The lower smoke box area,  2635 , is open to allow the speaker front wave sound to pass under the locomotive in the area of the steam chest. The area,  2601 , behind the speaker is acoustically sealed to prevent the back wave from mixing with the front wave. 
     While this method does allow the sounds to come from the correct area, the sounds are usually much tinnier and lower in volume than the same sounds produced in a suitable tender. A method that may solve both the problems of producing full-bodied sounds and correct sound location is also shown in  FIG. 26 . Here, a single large base speaker,  2632 , is placed in the tender and vented through chassis holes,  2634 , to the area under the tender. The electronics board,  2506 , supplies only low frequency sounds to the tender speaker and only mid and high frequency sounds to the locomotive speaker,  2633 . A simple crossover network could also be used. 
     Since the human ear is less sensitive to localizing lower frequency sound sources, the locomotive sounds will appear to be coming from the engine. In addition, the base speaker,  2632 , will create considerable presence and full-bodied sounds when used in conjunction with the engine speaker,  2633 . 
     Some tenders can be quite small or shaped in ways that can make installation difficult. When appropriate, some of the above methods may be useful for steam engines. In particular, some tender bodies curve in towards the bottom where they connect to the chassis (e.g., Vanderbilt types). In cases like this where speakers cannot be mounted flat on the chassis, speakers can be mounted at a tilt like the speaker shown in  FIG. 20 . In some rounded tenders, a full width speaker can be mounted on the flat partition that separates the fuel bunker from the water storage area. Either the area under the fuel bunker the water bunker can then be opened to allow sound to vent under the engine, depending on which method sounds better. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.
On-board model railroad speaker enclosure designs are presented that allow maximum sized speakers, improve impedance matching of sound to the outside of the locomotive, and isolate back and front speaker waves while maintaining the standard horizontal drive-train in model train locomotives.