The SoundTraxx Sound Primer


Installing Sound

Practical Tips for a Successful Sound Installation
As manufacturers, we are often asked, “How hard is it to install a sound system?”  The answer is, “It depends.” And what it depends on most is the skill of the individual doing the installation. The installer should have some basic modeling skills and be comfortable disassembling a locomotive, and be able to identify key wires such as power pickup and motor leads. An individual who has never taken apart a locomotive will have a harder time installing sound than someone who has taken his 2-6-0 completely apart and rebuilt it into a 4-8-4.

Modeling skills aside, the mechanics of installing sound is straightforward but often involves some cutting and drilling to the original model. If you aren’t comfortable with this idea, you should probably refer the work to a professional.

Easy Steps to Great Sound
Regardless of manufacturer, installing sound inside a locomotive generally requires the following steps:

  1. Select the model: Choose a locomotive appropriate to your installation skills.
  2. Plan the installation: Determine the location of the sound system, speaker, cam, and connectors.
  3. Modify the model shell or tender body and floor to accept the speaker and sound system components.
  4. Fit the speaker: Trim the speaker frame and/or add baffles.
  5. Install the exhaust cam (optional, steam only).
  6. Install and wire the sound system: Mount the sound system and connect.

Tools and Materials You Will Need
In addition to the common hand tools found on most modelers’ workbenches, you should have at your disposal:

  • Low-wattage soldering iron
  • Rosin-core solder
  • Hobby knife
  • Miniature screwdriver set
  • Diagonal wire cutters
  • High-speed motor tool such as a Dremel
  • Multimeter
  • Double-sided tape
  • Velcro
  • Heat-shrink tubing
  • Silicone RTV
  • Drill bit assortment

Step 1. Select your Locomotive
If this is the first time you have installed sound in a locomotive, then we suggest you choose your locomotive carefully. A few simple precautions will ensure that your first effort produces a great sounding locomotive instead of an intimidating ball of wires.

  • Pick a smooth-running locomotive that runs well on straight DC power. A smooth-running mechanism is vital for good throttle control and enhances the realism of the sound.
  • Pick a steam locomotive with a reasonably large, rectangular tender. The larger the tender, the more room you will have for installation of the sound system and speaker. It will also allow you to use a larger speaker, giving both better volume and fidelity. Reserve that favorite little Porter for a future installation when you have some experience behind you.
  • Start with an engine that is "sound-ready" if possible, such as an engine with pre-drilled speaker holes, or a diesel with a "‘B" unit. The simpler you can make your first installation, the better.

Step 2. Plan the Installation
You should give some thought as to where the installation of the various sound system components will be within the locomotive before you get started. Figure 5 shows a typical installation.

Speaker placement is critical to the success of your installation, which will depend on the size and type of the locomotive. But when considering the speaker’s location, remember that the volume of the speaker will be greatly enhanced when the speaker is fitted into a small, airtight enclosure with the front of the speaker open to surrounding air. Fortunately, most steam engines have a tender, and with a little work, the tender itself becomes a great speaker enclosure. The speaker can face up through the coalbunker or down through the floor; either way will provide equally good sound.

Typical steam installation

Figure 5- Typical Sound Installation

In small tenders, the speaker should be mounted forward in the coalbunker area, where higher walls are available to cover the speaker magnet. You may need to get clever and install the speaker inside the boiler or up in the smokestack. For other locomotives, the speaker enclosure need not be fancy and can be fabricated from sheet styrene, bass wood, cardboard, and plastic bottle caps! A 35mm film canister usually produces excellent results, as does the cardboard tube from the center of a roll of paper towels. We also offer snap-together speaker baffle kits.

Finally, in some cases you will need to decide whether or not to hardwire the electrical connections between the tender and the locomotive or use a plug-in connector. Using a connector will allow you to separate the engine and tender for storage, as well as make painting and service easier. However, this can also be unsightly in smaller models and also opens up the possibility of accidentally damaging the sound system by reversing the connector during reassembly. Hardwiring the sound system will prevent this possibility at the expense of making tender separation difficult.

Step 3. Modify the Tender Body or Locomotive Shell
In the case of a steam locomotive, you will probably be mounting the speaker facing down on the tender floor or facing up in the coalbunker, under the fan grilles, or in the cab for most diesels. In either case, a certain amount of "body work" may be necessary to accommodate the speaker and sound system.  This may include removing weights, mounting brackets, internal bracing, and other structural features.

Soundfig6Tender Body Modifications
On tenders with low sidewalls, the speaker is often best mounted facing up in the coalbunker to provide adequate clearance for the sound system, which will be mounted against the tender floor. In some cases, it may even by necessary to add a wooden "retaining wall" to build up the height (Fig. 6).

The simplest tenders have a flat deck across the coalbunker and are the easiest to deal with. Simply mark or scribe the speaker outline onto the tender deck and using a jeweler’s saw, cut out a circular opening about 1/16” inside the outline marks. Remove all sharp edges and burrs.  The speaker is then disguised by covering the opening with an acoustically transparent coal load. The coal load can be fabricated by contouring a piece of fine screen mesh or polyurethane foam, and then covering it with a thin layer of scale coal cemented in place with a thin wash of diluted white glue.

Plastic tenders often have a molded coal load already in place. Unfortunately, the inside surface of such tenders are contoured to match the coal load and do not provide a flat mounting surface for the speaker.  In such cases, you have two options. The first is to cut out and remove the coal load entirely and replace it with a flat sheet of 0.060” or thicker styrene and then proceed as described above for the flat-decked tenders.

Soundfig7Alternatively, a sub-deck can be fabricated from 0.060” sheet styrene such that it fits below the coal load tightly against the tender sides. Cut a large circular opening appropriate for the speaker diameter into the styrene sheet and then glue the sub-deck to the inside of the tender shell. Perforate the coal load by drilling a series of small holes (use a #50-#60 bit) at random angles between the coal "nuggets." This way, the sound will be allowed to escape yet the speaker remains relatively invisible regardless of viewing angle (Fig. 7).

Soundfig8The more realistic tenders will have fuel bunkers that prototypically extend to the tender floor. For larger tenders with such a feature, the speaker is best mounted on the tender floor. For smaller tenders, the slope sheet and interior walls will interfere with the speaker, as well as the sound system. It will usually be necessary to remove and replace them with sheet brass or styrene to create a flat-decked tender (Fig. 8).

Tender Floor Modifications
Soundfig9If you are mounting the speaker to the tender floor, first determine the exact speaker location within the tender and verify that there is adequate clearance between the tender body and speaker magnet. Use a marker or scribe to note the location of the speaker on the tender floor and then drill three to five rows of holes in the pattern shown in Figure 9 to provide an opening for the sound to escape. A drill diameter approximately 0.250” is a good size. Space the holes as best you can, taking into consideration the tender underbody detail. Although the spacing is not critical, you should attempt to provide at least this much of an opening. If necessary, use more holes of a smaller diameter. Keep all the holes within the expected cone area of the speaker. Take care not to place holes at the outer edge of the speaker assembly itself.

Once you have drilled your holes, remove all burrs and uneven edges. The speaker floor should be smooth, with nothing to physically interfere with the speaker’s ability to lay completely flat on the tender floor, such as solder joints, screw bosses, and mounting studs.
Step 4. Fit the Speaker
If the speaker is wider than the space in which you intend to install it, it will be necessary to reduce the speaker width to get a proper fit. Determine how much the speaker must be cut down and remove half of that amount from each side of the speaker. We recommend that you simply purchase a smaller speaker – we have a variety of speakers available to suit your needs. However, if you do need to file the speaker, be careful to remove only the speaker frame and outer edge of the diaphragm. Avoid cutting into the diaphragm itself.

Speakers with metal frames can generally have a larger amount of frame and speaker cone removed and still maintain a satisfactory sound. On the back of the speaker frame, draw a line where the first cut will be made. Draw a second, parallel line marking the other cut. Be sure that neither line cuts into the speaker coil wire.

Soundfig10A Dremel or similar motor tool with an abrasive cutoff wheel will make the job of cutting the speaker go quickly. Be sure to wear your safety glasses! Hold the speaker face down and make shallow cuts along each line until the frame has been cut through.  Trim the speaker cone with a sharp single-edged razor blade or hobby knife (Fig. 10).
Check the speaker’s fit into the installation site and trim the speaker frame and cone as needed until the desired fit is attained. For best performance, the speaker should fit snugly within the tender so that the edges of the speaker diaphragm are as close to the tender sides as possible without actually touching. Do not allow the speaker cone to rub the tender walls, as this will produce a "scratchy" sound.  Likewise, a large air gap will reduce the speaker’s bass response but can be filled in with cardstock if needed.

Secure the Speaker in Place
Once bodywork is complete and the speaker has been fitted in place, it must be secured tightly to the enclosure. For the best sound, an airtight seal is needed around the speaker edge. We have found the best way to hold the speaker in place is to use silicone RTV, which provides the airtight seal needed and allows the speaker to be readily removed in the future (unlike epoxy or other hard glues). Be careful that you don’t get any RTV onto the speaker cone, as this will severely distort the sound quality!

Step 5. Install the Exhaust Cam
If the sound system uses an exhaust switch or cam to control the chuff, now is the time to install it. Some systems use simple mechanical switches, while others use magnets or optical sensors. In any event, select a suitable mounting location and install the parts according to the instructions supplied with the sound system. For the most prototypical exhaust, you want four triggers per drive wheel revolution.  Sometimes you can get away with mounting the exhaust cam on a tender axle, especially if the tender wheels are an even multiple of the drive wheel diameter.

Step 6. Install and Wire the Sound System
Begin by securing the sound system in place. We like to use heavy-duty Velcro, as it allows the components to be easily removed later on. Double-sided foam tape also works, but is hard to remove once in place. Temporarily refit the body shell to ensure that adequate clearance still exists.

Next, wire the sound system according to the directions. When wiring the sound system, trim all wires to reduce unnecessary lead length. This will give your installation a neater appearance while also preventing wires from interfering with the drive mechanism and getting pinched between the frame and body shell.

It is also helpful to color code wires as to their function. For example, unless predetermined by the sound system’s wiring (i.e., SoundTraxx Digital Sound Decoders use purple for speaker connections), use yellow for speaker wires, orange for track pickups, and so forth. The key is to be consistent. To ensure long-term reliability, solder all connections and insulate with heat-shrink tubing. 

That’s it! You’re ready to go!