Double Stack Mini-Oval Speaker for Maximum Sound
Buy AHD Speakers Here
by Larry Maier
Larry Maier, our staff engineer and a model railroader, has double stacked two Mini-Oval speaker enclosures to get a lot more sound from this popular, great sounding small speaker!
Any moving diaphragm speaker requires an enclosure in order to perform correctly. The enclosure prevents the sound produced behind the speaker from slipping around the edge and cancelling the sound in front of the speaker. While enclosures come in many different forms, any given speaker’s sound will improve as the space behind the speaker is made larger. When it comes to adding sound to a locomotive, there is often little space available for the speaker, so it is difficult to get really good sound. The mini-oval speaker and associated enclosure available from American Hobby Distributors, http://www.amhobby.com/products/ahd.html provides a good combination of quality sound and small size. Where space is available, however, it is possible to improve on this combination by creating a larger mini-oval enclosure from two standard mini-oval enclosures.
The first step is to procure two mini-oval enclosures and set one aside temporarily. The second enclosure must have the bottom removed so that the speaker cutout hole goes all the way through. This can be done using a Dremel motor tool, the Dremel drill press stand, and an abrasive cutoff wheel. Figure 1 shows how to set this up. In this case, the cutoff wheel is set to be 0.20 inches above the work surface of the drill press. This will result in an added 0.20 inches to the enclosure. Check your installation, and make this cut as thick as possible consistent with fitting inside your locomotive. Remember that this cut piece will be stacked on top of a standard enclosure and that the mini-oval speaker sits somewhat above the top surface of the enclosure (see Figure 2). Once you have the drill press set up, carefully move the enclosure (sealed bottom side up) against the cutoff wheel. Be careful to keep the face of the enclosure firmly on the drill press worktable. Heat from the cutoff wheel may cause some melting of the enclosure plastic, so proceed slowly. You may need to make several passes to completely remove the bottom from the rest of the enclosure. When you are finished, you should have an open enclosure with one smooth surface and one surface still with the machined seat for the speaker itself.
Now place the smooth surface of the modified enclosure on the top surface (speaker cutout side) of the unmodified mini-oval enclosure. The two pieces should fit together with little if any gap. Carefully align them and then use some masking tape to hold them aligned. Mark one corner with paint, etc. so you can remember which way they were aligned when you take them apart to work on. Make a mark in each corner of the top of the enclosure 0.075 inches from each edge (see Figure 2 for screw drill locations). Drill each mark using a #56 drill. Go most of the way through the two pieces, but do not punch through the bottom of the enclosure. The Dremel drill press set so that the drill bit does not touch the work surface is a good way to do this. On the top piece only, enlarge these holes using a #52 drill, and then use a #33 drill to counter-sink the holes for the screw head. Thread the hole in the bottom piece using a 0-80 tap. These steps allow you to screw the enclosure together. Since the enclosure is made of Delrin (a very slippery plastic), it is hard to glue, but several people report that Walthers Goo can be used if you want to glue the assembly together instead.
If you are gluing the enclosure together, simply remove enough tape so that you can place the glue but still maintain parts alignment. If you are screwing the assembly together, then place some double-sided tape on the smooth surface of the top piece. This tape will help seal the enclosure. Use a razor knife to cut out the tape that is in the speaker hole and to clear the screw holes, and then align the pieces making sure your orientation marks (paint) line up with each other. Once together, make sure the pieces are evenly aligned all around and then screw the enclosure together using four 0-80 X 5/16 screws. The enclosure is now finished.
Orient the enclosure in your locomotive and determine the best location for the speaker wires to exit. Drill holes in the enclosure to pass the speaker wires. These holes should be just large enough that you can push the wires through. Drill one hole for each wire rather than a larger hole for both of them. This will help keep the enclosure sealed. Push your speaker wires into the enclosure and bring them through several inches so that you can work the free ends. Strip and tin the wires and then carefully solder them to the speaker terminals. Get a good solder joint, but use as little heat for as short a time as you can so that you don’t damage the speaker. If you are using more than one speaker, color code your wires and make sure to solder the same color to the same speaker terminal on each speaker. Now, carefully snap the mini-oval speaker into the seat machined for it on the top of your enclosure (remember that the top piece you made cut out the bottom leaving the top speaker seat intact). When you are done, your speaker(s) should look like Figure 2. Note the paint alignment marks, the tape seal between the enclosure pieces, and the four screws holding it all together.
Fig. 3 - Detail
The speaker(s) are now ready to install in your locomotive. The speakers shown in this project were installed in a New Haven EP4 made by Branford Hobbies. This engine has a reasonable amount of space in each end due to the dual cab design. This allowed the use of two speakers, which gives a further improvement in the sound quality and available volume. The other benefit of two speakers in this case is that the sound appears to come from the middle of the locomotive rather than from one cab or the other and gives a more prototypical feel. The final installation can be seen in Figure 3.
Frequency plots were made of the mini-oval in the standard mini-oval enclosure and in the extended enclosure. These plots are shown in Figure 4. The orange line is the standard mini-oval enclosure while the yellow line is the new extended enclosure. Note that the extended enclosure gives a much smoother response in the high frequency range and between 6 and 12 dB more output in the lower frequency range. Listening to the newly installed speakers confirms that the extended enclosure produces a much better sound than the standard enclosure. If you have the space available, this is definitely the way to go.
Generally, for all model railroad speaker applications the larger the enclosure the better the sounds.
Note: All the “Round” TDS Speaker Enclosures are furnished with an extender, use it if you can.