How Do I Get the Sound Out?

I installed a decoder and speaker in my loco and the sound is too weak!

Bruce hears this very often!

Most sound decoders have an adequate amount of electrical power to generate enough acoustic power to fulfill their needs. The issue is EFFECTIVELY converting the electrical power to acoustic power and then coupling the acoustic power outside the loco so that we can enjoy it. It is impossible in a few words to convey the decades of experience Bruce has in acoustic design. He has given many hour long clinics on the subject, which only scratch the surface.

You can now share the excitement of Bruce's clinics online.

The text is in the August 2012 Model Railroad Hobbyist magazine column (CLICK HERE) and the related video is on You Tube (CLICK HERE).

Here are some of the salient points.

  1. Sound comes off both sides of the speaker - for clarity, we'll call the sound coming off the cone side the positive pressure and that off the magnet side negative pressure.
  2. The positive and negative pressure waves will cancel each other out without some form of acoustic design. There are two basic ways to do this in our locos listed below. Once you review them, we recommend you cruise the sound installation examples on our web site and identify the design of each. 
  3. Using an enclosure - this is quick and easy, while adding expense to the installation. It still requires acoustic design thought on ducting the sound out of the locomotive. Also it wastes half of the acoustic energy from the speaker by containing it within the enclosure. This enclosure must be sealed and can be purchased ready to use or built out of styrene or other materials. An example of this is the install in a Life-Like PA, using an enclosure built out of a SoundTraxx kit. As a general rule, the larger free air volume in the enclosure, the more efficient the acoustic conversion. Thus, keeping the speaker magnet outside the enclosure will help the efficiency. For a quick way to show this effect for yourself is to take a SP-27RHB-08 speaker and put it into the end of a PVC pipe cap designed for 1/2 inch threaded pipe, as shown in this photo. INSTANT ENCLOSURE! Click on the photo to see a larger view. 
    • Use the loco itself as either a sealed box enclosure, as above, or as an infinite baffle.
    • One of the best examples of using the loco as a sealed box enclosure is where the speaker is mounted inside the tender of a steam locomotive, preferably facing up. The shell is relatively well sealed. The coal load is perforated with hundreds of very small holes (#70 drill is Bruce's favorite) from the INSIDE OUT. An example of this style of install is a Bachmann Consolidation .
    • An infinite baffle design is where you intentionally duct the positive pressure out one area of the loco and the negative pressure out somewhere else. These exit points need to be about 6 inches apart for our purposes. Frequently this is accomplished by mounting the speaker behind the fan grille on a diesel, allowing the negative pressure to travel through the shell and out through the trucks. An other method is to mount the speaker(s) so that the sound from one side of the speaker comes out through the fuel tank and the other half is ducted all the way to the roof an comes out the trucks. An example of this style of install is a Stewart FT A & B set with the speakers in the B unit 
  4. Since high frequency sound travels in straight lines, whenever possible, point the speaker directly at the listener. Through coal loads and fan grilles provide better bell sounds than fuel tanks and tender floors.
  5. Get the speaker as close as possible to the listener - up against the grille, etc.

With these basic ideas, the rest is EXPERIMENTATION!

Send us photos of your ingenuity!

Copyright © Bruce F. Petrarca 2007 - 2017; All Rights Reserved