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Sound Primer

Our Sound Primer will help you understand how sound works so you can get the most out of your SoundTraxx onboard sound system.

As model railroaders, we are trying to create a miniature world that is as realistic as our abilities allow. But no matter how detailed a model is, something is missing as our trains go silently down the track -- sound.

Real locomotives don't have Pittman motors in the cab. They are real working machines -- a symphony of sounds! Little kids realize this and try to make up for it by making "chuff, chuff" and "woo, woo" noises when they see a model train roll by. As adults this is: A) Not particularly satisfying and B) Potentially embarrassing if we are overheard!

Good news! We have designed and produced a wide range of options for creating realistic sound effects while preserving your dignity. However, a bad installation can cause problems, so listen up while we provide insights into SoundTraxx sound system technology and show you how to properly apply it.

Understanding Sound

What is Sound?

Simply put, sound is a pressure wave traveling through the air. A sound source, such as a speaker, vibrates air molecules back and forth to create small changes in air pressure and cause a sound "wave" to propagate to the listener’s ears. The eardrums pick up this vibration and turn it into a signal that the brain interprets as "sound."

Sound Volume

The amplitude of the air molecule vibrations determines sound volume. When the molecules vibrate a small amount, there is little change in air pressure, and thus, a low level of sound is produced that is discernable only to those near the sound source (Fig. 1). Similarly, when the vibration level is very high, the change in air pressure is high, and the sound is strong. When the sound is loud enough, we can actually feel the changes in air pressure!

Sound Frequency

Sound also is made up of various frequencies -- that is, the rate at which the molecules vibrate. Low frequencies (slow vibrations) produce deep bass sounds, such as the rumble of the firebox. High frequencies (fast vibrations) produce bright sounds, such as the dinging of the bell or hissing of steam.

Frequency is measured as Hertz (Hz), or cycles per second. Thus, if a sound causes an air particle to vibrate back and forth 100 times in a second, it is said to have frequency of 100Hz.

Three Ranges of Sound

Human hearing works in the range of 20Hz to 20,000Hz, which is divided into three categories: bass, midrange, and treble.

  • Bass sounds are characterized by their low frequency, typically from 20Hz to 500Hz, and are produced by such sounds as firebox blowers, exhaust chuffs, and diesel engine exhaust.
  • Midrange sounds fall in the range of 500Hz to 2500Hz and are produced by whistles, airpumps, radiator fans, and injectors.
  • Treble, or high-frequency sounds, have a range of 2500Hz and up and are made by bells, steam releases, dynamos, and turbochargers.

Some sounds, such as exhaust chuffs, are complex and are made up of many frequencies that span several sound categories.

Speakers and Sound

How a Speaker Works

Model railroad sound systems use what is called a dynamic speaker to produce sound. The structure of this speaker is fairly simple and consists of a metal or plastic frame to hold everything together (Fig. 2). Stretched across the frame is the speaker cone or diaphragm, which is usually made of a stiff paper or Mylar plastic. Attached to the center of the speaker cone is a voice coil, which is a bobbin wrapped with many turns of fine wire. A permanent magnet surrounds the voice coil.

When an electrical current is applied to the voice coil, a magnetic field builds up around it just like an electro-magnet. This magnetic field is either attracted to or opposed by the magnetic field of the permanent magnet. The effect is similar to that of bringing two permanent magnets together and turning one toward and then away. One way, the two magnets are attracted to each other; the other way, they are repelled.

When the audio signal is fed to the speaker, the varying magnetic field causes the voice coil to move back and forth. Since the voice coil is attached to the speaker cone, the speaker cone vibrates and produces a sound wave.

Getting Great Sound

The quality of the sound module will be a key factor in determining how good an installation will sound when finished. A bad sound system will still sound bad even when connected to the best speakers in the world. However, even a good sound system can sound bad when improperly installed.

Aside from the sound module itself, the most important factors in determining the successful outcome of a sound system installation are:

  • Speaker installation
  • Speaker type
  • Speaker size
  • Speaker quality

Speaker Installation
How the speakers are installed will have a dramatic effect on the overall sound quality. All things being equal, this makes the critical difference. A poor installation can make a good speaker sound bad, and a great installation can make a mediocre speaker sound pretty good.

Recalling our earlier physics lesson, sound is basically a pressure wave. The speaker cone makes sound by pushing against the air molecules to create a disturbance in pressure. The speaker, however, must be properly installed in an enclosure (or "housing") in order to build up any pressure. Otherwise, as the front of the speaker cone pushes out against the air, a vacuum is created on the backside of the speaker cone that absorbs the pressure from the front, so no sound wave is projected. Without an enclosure, an opposite pressure behind the speaker cancels any pressure developed by the front of the speaker. The enclosure isolates the front and back surfaces of the speaker, thereby increasing the pressure and hence, the volume.

It is important to properly size the enclosure to the speaker. If the enclosure is too small, it will interfere with proper operation of the speaker (it’s almost as bad as not having an enclosure in the first place). It’s hard to make the enclosure too large, although there is a point of diminishing return. As a rule of thumb for small speakers under 4” in diameter, the minimum length, width, and height of the enclosure should equal the speaker diameter. Thus a 1” diameter speaker should be housed in an enclosure that is at least 1” x 1” x 1”. Similarly, a 2” speaker would require a 2” x 2” x 2” enclosure. As this is a general guideline, exceptions can be made in many circumstances.

Note: The use of a proper speaker enclosure cannot be overemphasized and is almost always the cause of poor sound quality!

In addition to being the proper size, the enclosure should:

  • Be sturdy with stiff walls to avoid vibrations and buzzing.
  • Be airtight or nearly so. If a sound system is being mounted in a piece of rolling stock, it should be placed in boxcar and not a stock car, for example.
  • Have a speaker opening that is at least 50% of the speaker area. It is usually easier to drill a pattern of small holes than one large hole. None of the smaller holes should fall outside of the speaker face.

Speaker Type

A speaker is a speaker, right? Wrong! Every speaker has what is called its frequency response, which is a measure of its sound volume over a range of frequencies. The perfect speaker would produce the same volume level for all frequencies. Unfortunately, the perfect speaker is difficult to build and most have a limited range in which they work. Thus, speaker types are divided into four broad categories, each classified by the range of frequencies they can reproduce:

  • Woofer – designed to reproduce low frequency sounds
  • Midrange – designed to reproduce (you guessed it!) the midrange sound frequencies
  • Tweeter – designed to reproduce the high or treble range of frequencies
  • Full Range – designed to reproduce the most of the entire range of audible frequencies. Full range speakers usually represent some compromise at the extreme ends of the audio frequency spectrum.

High-fidelity speaker systems often use a combination of speakers to produce a finished speaker that evenly reproduces frequencies across the entire audio range. If size is not an issue, a simple cross-over network can be built using this theory (Fig. 3).

For locomotive sound systems, installing multiple speakers is usually impractical, so a full-range speaker is the best choice. When selecting a speaker, look for one with as wide a frequency response as possible.

However, as we shall see, a speaker’s frequency response is limited by its size, which is in turn limited by the size of the model, so often a compromise must be reached. Fortunately, we have built-in a 7-band equalizer to many of our Digital Sound Decoders so you can optimize speaker performance by cutting and boosting sound levels by +/- 12db over seven selected frequency ranges.

Speaker Size

When it comes to selecting a speaker for your sound system, remember: size matters and bigger is better! All other things being equal, a bigger speaker will:

  • Produce more volume. The larger speaker cone has more surface area and is thus able to vibrate more air particles creating larger pressure changes.
  • Produce a deeper bass sound for more realistic chuffs. The voice coil in a larger speaker can move back and forth along a greater distance than that of a small speaker, improving the low frequency response of the speaker.

When a large, single speaker is not practical, two or three smaller speakers can be used together to improve the frequency response and sound level.


Individual speakers in a multi-speaker system must be wired properly or the sound level will actually get worse. Speaker terminals are usually labeled plus (+) and minus (-). Be sure to connect plus to plus and minus to minus. If you are unsure which terminal is which, listen to the speakers when wired one way, then listen again with the speakers wired the other way and choose the arrangement that produced the best sound.

Additionally, the number and type of speakers used will affect the way multiple speakers are wired. See Fig. 4 and Technical Note No. 8.

Speaker Quality

Finally, the quality of the speaker should be considered. Despite appearances, speakers are not identical and there are a number of parameters that should be considered:

  • Frequency Range - Look for a speaker with as wide and flat a frequency response as possible.
  • Magnet Size - A larger magnet often allows the speaker designer to use heavier wire in the voice coiling, enabling the speaker to handle more input power. It does not necessarily mean the speaker will sound better, but it might last longer.
  • Power Handling - When possible, pick a speaker that has a power rating comparable to the sound system’s amplifier. If the power rating is too low, you risk eventually burning out the speaker.
  • Cone Material - Look for a speaker with a stiff cone and avoid ones that have a flimsy feel. For outdoor railroaders, a polypropylene or Mylar cone has the advantage of being moisture resistant.
  • Impedance - SoundTraxx Digital Sound Decoders work with 8-ohm speakers.

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.

Detailed instructions and wiring diagrams for installing Tsunami2 or Econami Digital Sound Decoders can be found in our Installation guide.