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:
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:
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:
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.
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:
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.
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: