Engine Two Way Speaker System

Testimony of the benefit

Here is an independent comment from a person who did this vary modification on 11/17/2008. The message was posted on the soundtraxx Yahoo group list.

"I have finished a so-far successful install into a brass streamlined steam locomotive and tender (the large Milwaukee Road Hiawatha 4-4-2)......A 3/4" speaker in a fabricated Strathmore paper "hatbox" enclosure was then placed in the smokebox facing upward toward the open stack. A 33µF bipolar capacitor was wired in series ....A 1.06" High Bass speaker was mounted facing down on the tender floor. Two parallel-connected 1000 uH chokes were wired top and tail in series....So far, the chuff sound and most other sounds are very, very clear, and very good, and for reasons that I cannot understand (perhaps I do not need to!), virtually all the sound now seems to be obviously coming from the stack, not to the rear of it- a tremendous advance!. Wow!"

What is he talking about? If your into audio and listened to a good stereo system properly setup, one can close their eyes and hear a "3D stage" of sound. You can here the violins playing on the right of the stage, drums in the center and the piano on the left front of stage. That is what a stereo speaker system can do as in create “atmosphere”. The goal were after here is the creation of the same atmosphere stage without the use of a true stereo setup. In other words, place the sounds closer to where they would come from on the prototype engine if you were standing near it.

What is the problem we are trying to solve?

The direction of the steam engine sounds are, UN-PROTOTYPICALLY and distractingly, comes from the tender. The larger the engine gets, the worse the problem. A multi speaker system offer the opportunity to create and control the sound image coming from a locomotive. What is not being discussed here is a true stereo sound syst

In a steam locomotive, it is easier to place the speaker in the tender since there is typically physically more room their than inside the steam engine itself. Unfortunately prototypically and physically the engine, which makes all the noise, it in front of the tender. So the sound we hear comes from the WRONG PLACE!

Diesels engines do not suffer from this problem as much but can still benefit especially on larger scales.

What is the solution?

Although most sound decoders do not have a stereo speaker support, you can create a "poor man's" or "virtual" version with any mono sound decoder. How? Install a two-way speaker system.

Why does this work? It turns out that most engine sounds have frequencies that are in the mid to low band sounds even with steam. Things like the initial crack of a Bell, whistle and the higher portions of the "hiss" sound have a lot more higher frequency content or energy that are efficiently produced by the tweeter. By placing the tweeter at the front of the locomotive, the sound moves toward the center-front of the locomotive. A two-way speaker system is able to make this "sonic shift" in sound happen without overloading the sound decoder despite the use of two speakers.

What is a Two Way Speaker System?

The HI-FI (stereo) industry typically defines the "full audio range" for humans as 20Hz to 20KHz (20,000 Hz). Think sub-bass earthquake rumble (20Hz) all the way up to sounds of cymbals or air (20KHz). For more information on the Audio range of sounds we hear, go here:Hearing Frequency Range

A given "Full Range" single speaker, as shown on the left, will typically have limitations on both the upper treble and lower bass extremes of the audiable frequency range.

"Full Range" here refers to a speaker that is able to produce a "very wide frequency range" or at least half or more of the 20 to 20KHz range. The point is that it cannot do a good job of the entire frequency range and what it does do it has to work harder at to do it. Think more distortion and less clarity.

Due to physical and acoustic properties, it is understood that the larger a "full range" speaker gets, the better it gets at produce bass typically at the expense of treble notes. Likewise smaller speakers get better at producing treble at the expense of bass notes. Always a compromise between the two.

A "Two Way" speaker system, as shown on the left, referes to the use of two speakers to reproduce sound in a divide and conquer way. One small speaker focuses on the high notes and called a "tweeter" while much larger speaker focuses on the low notes and is called a "Woofer". This allows both speakers to work well at the frequencies they do best which in turn leads to lower distortion and a wider frequency range than possible with a single speaker.

Many popular speaker system used for Stereo/HiFi sound systems start with at least a Two-Way speaker system.

We can use this "Two- Way" speaker system approach to help us get more realistic sound from steam locomotives in terms of better sound distribution over the length of the engine.