Engine Two Way Speaker System

3) What is the solution?

4) What is a Two Way Speaker System?

5) What are the Optimal Speaker Choices?

6) What if you already have a speaker in the locomotive?

7) How do you divide the Frequency Ranges between the two speakers?

8) What are the circuits and parts used to Build a Two-Way Crossover?

9) Table of Parts values for Crossover

10) Part Numbers for parts

Below is a discussion about installing a Two-Way speaker system into a locomotive.

This subject has multiple topics:

1) Testimony of the benefit

2) What is the problem we are trying to solve?

How do I install a 2 way Speaker system?

Place the small tweeter speaker in the engine and leave the woofer in the tender. The tweeters placement in the engine will "shift and stretch" the "sound stage" AWAY from the tender. The idle engine sounds will come from the rear of the engine while the whistle or turn on the bell, the sound stage will shift even more forward to the front of the engine as it should. A finer level of placement optimization can occur if you place the tweeter as far forward in the locomotive as possible. Sound wise given the tweeter production of treble sound, it will sound best if it fires its sound up the smoke stake of the locomotive into the air above the locomotive.

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.

What are the Optimal Speaker Choices?

1) Make the choice easy for yourself in terms of installation. Choose the largest "full range" speaker that one can fit inside the engine portion of the steam locomotive. It does not need to have any extended bass capability so it can also be thin. This speaker will be called the tweeter.

2) Choose the largest "Extended Bass" speaker that one can fit inside the Tender. This speaker will be called the woofer.

What if you already have a speaker in the locomotive?

Most locomotives that come with factory sound use a small single "full range" speaker, or if more than one speaker is used, both work together to form a virtual larger speaker that allow extend bass response. (For example: Any engine that comes with a QSI sound system.)

This is a tradeoff option. You can optionally choose to keep the existing loco speaker in the tender and use that as the woofer. Saves money and time especially if the speaker is a good one But you will typically get better sound by using a extended bass speaker in its place. There are many sounds, not as low in frequency as a Diesel Engine, but low enough to justify a extended bass speaker.

Want to learn more about Extended Bass Speakers? Go here: Extended (Hi) Bass Speakers

How do you divide the Frequency Ranges between the two speakers?

To make sure both speakers only focus on the frequency ranges they are good at, we need to add what is called an "Speaker Crossover" circuit. Since there are only two speakers involved, that terms becomes more specific as in a "Two-Way Crossover" A properly designed Two-Way crossover transfers the sound between the two speakers in such a way that at any time the two speakers do not cause any change in speaker impedance as seen by the amplifier. In other words, where as putting two speakers in parallel would normally cause the speaker impedance to be cut in half, that does NOT happen when a two-way crossover is used. Simplistically, any given frequency is only produced by one of the two speakers...not both. Hence the amplifier only sees just a one of the two speakers as the load for any given frequency it amplifies!

This works because of the properties of the two parts used to make up a crossover. A capacitor and an Inductor. These electrical devices have properties of allowing one band of frequencies to pass though it while blocking others. They are frequency selective based on their values.

What are the circuits and parts used to Build a Two-Way Crossover?

The circuit consist of two parts beside the two speakers. An Inductor and a Capacitor. See the schematic on the left. Top right is the tweeter and the woofer is on the bottom right.

Independent of the capacitor "C1" value, the construction is of a NON-POLARIZED or BI-POLAR type and is wired in series with the Tweeter.

For a given Inductor value "L1", the second property that need to be focused on is the wire resistance used in the construction of the inductor. This will allows minimize volume loss. Unfortunately the lower the resistance rating gets, the bigger the inductor gets!

You must find room for these parts in the locomotive.

The choice of the part values will depend on the crossover frequency you choose. A good frequency to start with is 1KHz. It offers a good even distribution of sound between the speakers.

Table of Parts values for Crossover

Below are the schematics and part values to use for different speaker impedances. That exact values need for the capacitor and inductor are typically not standard values for this class of parts. Hence there must be some value "rounding up or down" to ACTUAL or standard values that are available for purchase.

The tweeter speaker impedance does not have to be the same as the woofer. Just compensate by using the correct part value for the speaker you have.

Part Number information follows this section.

For 500Hz crossover frequency:

For 1KHz crossover frequency:

For 2KHz crossover frequency:

Part Numbers for parts

Capacitors: The part numbers for the capacitors and where to buy them is found here:

Bi-Polar Capacitors (Sound)


* Mouser Electronics Distributer specific Part Number.


1.0 6/06/2011

1.1 6/07/2011

1.2 6/10/2011

1.3 6/18/2011

1.4 6/19/2011


1) For calculation of C1 and L1 values for different speaker impedances or Xover frequencies, go here: