Sound decoder installation in N scale

Babbo_Enzo's picture

Hi all,

during my last decoders installation ( see the previous post here : ) I’ve drop down some notes, mainly taken from my past and present experience. Not really a “how-to” article, but a collection of suggestions to achieve the best results from sound decoders in N scale. Hope somebody is interested to hear about or at least agree or not: N scale sound is not so much discussed in Forums, and not all arguments that are valid for bigger scales are still useful in N. Enjoy?

Step-1: Speaker baffle.

First select the biggest speaker that can fit and the one that have best low frequency response. Steam sounds are full of low frequency ( not so much as Diesels ) but also a lot of hi-band “hiss, shish, chuff …) content. A bigger speaker is able to move more air (higher volume) and usually have better “sound-color” at same volume level.

For big steam tenders  one of the best 8 Ohm solution I’ve experienced was a rectangular 14x25mm Soundtraxx speaker ( ).

When the available space was not so much, a good round speaker can be the Soundtraxx ( or equivalent) 14-15 mm.( )

Low frequency response of both is around 500 Hz and this is one of the best value I was able to find.

When evaluating the speaker space, take in consideration to add a closed box on rear side. Again, bigger is better, but I’ve experienced not big difference from a deep dimension of 2-3 mm to one of 10-12 mm.

In N scale the space is limited, so increasing the speaker deep is always a tricky game. But the physics audio laws stated that the rear sound waves cancel the front pressure, so here some suggestions for N scale.

Cancel the rear audio wave is first rule. Somebody suggest to use the model body to do this job, but I’ve found very difficult to separate the two sides of the audio source and seal one without use a “real box”. I do usually this using 4 styrene square strips and a styrene cover. The rectangular speaker have a plastic chassis so I glue some styrene strips 2x2 mm on back.

The rear of the speaker was also filled with a tiny layer of phono-adsorbent foam that help to reduce the rear waves echo and case vibrations. A styrene cover will then seal the box. See the pictures:

- - - -

I shaped the strips with an Xacto cutter and a fine round file around the magnet area and the wires output and use the Faller plastic glue to seal everything. This is a solvent that melt the plastic sides so a very little quantity last long.

To experience how a sealed box can enhance your sound, just connect a speaker in open air to a sound decoder and use a piece of paper tape to cover the back: you will appreciate the increased volume and better sound “color”.

Step-2: How sound leave your model

From top or from bottom? What I can say is that the bottom side is where is simpler to hidden the eventual additional holes needed. Some modelers have do several tiny holes hidden in the coal tank, but, as I model oil tenders only… I’ve no coal around!

You don’t need to make a “strainer” in the tender body, in general a set of 4 or 6 holes ( 1-1,5mm) in-line along the car sill will suffice and are invisible when model is on tracks.

In addition I feel that sound reflecting on the ground result more “colorful” and “rounded” (less hi-frequency) specially in the “chuff” sequence.

How much volume is another common question, but here my answer is “depends”!

In a silent environment as a private train room where several motive power are running, usually I set the sound to a level than don’t disturb operators working in a different area of the layout. Think that a sound you can hear at 2 meters distance is corresponding to a scale 320 meters in 1:1. This is the max level I use and it can be regulated just setting the general volume CV. In some cases (meetings or show, big clubs layouts) where a noisy background is present, just move the same CV to higher value.

If you have the capability to connect a serial communication port to your system, then give a try to JMRI DecoderPro-3 software to program your roster: I’m sure you never come back! And it’s free of charge as MRHMag! ( )

Programming all variables that are in the Tsunami is a snap, the user interface is friendly and suggest text description for all intricate bit combinations. Setting and do a real time test of results is simple also for the band equalizer and related functions ( you know you can adjust the equalization to adapt to the speaker diameter ? ).

Step-3: Wiring

In case your loco is not DCC “friendly” …. this the most “creative” part of the job! In old steam engines, as well in brass models, DCC was long way in the future. Given that your model run well in DC, it can only run better in DCC. Other ways, I never hear of a bad running model will be cured installing a decoder. So, first check is related to motor, worms and other mechanical parts to have a good runner (low speed in particular, no wobbling, good “tractive force” , etc. ) The only adjustment you can obtain with a decoder is the limitation of the maximum speed ( without any mechanical change ) and the setting of a “prototypical” speed curve. BEMF technology additionally can help on grades ( but in most cases it will “disturb” in consists ).

Several times you’ve read “isolate the motor” from tracks…. Well, remember this is related to the two Motor Brush (Brush and Cap). No real reason to isolate the motor chassis from the engine.

In N scale Brass steams usually the right side drivers on engine takes one of the two polarity of the DC voltage and the left side wheel in the tender provide the second one. So, in this case ( or also if both polarity are taken in the engine and your decoder is in the tender ) you need to transfer one or two wires from engine body to tender. This is done via the drawbar and some contacts on it. My suggestion is to don’t use this tools! Just use wires and if you find a hidden place that is not disturbing too much the model, use a connector like the mini male-female by Miniatronics or similar show here.


( for example see here : )

Personally in most cases I think the additional mechanical work to fit the connectors give not so big additional value to your installation, and I just use thin and flexible wires ( paint it black with your brush or a fine felt pen) . The limit is that you can’t disconnect engine and tender, but, why you really need to do this? And the additional wires resemble the real connections too. A point of CA where wires leave tender and on the engine and leave it pending with an U curve to permit a good play on curves.

Another “big” invader in N scale is represented by the electrolytic capacitor supplied with the Tsunami or other sound decoders. This help to avoid a decoder reset when running on dirty tracks. An accidental  reset with these decoders is “very audible” ! Again the rule is “bigger is better”, but again my suggestion is don’t use the aluminum type supplied as standard ( well, it’s the cheapest model, but not the littlest one).

Try to find SMD Tantalum models as this one:   

Litchfield Station LLC ( ) as well Digikey will delivery it at your door.

Put some (Litchfield sell only 100 uF value, so connect at least 2, but up to 4 or 5 is better ) IN PARALLEL ( means connecting together the same polarity side ) to increase the total value.

That's all for now, friends.... any comment is only welcome.


skiloff's picture

Excellent summary

One of my (too many) projects is putting sound in a couple N scale diesels. The capacitor location has been one of my biggest issues so I will check out those SMD ones which should provide more options.


HO Scale '70s/80s era

Don't totally seal the back

You will actually reduce the sound power from a speaker if you totally seal the back of the speaker box.

The speaker makes sound by vibrating - essentially moving back and forth. If you totally seal the back, the speaker loses some energy having to compress or decompress the air in the box. Granted at these scales it might not make all that much difference but it does have an affect.

In the days of monster stereo speakers, this was solved by adding a tube that created an opening in the front of the speaker face plate until just before the back of the speaker box.That way the air pressure behind the speaker was easily adjusted (i.e. minimal compression and decompression) and the distance the sound wave traveled (to the back of the speaker and then out the front) was enough to remove the cancelation aspect. These were known as 'ported' speakers.

Who knew I would remember stuff from my "Physics of Stereo" lectures?


N scale, DCC-NCE, Switching, Operations

Babbo_Enzo's picture

>You will actually reduce the

>You will actually reduce the sound power from a speaker if you totally seal the back of the speaker box

David, sorry but the theory and practice (One of the activity in my pro-job is related on this ) say the opposite.

In open air, when the front of the speaker membrane push the air to create an audible wave, the rear of the same membrane is "sinking" (I don't have the English word for this, sorry, but hope give the idea) the air. So the two effects are conflicting and one cancel the other. The frequency are the same but the "phase" are 180° different.

In a perfect world .... you can't hear nothing! But as the reflections in the ambient acts , in reality you hear "less" volume.

The stereo speaker you refer are based on a different concept and have a different goal: enhance the lower band of frequency. In fact the simplest speaker design is based on "seal the back" concept. The air pressure on back side of speaker is partly "adsorbed" using fiberglass or better materials that don't reflect these frequencies and prevent the audio waves to push directly to rear wall of the box, this way decreasing also some mechanical unwanted vibrations of the cabinet.

In the "bass reflex"design ( I never hear your definition of "ported" ... interesting ) a forced path is created by use of a pipe that return "part" of the back sound pressure in front. The "path" or pipe have a shape (path and dimensions) calculated so to catch the low audible band and return it in front. Result is to increase the amount of sound on lower part of the audio range. This enable to use lower diameters "woofers" ( the lower frequency big speaker) that have less low frequency good response, and obtaining both a little cabinet and a cost effective (marketing definition for "cheaper") product.

If any doubt of mentioned "basics" .... connect a speaker without any cabinet to your radio, Iphone jack, decoder.... and compare the front volume when you cover the back with a paper tape or some other tool (if the speaker is a model speaker, just using the inside of your hand give significant difference)

One the popular claims for Plugin MRC sound decoders in N scale, is that he delivery very low volume of sound..... The cure is simple: seal the back! Well, sound quality or fidelity definitely don't change, but volume increase for sure.


I think we mostly agree

however, sealing the back does have an effect on the power.

The membrane of a subwoofer moves a lot. Without the hole in the back of the cabinet, it would compress the air in the cabinet. This would interfere with the movement of the speaker cone, just in the same way that it is difficult to compress a sealed plastic bag. Allowing the air pressure to equalize on both sides of the cone allows it to move farther (more volume to the sound) and with greater fidelity.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming

Now, I'm not suggesting that the micro-speaker you are using is anything like a sub-woofer in terms of cone travel but the physics says power is lost on a sealed back system and, in fact, it is lost to the work required to compress and decompress the air.

In all likelihood, you do not have a perfect seal and the speaker movement is minimal enough that the loss is acceptable. An interesting experiment would be to get a perfect seal and then break the seal to see what the difference is.


N scale, DCC-NCE, Switching, Operations

MRH advertiser Railmaster Hobbies has ported speakers.

"The speaker makes sound by vibrating - essentially moving back and forth. If you totally seal the back, the speaker loses some energy having to compress or decompress the air in the box. Granted at these scales it might not make all that much difference but it does have an affect."

Railmaster speakers are probably as small as the ones Enzo is using, and they come ported already.

Babbo_Enzo's picture

True... but ...

I don't want anyway to food this debate indefinitely, but consider the followings:

>however, sealing the back does have an effect on the power.

Yes, it's true. But the question is : on final balance sheet we have to list :

a)power loss (or additional power to compensate the loss?) and ...

b)gain in front wave pressure as result of elimination of the 180° phased wave of the rear side of the speaker.

As the final result is positive, and the additional power is minimal.... I think anyway the solution is win-win.

About the sub-woofer compression inside cabinet and "back hole", it's true but consider parameters are not comparable, as said, to values obtained in our models ( I guess from "N" up to "O" scale ):

- diameter of the sub-woofer speaker (>40-50 cm at least in home app) - air volume moved is huge and have to be compensate in some way.

- frequency response is < 500 Hz usually - these frequencies are omni-directionals (they "fill" the ambient and human ears can't distinguish the source location. For this reason a usual solution in home systems is to direct the front of the sub-woofer speaker to ground, and have rear waves redirected to same end using a bass-reflex configuration with a big channel ( because the very low frequency ). This way solve both internal air compression problem and reinforce also low frequencies generated (usually under the human audible threshold)

Aside this, I've noted a new thread about use of JMRI to setup Soundtraxx decoders CVs in general and the Equalizer in particular.

I've still to see the video but I'm sure is interesting : as I've said above.... If you try DecoderPro 3... you never come back.

Take care, friends, and do some experiments.


I'm no expert, but I've been

I'm no expert, but I've been a musician my whole life. There are 3 types of configurations. Closed back, open back, and ported/vented. A speaker as small as what we're dealing with here is not going to benefit from any type of porting what so ever. The physics is not going to allow it. Below 500 Hz is where a port will do the most good and these small speakers don't reproduce that range. A sealed enclosure does dampen some low frequencies depending on the box volume but it has a more directional throw and a tighter, cleaner sound and is going to enhance the frequencies allowed by the box which usually are lower mids. Too small and it gets very nasally, too big and you loose volume and definition. An open back design, where the back is enclosed but left with a small leak so sound can escape and get reflected around to the front, will have a broader response and a more omni-directional sound and these sounds 'can be' heard above 500 Hz. Box size is not as critical. So, for best results in a very small enclosure, if you want louder and more sound spread, use a box with an opening in the back, usually about 1/2 to 1/3 of the back being open. You want to separate the back sound from the front sound as much as possible so the speaker should be placed face up with the back opening facing down (you can reverse this).

On stage, when using open back cabinets, we place the speaker cabinet facing the audience with the back facing the wall behind us. The sound coming out the front is directional and the sound coming out the back is spread around the room giving you a more omni, open, and louder sound.


Mark R.'s picture

I've tried all kinds of

I've tried all kinds of installations and an air-tight enclosure gives the best sound reproduction in our models. The physics of home stereo sub-woofers and on-stage speaker do not translate to our models. Athearn themselves had a problem with one of their engines a while back in that the sound was terribly weak. The solution was to seal the enclosure (Athearn redesigned the enclosure to be air tight). So, even a small amount of leakage greatly destroyed the sound.

These tiny speakers vibrate more than anything - they don't "throw". The amount of air that is going to be compressed from a vibration is almost negligable.

That "tuned port" on the Railmaster speakers is nothing more than a gimmick if you ask me. Place your finger over the port - doesn't sound any different.

The biggest improvement I've found is what the enclosue is made of. I've resorted to making my enclosures from 3-ply craft plywood and have produced a much richer sound. For you guys that like to compare home audio speakers - how do you think they would sound in a plastic tote box ?

The only way to prove it yourself is to build both arrangements and listen for yourself.


Cube speakers

Have you compared the newer rectangular "cube speakers" such as from Knowles, Zimo, or even iPhones? I would be interested in seeing the frequency response comparison between cube speakers and traditional cone speakers. There are many Youtube videos of N scale diesels with cube speakers that give a surprising amount of volume. Thanks

Specs available...

Dear Johnny,

Spec sheets for the speaker drivers you mention are but a Google away,

some have had basic spectral analysis performed on them here onlist
(use the MRH search for "DSM EQ"),

and some of them can even get down <100Hz @ -3dB SPL...
(Remember, as per SMPTE/AES/ISO definition, "Bass" starts at 100Hz and only extends lower,
anything >100Hz is "low mid"...)

Happy Modelling,
Aim to Improve,
Prof Klyzlr

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