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Well-Known Member
Oval verses Round Speakers

I had occassion this morning to run a short experiment on a few speakers I have under consideration for use in HO trains. It was relatively simply experiement consisting of hooking up a 9 volt battery to a few speakers and momentarily energizing the coil of the speaker to visualize its amount of excursion in and out.

I tried three different oval speakers, two hi-bass round speakers, and one std round speaker. Based on these observations I am MUCH MORE inclinded to utilize the round hi-base speakers over all others. It appears to me that the oval speakers lack the significant excursion that the hi-bass round speakers have. That would seem to put them at an advantage, even while the oval ones have slightly more surface area. In other words the total volume of air moved by the smaller round speakers would still be greater than that of the bigger oval speakers.


Well-Known Member
Thinking Outside the Box (speaker box that is)

Sorry I've been absent for awhile on this subject, but I'm back looking at this technology of onboard locomotive sound again.

Just ran across a very interesting** video presentation on Youtube:
DCC speaker principles,Getting the sound out

...and here is an accompanying article:

Bruce Petrarca discusses his very interesting ideas of not always having to enclose our speakers in boxes....or at least the options that are available to try and make use of more than one-half of the sound projected from one side of the speaker, verses the full (combined) sound projected from both sides of the speaker

Mark R.

Custom Painter
I don't agree with many of Bruce's observations and installations. He may be the dean of DCC University, but I'm tried many of his methods and was very disappointed in the results.

Seeing as how we're talking "rules" of sound - a round speaker is much more efficient than a rectangular speaker due to the even stress on the cone surround. A rectangular speaker has considerably less flexibility on the sides than it does the ends. You would be much further ahead to use two round 16mm speakers instead of a single 16X32mm one.



Well-Known Member
I don't agree with many of Bruce's observations and installations. He may be the dean of DCC University, but I'm tried many of his methods and was very disappointed in the results.Mark.
Can you give any specific examples, and why you believe it may not have worked?

Mark R.

Custom Painter
In order for any speaker to gain, it has to have back pressure - and this obviously comes from an airtight sealed enclosure. Bruce's theory is to keep the sound waves from the front of the speaker from cancelling with the sound waves from back. That does in fact ring true, but when dealing with the tiny speakers we are, that theory doesn't hold water.

A number of years ago, I had a client that wanted me to take "normal" speaker installations to the next level for some really good sound. I personally conferred with Bruce in an attempt to kick it up a notch as I was under the impression he was the sound guru. The resulting installation is the dual speaker arrangement on my site under the DCC section. The speaker face fires down through the trucks and is completely sealed from the interior. The rear of the speaker was to use the interior of the shell as the sound chamber.

Well, guess what ? - It sounded terrible. Even with two speakers, the sound was weak. A real disappointment considering how much time was spent on it.

The interior volume of an engine shell is WAY to big to have an active effect for a good speaker installation. In an accoustic suspension enclosure (airtight), the speaker acts as a piston compressing and decompressing the air within the enclosure. This decreases the compliance of the speaker increasing its resonant frequency which improves bass response.

The size and shape of the enclosure is also important. The "golden rule" states that a ratio for an enclosure should be 0.6:1:1.6, meaning using those ratios for 0.6"depth:1"width:1.6"length would be the perfect enclosure (or any other measurements with those ratios. The rectangular design enhances sound quality. As I mentioned earlier, round speakers are much more efficient than a rectangular speake. Installing two round speakers in that rectangular enclosure will yield much better results (how many factory installs have you seen that use rectangular speakers ?). At the very least, avoid square or round enclosures that tend to exaggerate some frequencies over others.

Another problem we deal with is all that plastic. To make matters even worse, the speaker enclosure itself is made of plastic. I've done a lot of experimenting with enclosures and have been getting some really pleasant results making enclosures from wood veneer or sheet lead. The wood gives a much richer sound, but I'm still playing with different kinds of wood to find the balance between "warm" while trying to eliminate unwanted resonance. The sheet lead is probably the best so far as it will not resonate no matter what.

Then there's the issue with stuffing it in the shell - yet another plastic box. I've done like installations in both plastic and brass, and the brass engines always sound considerably better. The plastic shell picks up a resonance that can be very distracting. Again, still making progress on that front. I've had positive results by lining the interior of the shell with some thin peel-n-stick felt. Earlier attempts at brushing the interior of the shell with silicone helped to some degree as well.

All that being said - proper speaker design requires a PROPER sized enclosure (not the interior of an engine shell or tender). The second important factor we have to deal with is all that plastic. We need to try to avoid our sound waves from coming into direct contact with plastic as it will take on a resonance that, to me, is what makes it sound toy-like. Mounting your speaker to the shell is probably the worst case scenario as you are directly introducing your sound waves right into that plastic box.

These are my opinions and my findings through hundreds of installs and countless hours of experimenting. Take them for what you will.


One last comment .... those YouTube videos everyone seems to use to compare how good or bad an engine sounds like ? .... seriously ? The sounds you are listening to are recorded using who knows what for a microphone, and then played through your computer speakers - which I'm pretty sure won't fit in your engine.


Well-Known Member
As I re-read this PDF document several times I became aware of a possible speaker installation that was not covered in this document....excerpt:

"Separate back wave from front wave: It is important for sound quality that the inside cab area be sealed as much as possible to prevent the back wave from escaping through grills, vents, fans or other openings in the diesel or electric locomotive cab or through the openings in the chassis where the motors connect to the trucks. If there is any leakage of the back-wave to the outside, it will mix with the speaker front-wave and cause destructive inference in some base tones and perhaps constructive interference for some of the higher frequency tones. The respective path lengths for the front-wave and the escaping back-wave and the position of the listener will determine which frequency components are degraded or changed. Usually, since the acoustic chamber and path lengths are short, any back wave escaping from the locomotive causes degradation of the sound. If the back wave is allowed to escape close to the front-wave, the degradation is more severe. As the scale of the model decreases, this becomes more of a problem since the distances between front and back waves become smaller.

Vent the front-wave sound under the locomotive: Propagating sound upward into the open air seems to produce lower quality sound unless you are directly over the speaker. The sound has no opportunity to reflect against different parts of the layout such as buildings, mountains, etc. that add both volume and presence. Our experience is that the best design choice is to propagate the sound under the locomotive. The next best choice is out the sides of the locomotive through vents and grills. If sound is vented under the locomotive, always be aware of the affect of trucks and other obstacles and other factors that can either improve or degrade sound quality. Do not vent sound straight down too close to the track where it can be reflected back and decrease volume and sound quality. Venting the sound directly under the fuel tank usually does not produce good sound in smaller gauges (HO and N) since there is little space between the bottom of the fuel tank where the sound is vented, and the top of the track. Sound usually reflects back from the track resulting in poor volume and presence. Venting through the gear tower and chassis over the open truck areas seems to produce the best sound."

What I find interesting here is no mention of venting the 'front-wave' from the front of the tender, verses out the bottom, or out the coal load?? Why does this interest me? I have at least 4 locos with vandy tenders (C&O) that I wish to install sound into. He covers vandy tenders to some considerable degree, but does not mention this possibility.

I happen to have a friend who has a relatively cheap little IHC steamer that has its sound vented out the front bulkhead of the tender right into the cab area of the loco....the sound is GREAT. On the other hand I have a 2-8-8-2 with the sound vented out the coal load that I consider substandard. So I am giving real serious consideration to experimenting with one of my Spectrum C&O Heavy Mountains with their vandy tenders and venting the sound forward out of the tender.

Has anyone else seen such an installation? Does anyone have a photo and/or illustration of the 'factory sound' installation in these Spectrum models??
I am making good on my little experiment with the vandy tenders, and that 'sound out the front' theme, ...over here


Well-Known Member
Yes unless it contains pictures of actual installations where the writers have compared actual application it is just arm chair theory.

Like the illustration of having multiple smaller speakers will not improve the sound. The sound will be the same as a single speaker of that size, you need dissimilar speakers. I am told that is wrong, but I know it is true because I have an actual application. Speakers in locomotives isn't rocket science.

And I am making use of that suggestion(s) you have made on dissimilar speakers.

Iron Horseman

Well-Known Member
The real problem with sound in general is that everyone's ears are different. One could design the technically best reproduction (frequency wise) and it might still sound dull or flat to some people.


Well-Known Member
Staff member
The real problem with sound in general is that everyone's ears are different. One could design the technically best reproduction (frequency wise) and it might still sound dull or flat to some people.
Having hearing aids doesn't help either.


Well-Known Member
Sugar Cube Speakers

Interesting discussions concerning Sugar Cube Speakers. I wanted to post them here for others to see, as well as to help me remember them when I get time to get back to these installations. (soundtraxx) said:
Hello, all. Laurie had given me the idea of using Sugar Cube speakers in the "Speaker Enclosure" thread and I finally had a chance to experiment extensively with them, and they sure do sound amazing. I compared the sugar cube speaker to several Soundtraxx speakers (I had previously purchased every speaker Soundtraxx makes so that I could make and sell enclosures for them). I set up a lab by making an enclosed box lined with acoustic panels where I placed the speakers and a professional microphone. I used graphical frequency-response software and a pink-noise generator to get as close to a flat line as possible. I made a baffle and glued it to the speaker and got to work.

I would have to say that sugar cube speakers sound better than every speaker except the Mega Bass line of speakers, which are often too big to be practical. I was amazed how much better they sound than the ubiquitous 28mm(1") round speaker. Another popular speaker, the 35x16mm oval speaker, seems to be the closest in comparison. Although it has a lower frequency response, producing more in the ~750Hz range, it is hampered by less high end than the sugar cube. It really takes a nasty dip in the 4kHz range, and also it does not seem to produce quite as much volume as the sugar cube.

I further experimented with the physics of the speaker. I tried putting it in close proximity to a piece of paper, where I was able to create different shapes such as flat and curved surfaces. Of course I also experimented with a loco shell. I have reached the following conclusions:

Vastly different sounds can be achieved simply by placement. For example, placing it inside an HO diesel shell with some empty space will sound quite a bit different than an enclosed mounting. Some guys put it under/in the gas tank of a diesel facing down, which in my opinion should yield the loudest results because the sound waves will reflect off the layout. However, in such a case driving over a level crossing will produce noticeably different sound than over a trestle or even a heavily ballasted section.

Just like my other speaker enclosures, I have confirmed that the "baffle" acts as a method to prevent sound wave cancellation. Interestingly, the sugar cube is as near an example of an omni-directional speaker as I have ever experienced. At first I thought that somehow sound was emanating from the baffled side, but I realized that because of the high loudness-to-size ratio, sound waves are able to project in all directions within a short distance. What is nice about this is that you could use minimal mounting points or attach it from the side leaving most of the space around it open thus retaining it's omni-directional characteristics, if desired.

Laurie, I was not able to verify that "hard mounting" made any appreciable difference. Touching the speaker's baffle side to the outside of the loco shell should have made a difference in sound, but it did not. Only by placing it inside the shell and the sound waves thus reverberating was a difference heard. I even touched the speaker to my microphone which did not result in any change in frequency or amplitude.

My advice for mounting these speakers is if you are particular about the sound, do not decide where you are going to mount them until you hook them up to a decoder or amplifier and experiment. The speakers are small and the mounting options are numerous (at least in my scale of choice, HO), but they will sound a little different in every spot you place them. Personally, I am going to try mounting in the gas tank facing down next.

I realized that these things are better, smaller, and cheaper than pretty much anything you can fit in a HO or smaller gauge locomotive. Having been motivated thus, I immediately set out to acquire a bulk amount of those speakers and also make baffles for them. So in addition to my Soundtraxx speaker enclosures, I now have a sugar cube speaker/baffle kit available on my ebay store.

Laurie McLean (soundtraxx) said:
G'Day Jonathon,

Thank you for this information regarding Portable Media Device (PMD) or Sugar Cube speakers.
About 5 years ago I purchased some cell phone speakers of different sizes and set about testing.
After many disappointments with the very small PMD's (these were about 1/4" longand 1/8" wide) it was the larger (well not really large!) PMD's I got from Nokia that proved to produce pretty god sounds.
As time went on I got a few of these and a few of those to try and test.
Like the conventional speakers the material of a baffle or enclosure does make a big difference. The materials we can use will alter the sounds to be soft, medium or hard.
From my tests and experiences it is absolutely imperative to get a 100% seal just like conventional speakers. If a tiny gap exists then the sounds will be colored.
I now use latex to seal some of these PMD's as they have slots around the black plastic frame on some so beware.
Sony and Samsung have some of these they used on old tablet models.

As for mounting - try this with your diesel loco shell body -
1. Glue the speaker directly onto the plastic - try the hood first and have a listen, then place it up above the rear truck.
2. Now make an enclosure or use an enclosure that comes with the speaker. Mount in the same places as above and use very thin double sided sticky tape on the back of the enclosure adhered to the loco shell - and listen.
3. Now cut some double stick foam tape and place on the back of the enclosure and try in the same spots and listen.

In the 1. above I have used ACC glue (Loctite Gel) and no enclosure, mounting the speaker under the hood of a GP40.
The resonating sound waves transfer directly into the plastic loco body and act as an extension of the speaker making it "a bigger speaker?" (best way I can describe it).

Bottom line is what you have now discovered for yourself - try on the bench before actually installing. Once your ears like what they hear experiment in placement as this is VERY important. I like to allow the sound waves to exit as freely as possible with minimum defecting off loco internal parts.

Thanks for the testing you carried out - valuable information and good of you to share mate.

Regards, Laurie McLean MMR #417 NMRA AP Asst; Mgr Div.1 Queensland, Australia Home of Anim8FX ©


Well-Known Member
I'm investigating this whole subject of optimum sound installations in HO model trains. I ran across this PDF document with good explanations and nice illustrations. Thought this forum might find it interesting.

Wow, I was just going back to read thru this older speaker/sound subject I had written about a number of years ago,...and discovered this old link to that very good PDF is no good.

Would someone with more computer knowledge than I have know how to access that PFD document??
Last edited:


Well-Known Member
Staff member
I've been using the iPhone6s plus 5.5" speaker in several installs and there has been differences in the sound reproduction. These appear to come fully self sufficient i.e. enclosed etc, and by enclosed you include some padding or other content within the housing. (I have opened one of them and a couple of other types to see what is inside them) Small foam plastic pads and very fine white beads seem to be the materials of choice in most. Whether where they are mounted onto and how may also play a part too. I will be taking more notice of this in future installs to see what relevance there might be.


Well-Known Member
New link provided by a gentleman on another forum:

I have this file downloaded some 10 years ago, it is still on my PC. I have uploaded it here, link will expire in 29 days. I hope it will work.

I think this document is very useful for anyone trying to install sound in locomotives.


Well-Known Member
In reply to that older PDF paper I had referenced on speaker installations, JT Burke responded thusly,...

As to that Loco Acoustics pdf, he’s showing the principals that I teach are either impossible or impractical. Points below:

- I am glad that he showed the loco holding water and that the driver’s in-shell side needs to be water-tight. This is correct if using such an installation. It’s also impractical if not impossible. If you were even able to seal a shell this tightly, assembling the thing would be a challenge and any future maintenance a nightmare. I’ve had a couple of locos in my shop that had sound installed by some well-known installers, who did seal speakers into the shell. The sealed chambers interfered with the loco’s lighting, which needed replaced. The speakers didn’t even sound good. Removing them was a mess. Even a simpler shell, like a tender, is impractical to air/water-tight seal. ALL additional pieces like coal loads, bunker doors, etc need sealed up. Then, once you have the install on the frame or in the shell, you must seal the two together. IF you are actually successful at doing this, what happens when the decoder and/or speaker need servicing or replaced? A mess to get back apart. Furthermore, most shells will not provide the optimal resonance to enhance the driver’s loading; brass and metal are terrible resonant choices; plastic shells will vary from model to model and have additional parts attached that can/will rattle. I’ve had a lot of factory QSI-equipped locos in my shop and none of them have ever been sealed.

- He is wrong in that the larger the driver, the fuller sound. In a world where all things-are-created-equal, this would be true. We do not live in that world. In professional audio, I know of numerous 12” drivers that have much greater low-frequency output than some lesser 18” drivers. Cone size is only one, and not even the greatest, factor in a driver’s capacity to reproduce LF. Likewise with miniature drivers, there are many, many, many large drivers that cannot even come close to the best-performing smaller versions. Low-profile mini speakers (“cubes”), when designed well, will blow away most traditional cone-drivers anywhere near their size. As for cone-drivers, I’m currently testing some additional units in the 20-32mm diameter, but as of now the highest performing one is 25mm, reproducing greater LF than any of the 30mm sizes I’ve tested.

- Concerning the use of many small drivers, he states that using multiples of smaller drivers to equal the cone-area of one or two larger drivers will yield similar results. This also is only taking cone-area into consideration and there are many more factors in play with a driver’s performance. The complexities of electro-acoustics greatly escalates when you start adding more speakers, with the odds strongly against improvement.

The optimal solutions is to always use a self-contained speaker system mounted within the shell. The example I give during clinics discusses the impact a room has on a speaker system; If you have a home theater system in your living room, you probably enjoy a big, loud sound with thundering bass while watching movies. Why is this? You have a decently designed sound system interacting with room boundaries that are increasing the sound system’s efficiency. If you take that same sound system and place it in the middle of a field and listen, it won’t be able to get as loud and your bass-frequencies will be nearly gone. In concert sound, we can use fewer sub-woofers and less mid-high enclosures in an indoor venue than an outdoor venue of similar size. A room increases a speaker’s efficiency. Thus, I teach that the locomotive’s shell is the room in which your speaker lives; it is not the enclosure itself. I have found this to be true. A well-designed, self-contained speaker system that sounds good, will only sound bigger and better once it’s installed in a locomotive. It also makes installation easier, faster, cleaner and permits simple servicing in the future.

Here are a few more hidden YT videos if you’re interested.

I will say that I am pleased he wrote the article - even if it does have some misguidance. At least he was making an effort at improving on-board sound instead of just settling for whatever. Experimenting and tinkering toward improvement is never a bad thing. I never intend to minimize or ridicule anyone for their efforts. I will try to accurately educate if I can.

jt burke
Scale Sound Systems

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Preview YouTube video Scott Thornton's SW1 Speaker

Preview YouTube video Bigger is Not Always Better - DCC-Sound Speakers - Railmaster DLG-8 vs. Scale Sound Systems

Preview YouTube video Overland 2-8-0 Speaker Comparison for Dale Devene (private)


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