Improving the sound on an Intermountain SD40-2

[spookshow]

This seems crazy, but if you take the fuel tank off of an Intermountain SD40-2 you will see that there is a small hole smack dab in the middle of the plastic speaker enclosure. "Well, that seems dumb" I thought to myself and proceeded to cover up said hole with a piece of a tape. And wow, the volume and beefiness of the sound basically doubled after making that simple fix. In fact, it became so loud that I had to dial down the volume by lowering the value of CV63. Now, I can't have been the first person to ever figure this out, can I? And also, what the heck were they thinking? 



-Mark

[MK]

It's for the excess sound to drain, you know, like that overflow hole in the sink.   

[daniel_leavitt2000]

I really want to find the micro beads Apple and Samsung put in their cell phone speaker enclosures. I have not been able to find them though.

The beads are supposed to increase surface area of the enclosure interior and improve bass quite a bit.

[tehachapifan]

After all this time and all the discussions on the subject, you still can't convince certain people that you don't need holes to "let thew sound out". In this case, the hole is in the enclosure itself, which is a new one (well, sort of...see below **). Usually, the holes are on the fuel tank.

** I have experimented with placing an ultra-tiny hole (drilled with a #80 bit) in my custom N scale speaker enclosures for the purpose of allowing the speaker diaphragm to have more range of motion (a half-baked air pressure theory of mine). The thought is the tiny hole would not be big enough to foul the sound separation that the enclosure provides but will free up a little more diaphragm movement. It seems to work in some applications but not others. IDK what the purpose of the large hole in the pic above is. That will definitely foul the sound separation and result less robust sound as you've discovered.

[Chris333]

The beads are supposed to increase surface area of the enclosure interior and improve bass quite a bit.

Just like stuffing a woofer box full of poly fill.

[tehachapifan]

I really want to find the micro beads Apple and Samsung put in their cell phone speaker enclosures. I have not been able to find them though.

The beads are supposed to increase surface area of the enclosure interior and improve bass quite a bit.

Seems like the beads themselves would need to be hollow, Otherwise, the enclosure if effectively smaller.

[wmcbride]

Just like Scale Trains and all the holes in their fuel tanks to "let the sound out."

They seem impervious to commentary and suggestions on it -- and lacked the ability to look at what Atlas was doing with their Gold/Silver units that have the speaker in the fuel tank.

[Chris333]

The beads are probably some sort of foam.

The term “adiabatic” is used to illustrate the lack of heat transfer in the media. However, once the poly-fill is added, an isothermal process occurs. As air passes through the poly-fill material, its fibers wiggle and result in some form of heat dissipation.

This occurs when some of the energy which the air spring creates is converted into heat. This way, surrounding air molecules are heated up, and the air becomes less dense.

Because sound travels faster in denser media, the subwoofer relates with the system enclosure as though it is much larger than it is. Effectively, poly-fill increases the speaker enclosure size by over 40%!

[peteski]

Designing speaker systems (from concert size to a smart phone) is a complex science, and model RR model designers don't seem to have grasp of that fact.  They seem to just whip something up, hopping that it will work.

[spookshow]

This (the hole) just has to be something that changed with the most recent production run (which is what I have). Back when I reviewed the first production run in 2017 my comment was that the out of the box sound settings were so loud that I had to turn them down. Well, there's no way that could ever be the case now because with this hole the sound is barely audible, even with everything turned up to 11 (which is why I was inspecting the speaker in the first place). Does anybody have an older one they can check? I sold the one I originally reviewed so I can't.

Thanks,
-Mark

[robert3985]

I discovered this a while ago when installing sound decoders in brand-new Kato E-9's.  The speakers I was installing were circular, so I milled a hole clear through the rear of the chassis with a smaller diameter lip at the bottom to stop the speaker from falling out. 

I tested the first one I did without the body and man(!) the sound was barely discernible! Needless to say, I was highly disappointed, but, I decided to put the shell on (quite tight fitting) to make sure everything still worked, and much to my surprise, the sound (both volume and quality) was vastly improved.  I mean exponentially.

I posted what had happened here because it surprised me so much, and @peteski Peter suggested that I machine an airtight plug to see if the sound improved even more...so I machined a friction-fit tight plug out of brass for the top of the speaker hole , and yes, the sound improved even more!...and added some weight back to the chassis as an additional bonus.

Now, on ALL of my sound installs I make sure the speaker enclosure is airtight, even caulking the hole where the speaker wires come out and around the perimeter of the speaker frame if needed.

Photo (1) - Chassis mods for speaker installation on Kato E-9's on my workbench:


When somebody is having volume and quality problems with their sound installations, the very first thing I ALWAYS suggest is checking to make sure the model at least HAS a speaker enclosure and then, to make sure it's air tight.

Cheerio!
Bob Gilmore

(Edited for typos & grammar)

[peteski]

Yes, while I'm not an acoustic engineer I have learned that ideal speaker design would totally separate the air volume in front and behind the speaker cone (infinertely large baffle). Since that is not realistic, making the smaller enclosure airtight accomplishes that goal (with few small tradeoffs).

There are several alternate design which uses ported (partially open) enclosures to enhance certain low frequencies, but since air (as a gas) and its properties (like sound speed) do not scale, those are if limited  use for our miniature speakers.  But I recall someone here 3D printing some ported enclosure of N scale and  they seem to be somewhat successful.  However, a simple airtight enclosure will usually work fairly well and it is easy to make.

[kiwi_bnsf]

I do not understand why manufacturers do not pay attention to the fundamentals of speaker design — especially in N scale where it arguably matters even more given the tiny spaces available.

As part of a series of experiments with speaker enclosure design, I designed and tested a number of ported enclosures with differing lengths of port, differing numbers of ports, and different port geometry. In all cases these did not sound anywhere near as good as a sealed box (despite what the math promised).

I also categorically proved that 9x16mm and 8x12mm speakers sound better inward firing (wire terminals on the outside), rather than outward firing.

Finally, I did a number of tests with differing amounts of free space above my speakers with a sealed shell (no holes drilled), and the reinforcement possible with a balanced design that uses a sealed box with at least 2mm of free space delivered the best performance. I've now adopted this design philosophy for all my locos, and I've not been disappointed. The performance improvement against stock Atlas Gold or stock Scale Trains is like going from an AM radio to CD — the lack of distortion and a cleaner sound is the main benefit.

You can read more about this in my "New Adventures in Loco Hi-Fi" thread…

https://www.therailwire.net/forum/index.php?topic=55532

[Chris333]

I also categorically proved that 9x16mm and 8x12mm speakers sound better inward firing (wire terminals on the outside), rather than outward firing.

I have done this with woofers, but wired backwards so they fire out. The reason was the box was so small that poly fill wouldn't save it. So I put the speaker in backwards just to increase the size of the box. I worked just fine for many years.

I have no idea about other mid vocal type speakers, if the vocals will fire backwards, but bass will.

EDIT: non sealed ported enclosures. I always thought they were so you could tune the port to get a certain frequency. 

[jdcolombo]

I think I've been preaching about the need for a proper sealed enclosure since . . . a long time ago. 

In the high fidelity speaker world, ported enclosures are used to enhance bass output up to a specific cutoff point determined by the port and to increase sensitivity (so less power is needed to achieve a given SPL).  But the bass response of ported systems falls off a cliff below the port tuning frequency and even in the "prototype" hi-fi world, engineering a proper ported enclosure is very difficult.   Today it is easier with CAD and precision milling and manufacturing systems, but back in the early days of hi-fi, the only way to get good sound was to use monstrous drivers in a horn-loaded design with equally monstrous enclosures.  Altec Lansing's Voice of the Theater and Klipschorns were efficient and sounded great, but no normal person had room for them.  Which is why Acoustic Research created such a stir back in 1954 with the first successful sealed box "bookshelf" speaker design ("acoustic suspension") that used the air inside the enclosure as a piston to help drive high-excursion but relatively small speakers to produce decent bass.  Acoustic suspension (sealed box) designs are inherently less efficient than horns or ported designs but give relatively smooth frequency response with a slow roll-off in the bass.  With our tiny spaces, horn-loaded designs are impossible and engineering a working ported design would be a nightmare - it would require precision measurements and construction that most of us simply do not have the ability to create - and I'm not sure it would even work at all given the sizes of the enclosures we work with.  And since the speakers we use typically have a maximum power handling well below what modern sound decoders can output, there is really no need to seek maximum efficiency - most of us are turning down the volume on sealed box speakers anyway.

So, sealed boxes for us, folks.  Always.  As to why some manufacturers STILL seem to ignore physics, I suspect it has almost everything to do with assembly costs and complication.  I guess many customers are OK with some noise that sounds vaguely like a prime mover or horn or whistle or steam chuffs.  Thankfully, more manufacturers are selling "DCC ready" models that don't include a speaker or decoder, so if you want the best sound available, you can buy a "DC" version and then roll your own sound installation.   Which is what you should do if you are serious about getting the best sound you can.  Even then, you're not going to get any real bass since even the biggest speakers we use have almost no frequency response below 300hz (usually more like 500hz); but the rest of the audible spectrum will sound pretty good in a sealed box, and our brains are quite good at filling in the illusion of bass with good lower-midrange sound. 

John C.