Intermountain SD40-2 Speaker Transplant

[jdcolombo]

Hi everyone.

While I'm generally impressed with the sound implementation on the new IM SD40-2, after listening to the engine for a few minutes, I found that the sound seemed a bit "thin" to me - too much high frequency and not enough balance with lower midrange (we can't get true bass in the small speakers we use in N scale, so forget that; but a better lower mid, say down to 300hz, will help balance out the treble).  When I disassembled my engine to see what made it tick, I found that the IM speaker implementation, while better than some recent OEM efforts, still left a bit to be desired.  While IM did a good job putting in an 11x15mm cell-phone type speaker in a sealed enclosure (face down), the enclosure itself had no room left in it after the speaker installation.  For reasons I won't detail here, that's bad.  Speaker enclosures need some open internal volume of air space for speakers to sound their best; if you simply wrap a speaker with an enclosure that has no air volume after accounting for the speaker itself, it won't sound much better than using no enclosure at all (it will sound a little better, because you've eliminated the back wave cancellation, but it will still sound "thin").

So I decided to do a speaker transplant.  Before I get to the details, here's a YouTube video comparing the stock speaker to my custom effort.  At first listen, you may find the custom speaker sounds "muffled" - that is, far less bright with less treble energy than the stock speaker.  But that's not actually true.  If you listen to the bell comparison, you will see that my custom effort has plenty of real treble; the difference is that it also has plenty of lower midrange to balance out that treble, so the sound overall is much less bright, and more "full" (at least to my ears).  Also pay attention to the spitter valve on shutdown - my custom job has plenty of "spit" - which indicates that the treble energy is there when needed.  It just doesn't overwhelm everything else.  I think the stock speaker's lack of lower-midrange energy is what contributes to the fact that the turbo sound drives everyone insane - that's all you hear with the stock speaker.  Nevertheless, some folks LIKE a lot of treble energy, and if you like it that way, then don't even think of doing what I'm about to describe.  Here's the video:

Now here are some photos.  What I did was take an 8x12mm speaker, and build an enclosure around it that was larger than the speaker.  The "hole" in the fuel tank for the speaker enclosure is 13mm x 17mm (roughly), so I took some .020 styrene, and built a long enclosure with the speaker offset to one side.  This provided plenty of air space behind the speaker (enclosures don't have to be deep rectangles around the speaker - they can be any shape, as long as they have sufficient internal volume).  The speaker wires were routed through the "bottom" of the enclosure (the speaker fires down at the fuel tank, which IM was helpful enough to drill holes in!).  This enclosure is about 5.5mm high overall with the "bottom" (also of .020 styrene) attached and as you will see, fits almost perfectly in the stock cutout.





To do this, you also have to modify the plastic frame that surrounds the motor and cut back the two silver speaker leads that wrap around the motor to the top of the frame (where they contact the "speaker" pads on the LokSound decoder).  Here are photos of what the motor frame looks like after modification (I glued the cut speaker leads down to the side of the motor with a bit of CA).





Finally, I reassembled everything.  It all fit, and worked. 





The actual transplant took about an hour, once I'd figured out how I was going to do it.  I like this sound better - but YMMV!

John C.     

[wcfn100]

It's interesting that you can tell the new speaker is better, but not all the sounds are better.  I think the stock speaker bell is much closer to real life.

Jason

[AKNscale]

Did you find the spot to wire in the negative for the caps?

[jdcolombo]

Did you find the spot to wire in the negative for the caps?

Not yet.  Waiting for Peteski to weigh in.

John C.

[Santa Fe Guy]

I really like the new sound even if the bell is a tad off. I would much rather hear the motor do its thing.
Great video and description John. Thanks.
Rod.

[soo]

Nice work,, you can really tell the difference.

I will have to save this for when I do the units that I am going to get in the future.

Thanks for your hard work

Y-it

[powersteamguy1790]

Sounds good John.



Bob........................

[carlso]

John,

Thanks again for a valuable contribution to sound. I too think the second or new sound is the best.

I do have one question for you. Does the plastic IM fuel tank have any speaker material covering the holes they put into the bottom? Would it be good to cover those holes with speaker cloth or cheese cloth or something to keep the speaker from picking up crud from the right of way. As you know there can be almost anything laying between the rails.

Carl

[jdcolombo]

Hi Carl.

Nope, nothing covering the holes.  But nothing can get to the stock speaker, because there is nothing but a solid block of black plastic behind the holes. Take off the plastic fuel tank and you'll see what I mean.

John C.

[narrowminded]

The sound is distinctly better, more realistic.   Nice job.

[wmcbride]

Sounds so much better!

[drgw0579]

I am curious if replacing the shell makes any difference for either original or your modifications.

Bill Kepner

[jdcolombo]

I am curious if replacing the shell makes any difference for either original or your modifications.

Bill Kepner

Hi Bill.

In the factory configuration, the speaker fires upward (from its backside - the front faces down into the enclosure) into the chassis.  In theory, the shell could affect the sound some, but I didn't notice any major effects (maybe a little bit of reverb).  In my replacement, the speaker fires down at the track, so the shell has no effect in this case (the back of the enclosure is sealed.

Another way to do this, BTW, would be to use a 9x16mm speaker and orient it front-to-back in the same kind of enclosure.  This would leave 3mm of open space inside the enclosure on the long side, which would provide some of the air space needed for good speaker performance.  I might try this later this week just to compare results to the 8x12 (I've got a ton of Knowles "Fox" speakers still sitting around).   Another possibility is to mill a larger opening in the fuel tank, but I wanted to avoid doing something like that so that my modification could be copied by folks who didn't have milling equipment (or the patience to use a Dremel to accomplish the same thing).

John