MRC 4-6-2 and 2-8-2 Sound Decoder Selection and RS-11 Install

[jdcolombo]

A question for the speaker enclosure in the MRC 4-6-2 and 2-8-2 steamers.  Both tenders are the same so the method will be common.  The speakers will be 15 x 11 and the internal enclosure volume should come in right at 1000 cubic mm with any error in that estimate to the plus side.  I could just as readily make the enclosures from .020" brass or .040" styrene or ABS.  So which would you do, brass or plastic?

Use .040 plastic or abs rather than .020 brass.  Brass that thin will cause all sorts of weird vibrations.  In my steam locos, I typically use .040 styrene for the sides, and 1.5mm-thick lead sheet for the bottom to add weight.  My enclosures end up being 9.5mm high (8mm sides, plus 1.5mm lead sheet for the bottom = 9.5mm).  That works out to an internal volume of 1320 cubic mm (8x11x15); by the time you subtract the speaker superstructure, you're pretty close to 1000.

John C.

[peteski]

Lead would be an excellent material for these enclosures.  It has a "dead" feel to it and it would also add weight to the tender.  It can be put assembled using CA glue, Epoxy, or even possibly soldered/welded if someone is has a large enough soldering iron and a very light touch.

But of course it is a conductive metal, so that has to be considered.  ABS is a type of polystyrene, so both should have similar properties.

[jdcolombo]

Lead would be an excellent material for these enclosures.  It has a "dead" feel to it and it would also add weight to the tender.  It can be put assembled using CA glue, Epoxy, or even possibly soldered/welded if someone is has a large enough soldering iron and a very light touch.

But of course it is a conductive metal, so that has to be considered.  ABS is a type of polystyrene, so both should have similar properties.

Back before discovering the cell-phone speakers, I used lead for the entire enclosure.  But the cell-phone type speakers often have "spring" terminals that can touch the long sides of an enclosure, and that means lead would create a short if used as all four sides of the box (or two sides connected by a lead base).  So I started using .040 styrene where possible, with a lead base (for steam; for diesels, I can't use a lead base because that would short the frame).

While not as inert, .040 styrene does the job pretty well, particularly for smaller lengths (e.g. 15mm long).

John C.

[narrowminded]

Thanks for those replies.  I asked because I could use brass on this one but it occurred to me that it may tend to "ring" with the vibration.  Apparently that is the case.

I wasn't coming up with the depth of 9 mm you talked about but was wider and longer on the walls.  The depth is restricted by the drawbar, pin attachment, and wire way through the middle which I was going to leave intact, not modifying the tender beyond removing the original weights.  I may revisit that but know my current plan has 967 cubic mm inside the speaker enclosure.  That was arrived at by subtracting .040" on all walls and .050" for the speaker itself (1/2 of the speaker frame thickness).

For weight and mounting all of the components I planned a machined brass perimeter piece that's similar by style to the original weight but modified to allow fit of the decoder, capacitors (up to 10), with sufficient room for all of the excess wire.  That brass piece would serve as a weight but also mounting frame for all of the added components and install in the tender as a sub-assembly.

Considering adding a reverse light using one of Keystone Customs reverse light shells.  Need to figure placement.

[jdcolombo]

967 cmm will be just fine.

John C.

[narrowminded]

967 cmm will be just fine.

John C.

I just reviewed the drawing and I can add the little bit required to get 1000 cubic mm.  I was being overly cautious after the vibration in the RS-11.  All else seems good, I think?

[hminky]

Always found balsa wood isolated speakers from vibrating plastic:





Ever think about making the speaker enclosures from balsa wood, it is basically inert.

Harold

[narrowminded]

Ever think about making the speaker enclosures from balsa wood, it is basically inert.

Harold

It's funny that you mentioned that because in thinking about this recently, wood came to mind.  Not Balsa specifically but maybe MDF.  I don't know if it would hold up in that thin of a cross section but it would be easy enough to try.

[tehachapifan]

Thanks, @jdcolombo .  It wouldn't be this far without the good guidance I've received on this board.   I think you'll like the cleanness of the installation that the 73100 Direct decoder affords.  No grinding of the shell required.   But nothing is perfect.  The decoder overhang at each end consumes some of the space that would be nice to be able to use for the speaker but with some finagling, what's in these posts, I think I got that covered.  Time will tell.

Meanwhile, finished the pockets in the enclosure backs tonight.  All of the machining went well.  Tedious but well.   The enclosure material was secured to the fixture block with a glue applied at .002" thick utilizing a rubber glue type product (not sure what it is, just acts that way) that goes on like a double sided tape but is actually just the glue once the backing is removed.  The thickness of the glue apply is very accurate and can be predictably allowed for in the setup.  Still have to release the parts from the fixture block, clean them up, and then setup and finish one cut on each part from the opposite side.  Not sure I can get back to it for a few days but may.  Good news is, they look like they are going to work as planned. 

And here's a picture of those parts fresh from surgery.  This picture's only worth about 50 words.

(Attachment Link)

I am absolutely fascinated and at the same time completely bewildered with whatever you did here! Good grief those enclosures look great and man would I love to learn what the heck you did to machine those! That bench setup looks amazing too!

[jdcolombo]

It's funny that you mentioned that because in thinking about this recently, wood came to mind.  Not Balsa specifically but maybe MDF.  I don't know if it would hold up in that thin of a cross section but it would be easy enough to try.

MDF is used a lot by "real" speaker manufacturers for enclosures.  But it is THICK stuff (often 1" or more) and also has internal bracing in the enclosure itself to keep any vibrations at bay.

Balsa wood would be worse, I'd think than MDF, but no wood would be inert at an .040 thickness.  Even thick wood will vibrate at a certain frequency, which is why even MDF enclosures use internal bracing.  It's also why truly high-end speaker companies like Wilson Audio have spent a fortune coming up with special composites that they use for their speaker enclosures (and it's one reason why Wilson speakers start at $15,000/pr and go up from there).

However, certain woods can actually create pleasing "distortion" when they vibrate.  Think a Stradivarius or Guarneri violin or a Martin guitar.  The vibrating wood "box" that the strings sit on amplifies the sound made by the strings in a very pleasing manner.  So maybe the right wood could create the N-scale speaker Strad . . .

But perhaps what we really need is .040-thick carbon fiber sheet!  I'm sure BMW could manufacture some for us, since they've basically cornered the manufacturing market for their cars.

In the meantime, though, I'll use .040 styrene.  It works, it's easy to cut, sand, and glue, and it's cheap and non-conductive.

John C.

[Jbub]

perhaps what we really need is .040-thick carbon fiber sheet! 

John C.

I found this. 12x12 sheets in 1mm and even all the way down to .25mm. It seems a little pricey but it might be worth a try.

http://www.protechcomposites.com/high-gloss-carbon-fiber-sheet-12x12x-1-0mm-305mm-x-305mm/

[jdcolombo]

But perhaps what we really need is .040-thick carbon fiber sheet! 

John C.

I found this. 12x12 sheets in 1mm and even all the way down to .25mm. It seems a little pricey but it might be worth a try.

http://www.protechcomposites.com/high-gloss-carbon-fiber-sheet-12x12x-1-0mm-305mm-x-305mm/

Uh oh.  I can already see the gears turning in Narrowminded’s brain . . .

[Jbub]

Uh oh.  I can already see the gears turning in Narrowminded’s brain . . .

To help the gears go round and round I also found this on the interwebs.
The video is made by the same company that was linked to above.

[narrowminded]

I'm suspicious that carbon fiber would have issues similar to brass.  Fiberglass could be tried or then just epoxy casting.  Maybe a low temp cast metal?

[peteski]

I think there is some serious overthinking going on (for such a minuscule enclosure). Like with every question asked of the online collective, suggestions and ideas often get out of hand.