Large Scale Central

Railpro Installation

I’ve been doing a battery conversion/Railpro installation in an USAT GP38-2. 3D printed a mount for the Railpro and been toying around with speaker mounting options. Initially I made an adapter to mate the Visaton FRS 7 to the traditional speaker hole in the tank but then got the idea to make a box to invert it so the sound would come from inside instead of under the loco. I really love the way it sounds this way resonating through the shell. Anyone done this before? Any cons? It just doesn’t “seem right” to mount a speaker face up.

It will cut some of the high frequencies, but you lose a lot of them shooting them down into the ballast.

Have you ever thought to put 2 in the loco? Doesn’t that loco also have a smaller mounting location in the hood?

A smaller “tweeter” in the hood facing up, and the bass coming from underneath sounds great, have 2 speakers in all my USAT F units.

Greg

Greg Elmassian said:

It will cut some of the high frequencies, but you lose a lot of them shooting them down into the ballast.

Have you ever thought to put 2 in the loco? Doesn’t that loco also have a smaller mounting location in the hood?

A smaller “tweeter” in the hood facing up, and the bass coming from underneath sounds great, have 2 speakers in all my USAT F units.

Greg, I did think of installing a second speaker for bell and horn. The Railpro is very customizable but there is only one speaker output. When I read “tweeter” in your post it made me think this may be able to be done with a crossover. Are you using one?

No, I use QSI mostly and it has 2 independent speaker outputs.

But a crossover would be inexpensive and work very well… and you would be surprised how even with just a crossover how much more dimension the second speaker will give. Having higher frequencies appear to have the horn and bell come from one place on the loco, while the prime mover rumble will come from another.

Greg

For low-frequency emission, the most critical thing is to create a long or infinite path from the front of the speaker’s diaphragm to the back. This is the primary function of an enclosure. When the diaphragm moves, high pressure is created in front of it, and low pressure in the rear. At low frequencies, this transition is slow enough that the high pressure will just slosh around the edge of the driver to the low pressure, and not propagate away from the speaker. For speakers the USAT fuel tanks, the low frequencies are remarkably better if you create an airtight seal to the base of the locomotive and between the driver itself and mounting area.

Your design seems to put the front of the diaphragm very close to the base of the locomotive. I imagine that the air pocket created will have a higher impedance than the driver is designed for. What’s your motivation for mounting this way? I don’t know the GP38, so perhaps there’s a large opening into the locomotive body?

Hi Dan, glad to see you have your RailPro installed. The Visaton speaker FRS7 is a good speaker for this install. I have two mounted in the fuel tank of my SD-70. It is freaking impressive.

Based on Eric’s info , I believe it is best to mount the speaker with the cone facing down. You need the air pressure from behind to produce the best bass response and it goes with the physics of speaker design. You can try it both ways for your own info. I used rope caulking to seal around the speaker to the bottom of the fuel tank and around the inside of the tank where it meets the frame. This will prevent air leakage in your “speaker box”.

Using a second Visaton FRS5 2" speaker mounted to the opening under the top of the shell can be impressive. You definitely would need a passive crossover around 200hz to keep the low frequency below and might need to attenuate the volume of the smaller speaker for a good balance. Off the shelf crossovers seem to big and designed for to 300+ watts. That leaves making one from component parts.

Has anyone made a crossover?

Enjoy. Don

Another approach would be to mount the second speaker in the top of the locomotive on short standoffs to induce the pressure leakage I described above. This would theoretically HPF the driver acoustically.

Your design seems to put the front of the diaphragm very close to the base of the locomotive. I imagine that the air pocket created will have a higher impedance than the driver is designed for. What’s your motivation for mounting this way? I don’t know the GP38, so perhaps there’s a large opening into the locomotive body?

I’ve designed the box to be airtight and the foam gasket presses right up against the bottom floor of the loco. There is a round hole about 1" diam in the floor centered above the speaker. I’m considering enlarging it. I’m just experimenting but I find I really prefer the speaker mounted face up. It’s more realistic as the sound is trapped behind the doors. I noticed something interesting last night, the forwardmost fan spins!

Oh, got it. Disregard, then!

Dan Gilchrist said:

It just doesn’t “seem right” to mount a speaker face up.

Aristo mounted the speaker face up, under the top vents on the RS3. Mounted that way it seams to have less volume then my F3, because most of the sound is directed straight up, instead of bouncing off the ground back at me.

I noticed something interesting last night, the forwardmost fan spins!

Induced by the speaker? How high is the volume on that thing?

Don Sweet said:

Has anyone made a crossover?

Enjoy. Don

A simple High Pass filter (not a true crossover) can be made with a single electrolytic capacitor. The cut off frequencies for capacitor selection should be available on the net. The low-pass filter of a typical passive crossover is built using an air coil, or choke which can get pretty large in high power units. Again, the number of turns for specific cut off frequency should be on the net,. I suppose the air coil could be fairly small if it only needed to handle 1 watt or less.

EDIT to add - A true 2-way crossover is both a low and a high pass filter. The point at which the signal crosses over from the low pass side to the high pass side is the crossover frequency. 3, or even 4 way crossovers can be built using multiple filters to drive multiple transducers.

When mounting a speaker face up you need to use a fairly fine grill cloth to keep grit off the cone. Accumulated grit will vibrate and distort the sound. I don’t recall if the RS-3 had cloth over the cone, but I would think it would have to. I liked the sound from the top facing RS-3 speaker, but on my lash-up it was augmented by a downward facing Minimus 3 in a trail car.

For a capacitor in series with the speaker to act as a HPF, the corner frequency f=1/(2πRC)

so if R=8Ω, f=1/(16πR), so C=1/(16πf) or C=1/(50f)

For example, for 500 Hz, C=1/(50*500)=40 uF

Not disputing your math Eric, but isn’t a simple capacitor a High Pass filter?

The capacitor and speaker form a voltage divider. The reactance of the capacitor is frequency dependent, while the resistance of the speaker is (relatively) constant. As the frequency increases, the capacitive reactance (in ohms) decreases, dropping less and less of the total voltage. The speaker drops the remainder, and the more it drops, the louder it is. The reactance and resistance are equal at the corner frequency. So the capacitor is only a filter in concert with a resistive load.

The reactance Xc=1/(2πfC), so in my example above Xc=1/(2π(500)(40E-6))=8 Ohms, same as the speaker.

So the impedance of the speaker is an important part of the equation.

Induced by the speaker? How high is the volume on that thing?

Yep! Of course the fan is not spinning at prototype speed, at least not yet. Maybe if I install some ductwork! I had it at 50% volume. The amp is rated at 7.4 Watts into 8 ohm.

When mounting a speaker face up you need to use a fairly fine grill cloth to keep grit off the cone. Accumulated grit will vibrate and distort the sound.

This is something that concerns me. The speaker cone is not exposed to the outside, however it is directly below the smoke generators.

I’d like to keep the main speaker mounted face up as the engine sound is awesome but the horn and bell are too muffled. Going to pursue the 2 speaker route with some sort of filter/crossover/capacitor. I was looking at these Bass blockers today https://www.amazon.com/gp/product/B00A2680T0/ref=ox_sc_sfl_title_1?ie=UTF8&psc=1&smid=ATVPDKIKX0DER

That’s just a ~6.8 uF capacitor with shrink tubing over it a couple of decimal places added to the price.

Eric Reuter said:

That’s just a ~6.8 uF capacitor with shrink tubing over it a couple of decimal places added to the price.

That’s what I figured but a naked ~6.8 uF capacitor is like $5 .

Here’s one for 24 cents: https://www.digikey.com/product-detail/en/nichicon/UCS2G6R8MPD1TD/493-13293-3-ND/4332924

The shipping will get you, though! Several other options on eBay.

But in any case, you probably want your corner frequency to be a lot lower than 2.8 kHz.