Best sound-proofing techniques and recommended sound-absorbing materials? [closed]

Question

This is fundamentaly a question about soundproofing and sound absorbtion. When working this project I've realised I don't understand the concepts as much as I though I did.

I basically set up a shelf train set. The shelving is made out of fiberboard covered with melamine. And for the model base I chose extruded polystyrene foam (XPS), the kind used for insulation. Now, I was under the assumption that materials used for insulation were also good and used for sound-proofing... However, I found out after the fact that perhaps the XPS was the worst choice I could've made, soundwise, as the foam, contrary to what I expected from foam, at this density at least, becomes like the membrane of a drum and multiplies every noise! So now my mission is to quiet it down as much as possible.

At certain points the XPS sits right on top of the shelves. Here, I understand my best option is to glue it completely onto the melamine/fiberboard, thereby dampening its flexibility, which will prevent it from vibrating freely. Though here is where my knowledge of the concepts involved starts getting hazy... Would it be better to glue it entirely? Would it be better to glue it in certain parts and leave certain other parts unglued? Maybe even put some sound-absorbing material like cloth between the XPS and the shelf in those unglued parts? Is it better to glue it sparsely? Will any of this make a difference?

Then come other parts of the model where the foam needs to rise in an incline to meet another level of foam. I've got these set up as follows:

Where the incline is made by adding 3mm-thick pieces of MDF cut at intervals ranging from 0.5-4cm tall. In this case, my thougts are that leaving these empty "chambers" under the XPS will simply act as the body of a guitar and amplify everything even more. But what to do? Should I fill them with sound-absorbing material?

Should I glue in more incline intervals out of cardboard, for instance, to make it all more rigid?

Should I glue a more rigid base under the XPS? Like some MDF or even place some cloth?

What is my best option for achieving a good sound absorbtion here?

I can't get rid if the XPS because I already invested in a lot of it. I've already spentsoun way over what was planned, so I would greatly appreciate any solution that's rather on the budget side.

Thanks in advance for your input!

Answer 1

You're on the right track (as it were): isolate, dampen, and secure loose XPS. I would describe what's going on this way:

1. The vibration of the train's wheels on the track (plus its internal noise - gear, motor, etc.) is conducted through the track and ties to the XPS track bed, making it vibrate.
2. XPS is fairly rigid and vibrates like a sounding board, and apparently has enough exposed surface to couple well to the air and create nice strong sound waves, especially at the higher frequencies (i.e., "highpitched, styrofoamy sound"). The trellis supports also conduct vibrations to the XPS base which acts like another sounding board (something like the bridge on a string instrument).
3. Anything in contact with XPS that can move and rub against it will likely squeak (again, "highpitched, styrofoamy sound").

You can fix this by treating all three causes:

1. Isolation - Break the vibration path where you can, the closer to the source the better. You want fewer, not more points of contact between the track and XPS, attached by something that is compressible and lossy. Slip it between the ties and the bed (see #2 for an example). Ditto between the bed and the trellis, and the trellis and the base XPS.
2. Absorption - soak up the energy by covering the XPS with something dense but soft. HVAC, car audio system, and acoustical suppliers sell adhesive-backed "mass-loaded vinyl" in sheet form. For example. 1/8" thick of this(not an endorsement) reduces sound transmission by 10-45 dB (i.e., from half as loud to 1/20 as loud) from 100-4000 Hz. That would should really cut down on those annoying XPS squeaks.
3. Glue anything that contacts XPS securely so it cannot rub. Rubbing is like drawing the bow across a violin string; it makes it vibrate. If you have a glue that has some elasticity to it, so much the better as it will help isolate and absorb. (If you could sandwich in a layer of the stuff in #2, you would get bonus absorption and isolation).

You don't have to do everything, but the more you can, the better the result. I would start with the low-hanging fruit, whatever that would be for you, but as close to the source (wheels on track) as you can get. I don't imagine it would be much fun to disassemble a lot of the model to add the mass loaded vinyl. There's a trade-off between the work of installing little bits of the stuff at all the contact points and the cost of covering all the XPS with #2. The most effective place to put it would be under all the track because that's the noise source.