Benchmark

Actual testing. I love it!

3/4 plywood cut into sub-roadbed attached 16" centers with Midwest cork roadbed would make a good benchmark since it is a popular method.

pqe

I think that the track has to be physically or mechanically attached to the foam for the sound test to be valid. Just laying a piece of track on the foam will not be realistically representative of actual use. Sound transmission will not be the same.

Randy

Also...

Some sort of roadbed as well. The noise test would be good with track, cork and subroadbed, all glued down.

The Usual Suspects

I applaud your proposed testing as it will provide great data for modelers. It has been a while since I have conducted any serious acoustic testing, but I would start with the usual suspects including cork and foam (low and high density) roadbed, extra bracing under foam in line with track, 2" foam or two layers of 1" foam, etc. And since the sound will not radiate uniformly, I would also like to see some measurements taken from an "operators" position.

Benchmarks

The background noise in this room is about 42 dB SPL. I live in a fairly quiet neighborhood, at least as long as the students next door aren't throwing a party. Everything I expect to test should be at least 10 dB above that, so I don't need to find an anechoic space.

Testing the foam alone was revealing. This is 24" square, and as you would expect, pretty resonant. The fundamental resonant frequency is 80 hz, and a simple finger tap can produce 100 dB SPL at 6" above the center. The spectrum is strongly pitched, with harmonics in an exponential falloff. There is a fair amount of pink noise in the sound, about 10 dB below the harmonics. It's pretty typical of drums, especially square ones.

I tested the sound output of my Bachman 44 tonner by laying some track across a piece of acoustic foam. I ran the loco back and forth at speed step 16. The maximum reading I got with the mic 6" above the track was 58 dB with a flat spectrum slightly peaking at 2kHz. Stalling the loco didn't make it appreciably louder. 58 dB is typical conversation level, but the flat spectrum does not interfere with speech. (Much-- this has a reputation as a fairly loud loco.)

When I laid track directly on the foam the engine produced 70 db spread from 100 Hz to 2 kHz, with the peak at 125 Hz. 70 dB has been identified by the US EPA as a "stressful" level. In other words, loud enough to be annoying. Note that the track was not glued, just tacked in place with T pins. I'll experiment with glues later, as I expect such effects to be subtle.

I next laid the track on 1/4" cork/rubber roadbed. This reduced the SPL to 63, still in the 125 Hz band. This represents less than half of the worst case sound energy, but is still enough to interfere with conversation.

The final test today was to double the thickness of the foam by sticking two layers together with carpet tape. This had no significant effect. I suspect the tape holds with enough flex that the top layer is effectively on its own. I'll eventually try bonding the two layers with Elmer's (or just finding some thick foam), but I want to make more tests on the thin stuff first.

pqe

Something to consider ...

... "mechanical coupling" (i.e., the 'pins' that are being used to hold things in place) may be reducing the dampening effect of the cork.  I think a 'rigid' glue such as Elmer's would have similar results.  A more 'flexible' adhesive between the track and cork, and the cork and foam (or just the track and foam), may provide better sound suppression/isolation.

This is also something to consider when applying the ballast ... if the glue is mechanically coupling the track to the foam, any benefits in noise reduction from the sub-roadbed would be effectively bypassed as the sound will be transmitted directly from the track to the foam via the hardened glue.

A layer of felt between the foam and the roadbed (bonded with a flexible adhesive) may help.  The ballast would be bonded to the felt rather than the foam, retaining the desired sound isolation.

All very interesting ... lots of things to be learned here I think.  Thanks for sharing. 

PQE - thank you for your effort

From my own experience I have

From my own experience I have used both 2" foam with cork road bed (for the height and shape) and the usual plywood and cork road bed. The sound amplified once ballast was laid, and was more nosier with the foam, but from underneath the layout.  The foam acted as a speaker?  Future builds, revisions and expansions will be built with plywood and a homasote type sub bed. 

Steve

caulk and glue

Would be interesting to compare track attached with caulk versus glue.

If I was to rework my layout, I would use 1/2" for the base roadbed, then use 1/4" thick R1 foam for the top roadbed. Not so much for the sound but to give a better ballast profile.

My yard is 1/2" foam on top of 7/16" OSB, other sections have OSB with R1 foam for roadbed. There is a difference in the sound level.

Thanks for your suggestions

...

I've never experienced acoustic problems, but then I generally use at least no less than 2" of foam minimum, and I mostly use white bead board.  Yes, the evil white bead board stuff.

Dear PQ, Make sure you get a

Make sure you get a copy of the Kalmbach "track work special". It had a totally un-calibrated article RE acoustics and roadbed choices. In the article, the "gold standard" was flextrack on a thick soft bath towel... Would love to see that article's theories put up against an AP-1 fed with a B&K mic...

Caulked vs loose track

I caulked down my length of track and let it dry overnight. To keep everything consistent, I added a second piece of track, parallel to the first, but just pinned down as I had started. As Randy predicted, there is a difference of nearly 4 dB, so the worst case is now 74 dB. This is not so much a straight difference in level, but the caulk transmits a broader frequency spectrum, with more energy in the 50 Hz band as well as in the 4 and 8 kHz bands. The peak is now in the 250 Hz band, making it psychoacoustically louder as well as measuring louder.

I'll leave the caulked track alone for the next few experiments, then come back and make a comparison with hard glue.

pqe

A note on the termonology

I will be using some pretty technical terms in these posts, so feel free to ask me for definitions if I get too obscure. (Mail me directly if you like.) One term I use a lot that is often misunderstood is dB.

Decibels are a way of comparing sound levels as measured by a sound pressure meter. Every measurement is in relation to some standard level, which in the case of Sound Pressure Level or SPL is the quietest sound that can be heard, something like a gnat's snore. The measurement is a ratio-- 2x, 4x or 100x the standard power. Trouble is, the ear can hear such a fantastic range of sound power that the numbers would be huge. So we use logarithms instead, essentially counting the 0s in the ratio. The basic unit is the Bel, which is the log of the ratio of the measured power to the standard. This a bit coarse for everyday work, so we usually talk about tenths of a Bel, or decibels. At this point the explanation usually goes off into some formulas for obtaining decibels from pressure or power measurements, but I'm not going to bother since our meters give decibels directly. The important thing to remember is that the logarithmic scale matches our hearing pretty well, so a 6 dB difference between 80 and 86 dB sounds about the same as the difference between 58 and 64 dB.

3 dB is just about the smallest difference most people will notice, 6 dB means the energy has doubled. Most people describe 10 dB as twice as loud.

Here are some dB benchmarks:

• 20 dB - about as quiet as you are likely to experience without really special construction, like a deep mine.
• 40 dB - a pleasantly quiet living room
• 40-60 dB - typical polite conversation
• 70-80 dB - where most folks play their stereo
• 86 dB - loud in a movie theater
• 90-100 dB - loud concert
• 110 dB - professional hockey game

SPL isn't the only thing that affects the sense of loudness. Frequency is important in a complex way, but the essence is we are most sensitive to changes in level in the middle frequencies, 200 - 4,000 Hz. The meters try to take account of this by weighting scales. My measurements are all done with a weighting called C. 

Decibels are also used to measure electrical signal levels. They work the same way, but the reference level is often stronger than the measurement, so the ratio comes out a fraction. Since logarithms of fractions are negative numbers, audio heads often speak of -10dB.

pqe

More information please?

What type of 'caulk' was used?  How was it applied?  Was the track in contact with the foam or was there a layer of caulk between the track and the foam?  If so, what was the thickness of the layer of caulk between the track and the foam?  

Pictures would be very nice for these experiments as so much depends on actual materials used and the application of those materials.

Dear PQ, "acoustic loudness"

Would love to see your experiments include various other

types of roadbed, including Flexxbed ( http://www.hobbyinnovations.com/), Woodland Scenics' Track-Bed ( http://woodlandscenics.woodlandscenics.com/show/category/TrackBed) and (if you can find it) AMI Instant Roadbed (out of business) or an uncured butyl rubber equivalent ( http://cs.trains.com/mrr/f/11/t/107104.aspx).  Various types of underlayment might also be interesting as scratch-built roadbed or a buffer layer between roadbed and subroadbed?  http://www.homedepot.com/s/webapp/catalog/servlet/Search?keyword=underlayment&Ns=None&Ntpr=1&Ntpc=1&selectedCatgry=SEARCH%2bALL

Thanks for your notes

"when measured from a distance of 1 metre noisemaker"

   Yeah, distance has to be considered when talking about sound. Sound from a  shelf layout up high and in your face is gonna be more important than sound from a spaghetti bowl layout with trains far away at times. Jets are very loud up close but flying high they are not as obnoxious as those nasty little low flying prop planes that sound like they are out of tune and ready to quit at any time. Sound from a layout has never bothered me since I know real trains are loud so having loud toy trains just seems right. I can still recall the clatter of steam era freight cars over jointed rail as my dad paced the old SP freights along highway 99 and I hung out the window listening.........DaveBranum

pqe

I am interested in your testing. One thing that I think is important to note. When 'daping' your track, you shouls do it the same way that you would with your model railroad. A thin film spread with a putty knife so there is no oozing. Otherwise your experiment won't really give you a true indication as to real life usage of track on foam. You will have to properly install things with the same material as is done on a layout. If not, although interesting, your tests do not really give a true representation of things.  I would suggest properly attaching/gluing the track and re-run your sound test. I'm betting that the results will be different.  Connecting/attaching the track at a few spots will/should give different results than having it attached all the way. (as it is generally done).

Sound transmission will be totally dependent upon how the track is attached, and not just tacked in at a few places.

Randy

I have examples of both Flexxbed and WS Track-Bed

but have not yet had the opportunity to lay track on either.  They are definitely different, particularly if you get samples of the thicker versions of Flexxbed (which seems denser to me since you mention that).  Laid on top of pink or blue foam I would expect they'd make a difference and both are viable competitors to cork and both lay claim to quietness.  To me it would be incomplete not to include them.

Thanks for pointing that out

Some thoughts

First of all, I am very interested in seeing the results of these tests.  Anecdotal evidence is one thing, but actual data is hard to argue with.  I don't even see the need for calibration, since the point is to establish the relative performance of different combinations.

It would be interesting to learn why foam is supposed to work so well as a sound board, especially relative to plywood.  My guess is that it has to do with the mass of the foam, relative to the trains.  With a massive train and a super-light structure, it makes sense that the model would be better able to induce vibration in the foam.  In that case, the two ways to reduce the noise would be either to increase mass, or decrease the foam's ability to vibrate in the audible range.  The other option might be to somehow absorb the sound after it's generated.  Plaster scenery might add enough mass to reduce the problem, as could wood or metal bonded to the bottom of the foam in strategic (random, so they don't have their own harmonic frequency?) locations.  Various foam or wood ribs beneath the track, possibly perpendicular or at an angle to the track, might reduce the vibration.  And perhaps some foam rubber, crumpled newspaper, or poly-fiber might be suspended beneath the layout to absorb some of the sound.

Those are all just guesses, and I might be completely wrong.  Like I said, it will be interesting to see how this works out.

sound dampening

I believe the article along these lines in the MR "How to Build Realistic Layouts" issue found that cork roadbed on top of 3M camper tape was the quietest combination.