Keep the Noise Down In There! Designing a Layout to Limit High Speed Rolling Noise.

Transcription

1 Keep the Noise Down In There! Designing a Layout to Limit High Speed Rolling Noise. By David Baillie Introduction Have you ever been put off by the sound of a fast model train running on a layout that sounds as though it is amplifying the sound of the baseboard? Maybe not so much on a branchline layout, but if you are running express trains on a mainline layout, then the noise generated by the baseboard and track bed can become excessive, even more so if it is drowning out the sound of your new sound equipped loco. And with models of high speed DMUs, such as the XPT and Velocity about to hit the market, this noise will even become more of a priority to control. And what if you have constructed a track bed of cork, but were disappointed with the outcome? If so, read on to find out how your baseboard and track bed can be designed to minimise high speed rolling noise. Somerville Module My HO scale module, based on the VR station at Somerville, was my first attempt at building a layout. It was constructed on a pine ladder frame, topped with 7mm ply deck and a 30mm thick Insul-foam board. The track was glued directly on top of the insul-foam and ballasted. Although I don t run high speed trains down the line, there is still a surprising amount of noise generated by an express freight train rolling through Somerville on its way to the Westernport steel mill. While thinking about building a new module, I wanted to do-away with laying the track on the Insul-foam and plan to lay my track directly on to the 7mm ply deck. However, this got me thinking that the noise generated by a rolling train might be excessive. Time to run a few trials to check things out. First step was to measure the noise level generated from a train rolling through Somerville. Being a noise & vibration engineer by trade, I had a sound level meter set up ready in a flash. After a few runs of releasing a VR ELX bogie wagon down the grade, the maximum sound pressure level (SPL) consistently measured 69 dba. Hence my objective in building a new module was to be no louder than 69 dba for a fast rolling wagon. p 1

2 Above: My Somerville module tilted on a 1:15 grade. The sound level meter is resting on the ladder step ready to measure the noise of a bogie wagon rolling down the yard. Measuring Rolling Noise To measure the rolling noise, I needed a method to reliably run a wagon along the track at a repeatable speed. I could have run a locomotive at a set speed, but arguably on quiet sections of track, I might be measuring the motor whine as well as the rolling noise. So a foolproof method was to roll a wagon down a grade. Another noise I wanted to avoid measuring was the clickety-clack sound of the wheels across track joints and crossings. Although this is another noise we probably want to reduce, I think that best results to limit this noise is to take care during track laying by carefully aligning the track and not using excessive expansion joints between sections. For the rolling test, I settled on using a standard Lima VR ELX open wagon with steel wheels and a mass of 75 grams. When released from a standing start on a 1:15 grade, I timed the Lima wagon to take 1.8 seconds to cover the distance of 0.75m on a single piece of Peco code 100 flex track. After a number of goes down the ramp, I was happy that the wagon was constantly averaging a scale speed of 130km/h. This was more than enough speed to simulate noise from a rolling express passenger train. To measure the rolling noise, I used a sound level meter (SLM) set to record the maximum sound pressure level. The SLM was placed 1m from the track to represent the distance where an operator or viewer of the layout might be standing. p 2

3 Sound Level & Loudness Sound is normally measured and quoted as a Sound Pressure Level (SPL). Because the human ear hears certain frequencies better than others, the sound level meter can automatically apply what is known as an A-weighting to the measurement. Thus the result shown on the sound level meter is the sound level in the units of dba (known as A-weighted decibels). So how do we relate measured sound level (dba) to our perception of loudness? Mathematically speaking, double the sound pressure is an increase in 6 dba. However, our ears do not perceive a doubling of sound pressure as a doubling in loudness. So to compare the loudness of sounds, it is useful to compare the differences in sound pressure level based on the perceived loudness changes below: Difference of 3 dba = barely louder Difference of 6 dba = quite noticeably louder Difference of 10 dba = about twice as loud Difference of 20 dba = about four times as loud A New Module Now it s time to get around to building something. My new module is 2400mm long and 400mm wide. I find this module is of reasonable length to model a small country station and yard. At the same time, it can be stored upright leaning against the garage wall. The module is of simple ladder frame construction, using 90x19mm pine for the frame. The battens are at 600 mm spacing. The baseboard nailed to the top of the ladder frame is 7mm ply sheet. This construction makes a light weight module so that can be easily lifted and stored in the garage at home. For noise testing, a piece of test track (a piece of 3 foot Peco SL100 flex track) was positioned in the middle of the baseboard. This is where the baseboard is unsupported by the frame, thus providing an area of plywood that is free to resonate and radiate noise. Above: New module frame work (upside down) p 3

4 Noise Tests What would be the ideal set up to minimise noise from the rolling train? What about a track floating in the air, with no connection to the baseboard? I could start off with measuring this scenario and use it as a lower limit or baseline for other measurements. From the chart, you can see that the rolling noise level I measured for the floating track came in at 48 dba. So what happens when the track is laid on the baseboard, I hear you say? Once the track was laid loose on the baseboard (not glued or not nailed), the rolling test was performed, and the noise level measured was 56 dba, or 8 dba above the floating baseline. So already quite a noticeable increase in noise, but it seemed bearable. But hang on, my current Somerville module was 69 dba, or another 13 dba louder. Why is this so much louder than my newly constructed baseboard? Well, the answer was in the next test. In this case, the track was now glued to the new baseboard, and the rolling noise test repeated. The noise increased by 8 dba just by gluing it down to the baseboard. That s almost doubled the noise level, and comes close to the noise from my Somerville module. An unhindered path of vibration from the track to the baseboard is created by gluing or nailing the track down to the baseboard. This is the primary cause of high noise levels: see the diagram for an explanation. So if we need to fix the track our layout, we can place an isolating layer between the track and baseboard. Cork is commonly used and promoted as a noise isolator for model railways. The trick here is to ensure that the track is glued to the cork, and the cork is glued to the baseboard. Avoid nailing the track thru the cork and into the baseboard, as this will short circuit the isolation (see diagram for explanation). The result of the track glued to a 3mm cork underlay came in at 59 dba, or a noticeable reduction in noise compared to the track directly glued to the baseboard. An even better noise isolator is soft foam underlay. I tried gluing the track to a 3mm soft foam underlay that in turn was glued to the baseboard. The noise level came in at a whisper quiet 49 dba, that s about as good as having the track floating on air! Another underlay I tried was 2mm flooring vinyl. This is softer than cork, but holds its form, unlike the foam underlay. The result was a noise level as expected, somewhere in between cork and underlay. So you can see the general rule is that the softer the underlay, the quieter the ride. In fact, this principle is encapsulated in a rule of thumb I use in my job daily called the 10 to 1 rule. That is, adequate isolation can be achieved if the isolator is about ten times softer than the baseboard. Running on (upside down) flooring vinyl p 4

5 But often this result isn t achieved in practice, because if ballast is glued down as well, it can provide a short circuit between the tracks to the baseboard (see diagram for explanation). So ballast needs to be managed, otherwise money spent on noise insulating underlay is wasted. One method that looks promising is to use the underlay across the entire baseboard. This prevents short circuits with ballasting. It also helps by providing a deadening layer across the entire surface of the baseboard, also helping to cut down on noise radiated from the module. I think that a sheet of 2mm flooring vinyl might just be what would work well. In some respects, some scenery treatments might also help in this respect. Sound deadening of the baseboard could also be effected by bonding a sound deadening layer such as vinyl on the underside of the baseboard. Another solution I have heard that assists is a latex glue for ballasting. I have not tried it, so cannot report on it here. And finally, going back to the 10 to 1 rule, we could stiffen up the baseboard. This could be achieved by going for a thicker sheet of plywood, or adding more frame cross members. To test this, I glued the test track along the edge of the module so that it ran directly over the frame. The result was 5dBA quieter than running the track over the middle of the module. So it proves the point that a strong (and heavy) frame results in less rolling noise, but it s probably not the most efficient way to achieve a quiet railway, so I don t think concrete slab construction of indoor model railways will take off! A summary of how to use underlay: Without underlay, the vibration from the track is transmitted directly into the baseboard, which is then free to radiate noise. Using a soft underlay between the track and baseboard will prevent the track vibration reaching the baseboard. The soft underlay can be short circuited if nails are used to pin the track to the baseboard. The soft underlay can also be short circuited if ballast is glued between the track and the baseboard. One method to get around the ballast issue is to use a wide sheet of underlay. p 5

6 Full Set of Noise Results Lima ELX wagon rolling down a 1:15 grade Max SPL at 1m (dba) Delta SPL to floating track (dba) Floating track 48 0 (Baseline) Track laying on 7mm ply deck Track glued to 7mm ply deck Track glued to 3mm Card/Cork Track glued to 3mm soft open cell foam Track glued to 2mm Pegulan vinyl flooring Pegulan glued over entire board Track glued to table along crossmember /edge Somerville (track glued to 30mm styrene insul-foam and ballasted with white glue, on 7mm ply frame) What next? So, with these results to mind, what s installed for my new module? Well, I m sticking to the light weight ladder frame construction, because after all, at the end of the day I still have to store my module away against a wall in the garage. If I choose to build a mainline layout, I think I ll be investigating the use of wide strips of flooring vinyl as a way to cut down on rolling noise. Then again, if I decide to build a country branch line terminus, I ll probably not be running fast trains, and the choice will be mine as to whether I really need noise control. p 6