The Source, Of Course

by Lance Meister

(The first in an occasional series on the basics of noise and vibration as they relate to trains and transit.)

One of the basic things we do in our business is help to reduce noise or vibration levels from transportation sources.  At times, this can seem like a very daunting challenge, but it’s very helpful to look at all noise or vibration problems in a very simple framework.

The Source-path-receiver Concept

The Source-path-receiver Concept

When Carl Hanson and I teach the FTA Transit Noise and Vibration course around the country, we emphasize this concept to our classes, and try to show how any noise or vibration scenario contains all three pieces.

The first piece of the framework is the source, and that’s what I’m going to discuss in this post.  I’ll cover the path and receiver later, and also cover what to do with each piece of the framework to reduce the noise or vibration.

The source is what is generating the noise vibration.  And yes, this is very basic and obvious, you may be saying to yourself (quietly, I hope), but it is important.  To properly diagnose the problem, you need to understand the source of the noise or vibration.  (And really, noise and vibration are essentially the same phenomenon, just in different materials: noise  – air, vibration – ground.)

Think for a minute about taking your car in to a mechanic because you hear a noise.  What questions does the mechanic ask you?

  • Where is the noise coming from?
  • What does it sound like?
  • When does it occur?
  • How loud is it?

There are the same types of questions we ask in a noise or vibration assessment.  They may be a little bit more detailed, but we basically ask the same questions.  And just like with your car, the answers to those questions are very important, and help determine what needs to be done.

Here is a source I am very familiar with: a train.  There are many potential sources of noise and vibration on a train, and if you want to reduce the noise or vibration from a train, you need to understand where it is coming from.  Just like with your car, to say “the noise is coming from the train” may not be enough.  Depending on the type of train, there are many potential sources of noise and/or vibration, including:

Noise

  • Wheel/rail interaction
  • Brakes
  • Wheel squeal on curves
  • Horns and bells at grade crossings
  • Diesel exhaust and fans

Vibration

  • Vehicle suspension
  • Wheel type and condition
  • Track surface
  • Track support system
  • Transit structure
  • Depth of vibration source

And in addition to all of these, speed can have a dramatic effect on the source of noise or vibration.  In general, noise and vibration increase with increasing speed.  Certain sources increase more or less quickly with speed (and some not at all!), so knowing the speed is important for the source of noise and vibration.

Here’s something really interesting: with High Speed Rail (HSR), the speed actually changes the type of dominant source of noise!  At lower speeds, the power sources on the train dominate the noise (engines, cooling units, etc.).  At higher speeds, the wheel/rail noise dominates and at the highest speeds, (don’t worry, we’re not there yet in the US!) aerodynamics noise (airflow over the train itself) is the biggest source.

In addition to where the noise or vibration is coming from, you need to understand which sources of noise or vibration are contributing most to the specific scenario you are dealing with.  Knowing this helps to understand how you can address the problem for the maximum benefit.  Once you know what the source of the noise or vibration is, you need to know where it is going.

Next time: The Path “Where do we go from here?”

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