Speed of sound


M

modelbob

Administrator
As I mentioned in another post, I picked up a Broadway Limited PRR M1a, a 4-8-2. She's a nice loco, and will look really good hauling some hopper cars full of coal.

Naturally, I had to do a few test runs and the first thing I noticed was how quickly the exhaust turned into a near blur. It's hard to tell how fast your models are going by eye, but the sound tells the story really well.

I picked a nice speed by ear, say 30 mph or so, and it was moving a lot slower than what I would have guessed. I double checked the sounds visually (4 chuffs per revolution for anything that's not a geared loco or other oddity). The chuffs were right on.

It's a real eye opener (ear opener?) to see just how slow "scale speed" really is.
 
Last night at the club we ran only steam. About 3/4th of the locos had sound really adding to the atmosphere - you could almost smell the coal burning.

George
 
There's been some complaints over on the BLI and QSI Yahoo groups on the chuff sounds. Specifically, some models are set so that the chuff rate was slowed way down so you could hear the chuffs. Folks were complaining about this, and the response was that for something this small, it was very easy for the chuff sounds to blur into a mishmash, and they thought that was not a good thing. It's the right thing, but not a good thing.

Kennedy
 
> Specifically, some models are set so that the chuff rate
> was slowed way down so you could hear the chuffs.

This one isn't, it's got 4 chuffs per revolution. That's the easiest and most accurate way to check. Run it at a slow speed, and count the chuffs as the wheels turn. You should see one chuff every 90 degrees, for a total of 4 for each revolution of the wheels. Unless you're running a 3 cylinder compound, or mallet or geared loco, 4 chuffs per rotation is the right, and only right, answer.

What it tells you is that most model railroaders run their trains too fast. That's typically true. Want to prove it? Find a place you can see a train off in the distance, say from a high hill or other vantage point. Watch it go by, and notice how slow it seems to be moving, even if it's travelling at a high rate of speed.

I'm glad this one has the right number of chuffs. If it did not, that would really bother me. Since I know what a real steam loco should sound like, I simply use the sound to set my speed, and as such I'm running at a more realistic speed now.
 
I'm not sure if that was quite what I meant. What the discussion was referring to was that you had the 4-chuffs, but limited to 12mph, say, even though your loco is churning along at 60mph. Mainly because at 60mph, the 4chuffs blur together into one big sound.

But, running too fast is the generic problem. Though if it is a mainline, and the speed is prototypical, you should hear the blurred sound.

Kennedy
 
> What the discussion was referring to was that you
> had the 4-chuffs, but limited to 12mph, say, even
> though your loco is churning along at 60mph.

Apparently there were 2 related issues.

1) Some of the locos, I think maybe the 4-8-4's for example, had only 3 chugs per revolution so they could run faster before it all turned into a blur.

2) Other locos have 4 chuffs per revolution at slow speed, but as they get faster the chugs are reduced to 3 per revolution, again so they sound better at speed.

While I would prefer they be prototypical, I can understand the logic behind the second approach, and I guess I don't object to it too much.

I have noticed the chuffs are not always directly tied to the revolution of the wheels. For example, if my M1a is running at speed and I turn it off abruptly the engine will actual stop before the chuffing sound does. I like that idea, gives it more of a realistic sound, but I have to admit it was a bit disconcerting the first time I saw it happen. Kind of like "Uh, is that thing moving or not...."
 
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