train noises

[gusl]

Dear LJ Genie,

I'm very puzzled about something: if the nearest train tracks are 2 miles away, HOW is it possible that I hear the trains hooting so well?

Comments

[roseandsigil.livejournal.com]

I, too, have often wondered this.

[psifenix.livejournal.com]

Sound carries across water, which is the major component of Pittsburgh air?

[easwaran.livejournal.com]

In Berkeley, although the train tracks are on the far western edge of town by the marina, you can actually hear them more clearly as you move east. I imagine that it's something about the increasing slope of the hill, so that there's more of a direct line. And I guess train whistles just carry really really well.

[gustavolacerda.livejournal.com]

The "increasing slope" explanation applies to our case, but it's still *two miles* away. But why would any train need to whistle that loud? As you probably know, assuming there is no power loss (to heat, etc), the intensity of sound decays as the inverse of the distance squared (surface area of the sphere of radius r).

So, at 100m, the whistle sounds 1035 times as loud (as measured in Watts / m^2) as it does at home. (2 miles = 32.18 * 100m)

Noise complaints from train whistles

[easwaran.livejournal.com]

I believe the inverse square law applies when it's propagating in a full 3d area. However, there are sometimes phenomena where the sound gets focused in a particular layer of air (or water) and only decays as 1/r instead of 1/r^2. I think something like this is why it's so much harder to hear things outdoors than indoors. (Outdoor concerts are basically always amplified, but indoors you can have a solo singer be heard through the entire concert hall.)

[gustavolacerda.livejournal.com]

It doesn't need to be a full 3D area, as long as it's a constant "piece of the pie" (i.e. a constant proportion of the sphere). Concerts halls don't fall into this category unless they are shaped like an open cone.

In any case, the train sound is propagating through open 3D space.

[(Anonymous)]

You're forgetting the Dopler Effect.

[gustavolacerda.livejournal.com]

who is this? And why is the Doppler effect relevant here?

[gustavolacerda.livejournal.com]

OTOH, loudness perception is sort of exponential for some values. The Decibel scale is logarithmic, afterall.

[mpnolan.livejournal.com]

Because there are actually secret train routes running under the apartment.

Someone recently recommended I get "shooting earphones" -- those heavy-duty noise cancellors worn when shooting guns.