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Topic: Sound Propogation In The Open Air (Read 8818 times) previous topic - next topic
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Sound Propogation In The Open Air

I live in a very rural setting but have a major road running about a mile away in the next valley - IE there's a hill between my house and the road. Usually you can't hear the road at all but sometimes it's quite intrusive. Recently I've been wondering why the noise level varies so much and have made some informal observations hoping to understand why but they have not helped me at all. Basically the level of noise seems to be unrelated to atmospheric pressure, humidity, wind speed or direction, presence (or absence) of cloud cover. Any ideas?

Re: Sound Propogation In The Open Air

Reply #1 –
Sound can be diffracted, i.e. its path bent, if it passes from one air density to another. If there is a low-lying layer of dense air then it will tend to follow that layer, even as it passes over hills and through valleys.

This is similar to how light passes through a fiber, contained within the central core, even though the fiber is bent.

Re: Sound Propogation In The Open Air

Reply #2 –
Quote from: botface on
I live in a very rural setting but have a major road running about a mile away in the next valley - IE there's a hill between my house and the road. Usually you can't hear the road at all but sometimes it's quite intrusive. Recently I've been wondering why the noise level varies so much and have made some informal observations hoping to understand why but they have not helped me at all. Basically the level of noise seems to be unrelated to atmospheric pressure, humidity, wind speed or direction, presence (or absence) of cloud cover. Any ideas?

Concur with the other comment about sound being bent (refracted) or reflected by atmospheric layers, adding that the layers may differ only in terms of temperature, which you can't see.

When you can see these layers due to other differences such as moisture content, they are at times very distinct and doing seemingly crazy things like be going in different directions at fairly high speeds.

 You get clouds racing past each other going different directions, which makes sense particularly when you can see that they are at different altitudes. But, sometimes you can't see that.

Weather like this can accompany rain and particularly thunderheads. But it can also happen on seemingly clear, quiet days. My recollection is that I often hear it on overcast days that otherwise seem claim.

 The boundaries between the layers may be highly turbulent, leading to a potentially destructive effect called Wind Shear.

Related factoid - the air even just a few hundred feet up is often about twice the speed as the air next to the ground.  This may be  due to obstructions on the ground (trees, buildings, hills) blocking the wind.

Re: Sound Propogation In The Open Air

Reply #3 –
Thanks. Made sense on first reading but won't the densest air always be at ground level (ish)? So why wouldn't I always hear the road?

Re: Sound Propogation In The Open Air

Reply #4 –
These are other forces at work. On a windy day the layers mix and become more uniform. On a sunny day the air near the surface is heated so it rises and mixes with the air above.

Re: Sound Propogation In The Open Air

Reply #5 –
Quote from: botface on
Thanks. Made sense on first reading but won't the densest air always be at ground level (ish)? So why wouldn't I always hear the road?

If there was no sunlight heating the ground, yes. But due to the sun the atmosphere is constantly churning.

Re: Sound Propogation In The Open Air

Reply #6 –
Quote from: botface on
Thanks. Made sense on first reading but won't the densest air always be at ground level (ish)? So why wouldn't I always hear the road?

You would think so on first blush, but there is a weather phenomenon known as an "inversion" where dense cold air overlies a lower layer of less dense warm air.  This happens, usually, when a cold air mass approaches and mixes with a warm air mass.  When they mix the warm air is cooled at lower elevations but less than it is at higher elevations, so the air at height is denser and refracts sound waves back toward the ground.   It does other things, for instance inversions sometime cause "tropospheric ducting" at very high radio frequencies allowing long distance communications on frequencies that normally do not support this.  Also it's a recipe for turbulent air mixing and thunderstorms.

Also I invite you to do some internet research on "weather".  The Wikipedia article on the subject might be a good start.


Ed Seedhouse
VA7SDH

Re: Sound Propogation In The Open Air

Reply #7 –
Quote from: pdq on
Sound can be diffracted, i.e. its path bent, if it passes from one air density to another. If there is a low-lying layer of dense air then it will tend to follow that layer, even as it passes over hills and through valleys.
What you're describing is *refraction*. Diffraction is when the waves produce patterns near obstacles. To be honest I have no idea whether refraction is strong enough to produce noticeable effects, but would be interested in the answer.

Quote
This is similar to how light passes through a fiber, contained within the central core, even though the fiber is bent.
What would be equivalent to fiber would be a layer of *lower* density air (e.g. warmer). Fiber conducts light because it has a higher refraction index, which means that light travels more *slowly* than in a vacuum. In air, the higher the density, the faster the speed, so you need lighter air if you want a higher refraction index. That being said, I don't think you can get a relative refraction index (compared to the rest of the air around) much beyond 1.05 or so, whereas glass is around 1.5.

Re: Sound Propogation In The Open Air

Reply #8 –
Optical fibers typically include a transparent core surrounded by a transparent cladding material with a lower index of refraction. Light is kept in the core by the phenomenon of total internal reflection which causes the fiber to act as a waveguide. Wikipedia

And thanks, Armie had already corrected diffraction to refraction.

Re: Sound Propogation In The Open Air

Reply #9 –
Quote from: jmvalin on
Quote from: pdq on
Sound can be diffracted, i.e. its path bent, if it passes from one air density to another. If there is a low-lying layer of dense air then it will tend to follow that layer, even as it passes over hills and through valleys.
What you're describing is *refraction*. Diffraction is when the waves produce patterns near obstacles. To be honest I have no idea whether refraction is strong enough to produce noticeable effects, but would be interested in the answer.

Other readings in this area suggest that reflection (which is also stimulated by the interface between two objects with different refractive index/density may also be in play.


Quote
Quote
This is similar to how light passes through a fiber, contained within the central core, even though the fiber is bent.
What would be equivalent to fiber would be a layer of *lower* density air (e.g. warmer). Fiber conducts light because it has a higher refraction index, which means that light travels more *slowly* than in a vacuum. In air, the higher the density, the faster the speed, so you need lighter air if you want a higher refraction index. That being said, I don't think you can get a relative refraction index (compared to the rest of the air around) much beyond 1.05 or so, whereas glass is around 1.5.

The angles of reflection or refraction may not be much, but we are talking the atmosphere of the earth and the relevant dimensions and distances are huge. 

I have personally heard such dramatic differences in the loudness of distant noises, usually train whistles and traffic noises, that barring any convincing debunking, I'm going to keep believing. 

For example the nearest freeway (I-94) is busy 24/7, almost exactly a mile away and it is usually inaudible, even on the quietest night. However, when the weather is right, there is no doubt that it is there.  There is also a set of train tracks that is 4 or 5 miles away depending on direction, and it too is generally inaudible, but when the weather is right, that train whistle can be clearly heard.

Re: Sound Propogation In The Open Air

Reply #10 –
If sound meets an obstacle, such as a ridge, might it possibly be diffracted?

Re: Sound Propogation In The Open Air

Reply #11 –
Quote from: pdq on
If sound meets an obstacle, such as a ridge, might it possibly be diffracted?
It absolutely does, but no need to look that far. The sounds you hear get diffracted around you head and it's part of the way we localize sounds.

Re: Sound Propogation In The Open Air

Reply #12 –
Wet roads are a lot louder as well.

Re: Sound Propogation In The Open Air

Reply #13 –
Quote from: dhromed on
Wet roads are a lot louder as well.


The sounds that I hear the most often distinctly are truck engines. But on other occasions there is just the steady roar.

The point is that wet roads add a sort of spashing noise that has lots of high frequencies that don't travel well.

In areas where the freeway is close enough to be heard all of the time, that LF roar is generally what is heard.

Road noise (aside from exceptionally noisy engines) usually has its source in the flow of air, either around the vehicle and/or through the grooves in the tires. It is not unusual for worn tires to be noisier because the little circumferential air passages in the tread get skinnier.
 

Re: Sound Propogation In The Open Air

Reply #15 –
My friends live in a village and during late Autumn and Winter the loss of leaves from all the trees between their house and the railway makes passing trains sound louder.

Re: Sound Propogation In The Open Air

Reply #16 –
Quote from: Paul_ 2012 on
My friends live in a village and during late Autumn and Winter the loss of leaves from all the trees between their house and the railway makes passing trains sound louder.
Not an atmospheric phenomenon, though.  The sound causes the leaves to vibrate, that takes energy from the sound wave, some of which will be converted into heat, leaving less energy in the transmitted sound.  Enough leaves will  absorb all the sound eventually.
Ed Seedhouse
VA7SDH