Can ground albedo near a highway refract noise upward?

[Mike S.]

TL;DR Summary

Suppose you have a 200-meter strip of land near a highway to "paint" however you wish. For example, you could put black solar panels on part and white gravel on another part. Can you make it so that noise from the highway is diffracted upward much of the time by the temperature differentials, and never reaches the horizon?

I live near some highways that are roughly "a couple of miles" away. On some blessed days they are inaudible, and on other days they roar with a low frequency sound that penetrates into every corner of the basement. (If frequency matters, it would be most preferable to refract sound at 20Hz, but consider whatever works best) I know nothing more about why except "it's probably the atmospherics", and I know sound refracts. I was wondering though, could it work to intentionally refract sound with differential temperatures? I suggest creating these temperatures by albedo only because I can't think of much else affordable that would be able to produce that much heat over that large an area, unless you happen to live next to a power plant with cooling requirements. (It may also work to release water vapor, though I'm not so clear if there's any practical way to do that either) To be sure, I'm imagining "you" are an engineer for a state, etc. who can potentially arrange this over large distances of highways.

I know I can start with something like 700 W/m^2 insolation, and I could figure heat capacity of air for some environment, but I'm not sure how fast it would rise, and most importantly I don't know how to begin to calculate how much the sound would refract, and whether the refracted sound bent upward nonetheless reaches the horizon. (Say 2 km if the distance turns out to be important). I don't have a practical sense of how to set up the scenario to have the best odds of a successful result.

 

[hutchphd]

Interesting. My guess is that the more important effects are in the sky layers rather than the ground layers. I know the effects of explosions can be magnified by the presence of a cloud layer.

 

[Mike S.]

Interesting. My guess is that the more important effects are in the sky layers rather than the ground layers. I know the effects of explosions can be magnified by the presence of a cloud layer.

The problem I have with this explanation is that the top of the troposphere is more than 10 km above the surface. Sound would have to bounce back nearly 180 degrees, and I think it would sound like it was coming from above, for that to matter. Now it could be refracted in an arc much lower in the troposphere, perhaps by humidity or clouds, but I'm not sure that would be distinct from considering it as direct transmission except as a fine detail.

In the area near the highway, I'm assuming most of the relevant sound energy should be passing within tens of meters of the ground? A physical berm isn't very high.

 

[hutchphd]

Why is the "top" of the troposphere relevant?
Actually nothing in the above seems correct to me. Physics, please.

 

[tech99]

Sound, like radio waves, is bent slightly downward by a refractive index gradient in the lower atmosphere. This allows the waves to travel beyond the normal optical horizon. There is not much hope for altering this, but a wall, or upward reflecting panels near the sound source can reduce the sound intensity.

 

[Mike S.]

Sound, like radio waves, is bent slightly downward by a refractive index gradient in the lower atmosphere. This allows the waves to travel beyond the normal optical horizon. There is not much hope for altering this, but a wall, or upward reflecting panels near the sound source can reduce the sound intensity.

Is it possible to refract the sound waves downward then, so that they strike the ground near the highway as if striking a wall, and (perhaps with the help of some low barriers) are dissipated?

 

[Mike S.]

Why is the "top" of the troposphere relevant?

I must not have understood what you were saying. Can you clarify what you meant by "sky layers"?

 

[tech99]

Sound from vehicles will be greater at high angles because the ground reflection re-inforces it upwards. Atmospheric ray bending is very slight and not enough to do as you suggest. Another approach is to plant woodland along the highway as an acoustic screen.

 

[N1206]

It ain't albedo, but have a look at what they did at Schiphol
https://www.wired.com/2014/06/airport-schiphol/
In general, solid barriers don't do much as the waves simply refract around them, or if the wavelength are big enough, just go over them.
While this article is short, I've read ones that took the idea much farther.
Basically, that article (which I can't find) was talking about datacenter noise and clever, geometric landscaping that set up interference patterns, absorption and reflection to remove most of the coherent noise (specific frequencies) out of the mix and leave only white noise at lower volume when it was done without the use of plants or other high-maintenance items.
Long read here
https://www.theatlantic.com/magazine/archive/2019/11/the-end-of-silence/598366/

Trees and bushed are better than fences.
Plowed ground was better than unplowed ground.
You can actually do landscape design for noise control that utilizes reflection and interference as well as just absorption. It takes planning though as the wavelengths of the offensive sound and its point of origin need to be taken into account.

 

[bob012345]

Ground albedo I think would only work, if it did, during sunny days. I hear highways likewise a couple of miles away usually in the evenings at rush hour when there is rain or the roads are wet but sometimes there are different conditions though. If screening or sound insulation doesn't work as a first line of defense, how about noise cancellation technology applied inside your house or on your property since the highway noise is largely uniform?

 

[hutchphd]

Sound from vehicles will be greater at high angles because the ground reflection re-inforces it upwards. Atmospheric ray bending is very slight and not enough to do as you suggest. Another approach is to plant woodland along the highway as an acoustic screen.

But the effect, over longer distances, of atmospheric bending can be quite profound. I lived 8 miles from the highway ouside Ithaca (in Freeville) NY and some evenings one could hear the semi trucks on rt13 quite distinctly and sometimes not at all. I think this was usually because of a temperature inversion, but I never really pondered it.

 

[jrmichler]

The speed of sound in air increases with increased temperature. This suggests that an air temperature gradient will refract sound. So I searched air temperature gradient refract sound, and found this diagram:

This suggests some interesting experiments with a temperature sensor or data logger on, say, a drone.

A wind velocity gradient will do the same thing:

If there are buildings or trees between you and the highway, and wind blowing towards you, then there will be a wind velocity gradient at treetop level. This gradient is well known to pilots when landing in a crosswind with a tree line upwind. And it suggests some experiments with a kite.

 

[berkeman]

I live near some highways that are roughly "a couple of miles" away. On some blessed days they are inaudible, and on other days they roar with a low frequency sound that penetrates into every corner of the basement. (If frequency matters, it would be most preferable to refract sound at 20Hz, but consider whatever works best)

If the offending noise is mainly lower frequencies, you might consider active noise cancellation for some relief (plus, it sounds like a fun project!).

You could do something like two lines of fenceposts along that edge of your property -- the first line has microphones on the post sides facing the highway and the closer line of posts have medium size speakers facing your property. You would use the audio waveforms from the microphones to calculate the best speaker drive waveforms to cancel out the noise.

Have you looked into active noise cancellation for anything prior to this?

https://en.wikipedia.org/wiki/Active_noise_control

Active noise cancelling is best suited for low frequencies.

 

[Baluncore]

There are occasions when I can hear noise from dense slow traffic on a highway about 10 km away. Those occasions are in the morning, when there is absolutely no wind, and it has been a clear cold night. Only under those conditions can the sum of the noise of the many vehicles idling reach me. The air then is cooled near the ground with warmer air above, so the sound can be refracted over high ground between the highway and me. The thermal gradient needs to develop slowly and be very stable, which explains why having still air with no wind is required.

About 25 years ago, I estimated the distance to a “single virtual vehicle” with the same noise power as about 150 meters. Then from that I calculated the number of vehicles idling on the two lane highway, and concluded it was crowded, bumper to bumper in the morning rush hour.

I am about 22 km (as the crow flies) from the airport, near the navigation waypoint in line with the runway. On still days, aircraft descending on approach fly over me, then about 8 minutes later I hear the reverse thrust as they brake. From that, and the speed of sound, I can calculate their average airspeed between here and the middle of the runway. That situation also requires still air in the region.

Development over the years, neighbours and through traffic with duf-duf sound systems has raised the noise floor, so it is rare to hear those distant sound sources these days. Oh how I loved to be alone in the country.

 

[Baluncore]

Summary:: Suppose you have a 200-meter strip of land near a highway to "paint" however you wish. For example, you could put black solar panels on part and white gravel on another part. Can you make it so that noise from the highway is diffracted upward much of the time by the temperature differentials, and never reaches the horizon?

I live near some highways that are roughly "a couple of miles" away. On some blessed days they are inaudible, and on other days they roar with a low frequency sound that penetrates into every corner of the basement.

High albedo may cool an area exposed to sunlight, but not the objects nearby which will become hotter as they intercept some of the reflected energy.

Sunshine heats black bitumen roads. White painted signs on the bitumen remain cool. Look at the direction arrows on an exposed car park surface and notice how the arrows contain shrinkage cracks that are not healed by the heat from sunshine in summer.

If the verge of a highway had high albedo, it would remain cool relative to the black highway. That would tend to refract noise from the roadway back downwards, which is not what you want.

Any thermal differentiation of the air near a road is disturbed by the wind, and by turbulence induced by vehicle movement.

A solar powered hot air wall would need a face exposed to the sunlight. It would need a large heat exchange surface area in contact with air. That could be achieved with inefficient solar panels, but they would need to track the sun and even then would only work for part of the time.

A “thermal sound wall” would need a heat source along a line. Air would be heated at ground level, which would then rise, forming a warm air curtain moving upwards. Sound reaching the wall will be refracted slightly upwards in the wall of air, but there will be an equal and opposite refraction as it exits on the other side. In effect the wall would have very little change to noise on the other side.