Traffic noise: louder for those further away?

[Jonny R]

I live in a residential neighbourhood about a 5 minute walk away from a busy 4-lane road. I can normally hear the traffic from my house when I have the windows opened (not too loud but definitely noticeable sound of traffic). The other day I was going for a walk in the neighbourhood about 1 minute away from this street, and was surprised to hear how quiet it was. Noticeably quieter than around my house that's 5 minutes away actually (you only heard the traffic if you stopped walking and listened for a while). I told this to a friend (who's an electrical engineer) who told me that it is possible for the sound from the highway to hit the noise barrier and like a wave "rise up", bypassing the houses that are nearby, and come back down on the houses that are further away, where it will appear louder compared to the houses in the middle where the sound travelled over. Here's a drawing to help explain what he was trying to tell me. Pretend this is an aerial view from above:

----------------- H I G H W A Y

-----------------

/\/\/\/\/\/\/\/\ NOISE BARRIER \/\/\/\/\/\/\/\/

ROW 1 ROW 2

-(street)---- ROW 3 ROW 4

-(street)---- ROW 5 ROW 6

-(street)---- ROW 7 ROW 8

-(street)---- ROW 9 ROW 10

Everybody in the neighbourhood can hear the traffic (especially people in row 1 and 2). However it is quieter around row 3 and 4 compared to those around row 7 and 8 where the sound "lands".

Is what he is describing true? My friend is intelligent however he is an electrical engineer, not a civil engineer. But I do recognize that there is definitely something going on, given that the sound of traffic appears louder where I live (row 7, 8) VS around row 3 and 4.

[xrongor]

so what.... you dont believe your ears? you think its magic?

im kiddin with ya, but ya. sound bounces.

randy

[Edwin Pawlowski]

It can happen. I'm not an acoustics engineer so I don't know all the reasons but I've seen it a lot.

At work the shop has a radio blaring and the workers next to it complain they cannot hear it. On the other side of the building people complain how loud it is. They plainly hear the radio but not hte machine noise.

A neighbor three blocks away can hear our compressor running. No other house can, but it can be heard in his. Ed

[Charles Spitzer]

noise can be ducted in channels, especially if there's a parabolic shape anywhere nearby as it would tend to focus sound.

[Ron Hardin]

Depending on time of day, you get ducting (when the ground is cooler than the air). In that case sound travels huge distances because sound sent upwards comes back down, bounces off the ground, and relaunches itself for another bounce. Rather than just disappearing into the blue sky. So it weakens like 1/distance rather than

1/distance^2 which means it really hangs in there.

If you have a duct, you have something in the sky that reflects sound back down; and in particular sound that clears the sound wall comes back down some distance away.

If the sound wall weren't there, you wouldn't comment on it, because the close houses would also hear noise. But in this case they don't.

I have a highway a half mile away that I hear at night but not in the day; in the day, the ground is warmer than the air, and that anti-ducts the sound, sending horizontal sound upwards over my head.

[nospambob]

[Phisherman]

[Ron Hardin]

That's ducting in the ionosphere, but unfortunately a different effect, because the ionosphere always ducts AM signals. In the daytime, though, the sun ionizes the lower levels of the ionosphere as well as the reflecting higher layers, and AM signals lose energy to collisions in the denser lower layers, which attenuates the signals in the day. So they reflect, but don't survive, in the daytime. At night, they reflect as in the day but travel unhindered in the lower ionosphere and thus can be heard thousands of miles away. Except the FCC lets a thousand stations on the same frequency now, so you hear all thousand stations as well, except for a handful of comparative clear frequencies, and in short can't hear much but mush mostly.

It's called D-layer absorption.

Sound ducting is from sound travelling slower in cold air, and the ground cooling before the air after the sun goes down, creating a temperature inversion that bends sound downwards. In the daytime, there's no duct at all.

[jl_kirkby]

Ever hear of (heh...hear...of) ambient noise? This happens in the strangest places. You can stand on a street and hear just what is in front of you and except for that noise it is relatively quiet. If you move back from the street (i.e. a back yard or deck) you will hear noise from what is directly in front of you as well as all around you. Weird phenomenon... This is noticeable in high-rise buildings. Again stand on the street in front of a high rise building, you will hear noise from cars around you and any other things that may be in you immediate ear shot...people, dogs, etc. If you now move to a 20th floor balcony (just an example...could be the 9th floor...) you will not only hear the cars, people, dogs, etc in front of the building but you will hear things from many blocks around you.

meh...just my take on it. cheers. jl

[JerryMouse]

Spooky, isn't it? But yet, there it is.

[Rob Jones]

Ron,

Another factor that contributes to sound seeming louder at night is your brain (the 'device' that processes sound and light). When your eyes are receiving less light, a portion of the brain that normally processes light signals is diverted to other senses, such as hearing. This will also make sounds appear to be louder at night than during the day.

In a former life, I was a submarine sonar technician. We had mandatory hearing tests bi-annually, and we all learned early on that you can increase your score by closing your eyes during the test - same phenomenom as I described earlier.

rob jones

[SJF]

The way I see it, it's not so much due to ducting as refraction (really just a play on words). At night, the cooler (heavier) air near the ground retards the lower end of the wave front, redirecting it toward the ground. As you say, the reverse effect occurs in daytime.

There is a similar effect when there is a breeze blowing from the sound source. The slower air near the ground slows the lower part of the wave front with the same effect.

I have a church with chimes a quarter mile south of my yard and the sound is usually muffled by intervening trees. But some evenings they are very loud. A bit farther away is a dog pound which I seldom hear but sometimes the yapping comes in like it's next door.

[Joe C]

I thought that was Ron 'Jonsey' Jones. Oh, wait nevermind, that was Clancy....