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
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.
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.
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.
We heard a Fresno, California AM radio station in Yahats, Oregon in
the evening, probably 500 miles away. I used to listen to an AM
station in Salt Lake, Utah when I was in the San Francisco, California
area at night.
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.
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
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.
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.
Sound can be absorbed, reflected, or transmitted long distances. If
you cup your hands around your ears, more sound will be directed
toward your eardrum. A valley surrounded by hills could direct a weak
sound into a focal point where it is loud. Sound has "more problems"
traveling through less dense materials--and does not travel at all in
a vacuum. If you want a quieter property you can plant trees and
On 14 Sep 2004 11:10:17 -0700, jonny firstname.lastname@example.org (Jonny R)
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.
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