Does sound carry downwards?

August 8, 2008 8:04 PM

Does sound carry downwards?

I live on the 2nd and 3rd floors of a 2-family house. We've got neighbors downstairs, neighbors to the left (separated by a 10-foot driveway), and neighbors to the right (separated by a big ~50-foot yard). I can often hear quietish noises that my right-hand neighbors make in their yard, e.g. sweeping their deck, or their kids talking outside, pretty clearly. I can hear quietish noises from my left-hand neighbors' first-floor kitchen, e.g., dishes clinking in the sink as they get washed, or their radio while washing said dishes. If my downstairs neighbors are outside in our yard, I can hear them talking (though not clearly).

Obviously sound carries up. Does sound also carry DOWN? In other words, if I've got the TV on at moderate volume in my 2nd-floor living room and my windows are open, is it likely my neighbors can hear it clearly in our yard? If I'm talking in my 3rd-floor bedroom, or, say, having a lively debate with Mr. Informed, could people hear me if they're at ground level outside? Mr. Informed says no, but when I think about the science, it seems like sound should go down as well as up! I'm wondering if I have to start closing our windows for privacy, especially the bedroom one.

I'm talking about regular noise inside a room--not someone calling down from a window, or speakers on the windowsill. Can anyone offer an answer and an explanation of the physics?

posted by Ms. Informed to science & nature (11 answers total)

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/diffrac.html

The pictures here are in two dimensions (I mean, they are pictures after all...), but it applies for three dimensional situations, such as listening to sound from a source above you.
posted by Loto at 8:20 PM on August 8, 2008

yes, sound carries in every direction. sound is a vibration that you hear when the vibrating air reaches your eardrums. chances are the floor muffles it, and the speakers are pointed toward you, not down, so that also reduces the energy of the sound that travels down. but yes, your neighbors can probably hear your TV if you can hear their incidental noises.

if you are concerned about transmitting sound, carpeting with rubber padding underneath will help muffle it, as will heavy fabric hanging on the walls (probably overkill, although it could help save on energy bills). obviously, closing the window will help, too.

if you are simply concerned about your neighbors overhearing -what- you are watching (like porn or horror movies with lots of screaming--i keep wondering how many times police have been called on neighbors watching a slasher movie at high volume), not necessarily the noise itself, you can usually mask it with a box fan.
posted by thinkingwoman at 8:20 PM on August 8, 2008

Sound doesn't carry up. Or down. It can be directed this way or that, but it will dissipate in all directions equally.

There may be more ambient noise at ground-level (e.g., traffic), masking sound coming from above, but otherwise there is no difference.
posted by whiskeyspider at 8:21 PM on August 8, 2008

I don't know the physics of this, all I can offer is anecdotal experience. Often when I'm walking down the street in Brooklyn, I'll hear a conversation, clear as day, and think it's coming from behind me or the other side of the street - but when I look, no one is around... and I see that there are some people on a balcony above me or near an open window, often 2-3 stories up. Usually those that I hear are being slightly louder than normal.

I think from a psychological standpoint, being "on the ground" means that you are being somewhat active, are moving about through your environment, and are filtering out unnecessary audio information. If you're sitting in your quiet bedroom, you may be more primed to hear stray noises, since you're more likely to be dealing with less audio stimulation.

I would say... if you want privacy, safest bet is to close your windows.
posted by SmileyChewtrain at 8:22 PM on August 8, 2008

As anyone who's ever lived in a basement apartment can tell you, yes, sound certainly does carry down.

Explanation of the physics: sound is just vibrations carried through the air. When the vibrations travel through the air and hit a wall, they shake the wall slightly. The wall vibrates and that sends the air on the other side of the wall vibrating which produces a sound just like before, except slightly muffled, since the wall didn't vibrate as well as the air, so some of the sound energy was absorbed. When you make a sound it radiates outwards in a sphere so it's going to travel in every direction. The only reason it might not travel down is if there is really thick carpet on the floor which soaks up all of the vibrations.

That was off the cuff, I'm sure others can provide more detail. The gist of it is, if you can hear them, they can probably hear you.

But on preview I can see you're not worrying about sound carrying through the floor but rather from inside your apartment to outside. In that case I think the answer is yes it will carry, but not as loudly. When your neighbors talk outside your window, there is nothing but air between the sound source and you, so the sound waves travel quite well. When the sound source is inside your apartment, a lot of the waves are going to bounce off your walls and floor and stuff and a comparatively smaller amount of sound energy will make it out the window. But if I were you I would experiment.
posted by PercussivePaul at 8:23 PM on August 8, 2008

Just to clarify some of the things that have been said above: not only does sound reflect it also diffracts, or "bends", around objects and obstacles, interferes with other waves and it's own reflections, etc...

Simply, when the sound of your TV carries to the window, it is diffracted by the frame of the open window. It will be diffracted in all directions from that window and the intensity of that sound will die down as a function of the distance from the source (your tv) squared.
posted by Loto at 8:30 PM on August 8, 2008

I can hear the sound of airplanes that are very high above me.
posted by winston at 8:36 PM on August 8, 2008 [4 favorites]

If you can hear them, chances are they can hear you.
posted by flabdablet at 9:19 PM on August 8, 2008

Sound travels from its source as a spherical pressure wave. Imagine it like a bubble, with the sound source at the center. When the sound bubble encounters an object, like a wall or floor or ceiling, some of the energy in that sound is reflected away, but some passes through. The amount that passes through is highly dependent on the makeup of the object.

In short, direction doesn't affect how sound travels. Obstacles and even the air itself do.
posted by pmbuko at 9:27 PM on August 8, 2008

The popular belief that sound travels up may have something to do with refraction by the atmosphere. If the air well above the ground is cooler than the air below it, which it usually is, a sound wave will bend so it travels more and more straight up as it reaches cooler and cooler air. However, during an inversion, when the air above is warmer than the air below it, the sound wave will be bent more and more sideways, and can even be turned around and head back to the ground. Jearl Walker talks about this in his awesome book, The Flying Circus of Physics, 2nd ed., p. 153.

Atmospheric physics shouldn't be a factor for the case you asked about, though.
posted by lukemeister at 8:39 AM on August 9, 2008

This is not an answer, but I love the sequence between winston and flabdablet's replies.
posted by purplefiber at 11:56 AM on August 9, 2008 [1 favorite]