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I was in the shower while my roommate was listening to music and got to thinking about the fact that I could only hear the bass and lower drums through the walls. Why is this? The two possibilities I could think up were:

  1. For some reason, sound waves at low frequencies (if I remember this means a lower pitch) are better at traveling through solids.

  2. The bass sound waves are slightly vibrating the walls themselves, and this is somehow producing sound waves in the air on the other side of the wall.

I have no clue if either are on the right track, and I'd really love to know!

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In addition to CarlBrannen's answer, there is always the possibility that your roommate was listening to a drum and bass track ! –  twistor59 Dec 10 '11 at 8:09
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3 Answers

It's not so much that the bass frequencies go long distances as that the high frequencies get absorbed and don't.

Say that the dimensions of your room are 30 feet x 20 feet. Your room will be pretty good at scattering sound that has wavelength shorter (i.e. frequency higher) than $\lambda = 20$ feet. Since sound travels at around $c_s = 1000$ feet per second, this is frequency $f = 50\textrm{Hz}\;$: $$\lambda = c_s/f$$ $$ \textrm{20 feet = (1000 ft/sec) / (50 /sec)}$$ So you can expect that frequencies less than around 50Hz will escape your room better than the high frequencies.

When you see the sun go down the sky turns red because the red (low) frequencies get absorbed less than the blue (high). And as you'd guess, it's for the same reason. Except for things that are specially designed (or lucky), anything that absorbs a long wave length (low frequency) is big enough to also absorb the short wave lengths (high frequencies).

Of course, just as with colored glass, it's possible for matter to reverse the situation and have some low frequencies absorbed while the high frequencies penetrate. But it's not the way to bet.

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Sorry if I'm being thick here, but I have trouble understanding your answer. You are not making any distinction between "absorbed" and "scattered". Are the high-frequency waves absorbed or scattered? I also don't get why low frequencies will get out of the room (whose room?) easier. –  Arnoques Dec 10 '11 at 9:20
    
When I've heard the neighbors stereo, it doesn't matter much if the highs are scattered or absorbed. In either case, I can't hear them and so they're not at all annoying. If they're scattered, you'll hear white noise which is not so bad. –  Carl Brannen Dec 10 '11 at 23:29
    
Low frequencies go around corners better than high frequencies. To see this, consider the low frequency limit. Say you open up an oxygen tank in your room. This will increase the pressure not only in your room but also through the whole house (as it escapes). –  Carl Brannen Dec 10 '11 at 23:30
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There are two things at work here:

1) Scattering: The size of the particles in the walls etc. will determine what frequency they scatter. That is, if the size of the particles is larger than the wavelength, then those waves will be scattered. If on the other hand the particles are smaller than the wavelength then those waves will pass through without being scattered.

2) Resonance: The walls have their resonant frequencies and being large objects, these tend to be low frequencies. Therefore bass sound waves are better at triggering resonance in the walls, which helps their transmittance.

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As sound makes a wall move, we know one half wave of sound has momentum. A 100 Hz sound wave typically has 10 times the momentum of a 1000 Hz sound wave. (Because the 100 Hz wave is 1000 Hz wave enlarged by factor of 10)

The wall moves 10 times faster and for a 10 times longer time, and the displacement of the wall is 100 times larger, when the frequency of sound is 10 times lower.

Ten times larger displacement at ten times lower frequency would produce equal pressure changes, but it was a 100 times larger displacement, which is a 10 times "too" large a diplacement.

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