Why do bass tones travel through walls?

Question

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!

Answer 1

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

Answer 2

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.

Answer 3

Think about what happens when a soundwave "hits" a wall. Really what that means is that there's a high pressure area on one side of the wall (normal pressure on the other) followed by that high pressure area becoming a low pressure area.

So while the sound wave's pressure is high, the air is pushing the wall, causing it to move a bit. This stretches the elastic mediums within the wall (like pushing on a block of jello). Eventually these elastic forces cause the far side of the wall to move, which pushes on the air on the other side, transmitting the sound. When the low pressure region hits, the elastic energy pushes the wall back towards this low pressure area. Once again, this transmits the sound to the other side.

For low frequency sounds this is most of the story. The movement of the wall is relatively fast compared to the period of the sound wave. For high frequency sounds, however, it gets more interesting. With high frequency sounds, the low pressure trough might occur while much of the energy of the sound wave is still propagating through the wall (the jello is still squished, and hasn't had a chance to release outwards towards the other side). Now the elastic energy in this wall is "happy" to go in any direction, so when the low pressure wave starts to form in the air, some of the energy put into the wall in the high pressure phase doesn't ever get through the wall. It is instead "put to work" pulling the wall back to its original shape.

In general, this process has a tendency to invoke the non-ideal properties of the wall. It is not perfectly elastic. Some of that energy gets converted to heat. This "deadens" the sound. No more acoustic energy can be transmitted.

If you look at soundproofing a recording studio, the holy grail is "sprung mass." One common construction is to put up a layer of drywall followed by a layer of elastic and then another layer of drywall which "floats," not screwed into anything. When the soundwave hits this, it has a large mass which takes a great deal of time to accelerate, and a very nice elastic layer to soak up and dissipate the energy. This approach can deaden very low frequency sounds. In normal walls, these elastic effects are typically microscopic material properties which can't soak up as much of the energy into its elastic bonds before transmitting them all the way through.

This is also the source of "resonation." If you time the low pressure wave perfectly, just at the point where the wall is done transmitting energy to the air on the other side, you get to use all of the elastic energy stored up to move the wall, accelerating it a maximum amount. This can actually cause a sound to be louder than it was before, because the wall's movement makes it "easier" on the sounds source to generate higher pressures, providing some of the energy required.

Answer 4

Bass sound-waves are really big. The lower the bass, the bigger the wave. A single wavelength can go through a window, or door and enter back in through another window in a different room. They can go around walls and corners. They can also create resonance with large objects like walls, and this helps them to pass through because the wall is facilitating the wave by matching it's frequency. Plus, as was said, bass is usually cranked out with a lot of energy for the human ear to hear. The lower the frequency, the more energy needed. also, as a large wave, it fills more space, and is not so directional. That's why down shooting sub-woofers tend to work. A normal loudspeaker would sound terrible pointed at the floor. The sound would either get reflected off a hard surface, or absorbed by a soft surface, or both. Hope that helps.

Answer 5

The bass has longer wavelengths. Hence there is a huge probability that the particle velocity ,at the time it hits the wall, is large. The momentum transfer takes place at the wall interface. Hence it travels without much attenuation. It may also depend on the acoustic coupling of the wall & air.

Answer 6

The longer wavelengths aspect is still a little confusing without diagrams. I keep picturing a snake trying to slither between particles in a wall, and you'd think a smaller snake with shorter body ripples would have an easier time. But I do get it, overall.

A gut-level angle is that bass has more raw energy than higher frequencies and simply bullies its way through materials. If you move more air you move more total molecules.

Regardless of the science and acoustics, thousands of ghetto idiots are GLAD that bass transmits as far as it does! That's how they define their "territory."

Answer 7

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.