Now that we've talked about reflection within a room, I will talk a bit about the way sound travels through a structure. For this visual example, I like to use Newton's Cradle. This is where a number of stainless steel balls are hung inline with wires or strings shown to the left.

When one pulls one of the end balls away from the others and swings it toward the group, at the point of impact, the energy is transferred from the striking ball into the next. That energy is transmitted down the line until it comes to the last ball. Since the last ball has nothing else to pass the energy to, it is forced to swing up and away from the group. The process continues like this until the energy of the swinging has been exhausted.

For this example, please picture a standard, residential wall assembly: 5/8" rock, 2x4 stud, 5/8" rock.

If you think about the sound wave as a wave of pressure, as it hits the piece of sheetrock it some of it is reflected back into the room, but more importantly, some is transformed into a vibration. The vibration is going to naturally be transmitted from the sheetrock to the stud and again on to the sheetrock on the other side of the wall. Since there is nothing else for the vibration to travel through it will convert back into an airborne sound and continue away from the noise source.

During this process some of the energy is lost. Some of it is reflected back toward the noise source. Some of it is converted into heat by the vibration of the sheetrock. Some of it is lost as the sound wave is reflected around inside of the stud cavity. Some of it, however, is passed into the next room and perceived as a sound.

To reduce the amount of sound that makes it's way through the wall, one of two things need to happen. One would be to increase the mass and density of the wall assembly. The other would be to build a wall where the sheetrock on "side A" does not have any hard surface contact with the sheetrock on "side B". This can be done in a few ways, but we can get into that later.