The three most important things to understand about the path of sound is bounce, transmission, and flanking. As the sound moves away from the source it immediately begins to decay. When a person stands outside, the sound decays at about a rate of 6 db per every doubling of distance from the source. Inside a building, however, it is much more complex, because sound waves bounce. Sound does not lose energy when it hits a fully reflective surface, and as the sound continues to bounce there will be a buildup of reflected sound that will keep the magnitude from the source quite high.  This creates multiple sources of sound. Furthermore, the sound that bounces will be delayed, which at a certain point will become noticeable to the human ear. On the other hand sound that is absorbed on the surfaces of a room will begin to approximate an outdoor condition, where sound decays at a steady rate from the source. In theory, a fully absorptive room (which is nearly impossible) will approximate an outdoor condition where only the single source of sound remains, and that sound is decaying at a steady rate. It is important to note that it is only possible to get close to an outdoor condition; there is no way to improve on an outdoor condition. If something is loud outdoors it will be loud indoors, and there is no way to quiet a room beyond that point.

The second thing to understand about path is transmission. When sound waves impact a surface it will cause that surface to vibrate as well, that vibration will transmit its way through anything that connects to it and in turn this causes a chain reaction of air molecules on the other side of the surface. This is how sound makes its way through walls, ceilings, and glass surfaces. The more rigid the surface the less vibration there will be. Therefore masonry block walls stop sound better than gypsum partition walls. One of the most effective partitions for sound transmission attenuation is the double partition wall with batt insulation in between. The reason is that when there are two walls that are not connected the vibration cannot transmit through the structure to the other wall. This does not give you a completely sound proof wall because the vibrating air molecules that come off the first wall will strike the second wall and get it vibrating. This is where the batt insulation comes into effect. As the first wall vibrates the adjacent air molecules between the walls the insulation is there to trap those molecules before they strike the second wall. Wall and ceiling types are measured according to a sound transmission class or STC rating. Typical STC ratings range from 20 (poor) to 60 (very good).

As any chain is only as strong as its weakest link the same is true in sound attenuation. An extremely high STC-rated wall is relatively useless if the sound can find its way around the wall. This brings us to the third topic of flanking. Common flanking paths for sound occur at doors, windows, partition joints, and mechanical ducts. It is critical to seal the edges of a wall at floors, ceilings, and other joints. This alone can turn an STC 27 wall into an STC 50 wall. It is also critical to add gaskets and weather stripping to doors, or even better, a double door sound lock. Adding lamination to glass or using double panes can increase the STC rating considerably. Duct silencers may be needed between rooms as well. Furthermore all mechanical equipment vibrates, because there is no such thing as a perfectly balanced air handling unit. Isolation springs and flexible duct are the most common sound attenuation methods for this situation.

In conclusion, in order to control the path of sound all three movements of sound by bounce, transmission, and flanking must be addressed.

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