Windows in our home theaters, personal listening rooms, and professional recording studios are the weakest link in the whole acoustic soundproofing process. In order to examine how much of an issue they create in the total sound proofing process, we need to examine how weak they are in stopping outside noise from entering the environment we choose to soundproof. To show this weakness, we look at a number called a sound transmission class rating or STC.
Sound Transmission Class Rating
A sound transmission class rating is a number that tells us how much noise is held back or prohibited from transmitting through the particular structure. The STC scale is a special type of scale similar to the Richter Scale used to measure earthquakes. A existing sound transmission class rating of 28 is not satisfactory and is the STC rating for most single pane windows. If we increase the STC rating to 38 we have reduced the noise level by 90%. If we have an existing window that has an STC rating of 28 and we increase the STC rating to 43, we have reduced the noise levels by 95%. An increase from 90% to 95% means that 60% of the remaining noise has been reduced. There is also a difference between the measured STC rating and human perception. Typically, a 95% instrument measured STC, means that a human will perceive a 75% noise reduction. Small increases in the sound transmission class rating create a lot of noise reduction.
It is common to hear people say that single pane windows do not do much for outside/inside or inside/outside sound reduction. This is a true statement. This comment is usually followed by stating that a double or even a triple pane window will work much better. Lets examine the STC ratings for both of these scenarios. A single pane glass window has a STC rating of 26 – 28. A dual pane window has a STC rating of 26 – 33. The average STC rating then becomes 27 for a single pane and 28 for a dual pane window. Our acoustic goal is to have at least a 75% reduction in noise levels and neither a single or double pane window meets this requirement. Neither a single or double pane window accomplishes this goal.
Build A Dam
What is the solution? Do we replace our windows with walls? Do we add more glass or thicker glass? The answer goes back to using multiple layers of materials to stop vibrations from moving through the window and into our rooms. Remember from past discussions, that we are dealing with sound energy and its associated vibrations. We need to stop these vibrations by creating individual vibration “dams” if you will, that hold back parts of the vibration in one layer of material and then as that remaining vibration transmits to the next material layer, it is reduced even further. We need to put a series of “dams” between us and the vibration.
Use Multiple Layers
Thicker glass is an option, but it is not just thickness we are concerned with. It is the nature or the thickness and how the thicker glass is used in combination with other layers of materials. One option is to use laminated glass. Laminated glass is two pieces of glass sandwiched together with an inner layer of plastic between each layer. This three layer system follows our example of using multiple layers of materials to stop vibrations. The vibration hits the first layer of glass and is reduced by an amount. The vibration then goes through the layer of plastic and is reduced further. Finally, the vibration hits the second layer of glass and is reduced even further. This series of constrained layers of mass damped materials produces a better barrier to stop noise. A good glass thickness for each layer of glass is 1/4″. Greater thicknesses can be used but 1/4″ is a good starting thickness depending on existing noise levels.
Leave Existing Window In Place
We can leave our existing window in place and use it as another layer or “dam” in our vibration reducing link. We can add our new laminated glass in front of the existing single or double pane window. Air is also a material that we can use to our benefit. We can place an air space between the existing window and our new laminated window. For best results, the air space should be at least 5″. We can even repeat the process and add the same laminated window to the other side of our existing window, leaving the 5″ of air space between the existing window and our new laminated window. Now, we have a laminated window on both sides with a 5″ air space on each side of our existing window.
No Leaks In Our Dam
We must also make sure the window is sealed correctly with the existing structure. Using our example of a dam to hold back noise, we must not have a leak or hole in our “dam”. Even the smallest hole will let in noise and negate all the work we have gone through building our laminated window. We must make sure the window is sealed within the existing frame and that no air leaks are present. Another detail we can attend to making sure the window does not vibrate under any circumstances. Remember, vibration produces noise and that is not our goal. We can use a spring loaded seal to accomplish this.
Windows let the most noise through into our rooms. We do not need to replace our existing windows, we can simply add different materials to our existing windows and create a series of layers of materials with different densities to minimize noise transmission. Additional glass sections can be added with an air space between or we can opt for thicker laminated glass. Whatever the direction, all construction tasks must be taken seriously from beginning to end.