I wish I had known about this stuff years ago. It would have saved me time and money. It's a construction adhesive that turns regular drywall into one of the best sound-control building materials you can buy. Green Glue, as the name implies, is used to make a sandwich of two sheets of building materials. In the following examples, I often refer to drywall (gypsum board), but it can also be used in ceilings, floors, and other applications. How does it work? Without going into a physics lecture, Green Glue is a visco-elastic compound that assists in the damping process. Damping dissipates the energy (in our case, sound) by turning it into heat. The company's website has numerous animations, graphics, and white papers that do a better job illustrating this than I can here. So be sure to check it out. One popular misconception is the shoe analogy. Some people point out that visco-elastic fluid is found inside the soles of some tennis shoes. This is right and wrong. Those soles use a visco-elastic material that use extensional damping. This means the material has normal resting shape and damping occurs when this shape is compressed and flexes back to its original size. With Green Glue, the compound is compressed inside a stiff structure. This is called constrained layer damping. Damping occurs when the center of the wall is sheared. When bent, shear forces pull and stretch on the damping material, allowing the damping material to dissipate energy. Kudos to Bill Skibbe for turning me on to this distinction. How do you use it? After the first layer of drywall is erected, Green Glue is applied to the second layer just before it is hung. Just shoot the glue in a random pattern that covers the whole sheet. Optimally, use two tubes per 2 ft by 4 ft sheet of drywall. Then hang the second sheet with the glue facing the existing wall. How well does it work? To my ears, two sheets of drywall sandwiched with Green Glue are as effective as four sheets of drywall on a staggered stud wall (two sheets per side). We used a reclaimed skyscraper window in our live room (talk about soundproof). Along with the Green Glue, the single wall offered as much separation as if we put two separate walls up. (This will also give you a tough lesson about transmission via cable pass-throughs and electrical boxes, but that's another story.) I have a few notes about using Green Glue. You'll need a bigger caulk gun to hold the 29 oz tubes. (Most guns sold at local home improvement stores cannot hold these larger tubes.) Also, avoid getting it on carpet or clothing, as it is a bear to remove. Finally, the stuff performs better once it has a chance to cure, so expect isolation to improve over the first two weeks. However, the stuff is water-based and non-toxic, so it's much safer to work with than many items on the market today. In the last five years, the cost of lumber and other building materials has increased significantly. Having a non-toxic option that significantly reduces the quantity of raw resources required is an all-around good proposition. Studio owners, PA installers, and acoustic consultants/architects should check out Green Glue next time a project comes up. (\$177.48 for twelve 29 oz tubes; www.greengluecompany.com)