While large projects such as the new Goodman Theatre cost millions of dollars, researchers have been investigating less expensive ways to carve out smaller zones of quiet. In 1999, for example, engineers at the Georgia Institute of Technology (Georgia Tech) in Atlanta, reported that they had developed special curtains designed to act as a shield against sound. The curtains consist of two pieces of fabric with pockets between them. The pockets can be filled with various noise-absorbing materials, such as cardboard. The researchers found that a prototype of these “Quiet Curtains” reduced noise by about 7 decibels. If extensions are added to the top and bottom of the curtains so that the curtains reach from the ceiling to the floor, they cut noise by 12 decibels. The engineers said that Quiet Curtains could be used to reduce noise in many places, such as nursing homes, offices, libraries, and factories.

In 2001, researchers at the Hong Kong University of Science and Technology were developing a new class of materials called “resonant sonic materials” that served as a passive barrier against low-frequency noise. The material the researchers created consists of small lead balls about 1 centimeter (0.4 inch) in diameter, coated with soft silicone rubber and glued together with epoxy to form blocks about 13 centimeters (5 inches) on a side. A layer of these embedded spheres almost perfectly reflects sound waves of two specific frequencies—400 and 1,100 hertz—making the material potentially a highly effective noise shield against some types of sounds, such as the loud hum of a motor. The researchers expected that the materials would be ready for commercial use by 2003.

Scientists have also been investigating the sound-blocking potential of another new material, called polyamide foam. The foam, which was originally developed for use as insulation in submarines, could replace the fiberglass insulation that is wrapped around heating and air conditioning ducts in many buildings. As well as creating a barrier against the movement of heat into or out of the ducts, insulating materials cut down on the noise of the air moving through the ducts. Fiberglass insulation, however, has several drawbacks, including the fact that the glass fibers can damage the skin and that moisture often accumulates on the material, encouraging the growth of bacteria. Polyamide foam is a good insulator and absorbs the sound of rushing air as well as fiberglass, but without the drawbacks.

(Source: How stuff works?)