Passive noise control can be as simple as covering floors with thick carpets or as elaborate as designing a building from the ground up to shut out unwanted sound. One noteworthy example of the latter approach was the renovation of a 1920's-era building into a new home for the Goodman Theatre in Chicago, a project that was completed in December 2000. Architects for the project faced a challenging noise problem. Downtown traffic and the clatter of trains on elevated tracks outside the building created sound levels exceeding 100 decibels at the theater's front entrance. In addition, subway trains roared through a tunnel just 6 meters (20 feet) below the floor, creating vibrations that would ruin the delicate silence that a theater demands.

To isolate the theater from the subway, the structure had to be placed atop a novel layer-cake of sound-absorbing materials. Engineers first poured a slab of concrete made to be especially stiff. Atop that, they installed a layer of steel-reinforced rubber slabs 25 centimeters (10 inches) thick. The builders then poured another layer of concrete on top of the rubber isolators to support the building itself. This sequence of materials sets up what acoustic engineers call an “impedance mismatch.” The difference in the characteristics of the materials reflects sound waves, rather than permitting them to move through the materials to reach the interior of the structure.

To shield the interior of the theater from the traffic and elevated-train noise, the architects designed two sets of walls separated by an air gap 5 centimeters (2 inches) wide. Each wall rests on its own foundation, so there is no connection between the walls. The air space between the walls prevents sound from moving directly into the building. These construction techniques dramatically reduce the amount of noise that enters the theater, cutting the 100-decibel din outside the entrance to less than 20 decibels inside, about the level of a whisper.

(Source: How stuff works?)