HowStuffWorks "Blocking Sound Through the Walls": When contractors are building apartments or townhomes to sell, they're generally trying to keep construction costs as low as possible. To do this, they'll often construct walls in the cheapest way possible, with shared walls containing nothing more than some 2x4 studs with a layer of drywall on either side. In some buildings, they may add a layer of fiberglass batt insulation to help with noise and temperature control.