Acoustic shielding is the deadening of unwanted sounds, usually by introducing sound-absorbing materials in the path of the sound waves. Sound enters a room in three ways: through the air, through the structure, or by the diaphragm action of floors, walls, and ceilings. Airborne sounds are absorbed by using thick or absorbent walls in which there are no cracks or ducts; doors and windows should fit tightly and be seated in rubber or felt liners. Windows should be double-glazed. Structure-borne vibrations and diaphragm effects are reduced by using double walls with insulating material but as few ties as possible between them.