Airborne sound (or airborne noise) is sound that is transmitted through the air. Airborne Sound Insulation is the insulation against noise originating in air, e.g. voices, music, motor traffic, wind. The greater the airborne sound insulation between two rooms, the lower the resulting sound level in the receiving room.
This is unlike Impact Sound Insulation that protects against noise originating directly on a structure by blows or vibration e.g. footsteps above, furniture being moved, drilling and hammering the structure.