Noise isolation is an acoustical issue we regularly confront. The two primary methods for noise to be transmitted is airborne and structure-borne. Airborne sound sources pass sound by the air which causes the partition (wall or floor ceiling) to vibrate. The vibration is transmitted and produces noise on the other side. Airborne sound sources include: speech, TV (assuming the unit is not vibrating the partition), stereos, animals, etc.
Structure-borne sound is transmitted from a source vibrating the partition (wall or floor). The vibration of the partition produces sound on the other side. Examples of structure-borne sound sources include: sound from walking on a tile or wood floor, mechanical units (such as a roof top HVAC), plumbing in walls, etc.