Sound volume itself is measured in decibels (db), which can range from quiet voices in a library setting at 30db to a gunshot at 140db. At 180db, human hearing tissue begins to deteriorate.
The degree to which one desires to block sound depends on the context. For example, a recording studio in the middle of the woods may only need simple soundproofing techniques to keep the quiet, natural sounds from affecting the audio mix. However, in an urban setting, recording studios often require professional acoustics experts to install extra thick sound-proofing materials.
The sound reduction index is the international set of standards that determine sound insulation. The index is determined by taking into account the size of two environments, such as two neighboring rooms, their acoustic properties of reverberation, and the specific volume levels of a controlled sound in either room. Once these variables are calculated, the sound reduction index determines how easily noise bleeds from one room to the other.