Ambient sound consists of noises present i the enviroment.  
  
In Silence of the Lambs, when Agent Starling (Jodie Foster) is down with Lecter in the dungeon,  there were animal screams and noises built into the ambience.  (One element of the ambience is a guy screaming in pain.  The screaming was processed, slowed down and played in reverse)  

Subjective Ambience: 
In the trial scene of Philadelpia -  instead of using reverb to a voice as the konvention says for hallucinating  -  sound designer Ron Bochar used subjective ambience. He dropped the previous room tone and shifted the ambient sound.  He also changed the spatial placement of the ambient sound -  from left, right, and center speakers to surround speakers.  

R. L. Mott: Sound effects radio, TV, and film p 202 

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