• This involves using:
  - senses to detect audio output;
  - senses to detect visual output;
  - tactile feedback as well.
  
  
  
• Lots of factors affect this:
  - change in output (loudness/size);
  - max & min detectable levels;
  - field of perception (see display?);
  - fatigue, circadian rhythms;
  - problems with background noise.