History: What is the sustain phase? How do we measure it?

What is the sustain phase? How do we measure it?

• The sound increases that amplitude until release. Relative amplitude.
  Holds, not increases.
• The sound increases that amplitude until release. Time.
  Holds, not increases; wrong measure.
• The sound holds that amplitude until release. Relative amplitude.
  Yes!
• The sound holds that amplitude until release. Time.
  Not quite...