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

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

• The sound ramps up from zero amplitude to its highest amplitude. Relative amplitude.
  Wrong measure.
• The sound ramps up from zero amplitude to its highest amplitude. Time.
  Excellent!
• The sound ramps back down to a somewhat lower amplitude level. Relative amplitude.
  Decay; wrong measure.
• The sound ramps back down to a somewhat lower amplitude level. Time.
  That is decay.