There are four components in the life span of a tone: attack, decay, sustain and release. Sometimes some of these overlap or are eliminated. The overall term for the graph describing how a tone starts, sounds and ends is the envelope.

Attack – start of a tone (unit: time)
At the start of a tone, there is a very short period of much non-harmonic action or noise. For instance, when a piano hammer hits a string, the sound of the hammer hitting the string is very prominent. On the violin, the attack is the slight scrape heard when the bow engages the string.

The attack is in many cases how we recognise instruments.
For instance, a piano played backwards sounds like a harmonium. Listen!

Decay – attenuation of a tone (unit: time)
The attack dissipates quickly, and only then does the energy in it produce the actual tone with a clear pitch and timbre. This dissipation of the attack is called the decay. Note that in many instruments where the vibrating element is an air column, the decay is extremely short, because the tone sounds at once (the blowing continues).

Sustain – amplitude over time (unit: amplitude)
After the decay, the actual pitch and timbre of the tone can be determined as the partials settle in.

Release – end of a tone (unit: time)
When you stop playing a tone, vibrations continue for a while in the instrument and in the acoustic and then die out. This is where we can best recognise the space in which the instrument is being played.

Facts:

• The components of the envelope are: attack – decay – sustain – release (ADSR)
• The components of the envelope affect how we identify instruments, pitches and space