Mark Henderson answered
Attack and decay are two different characteristics of a sound. Attack refers to the time it takes from a note being played until it reaches peak volume. Decay is the time taken for a sound's level to reduce from the attack volume to the sustained level.
Attack, decay, sustain and release are the parameters of a sound and are collectively known as ADSR. The attack and decay of a sound have a great influence on an instrument's sound character. Attack and decay can be electronically altered when using an instrument such as a synthesizer or a digital audio work station.
- Attack time is the time taken for a sound to reach its peak level from nil.
- Decay time is the time taken for a sound to reach the designated sustain level from the attack peak.
This graph chorographs the ADSR of a sound. The sharp incline at the beginning is the attack time: The time taken from pressing a key on a piano to the sound reaching its peak volume.
The other side of the slope is its decay: the time it takes the pianos peak volume to decay to its sustained volume. The subsequent straight line is the sustained volume, and the release is the time it takes from that volume to reach zero again - for example from releasing a piano key to the point where no sound is audible.