The attack refers to the initial speed of a sound, before it reaches maximum amplitude.

* A softer attack is more ambient.
* A sharper attack is more energetic.

University of Michigan - Ann Arbor

The envelope of a sound refers to changes in a sound’s amplitude over time. An envelope is comprised of many technical components, but the two most basic are the rise (*attack*) and fall (*decay*) of a sound.


Envelope

Material Design Documentation

The decay refers to the amount and speed at which a sound’s amplitude decreases after its attack (until its amplitude is zero). The larger the decay, the longer and slower a sound will feel.

* A short decay feels small and fast.
* A longer decay feels large and slow.

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