Audio: envelope (0:08)
envelope plays two enveloped sounds in succession and the envelope figure shows their waveforms:
An envelope is a container to shape sound.
An envelope is shaped in segments. You can create as many segments in an envelope as you like with as many shapes as you wish.
Fade is a generic term for a segment. A fade-in is a segment at the beginning of a sound and a fade-out is a segment at the end.
An amplitude envelope has two purposes, it is a safety feature and it is a sound effect.
An amplitude envelope is a safety feature which should be applied to all music. It protects hearing and ensures that the output is within a comfortable listening range. It also protects speakers and headphones from damage.
All the music in the guide is protected by an ASR envelope. The attack segment is a linear fade-in with a duration between 10ms and 50ms, the release segment is a linear fade-out with a similar duration, and the peak amplitude of the sustain segment is normalised to -1dBFS.
An ADSR envelope is widely used in digital music to envelope the volume of a note. An ADSR envelope is an ASR envelope with an extra segment, Decay, added immediately after the attack segment. The decay segment has a short duration during which amplitude falls slightly from its peak attack value to its sustain value.
An amplitude envelope is also used as a sound effect. It adds dynamics to a single sound, it adds dynamics to a longer piece of music by creating loud and quiet passages, and it can be used solely to create effects such as that of the second sound in envelope. Bear in mind, however, that there is no agreed system for classifying unpitched sound, and you can easily change the description from a wave on a beach to the sound of Starship Enterprise flying overhead on the exoplanet Proxima Centauri b. Few could argue with that.