Envelope
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Amplitude Envelope
by J Marchand Knight
Timbre Lingo | Timbre and Orchestration Writings
In synthesis, sound recording, and mixing, the term envelope can be used to describe either how a sound’s amplitude (volume) changes over time (amplitude envelope), or the general shape of the frequency representation of the sound (spectral envelope). When recreating the timbre of an instrument (or other sounds such as a fire truck siren), it is equally important to get the overtone series right as it is to reconstruct or preserve the contour of the sound. In the world of synthesis and audio recording technology, the most commonly discussed type of amplitude envelope is ADSR (Attack-Decay-Sustain-Release).
Much of our timbral perception happens at the attack or onset of the sound. Imagine the quick crack of a snare drum as compared to the slow build of a bowed violin string. Consider the length of time it takes for each of these sounds to start, reach their loudest point, and level off. During this initial burst of sound, our brains receive a lot of acoustic information, both about frequency (having to do with pitch and quality) and amplitude (volume). As this information begins to decay, the volume drops a bit, some of the frequency information is lost, and we reach the sustain phase. While a saxophone and an electric guitar sound very different in real life, if you fail to capture the attack phase in sound synthesis or, if in audio recording and mixing, you chop off the attack or change it too much with digital intervention, listeners might actually fail to identify what they are hearing. This confusion can also be induced by reversing the tone’s playback, as shown in this piano example.
Finally, we come to the release phase, or how the sound ends. Some sounds stop abruptly and others fade slowly over time. Our brains process this information so fast that we are not conscious of all the parts that make up what we perceive as one sound or one thing. The same way a painter learns to see the depth of a face in terms of swaths of colour and shadow (and the rest of us just take in the face as a whole), in synthesis and audio engineering, it is crucial that we learn to separate the components of sound and work on them individually. Otherwise, the blaring sirens in your favorite episode of Chicago Fire (which are created in a lab) would be indistinguishable from the saxophone (also created in a lab) in that Rihanna tune you’ve been humming all week.