- Envelopes are a fundamental component in all synthesizers.
- Learn what ADSR is and how to use it to sculpt sound.
- Discover variations of ADSR you might have never heard of!
What are envelopes?
Envelopes in synthesis and DSP are used as sources for modulation. This is where the output of envelopes, LFOs, and other sources are used to animate parameters like volume, pitch, filter cutoff, and more.
(If you want to brush up on LFOs before you check out envelopes, head to What Are LFOs and How Do They Work?)
Modulation is an important part of synthesis that allows for movement, expression, and character in a synth.
Without modulation, synthesizers would become static tone generators, producing a sound that is precisely the same for the duration of an entire note. It’s impractical to manually move the filter cutoff or output volume as they play. It’s much easier to let envelopes do this movement for us every time we hit a key.
Envelopes are unique as a modulation source because they react to an entire key press – that is, “note on” (when the key is first pressed) and “note off” (when the key is released).
In any synth patch, there’s a high chance that an envelope is used to control the output volume. And if you encounter an envelope in a synth, there’s also a good chance it’s an ADSR envelope.
What Is ADSR?
ADSR (Attack, Decay, Sustain, Release) envelopes are the most common type of envelope in synthesizers. With ADSR envelopes, you have just enough controls to get a wide range of envelope shapes for modulation, letting you create many classic and familiar synth sounds.
To understand how ADSR envelopes work, we need to look at the four stages that form the name. As we step through each stage, imagine a line on a graph representing the envelope in the range of 0% (initial) to 100% (peak).
- Attack – The time for the envelope to go from the initial stage to the peak. Short attack times create snappy, responsive envelopes, while longer times create smooth fades useful for strings and pads. This is the first stage of the envelope, which begins when a key is pressed.
- Decay – The time the envelope takes from the peak (100%) to the sustain level. Short decay times are useful for plucks and stabs, while longer decay times create a sound with more body. Our perception of the decay stage is closely linked to the sustain level.
- Sustain – This is the level the envelope sits at while a key is held after the attack and decay stages are complete. It’s important to note that sustain is a level, not a time value, making it the odd one out in the ADSR acronym. If the sustain level is set to the max, the decay time becomes irrelevant because we cannot perceive a decay from the envelope’s peak to another point at the same level. Conversely, if the sustain level is set to the lowest value, we’ll only hear the attack and decay stages, with the envelope terminating the moment it hits the sustain stage.
- Release – Once a key is released, the envelope moves from its current level towards the initial stage (0%) according to the release time. Short release times mean the envelope finishes as soon as you let go of a key, while longer times create envelopes that continue to “ring-out” after a note is released.
Attack, decay, and sustain are usually measured in milliseconds, while sustain is measured in dbdBr, a value between 0 and 1.
It’s also important to note that the release stage does not necessarily follow the sustain stage. If the attack and decay stages have not been completed, the envelope will release from wherever it is when the note-off event is received.
What About ADSR Curves?
The curve here refers to the shape of the envelope between points.
Each synth will handle this differently. Many are fixed “under the hood” and may just use simple straight lines.
More powerful synths will let you tweak the curve for each stage, giving you convex, concave, and even spline shapes for a more complex envelope.
ADSR -> Volume
This is the most common scenario you’ll encounter in your synthesis journey. Using an ADSR to control the amplitude of your sound is a very effective way of creating articulation without overwhelming you with parameters.
By using an envelope to control the volume of a sound, we can create a wide range of contours, from sharp plucks to smooth pads that slowly fade in and out.
Let’s imagine the volume contour of a slow, sustained violin note. There’s a fade-in as the sound builds from silence (attack), the volume peaks, then drops to a comfortable level (decay and sustain), and then we return to silence as the string rings out after the bow is lifted (release).
How the envelope affects the amplitude of a synthesized sound is crucial for defining what category the patch belongs to the instrument it represents.
Without the proper ADSR settings, our violin patch would turn into a blaring synth lead. While this may also be useful, they are two sounds separated only by a few envelope parameters.
You might also use an envelope to create a sound that quickly dies out if the keys are held but slowly rings out if the keys are quickly tapped and released.
This trick is handy for bell and vibraphone synth patches or simulating a cymbal that can be choked only if the key is held. If you quickly hit these instruments, they resonate, but if you hold a mallet to them or grab them, they are quickly muted.
When you think about it, something has to control how the volume of your sound responds to your key presses. Without any volume modulation, your synthesizer would be a glorified tone generator that doesn’t shut up.
What Do ADSR Envelopes Sound Like?
Because they are a modulation source, envelopes themselves don’t sound like anything, and we don’t route them directly to the output for this reason. Instead, they work under the hood as a modulation source, just like LFOs.
If you’re asking how they sound as a modulation source, it depends on the ADSR settings. There is enough flexibility on a basic ADSR envelope to simulate the characteristics of many instruments and sounds in a broad sense.
Variations on ADSR
Once you know what ADSR stands for, you’ll have no problem figuring out how an AR or DSR envelope might work. These are simpler versions of ADSR envelopes that sacrifice one or more stages for the sake of simplicity.
Usually, this saves on components in a hardware synth or drum synths that don’t need a sustain stage.
In the case of an AR envelope, there’s no need for a sustain stage, the sound will just stay at the peak after the attack stage is finished. An envelope like this will work fine for dictating the volume of a lead or bass sound or for a simple riser.
AHDSR
This is an ADSR envelope with a “Hold” stage, meaning the envelope will stay at the peak for the hold time then start the decay stage.
Breakpoint Envelopes
Breakpoint envelopes are a more advanced alternative to ADSR envelopes. These envelopes allow you to specify the time and level of multiple points in an envelope, allowing for a much more precise envelope shape.
The best breakpoint envelopes will allow you to create as many points as you wish and control the curve between each point. You’ll also get control over where the sound sustains (which can involve cycling between a few moments in a loop) and how the envelope behaves after a note is released, for example, moving in a specific shape rather than simply fading out.
You’ll find breakpoint envelopes on “do-everything” softsynths like Vital, Pigments, and Serum. They’re also an essential feature in FM8 because having precise control over operators is very useful for FM synthesis.
One downside of breakpoint envelopes is that they can quickly become more fiddly than ADSR envelopes. Deleting, re-arranging, and undoing mistakes can be a pain when dealing with lots of dots on a grid.
This is one reason why breakpoint envelopes on early digital synths were limited to just a few points, but that was still considered a significant improvement over ADSR envelopes.
Envelope vs LFO
The truth is, these days, envelopes and LFOs are very similar, even interchangeable, depending on how you use them and what features your synth has. It’s easy to get into the weeds here, so let’s just take a generalized look at envelopes vs LFOs in their purest forms.
With this in mind, the main difference between an envelope and an LFO is that an envelope reacts to a key being held and released.
By comparison, LFOs are oscillators, cycling through a set shape ad infinitum with little regard for key presses.
At least, this is true in most cases, but the lines between LFOs and envelopes get blurry if you loop an envelope like an oscillator or use an LFO that doesn’t repeat.
Of course, not all synths will let you do this, particularly old analog ones, making the distinction between the two more apparent. So if you’re confused about why LFOs and envelopes are so similar on your fancy softsynth, spare a thought for the old times.
ADSR vs LFO
When comparing ADSR envelopes to LFOs, the differences become more apparent.
A basic ADSR envelope suffers from a lack of control over the peak level. That is, the envelope will always reach the maximum level after the attack stage. The only meaningful control you get over the shape of an ADSR envelope is the sustain level, and the rest is managed with time values only.
So, even if your synth lets you loop an ADSR envelope like an oscillator, it’s much harder to get a sine shape, for example.
What Is An Envelope Follower?
An envelope follower measures the level of a source audio signal (like a guitar) and creates a real-time envelope that can be used as a mod source in a synth.
Envelope followers aren’t super popular in software synths but are more common in the hardware world, particularly in modular synth rigs.
Envelope followers are used in auto-wah pedals to move the bandpass filter automatically according to the volume of your playing. This means strumming a chord loudly is like pressing the wah pedal all the way down, while soft strums go the other way and become muted.
Envelope followers may have attack, decay, or release settings, which work differently from ADSR envelopes. In this case, attack is how quickly the envelope reacts to increases in volume, while decay/release is how quickly the envelope reacts to decreases in volume.
So if you want a very snappy and responsive envelope that closely mimics the volume you’re playing at, use shorter times, but for an envelope follower that vaguely gestures towards the general level of your playing, use longer times.
Want To Learn More?
Producer Hive has many useful articles for beginners who are curious about synthesis and for more experienced users just looking to brush up on their existing knowledge.
Here are some to check out today:
