Sound Envelopes: Shape Your Sound Using ADSR

The basic definition of an envelope of a sound is that it’s an automation curve that is triggered with every MIDI note. In other words, it determines how the [volume of the] sound will move every time we press a key in our keyboard (and, as we’ll see, when we release that note).

Most of the time this term is related to the amplitude (volume — amp envelope), but the change can also be produced in terms of filters (the number of frequencies or harmonics) or pitch (the fundamental frequency of the sound: see waveforms). There are still some synthesizers like Serum that let you control virtually any parameter with the envelopes.

This automation is controlled by an envelope generator, that will give your sound wave a shape through the time. The most common one is the ADSR, which you’ll probably find in all synths. This stands for:

  • Attack
  • Decay
  • Sustain
  • Release
ADSR envelope drawing pink

In some synthesizers, you’ll find that there is more than one envelope generator, probably with the purpose of modulating different parameters (volume, pitch, etc.) with each of them.

The shape of this automation curve is extremely important to define the characteristics of a sound. For instance, sharp and quick adjustments may result in very percussive sounds, while slow and long curves will result in atmospheric pads.

Now, let’s see what this is all about, and how you can achieve the sound that you want using envelopes.

Attack

sound envelope attack drawing

The attack defines, when you first press a key, how long it will take for the audio signal to go from silence to its loudest level. It’s usually measured in milliseconds (ms).

Let’s take a look at how different attack times sound like. Here’s an example of a saw wave with a short (near 0 ms) attack time, followed by a sound with a “long” attack time:

That being said, it’s generally accepted in music that short attack times are better to create leads, basslines, pianos, plucked strings & brass, and percussive sounds (although there are also percussions with a not-that-short attack time, such as shakers; the same applies to leads and basslines). Meanwhile, long times are better for ambient pads, sound effects (such as risers and uplifters), and for some organic instruments like violins and cellos, if you also want them to play a long and slow melody.

Another use of the attack is to remove any sort of “click” that some synthesizers produce when you press a note (as we’ll see later, the same applies to the release parameter). Here is what a sine wave with an initial “click” sounds like:

For this purpose, you’ll have to set a really short attack time that is not 0 ms. This is the same sound, but with this problem fixed:

Finally, as we said, envelopes can also be used to modulate filters and pitch. And this is especially true for the attack since it’s the parameter of the ADSR that’s most commonly used to modulate that.

In the case of filters, it can be used to create some experimental effects. But the most common use is to apply a short attack modulating a low-pass-filter, which combined with a little bit of resonance, can create a sound that’s something like dreamy or spacey:

On the other hand, pitch it’s not a common parameter to be modulated by the attack, but still, it can achieve some interesting results:

Longer pitch attack times will result in weirder sounds that can be good for experimental purposes, but probably not useful enough for creating melodies.

Decay

sound envelope decay drawing

Once the sound has gone through the attack, the decay comes to play.

What decay does is that it determines how much time it takes to the sound to go back from its maximum point to the sustain point (which I’ll explain in a moment).

It’s basically the reduction of the value once the attack time has ended.

Here is an example of a saw wave with little-to-no attack time and 300 ms of decay, followed by a saw wave with 600 ms of decay.

The difference is very subtle, but as you can see, a short decay time combined with a short attack time (and, by the way, a low sustain level, which I’ll explain next) is very useful for creating percussive sounds with very highlighted transients.

The decay time has a very interesting effect when we are modulating the pitch of the sound. If we are trying to create a sound with high transients, modulating the pitch with a short attack and decay time will help with that. This is how it sounds like:

This last technique is also very useful for creating kick drums when playing a much lower note (kick drums are, essentially, waves pitching down very quickly).

Sustain

sound envelope sustain drawing

The sustain is the only parameter of ADSR that does not control time, but level.

It is the level to which the decay will drop. And it will stay there for as long as we keep the key pressed.

Here is an example of a saw wave with maximum, medium, and null sustain level respectively:

Note that the first sound has no transient at all because it goes from 0 to 100 instantly and stays there.

On one hand, the sound with no sustain sounds very plucky and percussive. Therefore, we can say that we can use low sustain levels to create percussive sounds (real percussions don’t have sustain at all). This, combined with short attack and delay times will create really good sounds.

On the other hand, lead instruments, basses, and pads usually have a very high sustain value (sometimes even 100). This depends on how much transient you want them to have, but you’d generally want to have a high sustain.

Note: if the sustain level is 100, there will be no decay. That is because, once the sound has gone through the attack and reached its highest value, it will have nowhere to drop to: it will remain at 100 until the release time.

Release

sound envelope release drawing

Once we release the note on our keyboard, the sound goes out from the sustain level and enters the release phase.

This final segment determines how much time it will take to the sound to go from the sustain level to 0.

This is an example of a saw wave with 0 release time, followed by a saw with 600 ms of release:

It just adds a little (or not-so-little) tail to the sound to make it smoother.

Short release times are commonly used for basslines and lead instruments because they usually play faster melodies. On the other hand, ambient pads usually have longer release times, since it helps create more atmosphere and background.

Note: if you release the key (or MIDI note) before the sound has reached the sustain point, there won’t be any sustain. The same applies to the decay: if you release the note before it has reached it, there will not be decay. It’ll just go from the point it has been left directly to the release.

Other parameters

So these are the most common parameters that you will find on the envelopes of almost any synthesizer.

Nevertheless, there are some synths that include other controls. These aren’t that common, but it’s important to know that they exist.

First, we have the hold control. This parameter is between the attack and the decay, and controls how much time the sound will stay at its maximum value before decaying, and it will keep playing even if we release the note.

This is mostly found in older synthesizers such as the Korg MS-20, but some modern VST plugin software such as Serum also include it. The envelopes that include this hold option are called AHDSR

Also, although this is even less common, some synths have a delay parameter. This one goes before the attack, and determines the length of silence between pressing a note and the attack.

The envelopes that include this delay option (such as the Prophet ’08) are called DADSR. Some synths even go further and include both hold and delay controls and have a DAHDSR envelope (such as Image Line’s 3xOsc included in FL Studio, which is surprising due to its general simplicity)

Conclusion

Understanding and implementing ADSR is crucial for shaping your sound in order to achieve the exact texture that you want.

What I recommend is to experiment with it, and see how each parameter sounds like. Also, try loading some presets on your favorite synthesizer and see what their envelopes look like, and try to think why they chose that particular shape.

Also, loading a preset and tweaking the ADSR may be enough for the sound to fit perfectly on your song.

So, this is it about the envelope technology. If you want to learn more about synthesis and sound design, you can check out our article about the basics where we go through the most important concepts, including a dedicated article for each of them.