What are Sound Envelopes? ADSR Explained

If you've ever dabbled in music production or sound design, you might have come across the term "sound envelope". In music production, sound envelopes are used to control the way a sound evolves over time. This can make a big difference in the overall flow and character of a piece of music. 

In this post, we'll explore what sound envelopes are, how they work, and focus specifically on the ADSR envelope.

What is a sound envelope?

A sound envelope is a visual representation of how a sound changes over time, from the moment the sound starts to the moment it ends. Sound envelopes show how a sound wave changes across several dimensions, including amplitude (volume), frequencies, and pitch. 

The envelope consists of four stages: Attack, Decay, Sustain, and Release (ADSR):

• Attack: the attack stage refers to the time it takes for the sound to reach its maximum level after it is triggered. 
• Decay: the decay stage comes after the Attack stage and refers to how quickly the sound fades after reaching its maximum level. 
• Sustain: the sustain stage occurs after the decay stage and refers to the sound's steady-state level after the initial attack and decay phases. 
• Release: finally, the release stage is the time it takes for the sound to decay to zero after the triggering event has ended. By adjusting the parameters of each stage, you can create a variety of sound textures and timbres.

Changing a sound envelope can significantly alter the character and impact of a sound. By manipulating the ADSR envelope, you can change how a sound evolves over time, which can result in different timbres, textures, and emotions – from sharp and percussive to smooth and sustained.

For example, if you increase the attack time and decrease the sustain level, you can create a plucked or percussive sound, like a guitar or harpsichord. Alternatively, if you increase the attack and sustain times, and decrease the release time, you can create a pad-like sound that swells and sustains.

How are sound envelopes generated?

Sound envelopes are generated through a process called amplitude modulation, which involves varying the amplitude or volume of a sound wave over time. There are several ways to generate sound envelopes, depending on the type of sound source and the desired outcome.

Some of the most common methods include:

• Synthesisers: most modern synthesisers have built-in envelope generators that allow you to shape the sound of the oscillator. These envelope generators are usually triggered by a keypress or a MIDI signal and are used to shape the ADSR envelope of the sound.
• VST instruments: it’s common for VST instruments to include built-in envelope modulation that allows users to shape the sound it produces.
• Sampling: when working with sampled sounds, such as drum hits or vocal samples, you can use digital audio workstations (DAWs) to manipulate the sound envelope. This can be done by using the volume automation feature in the DAW to adjust the envelope's parameters over time.
• Audio effects: some audio effects, such as compression and gating, can also be used to manipulate the sound envelope. Compression, for example, can be used to increase the sustain level of a sound, while gating can be used to create percussive sounds by cutting off the sound after a certain amount of time.‍
• Physical instruments: in the case of acoustic instruments, the sound envelope is generated naturally by the instrument itself. For example, the sound envelope of a piano is created by the hammer striking the string and the natural decay of the sound over time.

What are the 4 parts of a sound envelope?

At the heart of the sound envelope is ADSR, an acronym for Attack, Decay, Sustain, and Release. These 4 components control the sound wave from the instant you press a key. They can be used to control the loudness of a sound at different points in the soundwave, meaning sounds can essentially be shaped however you'd like them.

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Sound envelopes can be modulated to make a sound gently ease in or out, or sharply appear and disappear. 

Let’s take a closer look at ADSR:

Attack

The attack phase of the sound envelope is the initial part of the sound that occurs when a note is triggered. The length of the attack can be controlled to make the sound ease in slowly or kick in more suddenly. For example, a shorter attack time can create a percussive sound, while a longer attack time can create a smoother, more gradual sound.

• Short attack (0-100ms) will result in a more sudden sound, where the audio reaches its maximum amplitude much faster. A faster attack will add a sharper character to your sound. 
• Medium attack (100-250ms) will result in a slightly softer sound. Attack levels between 100 and 250 ms can help reduce the harshness of the sound envelope.
• Long attack (250ms+) will result in a much softer sound that eases in slowly. Long attack levels above 250 ms are often used for pad instruments, and commonly feature ambient music.

Decay

After the initial attack phase, the sound enters the decay phase. This is the period of time between the end of the attack phase and the beginning of the sustain phase. During this phase, the volume of the sound decreases from its peak level to a lower sustained level. The length of the decay phase can also be adjusted to create different effects. For example, a shorter decay time can create a staccato sound, while a longer decay time can create a more sustained sound.

The length of a "short," "medium," or "long" decay time is subjective and can vary depending on the specific sound and context. As a general guideline, the typical range of decay times are as follows:

• Short decay (0 ms to 250 ms):  generally used for percussive sounds, such as snare drums, hand claps, and rim shots.
• Medium decay (250 ms to 750 ms): generally used for instruments with sustained tones, such as pianos, guitars, and vocals.
• Long decay (750 ms to 2000 ms or longer): generally used for synth pads, string sections, and choir sounds.

Sustain

The sustain phase is the period of time in which the sound maintains a constant volume level after the decay phase. 

In essence, the sustain is the main part of audio that can be heard. This phase continues for as long as an instrument key is held down. The sustain is not a part of every sound envelope and instrument. If a key is released during the attack or decay phase, then it is not activated.

This phase is typically longer than the attack and decay phases and can be adjusted to create different effects. For example, a higher sustain level can create a louder, more sustained sound, while a lower sustain level can create a quieter, more muted sound.

Typical range of sustain times include:

• Short sustain: 50 ms to 250 ms
• Medium Sustain: 250 ms to 750 ms
• Long sustain: 750 ms to 2000 ms+

Release

The final phase of the sound envelope is the release phase. This is the period of time between when the note is released and when the sound fades away completely. During this phase, the volume of the sound decreases gradually until it reaches zero. The length of the release phase can also be adjusted to create different effects. A fast release will phase a sound out more suddenly, while a slow release will phase a sound out more gradually.

Typical release time ranges include:

• Short release: 0 ms to 100 ms
• Medium release: 100 to 250 ms‍
• Long release: 250 ms+

What's the difference between decay and release?

The decay and release stages of the ADSR envelope are often confused because they both involve a decrease in volume over time. However, they serve different purposes in shaping the overall sound of a note.

The decay stage controls how quickly the volume of a note decreases after it reaches its peak level during the Attack stage. The decay time determines how long it takes for the note to reach the Sustain level. Once the Sustain level is reached, the note will maintain a steady volume as long as the key is held down.

On the other hand, the release stage controls how quickly the volume decreases after the key is released. Once the key is released, the note enters the Release stage and the volume gradually decreases to zero. The release time determines how long it takes for the note to fade out completely after the key is released.

So, to summarise, the decay stage controls the volume of the note while the key is being held down, while the release stage controls the volume of the note after the key is released.

Examples of ADSR in use

Here are some examples of how different sound envelopes can be used in real-world songs to demonstrate the versatility of ADSR.

Pluck envelope

The pluck envelope is characterised by a short attack time, a short decay time, and a low sustain level. It is commonly used in creating percussive sounds such as plucked strings or metallic percussion. A great example of the pluck envelope in use is the synth lead in the song "Take On Me" by A-ha.

Pad envelope

The pad envelope is characterised by a slow attack time, a long decay time, and a high sustain level. It is commonly used in creating sustained sounds such as ambient pads or strings. A great example of the pad envelope in use is the intro to the song "Strawberry Fields Forever" by The Beatles.

Brass envelope

The brass envelope is characterised by a medium attack time, a medium decay time, and a medium sustain level. It is commonly used in creating brass-like sounds such as trumpets or horns. A great example of the brass envelope in use is the intro to the song "Careless Whisper" by George Michael.

Drum envelope

The drum envelope is characterised by a short attack time, a short decay time, and a short release time. It is commonly used in creating percussive sounds such as drums or percussion. A great example of the drum envelope in use is the drum beat in the song "Beat It" by Michael Jackson.

What is a synthesiser envelope?

A synthesiser envelope is essentially an envelope generator that is built into a synthesiser instrument. This controls the amplitude of the sound, and is commonly referred to as an amp envelope. Envelopes are built into a range of electronic music instruments and synthesisers, from hardware instruments to software VSTs.

Conclusion

Sound envelopes are an essential concept in music production, and understanding them can greatly improve the quality of your music. By mastering the ADSR envelope, you can create a wide variety of unique sounds that will set your music apart. With the tips and techniques outlined in this blog post, you'll be well on your way to creating professional-grade music. So experiment with different envelope settings, and have fun creating your own unique sounds!