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Sound Synthesis Tutorial

:: ADSR Envelope

The ADSR (Attack - Delay - Sustain - Release) is probably the most important aspect of shaping any given sound. The four components of the ADSR form what is known as the envelope of the waveform. The envelope represents the variations in a certain magnitude that a given sound experiments over time. In this case, I am referring to amplitude envelope, which dictates the volume (loudness) of a sound over time.

If we have a clear idea about what kind of sound we are trying to create in a synthesizer, after choosing the correct waveforms and additional settings in the oscillators, tweaking the ADSR values in the amplificator (AMP) is essential to find the right amplitude envelope for the sound. The sound of any musical instrument is characteristic not only because of the timbre, but also because of the envelope. Below, the image to the left shows a graphic representation on how Attack, Delay, Sustain and Release parameters work together to form the envelope of the sound, while the image to the right shows the ADSR controls (for both loudness and filter contours) in the MiniMogue Luxus analog synthesizer, which is available for download in this website.

ADSR MiniMogue Luxus ADSR

Seeing the ADSR graphic, it looks easy to understand that it represents a whole sound that we can differentiate in two parts: the first part (Attack and Decay) shows some kind of sound impact (like a percussive hit or a plucked string), brief but louder, while the second part (Sustain and Release) shows how the sound, after losing loudness, stabilizes its amplitude during a certain time and is finally extinguished with a gradual decrease on its amplitude. It is easy to understand that Sustain and Release are related with the amount of vibrations that a certain sound generator or musical instrument will produce just after a note is played (Attack and Decay). Some percussion instruments, such as a snare for example, would have insignificant amounts of Sustain and Release, while others that produce more vibrations when hit, such as a hi-hat for example, would have larger amounts.

Knowing this concept of ADSR, it is easy to figure how to built many kinds of known musical sounds. For example, to create one of those pad sounds that serve for creating background atmospheres, sonic textures that can give a deep or dramatic personality to a music piece, we would use a long Attack time, followed by a slow Decay, a pronounced Sustain level and a long Release time. As Attack is triggered when we hit a key in a keyboard, the note is sustained while pressing the key, and Release starts when we release that key, so a prolongated Release will allow to merge the successive notes that are played.

If we want instead to imitate the hit in a percussion instrument or the plucked effect of a stringed instrument, we would use a short Attack time for the hit/pluck effect, followed by a quick Decay, a minimal or relatively low Sustain level and a short Release time. In this case, being the sound much shortest, it would be quickly attenuated after pressing a key.

ADSR parameters allow for a great range of variations; they can be used to imitate known instruments or used in a creative way to create fantastic sounds. The following images show several waveforms as they are shown when loaded in an audio editor; these waveforms are samples of a single note played in different musical instruments.

Audio Sample

This waveform corresponds to a snare drum; it has a very fast Attack, and after a pronounced Decay it shows low amplitude vibrations. Sustain and Release are reduced in instruments that produce few vibrations after a note is played.

Audio Sample

This waveform corresponds to a mid tom, which being also a percussive instrument, produces a waveform that has resemblance to the one produced by the snare drum, but being less extreme in its parameters. Attack is fast, but not so instant; Decay is much less pronounced, and vibrations last a little longer, so Sustain and Release should have slightly larger values.

Audio Sample

The waveform produced by a hi-hat shows the ability of such instrument of keeping vibrating during a significant time; Decay is notably progressive, and nearly indistinguishable from Sustain and Release. Attack is short, but slightly longer than in the previous examples.

Audio Sample

This waveform corresponds to a turkish string instrument called saz; it has a short Attack for a string instrument, and a pronounced but relatively slow Decay.

Audio Sample

This waveform corresponds to another asian string instrument; the differences with the previous example are obvious, notably having a longer Attack and a less pronounced Decay.

Audio Sample

Audio Sample

These last two waveforms correspond to chimes (upper) and wind (lower) samples; they both have a prolonged Attack time and a slow, even indistinguishable, Decay.

The image below shows the ADSR Envelope function as it is included in an audio editor such as Cool Edit Pro 2.

ADSR Envelope

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