The way in which the level of a sound wave changes in time is determined by the envelope of the sound. It is the envelope of a wave, in conjunction with the timbre, which helps establish the sound’s unique individual quality; the envelope has a significant way on how we interpret sound...

The way in which the level of a sound wave changes in time is determined by the envelope of the sound. It is the envelope of a wave, in conjunction with the timbre, which helps establish the sound’s unique individual quality; the envelope has a significant way on how we interpret sound.

The envelope of a signal can be measured in four ways:

• Attack – The portion of the envelope that represents the time taken for the amplitude to reach its maximum level. Essentially it is the initially build up of a sound.
• Decay – The progressive reduction in amplitude of a sound over time. The decay phase starts as soon as the attack phase has reached its peak. In the decay phase, the signal level drops until it reaches the sustain level.
• Sustain – The period of time during which the sound is sustained before it begins to fade out. Many instruments do not contain a sustain phase.
• Release – The final fade or reduction in amplitude over time.
  

Sound envelope characteristics

Every envelope sound is different, for example, a percussion sound starts very suddenly, but then decays and releases quickly because no more energy is being applied to sustain the sound. A bowed string, on the other hand may build up with a slow attack, sustain for a short period and then release. Other sounds, such as an organ, can be sustained indefinitely by the player. Most natural sounds decay the higher frequencies faster than the low frequency components because high frequency energy is dissipated more rapidly than low frequency energy.

The concept of hearing envelopes relies upon peak to peak values of amplitude and not root-mean-square values (RMS) - which is similar to the human perception of loudness. This means that high peaks in the signal will not necessarily make an instrument sound loud unless the amplitude is sustained for a period of time. Short peaks tend to contribute to the ‘character’ of the sound rather than the loudness.

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