2.2 The Envelope

The sound envelope determines the duration, and the amplitude of the grain. The process of adding a grain envelope is known as windowing. Windowing is a type of amplitude modulation technique. The idea of windowing comes from an analysis process where small amounts of sound information can be put into a window frame, making it easier to analyse the sound. The idea of windowing also has a more closely related link to granular synthesis because of the practical nature in which it was first used.1 When the grain content is windowed it imposes an amplitude change over the content as was seen in figure 2.2 and 2.4. The amplitude follows the shape of the envelope. Figure 2.5 shows the most basic way to create a sonic grain.

       The envelope 

        The content 


        Figure 2.5 A method for producing a sound grain (Dodge & Jerse 1997: 263).

        This method involves multiplying the amplitude of the contents with the amplitude of the envelope.

Windowing may cause some slight convolution to the grain, but as a sonic grain is so small it is not noticeable.2 If the grain is constantly repeated in a synchronous manner the convolution will become noticeable. This suggests that the parameters of the envelope can be changed to alter the spectral composition of the grain and give extra control over the grain.

There are certain parameters that define an envelope. These are:


        Attack time
        Sustain time
        Decay time
These parameters can be summarised using the multidimensional 'parameter':


Fig 2.4 Diagram of a simple envelope.

2.2.1 Duration

The duration of the envelope determines the entire duration of the grain. Experimentation has shown that durations of between 10 milliseconds and 50 milliseconds are the best to work with.3 Obviously this is an extremely short duration, therefore only the most salient features would be immediately obvious.The duration determines the distinction of the contents of the grain. Grains with a shorter duration have a more ambiguous spectrum. This is due to the time-frequency relationship of sound. The shorter the duration, the less the spectrum is defined. A short grain duration creates a broader bandwidth making the grain noisier. This is very useful for creating a larger spread of frequencies. Another feature of using a short duration is that it is more difficult for the ear to decode the contents. This allows the composer to deceive the listeners perception of sound. An isolated grain with a very short duration produces a kind of buzzing or clicking sound. A longer duration causes less convolution in the contents of the grain. An isolated grain with a longer duration sounds more like when a woodblock is tapped on. Different durations become much more significant when there is more than one grain involved, as discussed later.

2.2.2 Amplitude

The amplitude refers to the peak amplitude. It determines the height of the envelope. This parameter may be applied directly by specifying the amplitude, or it may be implied indirectly by the other parameters. It is also influenced by the contents of the grain. The amplitude of the envelope dictates whether the amplitude of the contents is able to reach full capacity or not. The envelope amplitude as such, can be worked out as a percentage, 100% being loudest.

2.2.3 Attack time

The attack time determines how quickly the envelope reaches full amplitude. The attack is a necessary rounding tool. If a sound is immediately played at full amplitude, then there will be a loud crackling noise at the beginning. This occurs when a sound changes instantly from silence to full amplitude and causes extra distortion, which is usually not the desired result. Any sharp angles in the envelope will cause strong side bands in the spectrum. The side band is caused by convolution between the envelope shape and the contents of the envelope. Having no envelope is just like a vertical attack gradient, with essentially no duration. As stated previously there is a relationship between time and frequency. With an extremely short, or zero attack time, the resultant frequency would contain an extremely large bandwidth, resulting in the audible clicking noise. If the clicking sound is undesired then a significant attack time must be applied.
2.2.4 Sustain time
The sustain time determines how long the envelop stays at peak amplitude. This factor is also very important in determining the signal that is produced. For example if the grain has a long duration, but the sustain is only very short, with a very steep attack and release, such as with exponential curves, then it would treat the grain as if it had a short duration resulting in a buzzing or clicking sound in isolated instances. Using a shorter sustain is better suited to percussive type sounds. If the grain has a long sustain, the content becomes more significant allowing the content to play a more active role in the sonic effect. The sustain is not usually set as a parameter, it is normally a consequence of the overall duration of the envelope less the attack and decay time.


2.2.5 Decay time

The decay time determines how quickly the envelope drops back to zero amplitude. Different gradients of release will produce faster or slower cut off times. The release does not have to be at the same rate as the attack. When working with synchronous granular synthesis a different release time to the attack time will impose a different spectrum on the resultant sound. This is caused by convolution between the envelope and the contents. In an isolated grain the effect is not really noticeable. In order to percieve the resultant sound it must be repeated at a continuous rate, which is why it has less effect on asynchronous granular synthesis.

1In the earlier experiments of Dennis Gabor, he used a small glass window with the sides slightly shaded. The window was used in a film soundtrack (which is visual) where light passes through the soundtrack and is then converted to sound. See Chapter 3.5.2 for more information. (Gabor 1946: 446)
2See appendix 2 for more information about convolution.
3Gabor was the first to start experimenting and he came to this conclusion which has been reinforced by each successive exploration into granular synthesis. This will be dealt with in later chapters, starting with chapter 3.