Just as the oscillator will flow to the filter, so the filter flows to the amplifier section. (See diagram on the left.) This is often intrinsically linked with an element described as an envelope. These are incredibly useful and used to apply what is known as modulation to an element of your synth sound. (There’s more on modulation in the box on the previous page.) In the most common scenario, you may see the letters ADSR (Attack, Decay, Sustain, Release) residing in the amplifier section, which makes perfect sense, as they control the volume (or amplitude) of your sound as it is played.
Each of the ADSR phases occurs, one after another. The attack phase dictates how long it will take for your sound to increase to its maximum volume. This is then immediately followed by the decay phase, which is how long it takes for your sound to fade to nothing, while the synth key is being held. The decay phase can be interrupted by the sustain level; this is the only phase that applies a level, as opposed to a timing. The higher the sustain level, the louder the sound’s sustain is, interrupting the decay phase in mid-flow. Only once you release your keyboard/note, will the release phase begin, specifying the amount of time it takes for your sound to fade to nothing.