4. Envelope

The loudness of a sound changes over the time. There are two kinds of musical instruments: percussive and sustaining instruments.

Single excited instruments like piano, guitar or drums rise their loudness nearly instantly from zero to the highest level and then fade out slowly during the decay time. This are percussion instruments.

Continuously excited instruments like bowed strings, horns or organs produce sustaining sounds with the duration of any time. It may take a little time until they reach the steady state of the resonator, but afterward they produce a relative stable level of sound. The only modulation can be achieved by the level of excitation (air pressure of a saxophone).

Synthesizers use ADSR and LFO generators to modulate the loudness of a sound. ADSR stands for Attack, Decay, Sustain and Release. The loudness rises during the attack-time (0-50 ms ), decreases during the decay-time (100-2000 ms ), until it stays constant at the sustain level (0-100 %). Finally, after the key has been released, it fades out in the release-time. Sometimes there is also a hold-time to increase the “pressure” of a percussion sound.

With a LFO (Low Frequency Oscillator , 1-16 Hz), you can create a periodic tremolo-effect. This LFO-modulation can also rise or decay with the time.

Short attack-times (0-3 ms ) sound hard, while long times are lazy. Short decay-times sound staccato, while long fade out soft and harmonic. High sustain-levels (100 %) are good for sustaining instruments, while low sustain-levels (even zero) are used for percussions. A LFO modulation is used for violins and rock organs. There is an endless number of combinations possible.