Sound synthesis is the method by which base functions (sine wave, square wave, sawtooth, etc.) are defined and modulated to create new sounds. This is done entirely electronically, with no acoustic source. There are many popular methods by which sound is synthesized and each carry a characteristic feel and sound.
Components
Oscillators
These are the most basic forms of sound synthesis. The sine wave alone has been used in many popular basses, kicks, and sub-basses. These are all of the basic waveforms and often serve as building blocks for more complex sounds.
Filters
In the space of electronic signals, the time domain and the frequency domain are often discussed, modulated, and transitioned between. Filters often impose limits on the spectral content in the frequency domain. For example, a Low Pass Filter will allow frequencies below the designated corner frequency to “Pass” through unfettered. It, however, reduces the content of the higher frequencies above the corner frequency.
Envelopes
Envelopes define modulating behavior in the time domain. Thus, each envelope describes the behavior by which a single attribute (pitch, volume, etc.) change over time. This is most commonly defined in ADSR (Attack, Decay, Sustain, Release).
Effects
Effects are applied to the base sound to add complexity to the instrument. They come in many wildly differing forms such as Reverb, Delay, Chorus, Compression, etc.