The simplest form of sound synthesis is to combine two or more sine waves into a more complex wave. This process is called additive synthesis and can be used to create a wide variety of sounds by stacking layers of many individual sounds. The source of each wave can be modified independently. Earlier this was done by combining the sounds of several audio oscillators to create a new sound. For example, a triangular wave may be obtained from the assembly of many individual sine waves. A fundamental sine wave would be the loudest and additional sine waves would be added to build the overtone structure characteristic of a triangular wave.
Just as waveforms can be constructed by adding one sound to another, they can also be modified by eliminating certain components of the sound, such as overtones or the frequency of the fundamental tone. For this, sound filtering is used in a process called subtractive synthesis.