The process of constructing a complex sound using a series of fundamental frequencies (pure tones or sine waves). Each of the fundamental frequencies usually has its own amplitude envelope which allows independent control of each partial (harmonic). Pipe organs or Hammond organs are both instruments which are based on additive synthesis. Some modern synthesizers have employed additive synthesis techniques, but other techniques such as FM (see WFTD archive FM Synthesis) and physical modeling (see WFTD archive Physical Modeling Synthesis) have proven to be easier to develop and still very effective at producing a wide variety of sounds.