Additive synthesis is a method for generating musical tones/notes with mixing multiple sine-waves with different frequency together and therefore achieving the sound of a triangle,square and saw wave. The spectrum shows the different frequencies used for the sound, including their amplitude. Subtractive synthesis is basically just taking the frequencies and the amplitude and change it. That‘s what the ADSR-envelope-shaper does(It controlls the Amplitude. Attack,Decay,Sustain and Release). Frequencies can be cut off by different types of filters, which, as the name says already, filter frequencies at a specific point.