Additive synthesis   

In additive synthesis pure tones are added together to create more complex composite timbres. 
  harmonic series sweep 
In theory a complex timbre which has been analysed into its sinusoidal components by means of some kind of Fourier transform can then be reconstructed by means of additive synthesis.  This form of synthesis allows control over the individual simple components by means of individual envelopes for amplitude and frequency. 
Additive synthesis has the advantage that the many micro-variations in the frequency and amplitude of individual partials which make natural sounds so rich and lively can be recreated.  The disadvantage with this form of synthesis is its inefficiency in that a great deal of data must be specified to define a sound of any complexity of detail. 
See Also:
FM synthesis 
granular synthesis
subtractive synthesis 
wavetable synthesis