As mentioned in the introduction to this chapter, it takes many sinusoidal components to synthesize noise well (as many as 25 per critical band of hearing under certain conditions [#!Gerzso78!#]). When spectral peaks are that dense, they are no longer perceived individually, and it suffices to match only their statistics to a perceptually equivalent degree.
Sines+Noise (S+N) synthesis [#!SerraSmith!#] generalizes the sinusoidal signal models to include a filtered noise component, as depicted in Fig.10.7. In that figure, white noise is denoted by , and the slowly changing linear filter applied to the noise at time is denoted .
The time-varying spectrum of the signal is said to be made up of a deterministic component (the sinusoids) and a stochastic component (time-varying filtered noise) [#!SerraT!#,#!SerraSmith!#]:
Filtering white-noise to produce a desired timbre is an example of subtractive synthesis [#!Moorer77!#]. Thus, additive synthesis is nicely supplemented by subtractive synthesis as well.