Editing Additive synthesis (section)

===Time-dependent frequencies===
In the general case, the [[instantaneous frequency]] of a sinusoid is the [[derivative]] (with respect to time) of the argument of the sine or cosine function.  If this frequency is represented in [[hertz]], rather than in [[angular frequency]] form, then this derivative is divided by <math>2 \pi</math>.  This is the case whether the partial is harmonic or inharmonic and whether its frequency is constant or time-varying.

In the most general form, the frequency of each non-harmonic partial is a non-negative function of time, <math>f_k(t)</math>, yielding

{{NumBlk|:|<math>y(t) = \sum_{k=1}^{K} r_k(t) \cos\left(2 \pi \int_0^t f_k(u)\ du + \phi_k \right).</math>|{{EquationRef|3}}}}