The first-order case is very simple while enabling separate control of low-frequency and high-frequency reverberation times. For simplicity, let's specify and , denoting the desired decay-time at dc ( ) and half the sampling rate ( ). Then we have determined the coefficients of a one-pole filter:
The dc gain of this filter is , while the gain at is . From Eq. (3.9) (or Eq. (3.8)), we obtain two equations in two unknowns:
where denotes the th delay-line length in seconds. These two equations are readily solved to yield
The truncated series approximation
has been found to work well in practical FDN reverberators.