In [218], first-order delay-line filters of the form
are proposed. Clearly . This form has the advantage that the dc gain is always for all (stable) values of . Therefore, we can set to give a desired reverberation time at dc, and not have to change it when is varied to modulate the high-frequency decay rate. As in the previous section, from Eq. (3.9), we obtain
A calculation given in [218] arrives at
where