Flanging

Perhaps the simplest time-varying delay effect:

Used since the 1960s
Two identical tape machines mixed equally:

$\begin{figure}\centering \input fig/twotapemachines.pstex_t \\ {\LARGE } \end{figure}$
Touch the flange of one of the supply reels, slowing it
Touch the flange of the other supply reel to slow it
Repeat, creating an oscillating variable delay
Keep the maximum delay below 10 ms or so
Sound Examples