An equation which reveals the general relationship between the sensitivities of a wave digital filter and the associated reference filter is derived. With this result

the low sensitivity of wave digital filters

which was observed numerically and experimently

can be well explainted theoretically. The result also provides an efficient way of computing the sensitivity of a wave digital filter from that of the associated reference filter

and may be useful for optimization of wave digital filter hopefully. A numerical example for sensitivity computation is given to illustrate the use of the result.