The use of adaptive feed-forward controllers has proven to be a very successful
strategy for controlling noise and vibration in a variety of applications. One reason is that the
feed-forward controller is an open loop controller, which can be designed to cancel the
undesired noise in a position with any accuracy. However, the feed-forward controller
requires an input signal, called a reference signal, correlated to the noise source.
Consequently, a single reference controller can only reduce noise radiated from a single noise
source. In many applications, there is a need to attenuate noise produced by several noise
sources. In this paper, three different structures, single, modulating and individual controller,
for multiple input feed-forward controllers have been studied and some design aspects are
investigated.