Configuration of sensors and actuators for active noise control system is more difficult the more complex the system. There are several indicators for configuration of sensors and actuators, but they are fragmentary and the main indicators are different for each system or situation. Therefore, we would like to proposed a general index for configuration of sensors and actuators based on controllability, which is an important index for configuration of sensors and actuators basically. Most of the existing indicators are indicators that only consider control performance. However, from the viewpoint of optimal control, input energy is as important as control performance. In lumped parameter system, degree of controllability indicates minimum input energy for controllable system. However, in distributed parameter systems such as active noise control systems, there are not many completely controllable systems. So, in this paper concept of degree of controllability for approximately controllable systems.

active noise control,controllability,degree of controllability,input energy minimization