In designing a controller, one way to avoid an energy spillover is to use what is called the zero spillover scheme. However, practical limitations may make such a controller impossible to realize, and one will need to implement a relaxed version of this controller called a relaxed zero spillover controller (RZSC). Here, analytical and experimental investigations into a RZSC scheme are presented. This controller has been used for active structural acoustic control (ASAC) of sound transmission into an enclosure. Noise is transmitted through the flexible boundary of the enclosure, and piezoceramic patches, mounted on the flexible boundary, are used as actuators. Polyvinylidene fluoride sensors are used on the flexible boundary and condenser microphone sensors are used inside and outside the enclosure. The stability of the chosen RZSC scheme for a single input, single output system and the extension to multiple input, multiple output systems are discussed along with other issues.