ABSTRACT
Chapter 4 deals with passive control. In the traditional view, passive control involves the use of a sound-absorbing material whose parameters are selected according to frequency. In general, suitably thick and thus heavy materials are required for low frequencies. The authors have shown that by attaching appropriately optimized point masses and ribs to the casing walls, it is possible to fully shape the frequency response of the structure and thus provide high isolation for noise dominant frequencies. Examples of different quality functions are used to show how different functionalities can be achieved with this method.
An extension of this concept is the use of composite materials with variable properties along their cross section, which are known as functionally graded materials. Such an appropriately designed composite has more favourable sound insulation properties.
A particular type of passive reduction is the use of shunt circuits that collect energy from the surface of a vibrating plate, convert it into electrical energy and then dissipate it. The design of such a circuit is shown in detail.
