One of the first steps in the design of a system for noise reduction is to establish the acoustic criteria for the physical situation. This step is similar in principle to the determination of the failure mode in mechanical design. If rupture or breaking of the part constitutes failure, the ultimate strength of the material is used in the design of the part. The maximum stress to which the part is subjected is limited to a stress less than the ultimate strength of the material. There are different failure criteria for different cases. For example, if the part were subjected to dynamic loading (time-varying stress), the fatigue strength would be the material property that would be important in the design of the part. In some cases, excessive deflection may constitute failure, and stress is not involved in limiting the size of the part.