The function of vibration dampers may seem self-explanatory but they may be desirable for a number of reasons. The first is to protect a delicate item from external vibrations, such as where a control unit is mounted directly on an electric motor. The second reason would be when the item itself gives rise to vibration and the use of dampers is necessary to effectively provide mechanical isolation. In the case of large items, some degree of movement is allowed and this is limited by the use of springs, but these have virtually no damping effect and hydraulic dampers are usually employed. This system can be seen with car suspension or on a washing machine drum. For smaller items, rubber bushes, feet or dampers are employed. Rubber is ideal for this purpose due to its natural hysteresis or damping qualities. Without the provision of this mechanical isolation, even a relatively low level of vibration can be transmitted into the rest of the structure, and especially in box-type cabinets, this can result in unacceptably high noise levels. Small vibration dampers are usually in the form of a rubber element separating mounting bolts, or vibration-damping feet may be used.