Both of the transmission effects noted above may emanate either from building occupants or from building mechanical systems such as elevators, plumbing systems or heating, ventilating and air conditioning units. In some cases, it is merely necessary to specify the best available quieting technology in selecting such building hardware. In other cases, shock mounting of systems to control vibration may be in order. In the case of plumbing systems, there are specific protocols developed, especially for water supply lines, to create isolation clamping of pipes within building walls. In the case of central air systems, it is important to baffle any ducts that could transmit sound between different building areas.