Any noise problem may be described in terms of a source, a transmission path and a
receiver (in this context, a worker) and noise control may take the form of altering any one or
all of these elements. The noise source is where the vibratory mechanical energy originates, as
a result of a physical phenomenon, such as mechanical shock, impacts, friction or turbulent
airflow. With regard to the noise produced by a particular machine or process, experience
strongly suggests that when control takes the form of understanding the noise-producing
mechanism and changing it to produce a quieter process, as opposed to the use of a barrier for
control of the transmission path, the unit cost per decibel reduction is of the order of one tenth
of the latter cost. Clearly, the best controls are those implemented in the original design. It has
also been found that when noise control is considered in the initial design of a new machine,
advantages manifest themselves resulting in a better machine overall. These unexpected
advantages then provide the economic incentive for implementation, and noise control becomes
an incidental benefit. Unfortunately, in most industries, occupational hygienists are seldom in
the position of being able to make fundamental design changes to noisy equipment. They must
often make do with what they are supplied, and learn to use effective "add-on" noise control
technology, which generally involves either modification of the transmission path or the
receiver, and sometimes the source.