ABSTRACT
All sensors, whether active or passive, perform their function by detecting (and in most cases quantifying) the change in some specific property (or properties) of energy. Active sensors emit energy that travels away from the sensor and interacts with the object of interest, after which part of the energy is returned to the sensor. For passive sensors, the source of the monitored energy is the object itself and/or the surrounding environment. In the case of acoustical systems, it must be recognized that the medium of propagation can sometimes have significant influence, and such effects must be taken into account.
