Every noise control project starts with the identification and ranking of the noise sources. Several

methods have been proposed for the purpose and have proved to be useful and widely utilized. In this

chapter, sound intensity measurement and directional measuring devices such as the mirror-

microphone system and microphone array are introduced and their principles and applications are

described. Other useful measurements, such as acoustic holography method [1,2] and spatial

transformation of sound fields [3], are described in the literature.