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 , are described in the literature.