The Fourier transform has become one of the most analytical tools, which is used for the sound intensity analysis and acoustic measurement in many industrial areas. But this method has the limitation that sound sources to measure need to be stationary and it does not provide true characteristics of time varying signals. Instead of the Fourier transform, advanced analysis methods such as the short-time Fourier transform, the Wigner-Ville distribution and the wavelet transform were proposed. These instantaneous spectrum analysis methods can be applied for obtaining the instantaneous intensity of non-stationary signals. This book shows the basic concepts of the Fourier transform and other instantaneous spectrum analysis, and also gives the theory and applications of instantaneous intensity analysis methods including their advantages and disadvantages. The instantaneous structural intensity analysis and other acoustical analysis methods, which provide readers useful techniques for engineering acoustics, are also presented.