Abstract
The role of acoustics theory in Noise Control Research is reviewed. The classical sound theory of ∂2ρ/∂t2 − c2 ∂2ρ/∂xi2 = 0 which deals only with the wave propagation problem is compared with the unified acoustic theory derived by Lighthill, ∂2ρ/∂t2 − c2 ∂2ρ/∂xi2 = ∂Q/∂t − ∂Fi/∂xi + ∂2Tij/∂xi ∂xj, which deals with the sound propagation problems as well as the sound generation problems. With this unified theory, the problems of fan noise, axial compressor noise, jet noise, flow noise, and turbulent noise all become apparent. The unified theory clearly illustrates the limitation of the nearfield and farfield sound measurement that can be confidently performed. Examples of various types of noise sources and their radiation directivity will be discussed.
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