ABSTRACT
The process of propagation of sound within ducts is of practical concern in many areas of engineering acoustics and noise control, among which may be mentioned heating, ventilating and air conditioning (HVAC) systems, IC engine exhausts, gas turbine and turbo-jet intakes and exhausts, industrial pipework, wind tunnels, and fluid distribution networks. In ducts of small cross-sectional area, such as car exhaust pipes, sound propagates in the form of plane waves over most of the frequency range of practical interest, and theoretical and experimental techniques for the analysis of the behaviour of such systems have been under development for many years. 166 However, in larger systems, higher order (transverse) modes of propagation can also transport energy. Estimates of sound energy flux cannot then be made unless the duct is terminated artificially with an anechoic device, and then only by the employment of elaborate arrays of microphones, and complex signal processing techniques. Measurement of the axial component of sound intensity in such fields could, in principle, overcome this fundamental difficulty. We shall see, however, that formidable technical difficulties are created by the presence of mean fluid transport in a duct-difficulties which are currently the subject of basic research.
