Duct Acoustics

Abstract

This chapter deals with sound propagation in ducts. It begins with hard and smooth ducts, in which viscous and thermal losses at the walls are taken into account; this is important in narrow ducts (capillaries). The remainder deals with lined ducts of different cross sections and different linings. Sometimes the duct is assumed to be infinitely long, sometimes it has a finite length, but is still long enough to neglect reflections from the duct exit at the duct entrance. This assumption makes the contents of this chapter different from the contents of the chapter “K. Acoustic Mufflers”, where the reflections at both ends of duct sections play a dominant role. A section at the end of this chapter will discuss the influence of flow on sound attenuation in lined ducts in an approximation which is precise enough for most technical applications. A more sophisticated discussion of the influence of flow will be given in the chapter “N. Flow Acoustics”.