CHAPTER 11 What is a Loudspeaker? Loudspeakers by Philip Newell and Keith Holland A BRIEF LOOK AT THE CONCEPT Before answering the question posed by the title of this chapter, perhaps we had better begin with the question "What is sound?" According to Fahy and Walker (1998) "sound may be defined as a time- varying disturbance of the density of a fluid from its equilibrium value, which is accompanied by a proportional local pressure, and is associated with small oscillatory movements of the fluid particles."