Introduction

Noise and vibration are often treated separately in the study of dynamics, and it is sometimes forgotten that the two are inter-related – i.e. they simply relate to the transfer of molecular motional energy in different media (generally fluids and solids respectively). It is the intention of this book to bring noise and vibration together within a single volume instead of treating each topic in isolation. Central to this is the concept of wave–mode duality; it is generally convenient for engineers to think of noise in terms of waves and to think of vibration in terms of modes. A fundamental understanding of noise, vibration and interactions between the two therefore requires one to be able to think in terms of waves and also in terms of modes of vibration.

This chapter reviews the fundamentals of vibrating mechanical systems with reference to both wave and mode concepts since the dynamics of mechanical vibrations can be studied in terms of either. Vibration deals (as does noise) with the oscillatory behaviour of bodies. For this oscillatory motion to exist, a body must possess inertia and elasticity. Inertia permits an element within the body to transfer momentum to adjacent elements and is related to density. Elasticity is the property that exerts a force on a displaced element, tending to return it to its equilibrium position. (Noise therefore relates to oscillatory motion in fluids whilst vibration relates to oscillatory motion in solids.)