Ebook: An Introduction to Acoustics

Eindhoven University of Technology, 2016 (Copyright 2004), 288 p.Acoustics was originally the study of small pressure waves in air which can be detected by the human ear: sound. The scope of acoustics has been extended to higher and lower frequencies: ultrasound and infrasound. Structural vibrations are now often included in acoustics. Also the perception of sound is an area of acoustical research. In our present introduction we will limit ourselves to the original definition and to the propagation in fluids like air and water. In such a case acoustics is a part of fluid dynamics.A major problem of fluid dynamics is that the equations of motion are non-linear. This implies that an exact general solution of these equations is not available. Acoustics is a first order approximation in which non-linear effects are neglected. In classical acoustics the generation of sound is considered to be a boundary condition problem. The sound generated by a loudspeaker or any unsteady movement of a solid boundary are examples of the sound generation mechanism in classical acoustics. In the present course we will also include some aero-acoustic processes of sound generation: heat transfer and turbulence. Turbulence is a chaotic motion dominated by non-linear convective forces. An accurate deterministic description of turbulent flows is not available. The key of the famous Lighthill theory of sound generation by turbulence is the use of an integral equation which is much more suitable to introducing approximations than a differential equation. We therefore discuss in some detail the use of Green’s functions to derive integral equations.Next to Lighthill’s approach which leads to order of magnitude estimate of sound production by complex flows we also describe briefly the theory of vortex sound which can be used when a simple deterministic description is available for a flow at low Mach numbers (for velocities small compared
to the speed of sound).In contrast to most textbooks we have put more emphasis on duct acoustics, both in relation to its generation by pipe flows, and with respect to more advanced theory on modal expansions and approximation methods. This is particular choice is motivated by industrial applications like aircraft engines
and gas transport systems.This course is inspired by the book of Dowling and Ffowcs Williams: “Sound and Sources of Sound”. We also used the lecture notes of the course on aero- and hydroacoustics given by Crighton, Dowling, Ffowcs Williams, Heckl and Leppington.