# Acoustics for Engineers, Troy Lectures, 2e

** Acoustics for Engineers, Troy Lectures, 2nd Edition**Jens Blauert and Ning Xiang

Springer-Verlag, Berlin/Heidelberg, (2009)

253 pp., hardbound, 89.95 USD

ISBN: 978-3-642-03392-6

**Purpose**

This book concentrates on the classical aspects of engineering acoustics, with emphasis on physical acoustics and electroacoustics. According to the authors, it provides suitable material for an introductory course in engineering acoustics for students with knowledge in basic mathematics and is based on extensive teaching experience at the university level. The book may be sufﬁcient as the sole textbook for the subject of engineering acoustics; however, the authors recommend the accompanying guidance of an academic teacher.

Although the authors claim that this is an introductory course in engineering acoustics for students with basic knowledge in mathematics, this reviewer feels that the material and level of mathematics involved would be more suited to a third-year engineering student or graduate engineering student—one who has completed the requisite engineering calculus, physics, and mechanics coursework.

Each chapter deals with a well-deﬁned topic and represents the material for a two-hour lecture. The chapters alternate between more theoretical and more application oriented concepts.

**General Comments**

The 1st edition title is assumed to be *Acoustics for Engineers*. This 2nd edition has corrected a number of typos and ﬁgures, and notations and equations have been edited to increase clarity of presentation. Additionally, a collection of problems that was absent in the 1st edition has been included. However, solutions to the problems are not provided; instead, the authors include an Internet link and state that solutions will be provided on a peer-to-peer basis.

As part of this review, this Internet link was used in an attempt to ﬁnd solutions to each problem presented in the book. The website is not intuitive and it took considerable exploring to discover that one needed to click a “Log in” link in order to see the solutions, although no actual log-in or password was required. The solutions were laid out as a link for each chapter that opened a PDF containing the solution. Only the solutions to problems listed in Chapter 1 are provided on this website. The link for Chapter 2 is a broken link that returns a “ﬁle not found” error. The PDFs for Chapters 3–14 include only the text “Coming soon!” and “Solutions in German handwriting available upon request”. The solutions for Chapter 1 include questions and details that are worded sometimes quite differently than those presented in the book. In total, there are only six answers/solutions provided from a total of 58 questions (most of them multipart) presented in the book.

The author’s method of providing solutions to textbook questions is a puzzling choice. The best method would have been to include all solutions in an appendix within the text itself. The current method is wholly incomplete, confusing, and cumbersome.

Chapter 1, “Introduction,” provides a brief but detailed overview of the basics starting with terminology and quantities, moving through particle velocity and displacement, and ending with logarithmic frequency intervals and double-logarithmic plots. The chapter includes many relevant ﬁgures, tables, and formulas.

Chapter 2, “Mechanic and Acoustic Oscillations,” includes basic elements of linear, oscillating, and mechanic systems as well as acoustic systems, followed by free and forced oscillations of parallel mechanic oscillators. Discussion and mathematical details follow on energies and dissipation losses, and the chapter closes with a discussion and example of the Helmholtz Resonator.

Chapter 3, “Electromechanic and Electroacoustic Analogies,” deals with simple linear, time-invariant mechanic and acoustic networks and their electric analogies. The text includes descriptions and formulae for electromechanic analogies and the electroacoustic analogy, as well as levers and transformers. Very detailed diagrams are provided for the rules for deriving analogous electric circuits; schematics are provided for synopsis of electric analogies of simple oscillators; circuit ﬁdelity, impedance ﬁdelity, and duality are discussed; and, examples of mechanic and acoustic oscillators are provided.

Chapter 4, “Electromechanic and Electroacoustic Transduction,” presents the possibility of coupling electrical and mechanical domains, which results in a coupling of electric and mechanic energy and power. This topic is extremely important for modern acoustics and the authors restrict themselves to examples of practical importance. Included are sections on electromechanic couplers and two- or three-port elements, the carbon microphone—a controlled coupler, fundamental equations of electroacoustic transducers, reversibility, coupling of electroacoustic transducers to the sound ﬁeld, pressure and pressure-gradient receivers, further directional characteristics, and absolute calibration of transducers.

Chapter 5, “Magnetic-Field Transducers,” demonstrates that the force–law relationship between the mechanic force, *F*, and the coupled electric quantity, *I*, is either linear or quadratic, and provides methods of linearization in order to use quadratic force laws with linear transducers. Sections include the magnetodynamic transduction principle, magnetodynamic sound emitters and receivers, the electromagnetic transduction principle, electromagnetic sound emitters and receivers, the magnetostrictive transduction principle, and magnetostrictive sound transmitters and receivers.

Chapter 6, “Electric-Field Transducers,” concentrates on basic principles of electric-ﬁeld transducers and discusses some illustrative examples. Sections include the piezoelectric transduction principle, piezoelectric sound emitters and receivers, the electrostrictive transduction principle, electrostrictive sound emitters and receivers, the dielectric transduction principle, dielectric sound emitters and receivers, and further transducer and coupler principles.

Chapter 7, “The Wave Equation in Fluids,” focuses on waves—those processes that vary with both time and space. Unlike the preceding chapters dealing with vibrations, which are a function of time and can be expressed with common differential equations, this chapter uses partial differential equations to describe waves in ﬂuids. Sections include derivation of the one-dimensional wave equation, three-dimensional wave equation in Cartesian coordinates, solutions of the wave equation, ﬁeld impedance and power transport in plane waves, transmission-line equations and reﬂectance, and the acoustic measuring tube.

Chapter 8, “Horns and Stepped Ducts,” considers one-dimensional propagation in a tube where the diameter varies with *x*. Sections include Webster’s differential equation—the Horn Equation, discussions of conical and exponential horns, radiation impedances and sound radiation, steps in the area function, and ﬁnally, a discussion of dealing with stepped ducts by means of electric analogies.

Chapter 9, “Spherical Sound Sources and Line Arrays,” discusses the basic solutions of the wave equation in spherical coordinates. Comparisons are made between periodical time signals that can be decomposed into Fourier harmonics, with spherical sound waves that are decomposed into spherical harmonics. Sections include spherical sound sources of the 0th and 1st orders, higher-order spherical sound sources, line arrays of monopoles, analogy to Fourier transforms as used in signal theory, and directional equivalence of sound emitters and receivers.

Chapter 10, “Piston Membranes, Diffraction and Scattering,” focuses on line arrays from point sources where reﬂection and diffraction do not occur, which is a ﬂat membrane in an inﬁnitely extended, rigid plane bafﬂe. Sections include the Rayleigh integral, Fraunhofer’s approximation, the far and near ﬁelds of piston membranes, and general discussion on diffraction and scattering.

Chapter 11, “Dissipation, Reﬂection, Refraction, and Absorption,” discusses the fact that a lossless medium is an idealization, and dissipation occurs in sound waves due to sound propagation through real media. Topics of discussion include dissipation during sound propagation in air, sound propagation in porous media, reflection and refraction, wall impedance and degree of absorption, porous absorbers, and resonance absorbers.

Chapter 12, “Geometric Acoustics and Diffuse Sound Fields,” deals with sound ﬁelds inside rooms with complicated shapes, like concert halls or churches, using an approximate method called geometrical acoustics. Chapter sections include mirror sound sources and ray tracing, ﬂutter echoes, impulse responses of rectangular rooms, diffuse sound ﬁelds, reverberation time formulae, and application of diffuse sound ﬁelds. This chapter has many practical applications in building and room design and provides many useful diagrams, ﬁgures, and formulae for the construction of habitable spaces.

Chapter 13, “Isolation of Airand Structure-borne Sound,” discusses sound isolation and the conﬁnement of sound to a space in such a way that transmission to neighboring spaces is totally or partially prevented, and differentiates this from sound damping. Sections include sound in solids—structure-borne sound, radiation of airborne sound by bending waves, sound transmission loss of single-leaf and double-leaf walls, the weighted sound reduction index, isolation of vibrations, and isolation of ﬂoors with regard to impact sounds.

Chapter 14, “Noise Control—A Survey,” is a discussion of the nature of noise, measurement techniques, and method of noise abatement. Sections include origins of noise, radiation of noise, noise reduction as a system problem, and noise reduction at the source, along the propagation paths and at the receiver’s end.

Chapter 15, “Appendices,” includes expanded details on previous topics, additional formulae, and sample acoustic engineering problems. Sections include complex notation for sinusoidal signals, complex notation for power and intensity, supplementary textbooks for self-study, exercises, and letter symbols, notations and units.

**Summary and Recommendation**

Blauert and Xiang’s enlarged 2nd edition of *Acoustics for Engineers *provides comprehensive material for an introductory course in engineering acoustics for students with knowledge in engineering calculus, physics, and mechanics. The text is designed for extensive teaching at the university level under the guidance of an academic teacher. Chapters progress from linear mechanical and electromechanical sound propagation through air, to more complex wave acoustics in varying dimensional conﬁgurations, and provide very clear schematics, ﬁgures, formulae, and conversion equations. This book would be a welcome addition to an educator on the topic or to a practicing acoustic engineer. One downside to the text is the omission of solutions to the included exercises for each chapter.

Chuck H. Perala

Aviation and Aerospace Industry

Washington, DC, USA

cperala@yahoo.com