Engineering Acoustics and Its Control






Course Contents:

Introduction to wave phenomenon, Acoustic variables, Sound speed, Concepts of wavefronts, Progressive and standing waves, Noise control strategies: Sound source modification, Control of transmission path, modification of receiver path, Airborne and Structure-borne noise, 1-D Acoustic wave equation, Helmholtz equation, Boundary conditions, Resonance frequencies of a closed/open tube, Sound Pressure Level, Sound Intensity Level and Sound Power Level, A-weighting, Simple (monopole), dipoles, lateral and longitudinal quadrupole sources, Directivity, Near- and far-field, Piston in a baffle, Reflection and transmission of normally incident wavefronts, Outdoor sound propagation: Analysis of barriers, Wave propagation in waveguides: Muffler analysis, General noise control methods, Noise estimates for typical engineering applications

Lecturewise breakup: (number of 50 minutes lectures indicated in brackets)

Introduction to wave phenomenon:

Forward and backward propagating pulse, Sound or wave speed


Development of one-dimensional acoustic wave equation:

Derivation of continuity and momentum equation based on control volume approach, Isentropic (State) relationship, Acoustic state variables: perturbation pressure, density and velocity, Planar wavefronts in an infinite waveguide, D-Alembert’s solution, Helmholtz equation (frequency-domain), Backward and Forward propagating waves, Boundary conditions: rigid, open and impedance conditions, Resonance frequencies of an open or a closed duct, 1- D acoustic waves in an infinite duct with a uniform moving medium


Acoustic Terminology (Measurement Units):

Acoustic pressure, velocity, density (revise), Impedance, Acoustic power expression, Sound Pressure Level (SPL), Sound Intensity Level, Sound Power Level, Spectrum and Frequency bands, Z-, A- and Cweighting


Noise Control Strategy (Introduciton concepts):

Sound source modification, Control of transmission path, Modification of the receiver path, Airborne and Structure-borne noise


Reflection and Transmission of Normally Incident Plane waves:

Reflection and Transmission coefficients, Reflection of plane wave at infinite plane boundary, special cases of rigid-wall (termination), pressure release, matched-impedance conditions, general resistive or impedance termination, Reflection at finite interface: change in cross-sectional area, RayleighSommerfeld radiation formulation


Canonical Acoustic Sources:

Simple (monopole), dipoles, lateral and longitudinal quadrupole, Array of N sources, continuous line array of sources, near- and far-field spectrum, Source directivity, Real-world examples (Approximations) for outdoor sound propagation such as barriers, Piston in an infinite baffle and related cases


Wave propagation in waveguides:

Waveguides of rectangular, circular cylindrical geometry: Solution of Helmholtz equation in Cartesian and cylindrical polar co-ordinates, normal (rigid-wall) modes and natural frequencies, cut-on and cut-off frequencies, evanescent modes, Gradually varying area ducts: Webster’s horn equation, Ducts with compliant walls


Mufflers (Silencers) for controlling emissions from Internal Combustion Engines:

What is it? Types (classification): Reactive and Dissipative Mufflers, Measures of Muffler performance, Lumped element analysis and Electro-acoustic analogies, Impedance at the duct termination, Endcorrection, Reactive mufflers (1-D or plane wave approach): Sudden-area discontinuity, Simple expansion chamber (SEC), Extended-inlet and extended-outlet (quarter-wave) resonator types, straightthrough silencers with perforated airway (pipes), plug-muffler, flow-reversal configurations, mufflers with multiple propagation paths, side-branch resonator, Aeroacoustic state variables and analysis, Perforated mufflers: Acoustic impedance of perforates, Concentric Tube Resonator, Cross-flow expansion and contraction elements, conical-concentric tube resonator (Matrizant analysis), Perforated elements with three-interacting ducts, Commercially used perforated duct mufflers, 3-D analytical modelling for muffler performance evaluation, Dissipative mufflers: Lined duct silencers, Introduction to numerical approaches in muffler modelling


Noise Control Strategies:

Control of noise at source: Select a quieter machine, lossy materials, quieter processes or tools, Reduce radiation efficiency, Maintenance and health-monitoring, Estimation and Control of Compressor, Fan and Blower noise, packaged chillers and cooling towers, pump noise and jet noise


Suggested Texts:

  • Fundamentals of Physical Acoustics, David T. Blackstock, John Wiley (2000)

  • Fundamentals of Acoustics, Lawrence E. Kinsler et al., John Wiley and Sons – can buy this one also (Indian edition available).

  • Engineering Noise Control, David A. Bies & Colin H. Hansen, CRC Press 2009

  • Engineering Principles of Acoustics, Noise and Vibration Control, Douglas D. Reynolds (1981), Allyn and Bacon

  • Theoretical Acoustics, Philip M. Morse and Uno Ingard, Princeton University Press

Reference Texts(Waveguide and Silencer analysis):

  • Acoustics of Ducts an Mufflers, M. L. Munjal, Wiley, 2nd Edition 2014

  • Acoustic Analysis and Design of Short Elliptical End-chamber Mufflers, A. Mimani, Springer 2021