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Acoustics of Ducts and Mufflers von Munjal, M. L. (eBook)

  • Erscheinungsdatum: 04.02.2014
  • Verlag: Wiley
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Acoustics of Ducts and Mufflers

Fully updated second edition of the premier reference book on muffler and lined duct acoustical performance Engine exhaust noise pollutes the street environment and ventilation fan noise enters dwellings along with fresh air. People have become conscious of their working environment. Governments of most countries have responded to popular demand with mandatory restrictions on sound emitted by automotive engines, and a thorough knowledge of acoustics of ducts and mufflers is needed for the design of efficient muffler configurations. This fully updated Second Edition of Acoustics of Ducts and Mufflers deals with propagation, reflection and dissipation/absorption of sound along ducts/pipes/tubes, area discontinuities, perforated elements and absorptive linings that constitute the present-day mufflers and silencers designed to control noise of exhaust and intake systems of automotive engines, diesel-generator sets, compressors and HVAC systems. It includes equations, figures, tables, references, and solved examples and unsolved exercises with answers, so it can be used as a text book as well as a reference book. It also offers a complete presentation and analysis of the major topics in sound suppression and noise control for the analysis and design of acoustical mufflers, air conditioning and ventilation duct work. Both the fundamentals and the latest technology are discussed, with an emphasis on applications. Deals with reactive mufflers, dissipative silencers, the frequency-domain approach, and the time-domain approach. Fully updated second edition of the premier reference book on muffler and lined duct acoustical performance, in one complete volume Presents original new research on topics including baffle silencers and louvers, 3D analytical techniques, and flow-acoustic analysis of multiply-connected perforated-element mufflers Includes a general design procedure to help muffler designers in the automotive industry, exhaust noise being a major component of automobile and traffic noise pollution Written by an expert with four decades' experience in teaching to graduate students, publishing extensively in reputed international journals, and consulting with industry for noise control as well as designing for quietness
Dr. M. L. Munjal is a Mechanical Engineer specializing in Engineering Acoustics. Over a four-decade career at the Indian Institute of Science, he has published over 190 papers in refereed journals, carried out over 100 industrial consultancy projects, and has run graduate courses on 'Industrial Noise Control' and 'Acoustics of Ducts and Mufflers'. He is Chairman of the National Committee for Noise Pollution Control that advises on noise control measures and legislations in India. He is also a Distinguished International Member of the Institute of Noise Control Engineering, USA, and Honorary Fellow of the International Institute of Acoustics and Vibration. He has been a member of the Editorial Board of several journals including the International Journal of Acoustics and Vibration .

Produktinformationen

    Format: PDF
    Kopierschutz: AdobeDRM
    Seitenzahl: 417
    Erscheinungsdatum: 04.02.2014
    Sprache: Englisch
    ISBN: 9781118443088
    Verlag: Wiley
    Größe: 8015 kBytes
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Acoustics of Ducts and Mufflers

1

Propagation of Waves in Ducts

Exhaust noise of internal combustion engines is known to be the biggest pollutant of the present-day urban environment. Fortunately, however, this noise can be reduced sufficiently (to the level of the noise from other automotive sources, or even lower) by means of a well-designed muffler (also called a silencer). Mufflers are conventionally classified as dissipative or reflective, depending on whether the acoustic energy is dissipated into heat or is reflected back by area discontinuities.

However, no practical muffler or silencer is completely reactive or completely dissipative. Every muffler contains some elements with impedance mismatch and some with acoustic dissipation. In fact, combination mufflers are getting increasingly popular with designers.

Dissipative mufflers consist of ducts lined on the inside with an acoustically absorptive material. When used on an engine, such mufflers lose their performance with time because the acoustic lining gets clogged with unburnt carbon particles or undergoes thermal cracking. Recently, however, better fibrous materials such as sintered metal composites have been developed that resist clogging and thermal cracking and are not so costly. Besides, long strand unglued glass fibers can stand high temperatures. Nevertheless, no such problems are encountered in ventilation ducts, which conduct clean and cool air. The fan noise that would propagate through these ducts can well be reduced during propagation if the walls of the conducting duct are acoustically treated. For these reasons the use of dissipative mufflers is much more common in air-conditioning systems.

Reflective mufflers, being nondissipative, are also called reactive mufflers. A reflective muffler consists of a number of tubular elements of different transverse dimensions joined together so as to cause, at every junction, impedance mismatch and hence reflection of a substantial part of the incident acoustic energy back to the source. Most of the mufflers currently used on internal combustion engines, where the exhaust mass flux varies strongly, though periodically, with time, are of the reflective or reactive type. In fact, even the muffler of an air-conditioning system is generally provided with a couple of reflective elements at one or both ends of the acoustically dissipative duct.

Clearly, a tube or pipe or duct is the most basic and essential element of either type of muffler. A study of the propagation of waves in ducts is therefore central to the analysis of a muffler for its acoustic performance (transmission characteristics). This chapter is devoted to the derivation and solution of equations for plane waves and three-dimensional waves along rectangular ducts, circular tubes and elliptical shells without and with mean flow, without and with viscous friction, with rigid unlined walls and compliant or acoustically lined walls. We start with the simplest case and move gradually to the more general and involved cases.
1.1 Plane Waves in an Inviscid Stationary Medium

In the ideal case of a rigid-walled tube with sufficiently small cross dimensions filled with a stationary ideal (nonviscous) fluid, small-amplitude waves travel as plane waves. The acoustic pressure perturbation (on the ambient static pressure) p and particle velocity u at all points of a cross-section are the same. The wave front or phase surface, defined as a surface at all points of which p and u have the same amplitude and phase, is a plane normal to the direction of wave propagation, which in the case of a tube is the longitudinal axis.

The basic linearized equations for the case are:

Mass continuity

(1.1)

Dynamical equilibrium

(1.2)

Energy equation (isentropicity)

(1.3)

wh

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