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Increased national and international travel over the lastdecades has caused an increase in the global number ofpassengers using different means of transportation. Greateffort is being directed to improving the noisy environment inthe residential community. This is to face the growing strictnoise requirements which are implemented by international noiseregulatory authorities, governments, and local airports. Thereis also a strong competition between different manufacturers tomake their products quieter. The propulsion system in anaircraft is the major source of noise during relevant flightconditions. The engine noise in a vehicle dominates the totalradiated noise at low speeds especially inside cities. Manyrecent studies on noise reduction involve the use of perforatedplates in the air and gas flow ducting connected to the engine.This thesis deals with the modelling of perforates as anabsorbent. There are many difficulties in using liners in theseapplications. The most important is that there is no largesurface area to which the linings may be applied. Equally, theenvironment in which linings have to survive is hostile.Therefore, liners have to be carefully tailored in order toachieve the most efficient attenuation. The full-scalesimulation testing, which is usually necessary to define thenoise attenuation produced by a liner installation, is bothtime-consuming and expensive. Therefore, a need for accuratemodels is a must. This thesis fills some gaps in the impedancemodelling of perforated liners. It also concentrates on thosecomplicated situations of sound propagation in ducts that weresolved earlier using Finite Element Methods. Alternateanalytical solutions to these problems are developed here,which gives more physical insight into the results. The key design parameter of perforates is the acousticimpedance. The impedance is what determines their efficiency toabsorb sound waves. A semi empirical impedance model wasdeveloped to be capable of accurately predicting the linerimpedance as a function of its physical properties and thesurrounding conditions. It was compared to all previous modelsin the literature. Nothing in the literature has been reportedon the effect of temperature on the perforate impedance,therefore a complete study was performed. A new inverseanalytical impedance measurement technique was proposed. It isbased on educing the impedance value based on the measurementof the attenuation across a lined duct section. Twoapplications were further considered: The effect of hard stripsin lined ducts on there attenuation properties; and themodelling of perforations in a complicated automotive mufflersystem. Keywords:Perforates–Liners–Acousticimpedance–Hot stream liners–Hard splices–Mufflers–Lined ducts–Collocation–Flowduct.

The currently low requirements on sound emissions for industrial trucks are expected to be raised in the near future. The company Kalmar Industries AB, which develop, produce and market industrial trucks, want to improve their truck silencers as a precaution to the future harder restrictions and also to improve their competitiveness. The sound emission produced by a vehicle depends on type and range of application it is designed for but the dominant part of the sound is usually produced by the engine and silencer. A new measuring method is developed for studying sound emanating through the silencer system. The analysis of the measurement data establishes that the silencers are not working well. The simulations made with SIDLAB, which is a computer programme for calculating the propagation of sound in ducts, confirms that the silencers are inefficient and that they are simply too small. A simulation which implements a parallel resonator in the main silencer shows that it is possible to make great improvements in reducing the noise from the truck as well as meeting requirements on space. Mathematical modelling based on linearity and one-dimensional interaction between the silencer elementsis advantageous and gives very good results when understanding, analysing and simulating the silencer. The simulation tool SIDLAB works well and saves a lot of time by its fast modelling and easy interface.

Noise between 25-500 Hz is a common problem in Heating, Ventilating, and Air Conditioning (HVAC) systems. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Handbook lists values of end reflection loss (ERL), a frequency dependent parameter describing energy reflected back up a duct at a termination impedance, to help engineers design and account for noise. The ASHRAE Handbook does not account for common termination variations and only lists ERL values using octave bands down to 63 Hz. This thesis experimentally determined the ERL of a variety of rectangular duct configurations and termination conditions between 25-500 Hz. This research also compared experimental ERL results with analytic predictions and ASHRAE Handbook values. Seven duct sizes were tested, from 6X6 to 18X54 inches. Duct termination baffle hardness was varied between acoustically hard (plywood) and soft (ceiling tiles) for the 6X6, 6X10, and 6X18 ducts. Five duct termination distances above the termination baffle were tested, between flush and 1D for the 6X10 and 6X18 ducts and between flush and 5D for the 6X6 duct, where D equals the duct s effective diameter. Diffusers and flex duct configurations were installed at the end of the rigid duct to test their effect on ERL on the 6X6, 6X10, and 6X18 ducts. ERL was determined using an adaptation of the ASTM E1050 Standard, an application of the two-microphone impedance tube method. Experimental results closely conformed to analytic predictions and are an improvement over ASHRAE Handbook ERL values. The results indicate that baffle hardness has a negligible impact on ERL, which contradicts the ASHRAE assumption that diffusers that terminate in a suspended lay-in acoustic ceiling can be treated as terminating in free space. Termination distance above the baffle has a negligible impact on ERL at distances less than six inches for the 6X6 duct. Termination distances above the baffle greater than six inches exhibit limited free space ERL behavior for the 6X6 duct. The use of flex duct greatly reduces low frequency ERL and this is not accounted for by the ASHRAE Handbook. The impact from flex duct usage also negates any influence from downstream termination variations.

<p>Increased national and international travel over the lastdecades has caused an increase in the global number ofpassengers using different means of transportation. Greateffort is being directed to improving the noisy environment inthe residential community. This is to face the growing strictnoise requirements which are implemented by international noiseregulatory authorities, governments, and local airports. Thereis also a strong competition between different manufacturers tomake their products quieter. The propulsion system in anaircraft is the major source of noise during relevant flightconditions. The engine noise in a vehicle dominates the totalradiated noise at low speeds especially inside cities. Manyrecent studies on noise reduction involve the use of perforatedplates in the air and gas flow ducting connected to the engine.This thesis deals with the modelling of perforates as anabsorbent.</p><p>There are many difficulties in using liners in theseapplications. The most important is that there is no largesurface area to which the linings may be applied. Equally, theenvironment in which linings have to survive is hostile.Therefore, liners have to be carefully tailored in order toachieve the most efficient attenuation. The full-scalesimulation testing, which is usually necessary to define thenoise attenuation produced by a liner installation, is bothtime-consuming and expensive. Therefore, a need for accuratemodels is a must. This thesis fills some gaps in the impedancemodelling of perforated liners. It also concentrates on thosecomplicated situations of sound propagation in ducts that weresolved earlier using Finite Element Methods. Alternateanalytical solutions to these problems are developed here,which gives more physical insight into the results.</p><p>The key design parameter of perforates is the acousticimpedance. The impedance is what determines their efficiency toabsorb sound waves. A semi empirical impedance model wasdeveloped to be capable of accurately predicting the linerimpedance as a function of its physical properties and thesurrounding conditions. It was compared to all previous modelsin the literature. Nothing in the literature has been reportedon the effect of temperature on the perforate impedance,therefore a complete study was performed. A new inverseanalytical impedance measurement technique was proposed. It isbased on educing the impedance value based on the measurementof the attenuation across a lined duct section. Twoapplications were further considered: The effect of hard stripsin lined ducts on there attenuation properties; and themodelling of perforations in a complicated automotive mufflersystem.</p><p><b>Keywords:</b>Perforates–Liners–Acousticimpedance–Hot stream liners–Hard splices–Mufflers–Lined ducts–Collocation–Flowduct.</p>

<p>The currently low requirements on sound emissions for industrial trucks are expected to be raised in the near future. The company Kalmar Industries AB, which develop, produce and market industrial trucks, want to improve their truck silencers as a precaution to the future harder restrictions and also to improve their competitiveness. The sound emission produced by a vehicle depends on type and range of application it is designed for but the dominant part of the sound is usually produced by the engine and silencer.</p><p>A new measuring method is developed for studying sound emanating through the silencer system. The analysis of the measurement data establishes that the silencers are not working well. The simulations made with SIDLAB, which is a computer programme for calculating the propagation of sound in ducts, confirms that the silencers are inefficient and that they are simply too small.</p><p>A simulation which implements a parallel resonator in the main silencer shows that it is possible to make great improvements in reducing the noise from the truck as well as meeting requirements on space.</p><p>Mathematical modelling based on linearity and one-dimensional interaction between the silencer elementsis advantageous and gives very good results when understanding, analysing and simulating the silencer. The simulation tool SIDLAB works well and saves a lot of time by its fast modelling and easy interface.</p>

A wooden, perforated, uniform cross-section duct was examined to determine the optimum levels of aperture ratio and fan speed with respect to uniformity of discharge. The optimum aperture ratio for the 8.54 m long duct was 1.0 with a uniformity coefficient of 90.28%. The fan speed had little effect on the uniformity of discharge. The friction factor was experimentally determined to be 0.048 for a non-perforated duct and this value was assumed to be the same for a perforated duct of similar construction. A kinetic energy correction factor was used to analyze the flow in the duct. Values for this correction factor were determined from experimental data. Values of the coefficient of discharge and the total duct energy were calculated. A mathematical model was proposed based on the conservation of momentum and the Bernoulli's equation. The model responded favourably and predicted the duct velocity nearly perfectly and slightly underestimated the total duct energy.

Air distribution ducts are used in the environmental control of livestock and poultry building as well as the conditioning of most agricultural produce. / In order to simplify the approach to the design of ventilation ducts, a mathematical equation has been derived to describe the average air velocity of a duct. / The primary objective of the research work was to test goodness of fit of an equation describing the average air velocity of perforated ventilation ducts, under balanced as well as unbalanced air distribution: $V = H sb{o}{X over L} + (V sb{L}-H sb{o}) {X sp2 over L sp2}$. / This equation was successfully tested using data measured from 14 ducts of constant cross-sectional area, built of wood or polyethylene with outlets of various shapes and aperture ratios. Results indicated that aperture ratio and distance along the duct are the two most significant factors influencing the average duct air velocity values, but material and outlet shape had little effect.

Includes bibliographical references (leaves 287-305) / 310 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Mechanical Engineering, 1991

Thesis (Ph. D.)--University of Washington, 1997. / Vita. Includes bibliographical references (leaves [92]-106).