47,068 research outputs found

    Characterizing and Classifying Acoustical Ambient Sound Profiles

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    Acoustical modeling focuses on the sound pressures generated by the operation of some system of interest, the propagation of the sound through some medium (the atmosphere) and the sound pressure levels picked up by the receiver (listening device or a human). Ambient noise, or the background noise occurring naturally in an environment, affects the ability of the receiver to identify the sound from the system of interest. This work examines nominal logistic modeling of human performance data, methods to remove the time-dependency aspect of ambient sound profiles, and develops a method with which to classify sample ambient noise profiles against one of nine standard ambient noise profiles. The application of these methods to a notional scenario is provided

    Dispersion of sound in a combustion duct by fuel droplets and soot particles

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    Dispersion and attenuation of acoustic plane wave disturbances propagating in a ducted combustion system are studied. The dispersion and attenuation are caused by fuel droplet and soot emissions from a jet engine combustor. The attenuation and dispersion are due to heat transfer and mass transfer and viscous drag forces between the emissions and the ambient gas. Theoretical calculations show sound propagation at speeds below the isentropic speed of sound at low frequencies. Experimental results are in good agreement with the theory

    Defining true propagation patterns of underwater noise produced by stationary vessels

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    The study of underwater vessel noise over the past sixty years has predominantly focused upon the increase in ambient noise caused by the propulsion mechanisms of large commercial vessels. Studies have identified that the continuous rise of ambient noise levels in open waters is linked to the increase in size and strength of anthropogenic sound sources. Few studies have investigated the noise contribution of smaller vessels or ambient noise levels present in coastal and in-shore waters. This study aimed to identify the level of noise common to non-commercial harbors by studying the noise emissions of a diesel generator on board a 70m long sailing vessel. Propagation patterns revealed an unconventional shape (specific to the precise location of the noise source on board the vessel), unlike those of standard geometric spreading models, as typically assumed when predicting vessel noise emission. Harbor attributes (including water depth, ground sediment and structural material components) caused for altered level and frequency characteristics of the recorded underwater noise, and were correlated to the sound measurements made. The measurements (taken in eight harbors around Northern Europe) were statistically analyzed to identify the primary factors influencing near-field sound propagation around a stationary vessel

    Velocity relaxation of a particle in a confined compressible fluid

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    The velocity relaxation of an impulsively forced spherical particle in a fluid confined by two parallel plane walls is studied using a direct numerical simulation approach. During the relaxation process, the momentum of the particle is transmitted in the ambient fluid by viscous diffusion and sound wave propagation, and the fluid flow accompanied by each mechanism has a different character and affects the particle motion differently. Because of the bounding walls, viscous diffusion is hampered, and the accompanying shear flow is gradually diminished. However, the sound wave is repeatedly reflected and spreads diffusely. As a result, the particle motion is governed by the sound wave and backtracks differently in a bulk fluid. The time when the backtracking of the particle occurs changes non-monotonically with respect to the compressibility factor and is minimized at the characteristic compressibility factor. This factor depends on the wall spacing, and the dependence is different at small and large wall spacing regions based on the different mechanisms causing the backtracking.Comment: 8 pages, 9 figure

    Green's Function Applicable to Turbofan Exhaust Noise in Jets with an External Center-Body

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    The problem of propagation of sound across the shear layer in a turbofan jet exhaust with an external center-body is discussed. The wave equation of interest is the compressible Rayleigh equation. Two forms of the equation are considered, and the Green's function solutions, subject to appropriate surface conditions on the center-body and flight condition in the ambient, are presented. Directivity studies in a heated exhaust at temperature ratio of 2.0 and Mach number 0.90 indicate that a rigid center-body tends to increase the sound propagation at forward angles relative to an exhaust without a center-body, while application of suitable surface liner may significantly reduce this enhancement

    Transmission of light and audible sound in a synthetic fog medium

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    The primary goal of the thesis was to study the propagation of visible light and auditory sound through a synthetic fog medium compared to an ambient air environment. It is known that the fog substantially decreases the visibility however; this has not been studied quantitatively. Further information regarding other energies such as sound is also needed to understand how the energy reacts in the fog medium. The extent of visual and auditory degradation in humans needs to be investigated. Researchers have studied light transmitted through water, air; however, no one has studied how light or sound is transmitted through a synthetic fog medium. The first aspect of this thesis was to build the appropriate environment for the experiment, which used light sensors to detect the intensity of the light, and a sensitive microphone to detect the frequency of sound in an unknown environment. Lab-VIEW, a graphical programming language, was used to gather data for the sound experiment. Data were then analyzed by graphing the relationship of intensity of sound vs. distance vs. different production level of fog and frequency vs. distance vs. different production level of fog in the varying density of the synthetic fog medium. The data, which were collected from the light meter, in the fog medium, were then compared with the data collected in the room filled with ambient air. Similarly, the sound energy was detected using a microphone, in the synthetic fog medium, which was compared with the sound signal transmitted in an ambient air environment
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