In muffler design, concentric-tube resonators are often used to provide high-frequency attentuation. These resonators are constructed by shaping a rigid shell around a length of perforated tube, forming an unpartitioned cavity. Often the entire length of tube is perforated. Because of the length of the assembly, it is not possible to use the simple Helmholtz resonator theory to predict the attentuation. Beginning with a one-dimensional control volume, a mathematical model was derived which accounted for mean flow in the tube and for the wave motion in the cavity and the coupling between cavity and tube via the impedance boundary of the perforate. For impedance in the linear regime, a closed-form solution for the resonator transmission loss was obtained. With this restriction and for the case of zero mean flow, excellent agreement was obtained with experimental results. Limited parametric studies suggest that the performance of this type resonator is quite sensitive to the porosity of the perforate, and that unusually large bandwidths of attenuation may be obtained for select geometries at zero or near-zero mean flow conditions. As mean flow is increased, however, the model suggest that such large bandwidths may not be attainable in practice.
The U.S. Army has been using the day-night average sound level (DNL) to manage the community noise impact from heavy weapons noise since the late 1970's. In this case, the DNL is C-weighted as recommended by the National Academy of Sciences-National Research Council Committee on Hearing, Bioacoustics and Biomechanics (CHABA). CHABA's recommendation was justified by the fact that C-weighting (originally developed for the loudness of intense sounds) measures lower frequency sound energy in large guns which is otherwise missed by A-weighting. When the CHABA methodology was first adopted, there was relatively little night firing, but, today, night vision technology makes firing during darkness an absolute necessity for military readiness. Recognizing that the 10-dB penalty incorporated in the DNL methodology was not intended to predict sleep disturbance and that sleep disturbance may be a function of discrete noise event levels rather than annualaverage noise levels, the U. S. Army Engineer Research and Development Center initiated a project to measure sleep disturbance among people living near tank gunnery ranges. A first step in this project was to evaluate whether a commonlyused instrument for measuring sleep disturbance, the actimeter, would be sensitive to awakenings from blast noise. After preliminary screening of three designs of actimeter, the preferred design was tested in cooperation with the Army Center for Health Promotion and Preventive Medicine with subjects sleeping inside the Army Research Laboratory's Hostile EnvironmentSimulator. Subjects were exposed to nighttime blasts at two linear peak sound pressure levels (110 dB and 120 dB). The results confirmed that the preferred design would be a reliable and rugged instrument for the actual field study of awakening from live fire. This article is a government work and as such, is in the public domain and not subject to copyright. © 2008 Institute of Noise Control Engineering.
Statistical energy analysis is used to study the transmission of random-incidence sound waves through two independent panels separated by an air space. The analytical model consists of five linearly coupled oscillators arranged, room-panel-cavity-panel-room. Both nonresonant and resonant transmission for the panels are included. The cavity is considered to behave as a resonant system and its modal density and loss factor are determined, analytically. Absorption material is placed around the edges of the cavity. The sound energy transmitted is found to be strongly dependent upon the radiation resistance of the panels, the panel spacing, and the panel and cavity loss factors. Agreement between the theoretical results and experiments for several different double panel systems is found to be good.
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