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In this paper procedures for estimating the sound absorption coefficient when the specimen has inherently low absorption are discussed. Examples of this include the measurement of the absorption coefficient of pavements, closed cell foams and other barrier materials whose absorption coefficient is nevertheless required, and the measurement of sound absorption of muffler components such as perforates. The focus of the paper is on (1) obtaining an accurate phase correction and (2) proper correction for tube attenuation when using impedance tube methods. For the latter it is shown that the equations for tube attenuation correction in the standards underestimate the actual tube attenuation, leading to an overestimate of the measured absorption coefficient. This error could be critical, for example, when one is attempting to qualify a facility for the measurement of pass-by noise.

The sound attenuation performance of microperforated panels (MPP) with adjoining air cavity is demonstrated. First of all, simulated results are shown based upon Maa's work investigating the parameters which impact MPP performance [1]. It is shown that the most important parameter is the depth of the adjoining cavity. Following this, an experimental study was undertaken to compare the performance of an MPP to that of standard foam. Following this, two strategies to improve the MPP performance are implemented. These include partitioning the air cavity and having a cavity with varying depth. Both strategies show a marked improvement in MPP attenuation.

A process for predicting the transmission and insertion losses of multi-component exhaust systems is detailed in this paper. A two-tiered process incorporating boundary element analysis to evaluate multi-component systems is implemented. At the component level, the boundary element method is used to predict the transfer matrix for larger components where plane wave behavior is not expected within the component. The transfer matrix approach is then used to predict insertion loss for built-up systems with interconnecting duct or pipe work. This approach assumes plane wave behavior at the inlet and outlet of each component so it is limited to the low frequency regime. Results are compared with experimental results for HVAC systems.

This paper considers the use of partial enclosures and absorbing materials inside those enclosures to dissipate energy. Several experiments were conducted where various parameters of an enclosure were altered and the effect on the noise radiating through the opening was measured. From these results, the parameters that play the most important role in sound radiation through the opening of an enclosure were determined. The two-point method and decomposition theory were used to calculate the transmission loss, which was used as the primary variable to analyze the enclosure's performance; the transmission loss is shown to be a better variable than sound pressure or output sound power for this purpose. Numerical simulations were conducted using the indirect boundary element method, and the results were compared with experimental results.

An energy source simulation method (ESSM) has been developed to determine sound energy density. Using this approach, a specified intensity boundary condition on the surface of a vibrating body is approximated by superimposing energy density sources placed inside the body. The unknown strengths for these sources are then found by minimizing the error on the boundary, using a least squares technique. The superposition of these energy density sources should then approximate the sound radiating from the body. The approach was evaluated in two-dimensions for a circle, square, and a more general geometry. The ESSM proved an excellent tool for predicting the energy density provided that power radiated uniformly in all directions. However, the ESSM could not accurately predict the directional characteristics of the energy density field if the power radiated significantly higher from one side of an object than other sides.

This paper considers two questions: how does one know when a boundary element mesh is reliable, and what are the advantages and potential pitfalls of various methods for sound radiation prediction. To answer the first question, a mesh checking method is used. With this method velocity boundary conditions are calculated on the nodes of the mesh using a point source or sources placed inside the mesh. A boundary element program is then used to calculate the sound power due to these boundary conditions. The result is compared to the known sound power of the point source or sources. This method has been used to determine the maximum frequency of a mesh, how many CHIEF points to use, etc. The second question is answered by comparing the results of several numerical methods to experimental results for a running diesel engine. The methods examined include the direct and indirect boundary element methods and the Rayleigh integral.

The most common approach for measuring the transmission loss of a muffler is to determine the incident power by decomposition theory and the transmitted power by the plane wave approximation assuming an anechoic termination. Unfortunately, it is difficult to construct a fully anechoic termination. Thus, two alternative measurement approaches are considered, which do not require an anechoic termination: the two load method and the two-source method. Both methods are demonstrated on two muffler types: (1) a simple expansion chamber and (2) a double expansion chamber with an internal connecting tube. For both cases, the measured transmission losses were compared to those obtained from the boundary element method. The measured transmission losses compared well for both cases demonstrating that transmission losses can be determined reliably without an anechoic termination. It should be noted that the two-load method is the easier to employ for measuring transmission loss.

The two-source method was used to measure the bulk properties (complex characteristic impedance and complex wavenumber) of sound-absorbing materials, and results were compared to those obtained with the more commonly used two-cavity method. The results indicated that the two-source method is superior to the two-cavity method for materials having low absorption. Several applications using bulk properties are then presented. These include: (1) predicting the absorptive properties of an arbitrary thickness absorbing material or (2) layered material and (3) using bulk properties for a multi-domain boundary element analysis.

This paper revisits the popular Rayleigh integral approximation, and also considers a second approximation, the high frequency boundary element method which is similar to the Rayleigh integral. Both methods are approximations to the boundary integral equation, and can solve problems in a fraction of the time required by the conventional boundary element method. The development of both methods from the Helmholtz integral equation is demonstrated and the differences between the two methods are delineated. Both methods were compared on practical examples including a running engine, gearbox, and construction cab. It was concluded that both methods can reliably predict the sound power for many problems but are inaccurate for sound pressure computations.

This paper explores the use of inverse numerical acoustics to reconstruct the surface vibration of a noise source. Inverse numerical acoustics is mainly used for source identification. This approach uses the measured sound pressure at a set of field points and the Helmholtz integral equation to reconstruct the normal surface velocity. The number of sound pressure measurements is considerably less than the number of surface vibration nodes. A brief guideline on choosing the number and location of the field points to provide an acceptable reproduction of the surface vibration is presented. The effect of adding a few measured velocities to improve the accuracy will also be discussed. Other practical considerations such as the shape of the field point mesh and effect of experimental errors on reconstruction accuracy will be presented. Examples will include a diesel engine and a transmission housing.

Large reciprocating engines produce a tonal spectrum of sound radiating from their exhaust. Even after standard reactive mufflers and after-treatment devices are added, and the target A-weighted sound level has been achieved, very audible low frequency tones can remain, and their levels are sometimes even enhanced by the exhaust system, creating potential annoyance problems in neighboring communities. This paper describes a practical design approach to such a problem and demonstrates variation in critical system parameters that affect acoustical performance. These parameters include temperature, source impedance, end impedance, flow, and pipe lengths, which are explored through practical models. The results of field measurements before and after installation of a final design are included and demonstrate a significant reduction in the sound level at the frequencies of interest.

This paper explores the use of inverse boundary element method to identify aeroacoustic noise sources. In the proposed approach, sound pressure at a few locations out of the flow field is measured, followed by the reconstruction of acoustic particle velocity on the surface where the noise is generated. Using this reconstructed acoustic particle velocity, the acoustic response anywhere in the field, including in the flow field, can be predicted. This approach is advantageous since only a small number of measurement points are needed and can be done outside of the flow field, and a relatively fast computational time. As an example, a prediction of vortex shedding noise from a circular cylinder is presented.

Numerical acoustics has traditionally been relegated to a prediction only role. However, recent work has shown that numerical acoustics techniques can be used to diagnose noise problems. The starting point for these techniques is the acoustic transfer vector (ATV). First of all, ATV's can be used to conduct contribution analyses which can assess which parts of a machine are the predominant noise sources. As an example, the sound power contribution and radiation efficiency from parts of a running diesel engine are presented in this paper. Additionally, ATV's can be used to reliably reconstruct the vibration on a machine surface. This procedure, commonly called inverse numerical acoustics (INA), utilizes measured sound pressures along with ATV's to reconstruct the surface velocity. The procedure is demonstrated on an engine cover for which the reconstructed vibration had excellent agreement with experimental results.

Micro-perforated panels have tiny pores which attenuate sound based on the Helmholtz resonance principle. That being the case, an appropriate cavity depth should be chosen to fully capitalize on the attenuation potential of the panel. Generally, the panel's sound absorbing performance can be predicted by Maa's theory given information about the panel and the cavity depth. However, in some cases, one cannot use the theory to predict the panel's performance precisely, especially when the micro-perforate has varying diameters and/or irregular hole shapes. In these cases, the sound-absorbing performance of the micro-perforate is different from that of a uniform pore diameter perforate. This paper presents an alternative method to predict the micro-perforated panel's performance precisely. As a first step, the transfer impedance of the micro-perforate should be measured.