Ralph T. Muehleisen

Measurements and empirical model of the acoustic properties of reticulated vitreous carbon

Reticulated vitreous carbon (RVC) is a highly porous, rigid, open cell carbon foam structure with a high melting point, good chemical inertness, and low bulk thermal conductivity. For the proper design of acoustic devices such as acoustic absorbers and thermoacoustic stacks and regenerators utilizing RVC, the acoustic properties of RVC must be known. From knowledge of the complex characteristic impedance and wave number most other acoustic properties can be computed. In this investigation, the four-microphone transfer matrix measurement method is used to measure the complex characteristic impedance and wave number for 60 to 300 pore-per-inch RVC foams with flow resistivities from 1759 to 10,782 Pa s m(-2) in the frequency range of 330 Hz-2 kHz. The data are found to be poorly predicted by the fibrous material empirical model developed by Delany and Bazley, the open cell plastic foam empirical model developed by Qunli, or the Johnson-Allard microstructural model. A new empirical power law model is developed and is shown to provide good predictions of the acoustic properties over the frequency range of measurement. Uncertainty estimates for the constants of the model are also computed.

Modal coupling in acoustic waveguides: planar discontinuities

Abstract A new derivation for a scattering matrix for reflection and transmission of higher order modes at the planar junction of two waveguides is presented. The derivation is extended to include finite junction wall impedance and offset waveguides. The resulting matrix equations are analyzed and the physical significance of the matrices is explained. As an example of the theory, analytical expressions for the coupling coefficients at a size change in a rectangular duct are developed and the resulting reflection and transmission coefficients are computed. The results should be of interest to the HVAC noise control community. The paper also shows the effects of modal truncation on the accuracy and convergence of the solution. It is shown that the proper selection of the ratio of the number of modes on either side of the discontinuity is related to the ratio of the characteristic sizes of the waveguides. Finally it is shown that at least one higher mode should be included for reasonable accuracy in the computation of plane wave reflection and transmission coefficients except at the very lowest frequencies.

Acoustics of green buildings

As the world realizes the considerable environmental impacts of building construction and operation, a strong movement to create more sustainable buildings, i.e., “green” buildings has developed. While energy and resource use of green buildings is reduced and some indoor environmental conditions are improved, the acoustics of green buildings are often ignored and are frequently worse than in conventional buildings. In this tutorial presentation, the sustainable building movement is reviewed, the major design differences between green and conventional buildings are explained, and the impact of green design on acoustics is explored. There will be animations and auralizations to help attendees better understand acoustic impacts of green building design choices. Finally, lest ye think all is bad, the presentation will discuss solutions to the green building acoustic problem and present some of the positive impacts of green buildings can have on acoustics.

Comparison of errors in the three- and four-microphone methods used in the measurement of the acoustic properties of porous materials

The three-microphone and four-microphone methods have been shown useful for measuring the characteristic impedance and propagating wavenumber of porous materials. In this paper the effects of uncertainties in the apparatus and pressure transfer function measurements are analyzed using Monte Carlo methods. It is shown that for materials with low flow resistivities the four-microphone method is appreciably more accurate than the three-microphone method at low frequencies. The dominant contributors to the overall error are uncertainty in the magnitude and phase of the transfer function measurement.

Using COMSOL multiphysics software to investigate advanced acoustic problems

Numerical simulations provide a valuable tool for students to investigate complicated behavior found in many applications of acoustics. Computational “experiments” can be conducted quickly for a large number of parameter values, enabling students to visualize abstract quantities and to grasp causal relationships not easily recognized when looking at equations. The COMSOL finite-element multiphysics software package provides an integrated workspace in which the user defines a problem, meshes the geometry, and plots the solution(s). A brief overview of COMSOL will be presented, along with three examples of how it can be used to model advanced acoustics problems often encountered by students. The example problems involve musical acoustics, fluid-loaded shell vibrations, and flow resistance in porous materials. Also discussed will be the educational benefit of examining how choices made setting up the model can affect the integrity of the solution.

Fundamental azimuthal modes of a constricted annular resonator: Theory and measurement

The fundamental azimuthal modes of a constricted annular resonator are investigated. It is found that a given mode of an unconstricted resonator splits into two separate modes in the constricted resonator. One mode is of a higher frequency and has a pressure antinode centered in the constricted region. The other mode is of a lower frequency and has a pressure node centered in the constricted region. The resonance frequency of the higher-frequency modes increases linearly with a decrease in the constricted to unconstricted area ratio, whereas the lower frequency drops nonlinearly. Measurements and theory match to within 0.5% when end corrections and thermo-viscous losses are included in the system model. It was found that end correction impedances derived by mode-matching techniques were the only ones accurate enough to match the measurements and computation to within the error bounds.

Simple model for temperature gradient formation in a short stack

There has been much analytical, numerical, and experimental work investigating steady‐state thermoacoustic phenomenon. However, much less work has been done on time‐dependent problems such as the evolution of the thermoacoustic temperature gradient in a short stack of plates. If the time scale of the acoustics is much shorter than that of the thermodynamics of the stack itself, the acoustics can be assumed to be in steady state while the temperature gradient in the stack is formed. With this assumption, the thermoacoustic heat flux between the stack plates will be of the form Q=(C1+C2∂T/∂x)p2. This gives rise to a temperature gradient that rises exponentially in time with a time constant proportional to the thermal conductivity of the stack and the acoustic pressure squared. There is an additional exponential temperature rise from viscous heating. The model is shown to match measured temperature gradients very well. [This work was supported by the Office of Naval Research and the American Association for ...

Enhanced nearfield acoustic holography for larger distances of reconstructions using fixed parameter Tikhonov regularization.

This paper evaluates the performance of various regularization parameter choice methods applied to different approaches of nearfield acoustic holography when a very nearfield measurement is not possible. For a fixed grid resolution, the larger the hologram distance, the larger the error in the naive nearfield acoustic holography reconstructions. These errors can be smoothed out by using an appropriate order of regularization. This study shows that by using a fixed/manual choice of regularization parameter, instead of automated parameter choice methods, reasonably accurate reconstructions can be obtained even when the hologram distance is 16 times larger than the grid resolution.

Steady state acoustic radiosity for the prediction of sound pressure levels in enclosures with diffuse surfaces

This paper presents a derivation for and shows practical application of the steady-state diffuse acoustic radiosity method to the prediction of steady state sound levels in rooms. The paper begins with a brief review of the general radiosity literature and a more thorough review of the radiosity method in acoustics and proceeds to derive the general steady state acoustic rendering integral equation applicable to radiative energy exchange enclosures with arbitrary surface reflection properties. The rendering equation is simplified assuming diffuse reflections and the enclosure surface is discretized to convert the integral equations into summations. A closed form matrix solution to the discrete equations and another matrix equation for computing mean square pressure at multiple locations from multiple sources are presented. The computer code used for further analysis is verified by comparison to a closed form solution for a special case. Convergence of the discretized solution is investigated and the method is shown to converge linearly as the patch size decreases. A practical implementation of the method for predicting of sound levels in a classroom is shown. Predictions using radiosity are compared with simple diffuse field theory and measurements of an actual room. The radiosity method is shown to match diffuse ray tracing to within a fraction of a dB and predictions with a discretization with patches as large as 2 m are shown to be within a fraction of a dB of the asymptotic values

Effects of common indoor air pollutants on the speed of sound

With growing concern for air quality in residential, commercial, and industrial settings, the need for inexpensive and reliable air quality sensors is growing. Since the speed of sound can be measured very precisely and reliably and the speed of sound of air is directly related to its chemical composition, measurement of the speed of sound can be used as a method to detect small changes in gas composition. The effects of small concentrations of fourteen common indoor air pollutants on the speed of sound of air are investigated. The deviations in the speed of sound from the pollutants at the maximum levels suggested by the World Health Organization (WHO), minimal risk levels developed by the Agency for Toxic Substances and Disease Registry (ATSDR), and the occupational safety levels developed by the National Institute for Occupational Safety and Health (NIOSH) are computed. The computations show that, at this time, a low cost acoustic resonant sensor is probably not sensitive enough to be used as an indoor...